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description | This application is a continuation of and claims the benefit of priority to U.S. application Ser. No. 13/502,946, filed Apr. 19, 2012 (pending), which is a continuation of PCT Application PCT/EP2010/065707, filed Oct. 19, 2010, which claims priority to European Application EP09173989.6, filed Oct. 23, 2009. The contents of the above-referenced applications are expressly incorporated herein by reference in their entireties. The present invention relates to a charged particle therapy apparatus used for radiation therapy. More particularly, this invention relates to a rotatable gantry designed for receiving a charged particle beam in a direction substantially along a rotation axis of the gantry, for transporting and for delivering said beam to a target to be treated. Radiotherapy using charged particles (e.g. protons, carbon ions, . . . ) has proven to be a precise and conformal radiation therapy technique where a high dose to a target volume can be delivered while minimizing the dose to surrounding healthy tissues. In general, a particle therapy apparatus comprises an accelerator producing energetic charged particles, a beam transport system for guiding the particle beam to one or more treatment rooms and, for each treatment room, a particle beam delivery system. One can distinguish between two types of beam delivery systems, fixed beam delivery systems delivering the beam to the target from a fixed irradiation direction and rotating beam delivery systems capable of delivering beam to the target from multiple irradiation directions. Such a rotating beam delivery system is further named gantry. The target is generally positioned at a fixed position defined by the crossing of the rotation axis of the gantry and the central treatment beam axis. This crossing point is called isocenter and gantries of this type capable of delivering beams from various directions to the isocenter are called isocentric gantries. The gantry beam delivery system comprises devices for shaping the beam to match the target. There are two major techniques used in particle beam therapy to shape the beam: the more common passive scattering techniques and the more advanced dynamic radiation techniques. An example of a dynamic radiation technique is the so-called pencil beam scanning (PBS) technique. In PBS a narrow pencil beam is magnetically scanned across a plane orthogonal to the central beam axis. Lateral conformity in the target volume is obtained by adequate control of the scanning magnets. Depth conformity in the target volume is obtained by adequate control of the beam energy. In this way, a particle radiation dose can be delivered to the entire 3D target volume. The particle beam energies required to have sufficient penetration depth in the patient depend on the type of particles used. For example, for proton therapy, proton beam energies are typically ranging between 70 MeV and 250 MeV. For each required penetration depth the beam energy needs to be varied. The energy spread of the beam should be limited as this directly influences the so-called distal dose fall-off. However, not all accelerator types can vary the energy. For fixed energy accelerators (e.g. a fixed isochronous cyclotron) typically an energy selection system (ESS) is installed between the exit of the accelerator and the treatment room as shown in FIGS. 1, 2 and 3. Such an energy selection system is described by Jongen et al. in “The proton therapy system for the NPTC: equipment description and progress report”, Nuc. Instr. Meth. In Phys. Res. B 113 (1996) 522-525. The function of the Energy Selection System (ESS) is to transform the fixed energy beam extracted from the cyclotron (e.g. 230 MeV or 250 MeV for protons) into a beam having an energy variable between the cyclotron fixed energy down to a required minimum energy (for example 70 MeV for protons). The resulting beam must have a verified and controlled absolute energy, energy spread and emittance. The first element of the ESS is a carbon energy degrader which allows to degrade the energy by putting carbon elements of a given thickness across the beam line. Such an energy degrader is described in patent EP1145605. As a result of this energy degradation, there is an increase in emittance and energy spread of the beam. The degrader is followed by emittance slits to limit the beam emittance and by a momentum or energy analysing and selection device to restore (i.e. to limit) the energy spread in the beam. A layout of such a known energy selection system 10 is shown in FIG. 1 together with a stationary, fixed energy accelerator 40 (in this example a cyclotron). After the degrader and emittance limiting slits, the beam passes through a 120° achromatic bend made up of two groups of two 30° bends. To meet the specification for the distal fall off, the momentum spread or the energy spread in the beam is limited by a slit placed at the center of the bend. The beam is focused by quadrupoles before the bend and between the two groups of two 30° bending magnets so that the emittance width of the beam is small and the dispersion is large at the position of the slit. The entire beam line starting at the energy degrader 41 up to the treatment isocenter 50 forms an optical system that is achromatic, i.e. a beam-optical system which has imaging properties independent from momentum (dispersionless) and independent from its transverse position. The beam line can be divided in multiple sections and each section is forming itself an achromat. As shown in FIG. 2, the first section is the ESS 10 followed by an achromatic beamline section that brings the beam up to the entrance point of a treatment room. In the case of a gantry treatment room, this entrance point is the entrance point or coupling point of the rotating gantry 15. The gantry beam line is then forming a third achromatic beam line section. In the case of a single treatment room particle therapy configuration, as shown in FIG. 3, the beam line comprises two achromatic beam line sections: a first section is the ESS 10 that brings the beam up to the gantry entrance point and the second achromatic section corresponds to the rotating gantry 15 beam line. At the gantry entrance point, the beam must have the same emittance in X and Y in order to have a gantry beam optics solution that is independent from the gantry rotation angle. The X and Y axis are perpendicular to each other and to the central beam trajectory. The X axis is in the bending plane of the dipole magnets. A disadvantage of the use of such a degrader and energy analyser is that this device requires a relative large space area as shown in FIG. 1 and hence a large building footprint is required. The installation of an ESS results also in an extra equipment cost. The present invention aims to provide a solution to overcome at least partially the problems of the prior art. It is an objective of the present invention to provide a charged particle therapy apparatus that has a reduced size and that can be built at a reduced cost when compared to the prior art particle therapy apparatus. The present invention is set forth and characterized by the appended claims. In the prior art particle therapy configurations as shown for example in FIGS. 1 to 3, the functionalities of limiting the momentum spread (or energy spread, which is equivalent) and the emittance of the beam is performed by a separate device, namely with the energy selection system (ESS) 10, which is installed between the stationary accelerator 40 and the rotating gantry 15. As shown on FIG. 1, a first element of the ESS is an energy degrader 41 which is used to degrade the energy of the particle beam of the fixed-energy accelerator 40. With the present invention, a rotatable gantry beam delivery system is provided having a gantry beam line configuration which fulfills multiple functions: The known function of transporting, bending and shaping an entering particle beam in such a way that a particle treatment beam can be delivered at a gantry treatment isocenter for use in particle therapy; The additional function of limiting the energy spread of the entering particle beam to a selected maximum value. With the present invention, the ESS functionality of limiting the energy spread or momentum spread of the beam to a selected value is performed by the gantry system itself. Hence the size and cost of a particle therapy facility can be reduced. In the context of the present invention, the momentum spread is defined as the standard deviation of the momentums of the particles at a given location and is expressed as a percentage of the average momentum of all particles at this location. Whatever the location of the means for limiting the momentum spread in the gantry, these means are preferably designed for limiting said momentum spread to 10%, more preferably to 5%, and even more preferably to 1% of the average momentum of all particles. Preferably, the gantry also fulfills a second additional function of limiting the transverse beam emittance of the entering particle beam to a selected maximum value, which further reduces cost and size of the particle therapy facility. More preferably, the gantry according to the invention also comprises a collimator installed in-between the gantry entrance point and a first quadrupole magnet in the gantry. This collimator is used for reducing the emittance of the beam before the beam is arriving at the first magnet in the gantry beam line. In an alternative preferred embodiment, the above mentioned collimator is installed outside of the gantry, i.e. in-between the energy degrader and the entrance point of the gantry. According to the invention, a particle therapy apparatus is also provided comprising a stationary particle accelerator, an energy degrader and a rotatable gantry having means to limit the momentum spread of the beam. Preferably said gantry also comprises means to limit the emittance of the beam. Alternatively, a particle therapy apparatus is provided comprising a stationary particle accelerator, an energy degrader, a rotatable gantry comprising means to limit the momentum spread of the beam and a collimator installed in-between said energy degrader and said gantry for limiting the emittance of the beam. More preferably, said gantry comprises additional means to limit the emittance of the beam. The present invention will now be described in detail in relation to the appended drawings. However, it is evident that a person skilled in the art may conceive several equivalent embodiments or other ways of executing the present invention. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. A exemplary particle therapy configuration according to the invention is shown in FIG. 4. In this example, the rotatable gantry according to the invention is coupled with a stationary-, fixed energy-particle accelerator 40 to form a single room particle therapy apparatus 100. An example of a particle accelerator for protons is a superconducting synchrocyclotron which has a compact geometry (e.g. with an extraction radius of 1.2 m). The gantry according to the invention is installed in the gantry room and a shielding wall (e.g. a 1.7 m thick concrete wall) separates the gantry room from the accelerator room. An energy degrader 41 is installed between the accelerator 40 and a gantry entrance point 45 (coupling point). This energy degrader 41 is positioned within the accelerator room just in front of the shielding wall 52 separating the accelerator room from the gantry room. The gantry entrance point 45 is located after the degrader 41 and is an entrance window for the beam line of the gantry. This entrance window 45 is the first part of a gantry beam line section where the beam is entering the gantry in a direction substantially along the rotation axis of the gantry. The rotation axis of the gantry is indicated by a horizontal dash-dotted line passing through the isocentrer 50 and the entrance point 45. As shown in FIG. 4, there is no momentum or energy analyser device installed between the degrader and the gantry entrance point as is the case in the prior art systems (FIGS. 1 to 3). Similar as in the prior art configurations shown in FIGS. 1 to 3, there is a short beam line section between the exit of the accelerator and the degrader 41, where for example two quadrupole magnets 44 are installed for transporting and focussing the beam into a small spot (for example between 0.5 mm and 2 mm one sigma) at the energy degrader. The energy degrader 41 is for example a rapidly adjustable, servo controlled, rotating, variable thickness, cylinder of degrading material (as disclosed in EP1145605). The distance between the exit of the accelerator and the degrader can be about 2 m. Other types of energy degrading systems, e.g. lateral moving wedge shaped based degraders can be used as well. The energy degrader currently used by the applicant has at its entrance an integrated horizontal-vertical beam profile monitor which allows measurement of the size and position of the beam spot and, through a control system algorithm, means for automatic tuning of the up-stream beam optics. Hence, the beam at the degrader 41 can be well defined, for example, the beam is focused into a small waist with a half width not exceeding 2 mm in both planes. With these input beam conditions, the output emittance of the beam degraded in energy is dominated by multiple scattering in the degrader and is relatively independent from the input conditions. The resulting beam after energy degradation can be considered as a diverging beam from a virtual waist in X and Y at the degrader with a given size and divergence. The two orthogonal coordinate axis X and Y are perpendicular (transverse) to the central beam trajectory. The emittances in X and Y (also called “transverse emittances”) can be considered to be substantially identical at this point. The larger the energy reduction introduced by the degrader, the larger will be the transversal emittance in X and Y and the larger will be the momentum spread of the degraded beam. The embodiment of the invention is a gantry configuration comprising means 43 to limit the momentum spread of the incoming beam. A beam entering the gantry comprising particles having an average momentum value and a momentum spread. To limit the momentum spread of the incoming beam, a pair of momentum analysing slits 43 are installed in the gantry. These momentum analysing slits 43 are preferably located at a position along the beam path where the particles of the beam are dispersed according to their momentum. More preferably, these slits are installed at a position where the nominal dispersion is larger than the nominal beam size. The nominal dispersion is defined as a transversal displacement of a particle whose momentum differs by 1% (one percent) of an average momentum P of all particles of the beam. The nominal beam size is defined as the one sigma beam size value in X of a mono-energetic particle beam having the average momentum P. Suppose that the nominal dispersion is 2.5 cm: this means that a particle having a momentum P′=1.01·P will be displaced by 2.5 cm in X from a particle having momentum P. In this example, a particle having a momentum P′=0.99·P will also be displaced in X by 2.5 cm but having an X coordinate with an opposite sign. The momentum limiting slits can for example be installed at a position where the nominal beam size in X is between 0.2 cm and 1 cm and the nominal dispersion in X is between 1 cm and 3 cm. By opening or closing the slits, the maximum momentum spread that is required (selected) can be obtained. One can for example select to limit the maximum momentum spread to 0.5% of the average momentum by adjusting the slits correspondingly. If one wants to limit the maximum momentum spread to 0.4% of the average momentum, then one has to close the pair of momentum slits more. For this purpose a calibration curve can be established, defining the slit opening as function of the required momentum spread. In the configuration of FIG. 4, the nominal dispersion is large in comparison with the beam size at a position in-between gantry quadrupole magnet number seven and the second dipole magnet 48 and hence this is a preferred position to install the momentum spread limiting slits. These slits can for example be installed just before the second dipole magnet 48. The exact position can vary depending on the detailed gantry configuration. Instead of using a pair of slits as means for reducing the momentum spread of the beam, other means can be used as well. For example one can use apertures or collimators with various diameters which can be put in the beam line, preferably at the above discussed positions. In the example shown in FIG. 4, a gantry for delivering scanning beams at the treatment isocenter 50 is presented and the beam line of this gantry comprises three dipole magnets 47,48,49 and seven quadrupole magnets 44. In this gantry configuration, scanning magnets 46 are installed upstream of the last dipole magnet 49. Between the gantry entrance point 45 and the first dipole magnet and in between the first and second dipole magnet there are respectively, two and five quadrupole magnets. Preferably, in addition to the means 43 to limit the momentum spread of the beam, also means 42 to limit the transverse beam emittance can be installed in the gantry 15. For this purpose, two pairs of slits (in X and Y) limiting the beam divergence can for example be installed in-between the second quadrupole magnet and the first dipole magnet 47. Hence, by limiting the divergence of the beam, the transverse beam emittance, which is proportional to the beam divergence, is limited. The first two quadrupoles installed in the gantry in-between the entrance point 45 and the first dipole magnet 47 serve to focus the divergent beam, originating from the degrader, before the beam reaches the divergence limiting slits. To what extent the beam emittance needs to be reduced will depend on what the maximum emittance the gantry can accept to efficiently transport the beam and it will also depend on what the beam requirements are at the treatment isocenter (such as for example the beam size required at the treatment isocenter). Acceptable beam emittances and beam sizes may depend on the technique used for shaping the beam (e.g. pencil beam scanning or passive scattering). The example given in FIG. 4 is for a scanning beam delivery system. For a pencil beam proton scanning system the beam emittance can for example be limited to 7.5 Pi mm mrad in both X and Y. For practical beam tuning purposes, just in front, downstream, of the divergence limiting or emittance limiting slits, a beam profile monitor can be installed (not shown on FIG. 4). Instead of using a pair of slits in X and Y as means for reducing the divergence of the beam, other means can be used as well. For example one can use apertures or collimators with various diameters which can be put in the beam line. If the energy reduction of the beam is very large (e.g. reduction of 250 MeV protons down to 70 MeV), the emittance and divergence of the beam becomes very large and the diameter of the beam, just before the first quadrupole magnet in the gantry, can become larger than the diameter of the beam line pipe. For this purpose a collimator (not shown in FIG. 4) can furthermore be installed upstream of the first quadrupole magnet in the gantry 15 to cut off already a part of the beam. This collimator can be installed in the gantry 15 in-between the entrance point 45 and the first quadrupole magnet of the gantry. Alternatively, such a collimator can be installed outside the gantry, i.e. in-between the degrader and the entrance point 45 of the gantry 15. When such a collimator for limiting the emittance of the beam is installed in either of the two positions mentioned above, in an alternative gantry embodiment the means 42 for limiting the emittance can be omitted. When a particle beam hits divergence and/or momentum limiting slits, neutrons are produced. To limit the neutron radiation at the level of the treatment isocenter 50 where the patient is positioned, adequate shielding need to be provided. As neutrons are mainly emitted in the direction of the beam, one can install just after the first dipole magnet, across the axis of rotation of the gantry, a neutron shielding plug 51 to shield the neutrons produced on means to limit the emittance of the beam installed upstream of the first dipole magnet 47. As the neutrons are mainly emitted in the direction of the beam, neutrons produced at the momentum limiting slits 43 are not directing to the patient. Nevertheless, a local neutron shielding (not shown on FIG. 4) can be installed around the momentum limiting slits 43 in order to reduce overall neutron background radiation. In order not to overload FIG. 4, details of the mechanical construction of the gantry have been omitted on purpose. Examples of such mechanical elements not shown on FIG. 4 are: two spherical roller bearings for rotating the gantry by at least 180° around the patient, a gantry drive and braking system, a drum structure for supporting a cable spool, a counterweight needed to get the gantry balanced in rotation, etc. When designing a gantry for particle therapy, several beam optical conditions need to be fulfilled. At the gantry entrance point 45 the beam must have identical emittance parameters in X and Y in order to have a gantry beam optics solution that is independent from the gantry rotation angle. As discussed above, these conditions are naturally fulfilled when placing the energy degrader just in front of the gantry entrance point. In addition, the following beam optical conditions need to be met: 1. The gantry beam-optical system must be double achromatic, i.e. the beam imaging properties must be independent from momentum (dispersionless) and independent from position. 2. The maximum size of the beam (one sigma) inside the quadrupoles should preferably not exceed 2 cm in order to keep a reasonable transmission efficiency in the gantry.There is also a third condition that however can vary depending on the technique used for shaping the beam as discussed above. For a scanning system this third condition can be described as follows: 3. At the isocenter 50 the beam must have a small waist, of substantially identical size in X and Y.For a scattering system, required beam sizes can be specified more upstream of the isocenter (for example at the exit of the last bending magnet) and the acceptable beam sizes for scattering are in general larger than for scanning (for example 1 cm at the exit of the last bending magnet).In addition to these three conditions (1 to 3), new requirements are introduced resulting from the current invention: 4. At the position of the energy spread limiting slits 43, the nominal dispersion in X should preferably be large in comparison with the nominal beam size in X (for examples of values see discussion above).Preferably, a gantry according to the invention also comprises means to limit the emittance of the beam. This results in an additional requirement: 5. At the position of the emittance limiting slits 42, the beam must have beam optical parameters (size and divergence) in X and Y that allow to cut the divergence. This means for example that the beam must have a reasonable size (e.g. 0.5 cm to 2 cm, one sigma). The gantry configuration shown in FIG. 4 is based on a beam optical study performed with the beam optics “TRANSPORT” code (PSI Graphic Transport Framework by U. Rohrer based on a CERN-SLAC-FERMILAB version by K. L. Brown et al.). The beam envelopes in X and Y in the gantry beam line for an entering proton beam of 170 MeV are shown in FIG. 5 as an example. The beam envelopes are plotted for the X direction and Y direction in the lower panel and upper panel, respectively. In this example the emittance of the final beam is 12.5 Pi mm mrad. This corresponds to a situation where the divergence of the incoming beam has been limited to 6 mrad in X and Y. The beam transported through the system can then be considered as a beam starting at the degrader with a small beam spot of 1.25 mm and a divergence of 6 mrad. With this beam optics a beam size at the treatment isocenter of 3.2 mm (one sigma value) is obtained which is an adequate value for performing pencil beam scanning. The positions of the quadrupole magnets and dipole magnets are shown on FIG. 5. The transversal positions of the dipole magnets (the vertical gaps) are not shown on scale in this figure and the purpose is only to indicate their position along the central trajectory. Especially the gap in X an Y of the last bending magnet 49 are much larger than on the scale of FIG. 5 as a large opening is needed because the scanning magnets are positioned upstream of this dipole magnet and a large scanning area need to be covered at isocenter. The position of the scanning magnets along the beam path is indicated by a vertical line. The dotted line represents the nominal dispersion in X of the beam. As shown, just before the second dipole magnet 48 a large nominal dispersion value is obtained and this is the position where the momentum limiting slits 43 are preferably installed. The position along the central beam trajectory of the momentum limiting slits 43 is indicated by a vertical line on FIG. 5. The nominal beam size in X at the momentum limiting slits is about 0.23 cm while the nominal dispersion in X at this position is about 2.56 cm, hence obtaining a good momentum separation of the incoming beam. Preferably, also divergence limiting slits 42 are used. A good position for these slits 42 is indicated on FIG. 5 by a vertical line. At this position, the beam size in X and Y is about 1.8 cm and 0.6 cm, respectively. This beam optical solution presented fulfills the conditions of a double achromat. In the example shown in FIG. 4 and FIG. 5, a three dipole gantry configuration was used with dipole bending angles of respectively 36°, 66° and 60°. However, the invention is not limited to a specific gantry configuration for what concerns number of dipoles or bending angles of the dipoles. The invention is neither limited to the number of quadrupole magnets and the relative positions of the quadrupoles with respect to the dipole magnets. As a second example, the invention has been applied to a conical two dipole large throw gantry. This corresponds to the gantry configuration shown on FIG. 2 and FIG. 3. These large throw gantries have been built by the applicant and are discussed by Pavlovic in “Beam-optics study of the gantry beam delivery system for light-ion cancer therapy”, Nucl. Instr. Meth. In Phys. Res. A 399(1997) on page 440. In these gantries a first 45° dipole magnet bends the beam away from the axis of rotation of the gantry and the beam then further follows a second straight beam line section before entering the second 135° dipole magnet which is bending and directing the beam essentially perpendicular to the axis of rotation. The straight beam line section between the gantry entrance point and the first 45° dipole magnet comprises, in the original gantry design, four quadrupole magnets (FIG. 2 is a configuration having only two quadrupole magnets installed in this beam line section), and the second straight section between the first and second dipole magnet comprises five quadrupole magnets. With this gantry the distance between the exit of the last bending magnet and the treatment isocenter is 3 m and the beam shaping elements configured in a so-called nozzle are installed upstream of the last bending magnet. This nozzle uses either the passive scattering technique or the scanning technique for shaping the beam conform the treatment target. The scanning magnets are part of the nozzle and are hence installed downstream of the last gantry dipole magnet. A beam optical analysis has been performed for this two dipole gantry configuration. The same conditions and requirements as discussed above have been respected. The resulting beam envelopes in this gantry are shown in FIG. 6 for a proton beam of 160 MeV. The beam envelopes are plotted for the X direction and Y direction in the lower panel and upper panel, respectively. The positions along the central beam path of the 45° dipole magnet 67, the 135° dipole magnet 68 and the various quadrupole magnets 44 are indicated in FIG. 6. Also here the energy degrader is installed just before the entrance window of the gantry and, as an example, in this calculation the divergence was cut at 8 mrad and the emittance of the final beam is 10 Pi mm mrad both in X and Y. The beam envelope as shown in FIG. 6 starts at the gantry entrance window and the beam has a size of 1.25 mm (one sigma value). In this gantry configuration the first straight section between the entrance window and the first 45° gantry bending magnet 67, comprises four quadrupole magnets 44. Divergence limiting devices 42 are installed in between the second and third quadrupole magnet and are indicated by a vertical line on FIG. 6. The momentum spread limiting slits 43 are installed at a position where the nominal dispersion in X is large compared to the nominal beam size. The dotted line on FIG. 6 represents the nominal dispersion in X of the beam. The position of the momentum spread limiting slits 43 are indicated by a vertical line on FIG. 6. At this position the nominal dispersion is about 2.6 cm in X and the nominal beam size in X (one sigma value) is about 0.6 cm which is adequate for analysing the incoming beam according to momentum and limiting the momentum spread to a given value by setting the slits at the corresponding position. The beam envelope shown in FIG. 6 is a tuning solution for a nozzle using the scanning technique (the scanning magnets are installed downstream of the 135° dipole magnet but are not shown on FIG. 6). This gantry configuration used in this beam optics study also comprises two quadrupole magnets installed upstream of the 135° last dipole magnet 68 as indicated on FIG. 6. With this tuning solution, a double waist in X and Y is obtained at isocenter having a beam size of 4 mm (one sigma value), which is suitable for performing pencil beam scanning. This beam optical solution fulfills the conditions of a double achromat. A particle therapy apparatus 100 can be formed by combining a stationary, fixed energy particle accelerator, an energy degrader and a rotatable gantry according to the invention, i.e. a rotatable gantry comprising means for limiting the energy spread or momentum spread of the beam and preferably also comprising means for limiting the emittance of the beam. As shown on FIG. 4, which is an example of a proton therapy apparatus, a compact geometry can be obtained and the building footprint that is needed to install this apparatus is smaller than with a separate energy selection system. Although the embodiments described are focussing on proton gantries, the invention is not limited to proton gantries. The person skilled in the art can easily apply the elements of the invention, i.e. means for analysing the beam (limiting the emittance and limiting the energy spread), to gantries for use with any type of charged particles such as e.g. a gantry for carbon ions or other light ions. Gantries for particle therapy have been designed since many years and, in combination with stationary, fixed energy particle accelerators, a separate energy selection system was always installed in the beam line between the accelerator and the gantry. According to the present invention a new gantry configuration is provided comprising means for limiting the energy spread or momentum spread of the beam and preferably also comprising means for limiting the emittance of the beam. Hence the gantry itself comprises functionalities of the standard prior art energy selection system. By designing a gantry with these means to analyse the beam as described, a more compact particle therapy apparatus can be built. |
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abstract | A grating with a large aspect ratio is disclosed, in particular to be used as an X-ray optical grating in a CT system and in particular produced by a lithography method. In at least one embodiment, the grating includes a multiplicity of recurring alternating grating webs and grating gaps with a height, and a multiplicity of filler beams, respectively arranged in the grating gaps with a spacing from one another in the direction of the gaps, which beams connect respectively adjacent grating webs over their height. In at least one embodiment, the grating webs and the grating gaps run from a first to a second side of the grating, and a filler beam has a width in the direction of the gaps and this width is at most 10% of the spacing between two adjacent filler beams. In at least one embodiment, the spacings between respective adjacent filler beams in a grating gap do not vary by more than 10% in the entire grating. At least one embodiment of the invention furthermore relates to a CT system containing at least one grating according to at least one embodiment of the invention. |
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056446144 | claims | 1. A pre-patient collimator for controlling the shape of a collimated fan beam for use in a computed tomography system, the computed tomography system including a detector array having a plurality of rectangular shaped detector cells and an x-ray source, said collimator comprising x-ray absorbing material having an aperture therein for restricting the collimated fan beam, said aperture contour providing that a fan beam umbra of a beam passing therethrough has a substantially rectangular cross sectional shape. 2. A pre-patient collimator in accordance with claim 1 wherein the system is configured to scan an object, and wherein the dimensions of said contoured aperture may change contour. 3. A pre-patient collimator in accordance with claim 1 wherein said aperture is contoured with a fixed linear ramp. 4. A pre-patient collimator in accordance with claim 3 wherein said fixed linear ramp has a ramp slope of 0.2 mm per 100 mm. 5. A pre-patient collimator in accordance with claim 1 wherein the detector array comprises at least one generally rectangular detector element, and wherein said aperture is contoured so that a fan beam umbra of a beam passing therethrough is substantially fitted to the rectangular detector element. 6. A pre-patient collimator in accordance with claim 1 wherein said collimator is a double cam collimator comprising a first cam and a second cam, at least said first cams being movable relative to said second cam. 7. A pre-patient collimator in accordance with claim 1 wherein said collimator aperture can be adjusted as a function of a focal spot size of the x-ray source. 8. A pre-patient collimator in accordance with claim 1 wherein the system is configured to reconstruct slices of varying configurations, and wherein said aperture is contoured as a function of the slice configuration. 9. A pre-patient collimator for controlling the shape of a collimated fan beam for use in a computed tomography system, the computed tomography system including a detector array and an x-ray source, said collimator comprising x-ray absorbing material having an aperture therein for restricting the collimated fan beam, said aperture having a contour substantially according to: EQU c(.alpha.)=(Z-f)s(.alpha.)/d(.alpha.)+f .alpha.=fan beam angle; Z=position of beam on detector; f=position of focal spot in z axis; c=position of collimation point in z axis; d=source to detector distance; and s=source to collimation distance. .alpha.=fan beam angle; Z=position of beam on detector; f=position of focal spot in z axis; c=position of collimation point in z axis; d=source to detector distance; and s=source to collimation distance. 10. In an imaging system including an x-ray source and a detector array having a plurality of rectangular shaped x-ray detector cells, a collimator comprising x-ray absorbing material having an aperture therein for restricting the collimated fan beam, said aperture having a contour to form a generally rectangular collimated fan beam and said aperture contour providing that a fan beam umbra of a beam passing therethrough is substantially fitted to the rectangular detector cells. 11. A collimator in accordance with claim 10 wherein said aperture is contoured according to: EQU c(.alpha.)=(Z-f)s(.alpha.)/d(.alpha.)+f 12. A collimator in accordance with claim 10 wherein said aperture is contoured with a fixed linear ramp. 13. A collimator in accordance with claim 12 wherein said fixed linear ramp has a ramp slope of 0.2 mm per 100 mm. 14. A collimator in accordance with claim 10 wherein the system is configured to scan an object, and wherein the dimensions of said contoured aperture are adjustable. 15. A collimator in accordance with claim 10 wherein said collimator is a cam collimator. 16. A collimator in accordance with claim 10 wherein said collimator aperture can be adjusted as a function of a focal spot size of the x-ray source. 17. A collimator in accordance with claim 10 wherein the system is configured to reconstruct slices of varying configurations, and wherein said aperture is contoured as a function of the slice configuration. |
claims | 1. A method of preparing high-specific-activity 195m Pt comprising the steps of: a. exposing 193 Ir to a flux of neutrons sufficient to convert a portion of said 193 Ir to 195m Pt to form an irradiated material; b. dissolving said irradiated material to form an intermediate solution comprising Ir and Pt; and c. separating said Pt from said Ir by cation exchange chromatography to produce a product comprising 195m Pt. 2. A method in accordance with claim 1 wherein said dissolving step is carried out at a temperature of at least 210xc2x0 C. claim 1 3. A method in accordance with claim 2 wherein said dissolving step is carried out at a temperature of at least 217xc2x0 C. claim 2 4. A method in accordance with claim 1 wherein said intermediate solution further comprises aqua regia. claim 1 5. A method in accordance with claim 1 wherein said separating step further comprises the steps of: claim 1 a. loading said intermediate solution onto a cation exchange column; b. eluting said Pt with a first eluent solution comprising HCl and thiourea. c. eluting said Pt with an essentially thiourea-free second eluent solution comprising HCl. 6. A method in accordance with claim 1 wherein said 195m Pt product is characterized by a specific activity of at least 30 mCi/mg Pt. claim 1 7. A method in accordance with claim 6 wherein said 195m Pt product is further characterized by a specific activity of at least 50 mCi/mg Pt. claim 6 8. A method in accordance with claim 7 wherein said 195m Pt product is further characterized by a specific activity of at least 70 mCi/mg Pt. claim 7 9. A method in accordance with claim 8 wherein said 195m Pt product is further characterized by a specific activity of at least 90 mCi/mg Pt. claim 8 |
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claims | 1. A mixed-layered bismuth oxy-iodine material, comprising a chemical assemblage at an atomistic scale of at least two different Bi—O—I lattice phases. 2. The material of claim 1, wherein the at least two different Bi—O—I lattice phases comprise BiOI and Bi507I lattice phases. 3. The material of claim 2, wherein the relative proportions of the BiOI and Bi5O7I lattice phases is 85-80 mole % BiOI and 15-20 mole % Bi507I. 4. The material of claim 1, wherein the mixed-layered bismuth oxy-iodine material has an elemental composition comprising 17-22 wt % iodine, with the balance comprising primarily bismuth and oxygen. 5. The material of claim 1, wherein the mixed-layered bismuth oxy-iodine material 20 has a bismuth-to-iodine (Bi:I) molar ratio of between 1.64 and 2.13. 6. The material of claim 1, wherein the mixed-layered bismuth oxy-iodine material has a iodine solubility equal to or less than 15 ppb, after exposing the material for 3 days to deionized water at a temperature of 25° C. or lower. 7. The material of claim 1, wherein the mixed-layered bismuth oxy-iodine material has a iodine solubility equal to or less than 100 ppb, after exposing the material for 3 days to deionized water at a temperature of 94° C. or lower. 8. The material of claim 1, wherein the mixed-layered bismuth oxy-iodine material has a iodine solubility equal to or less than 100 ppm, after exposing the material for 3 days to deionized water plus 0.005 molar sodium sulfate at a temperature of 25° C. or lower. 9. The material of claim 1, wherein the mixed-layered bismuth oxy-iodine material has a iodine solubility equal to or less than 1000 ppm, after exposing the material for 3 days to deionized water plus 0.005 molar bicarbonate at a temperature of 25° C. or lower. 10. The material of claim 1, wherein the mixed-layered bismuth oxy-iodine material has a iodine solubility equal to or less than about 1 ppm, after exposing the material for 3 days to deionized water plus 0.005 molar chloride at a temperature of 25° C. or lower. 11. The material of claim 1, wherein the iodine in the mixed-layered bismuth oxy-iodine material comprises radioactive 129I. |
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abstract | A thin GaAs Substrate can be provided with a copper back-metal layer to allow the GaAs Substrate to be packaged using conventional plastic packaging technologies. By providing the GaAs Substrate with a copper back-metal layer, the GaAs Substrate can be made thinner than 2 mils (about 50 microns), thereby reducing heat dissipation problems and allowing the semiconductor die to be compatible with soft-solder technologies. By enabling the semiconductor die to be packaged in a plastic package substantial cost savings can be achieved. |
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048511870 | summary | The invention relates to a nuclear reactor fuel assembly including a fuel assembly box having sides, an upper end and corners, a head plate disposed in the fuel assembly box, fuel rods containing nuclear fuel being disposed in the fuel assembly box and guided in leadthroughs formed in the head plate, at least some of the fuel rods being secured to the head plate in the leadthroughs, a corner bolt standing on top of the head plate, a cross bar disposed inside one of the corners at the upper end of the fuel assembly box on the corner bolt, an angle element having an outer surface and being adapted to the fuel assembly box, two leaf springs each being disposed on the outer surface of the angle element at a respective one of the sides of the fuel assembly box and extending in longitudinal direction of the fuel assembly box, a screw bolt firmly screwed to the fuel assembly box and the angle element at the corner bolt, the screw bolt having an expansion shaft with a reduced diameter, a bolt head having two ends and being disposed on top of and supported on the angle element, a constriction disposed between the ends of the bolt head defining a coaxial bolt head shaft with a reduced diameter, the constriction being disposed in a bore formed in the angle element between the ends of the bolt head, and a transverse pin being disposed in the bore and engaging the constriction. A nuclear reactor fuel assembly of this type is known from European Patent Application No. 0 142 778, corresponding to U.S. application Ser. No. 120,725, filed Nov. 13, 1987. In this known fuel assembly, the transverse pin, which engages the constriction location between two ends of the bolt head, is secured at both ends on the angle element, and acts as a retainer in order to associate the screw bolt with the angle element in captive fashion. The screw bolt will break only at the expansion shaft and nowhere else whenever expansion forces in the screw bolt occurring in the loosening direction become too great, such as when the fuel assembly is used in a boiling water reactor. Meanwhile the constriction location of the bolt head, or in other words the bolt head shaft having a reduced diameter, is not stressed at all by these expansion forces. The transverse pin secured with both ends to the angle element thus prevents the fragments of the screw bolt from being able to become detached from the fuel assembly, when the expansion shaft breaks. Nevertheless, the screw bolt is rotatable about the longitudinal axis thereof. There is no provision, however, for replacement of the screw bolt, such as after the fuel assembly has been placed in the nuclear reactor. It is accordingly an object of the invention to provide a nuclear reactor fuel assembly, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which permits easy replacement of the screw bolt. With the foregoing and other objects in view there is provided, in accordance with the invention, a nuclear reactor fuel assembly, comprising a fuel assembly box having sides, an upper end and corners; a head plate disposed in the fuel assembly box; fuel rods containing nuclear fuel being disposed in the fuel assembly box and guided in leadthroughs formed in the head plate, at least some of the fuel rods being secured to the head plate in the leadthroughs; a corner bolt standing on top of the head plate; a cross bar disposed inside one of the corners at the upper end of the fuel assembly box on the corner bolt; an angle element having an outer surface and being adapted to the fuel assembly box; two leaf springs each being disposed on the outer surface of the angle element at a respective one of the sides of the fuel assembly box and extending in longitudinal direction of the fuel assembly box; a screw bolt firmly screwed to the fuel assembly box and the angle element at the corner bolt, the screw bolt having an expansion shaft with a reduced diameter, and a bolt head having two ends and being disposed on top of and supported on the angle element, the bolt head having an outer surface with an annular recess formed therein between the ends of the bolt head defining a coaxial bolt head shaft with a reduced diameter, the coaxial bolt head shaft being disposed in a bore formed in the angle element between the ends of the bolt head; and a transverse pin being disposed in the bore and having one end protruding into the annular recess in the bolt head. The transverse pin protruding with one end into the annular recess or constriction location of the bolt head is removable from the angle element whenever the screw bolt is to be replaced. In accordance with another feature of the invention, the transverse pin extends in radial direction of the bolt head shaft. In accordance with a further feature of the invention, the transverse pin is a screw. In accordance with an added feature of the invention, one of the leaf springs is firmly screwed to the angle element with the screw. In accordance with a concomitant feature of the invention, the expansion shaft has a smaller diameter than the coaxial bolt head shaft. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a nuclear reactor fuel assembly, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. |
abstract | Method including: inducing, with antiprotons, nuclear fission in a material, such as depleted uranium; measuring leakage of radioactive byproduct produced by the fission; and producing, responsive to the measuring, a design for the nuclear fuel element. Apparatus, manufactures, and products produced by the method can be encompassed. |
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050680803 | summary | MICROFICHE APPENDIX A microfiche appendix is included herewith which contains one fiche and nineteen total frames. CROSS REFERENCE TO RELATED APPLICATIONS The present invention is related to allowed U.S. Pat. Nos. 4,803,039 and 4,815,014 respectively filed on Feb. 3, 1986 and Feb. 27, 1987 both of which are incorporated by reference herein. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to a digital computer based system for selecting information to be presented to an operator of a processing facility as he executes complex operations and, more particularly, to a system in which the automatic selection of new displays of information is internally triggered by detected changes in plant state that occur as a result of the ongoing operations and where the selection process is guided by the observed sequence of such plant state changes. 2. Description of the Related Art U.S. Pat. Nos. 4,803,039 and 4,815,014 teach the basic concepts underlying a system for computer based monitoring of the execution of complex procedures. It is recognized that such a system is primarily intended for use under circumstances in which the operator of a complex processing facility is faced with an unusual, adverse situation with little or no prior experience therein. Under such circumstances the operator has little choice but to rely heavily on, and follow closely, pre-planned, written procedures, defined usually by the systems' designers, in order to attempt to restore critical system functions. In such cases frequent interaction between the operator and a computer based monitor of procedures execution, such as the system referenced above, is highly desirable if undesirable consequences are to be avoided. In another class of more commonly encountered operating situations the operator of a complex processing facility performs a nearly routine operation. The startup after shutdown of a large electrical generating plant, which might typically occur several times a year, is representative of this class of situations. These situations inevitably involve programmed sequences of changes in the state of the plant. These changes in state may be as pervasive as changes in temperature or pressure of a working fluid or as simple as the opening or closing of a valve, or the starting or stopping of a pump. By ordinary and broadly accepted industry standards and practices such changes in state are monitored and may serve to trigger the next step in the human controlled evolution of the operation being carried out. The operator usually has had prior experience in such situations, but by reason of infrequency of occurrence he may not remember every detail of the sequence of operations to be performed. Such situations are not hazardous and in general pose no threat to the health and safety of either the operators or the general public. They are of interest to the owners and managers of the affected facilities because errors made in the execution of the operations can be quite costly in terms of lost production and in equipment damage resulting from incorrect sequencing of consecutive operations. Written procedures or a preplanned sequence of operations to guide the operator in efficiently executing the desired nearly routine sequence of operations commonly exist in complex processing facilities. Nonetheless, costly operator errors occasionally occur as a result of an operator's misunderstanding of particular parts of the applicable procedure or of inadvertent omission of one or more steps of the procedure. Hence, the use of a computer based system for monitoring the execution of such nearly routine sequences of operations can again offer valuable benefits. Attempts to apply the system of the above-identified applications to this class of operations where the operator is nominally familiar with the course of actions to be followed have consistently shown that such systems are too rigorous and too demanding of operator interactions to be fully practical. SUMMARY OF THE INVENTION It is an object of the present invention to provide a computer based system which has the ability to utilize internal recognition of successive changes in monitored system state to stay in step with the actual execution of a currently applicable sequence of operations. It is a further object of the present invention to provide a system which detects impending or just committed operator errors and automatically provides output displays alerting the operator to the deviation from the sequence of operations along with other displays showing the evolution of the affected plant systems. It is an additional object of the present invention to provide a monitoring system which automatically generates a textural display indicating a deficiency in the accomplishment of a step in an operation, detailing the evidence that identifies the deficiency and setting forth the actions needed to rectify the situation. It is also an object of the present invention to provide a system which when the operator has completed corrective actions, confirms completion of the previously deficient step and automatically passes to consideration of the next step in sequence. It is an object of the present invention to provide a computer based system which effectively assists in the execution of nearly routine operations of a plant which provides minimally obtrusive support to the plant operators in carrying out a prescribed evolution of plant states. It is still another object of the present invention to transfer at least part of the burden of continuously monitoring peripheral aspects of the evolution of a plant from the human operator to a computer. It is also an object of the present invention to automatically collect, organize and present information relative to the current transition in plant state in a single convenient location and to automatically document the process as it evolves. The above objects can be accomplished by a computer based system for supporting plant operators in carrying out prescribed nearly routine procedures or a preplanned sequence of operations. The computer based system monitors available plant instrumentation signals and processes the gathered information to detect successive changes in plant state. The system then compares the sequence of observed changes in plant state with a preprogrammed sequence and draws the operators attention to any undesirable deviations from the preplanned sequence by providing appropriate displays on a system monitor. The system requires no input from the operator when the operation underway is following the prescribed sequence and minimal input from the operator when a deviation is detected. The system internally and automatically tracks the evolution of plant states during the nearly routine operations. These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. |
claims | 1. An optimized laser cutting method for cutting out a piece from a material by a cutting system comprisinga laser source for producing a laser beam having a power, anda cutting head comprising an end nozzle for the passage of the cutting laser beam,wherein the method is characterized in that it comprises a step of determining a cutting power Pd such that:Pd=Max(Popt;λe)where Max is the mathematical operator of the maximum,Popt is an optimal power of the laser beam of the cutting system, determined in accordancewith the piece to be cut out, and/orwith cutting parameters and/orwith the type of system,in order to minimize the mass defect per unit length of the piece during a cutting of the piece,λ is a leading coefficient representing the number of kW required for cutting out the piece per mm of thickness of the piece, ande is the thickness of the piece, in mm. 2. The method according to claim 1, wherein the cutting power Pd is of the form:Pd=Max(Λ;λe)where Λ is a predetermined constant,λ is a leading coefficient representing the number of kW required for cutting out the piece per mm of thickness of the piece, ande is the thickness of the piece, in mm. 3. The method according to claim 2, wherein, for a cutting system comprising a laser source of the yttrium aluminium garnet YAG type for the production of a laser beam having a wavelength of the order of 1 μm, the power Pd is of the form:Pd=Max(4.75;0.1·e). 4. The method according to claim 1, comprising an initial step of determining an expression of the power of the laser beam in accordance with the piece to be cut out, and/or with cutting parameters, and/or with the type of system, initial step according to which:the system performs a plurality of test cuttings of a piece while varying the power of the beam, and/or the piece to be cut out, and/or the cutting parameters and/or the type of system;a sensor performs a plurality of corresponding measurements of the mass defect during each test cutting of the piece,a computerexpresses the mass defect per unit length during each test cutting of the piece in accordance with the power of the beam, and/or with the piece to be cut out, and/or with cutting parameters and/or with the type of system;performs a partial derivative of the expression of the mass defect per unit length, with respect to the power of the laser beam, and determines the expression making it possible to cancel out said partial derivative in accordance with the piece to be cut out, and/or with cutting parameters and/or with the type of system. |
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description | The present application claims priority from and fully incorporates herein, U.S. Provisional Patent Application No. 60/676,062 entitled “TREATING MATERIALS AND HAZARDOUS MATERIALS”, filed on Apr. 29, 2005. The U.S. Provisional Patent Application No. 60/676,062 is incorporated herein by reference. 1. Field Embodiments of the invention pertain to methods and compositions for treating hazardous materials. In particular, embodiments of the invention pertain to methods and compositions for treating radioactive materials. 2. Background Information Radioactive isotope laden waste has been generated from the use of nuclear fuel, from the medical industries, and from weapons manufacture. It is often desirable to treat the radioactive isotope laden waste and contain the radioactive isotopes prior to long-term storage. One approach that has been used to treat certain radioactive materials, such as, for example, cesium from spent fuel rods, includes adding the radioactive materials to borosilicate glass and melting the glass. However, this approach may offer a number of potential disadvantages. One potential disadvantage is the generally high capital investment cost to build the glass plants. Another potential disadvantage is the high operational costs due to such factors as energy consumption and the high replacement cost of the refractory blocks of the glass plant. Thus there is a general need in the art for new and useful approaches for treating radioactive isotope laden wastes. Embodiments of the invention pertain to methods of treating radioactive materials. Other embodiments of the invention pertain to compositions to treat the radioactive materials. Further embodiments of the invention pertain to methods of making the compositions or methods of using the compositions. In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known structures and techniques have not been shown in detail in order not to obscure the understanding of this description. Radioactive isotopes, such as, for example, cesium, sodium, strontium, plutonium, uranium, actinides, and other radioactive isotopes, and combinations thereof, may be separated from spent fuel rods, medical wastes, or other nuclear wastes, or otherwise provided. The scope of the invention is not limited to how the radioactive isotopes are obtained. The radioactive isotopes may be diluted in a filler material and then treated and contained within a multi-layered containment structure. FIG. 1 conceptually illustrates a treated radioactive material 100, such as, for example, radioactive isotopes, within a containment structure 110, 120, 130, according to one or more embodiments of the invention. The containment structure includes a zeolite (or other sorbent) 110, a salt crystal/filler matrix 120, and a cement (or other inorganic binding agent or hardening material) 130. The filler material may be used to dilute the radioactive isotopes prior to treatment, such as, for example, to a dilution level that meets criticality standards. A potential advantage of the multi-layered containment structure is that radioactive isotope may be sealed in the zeolite or other sorbent and contained by multiple different layers and mechanisms of containment. The zeolite or sorbent may provide a first mechanism and material of containment for the radioactive isotopes. The radioactive isotopes may be selectively drawn into or sorbed and chemically held or retained in cavities or pores of the internal structure of the zeolite or other sorbent. Chemical bonds or interactions may be used to hold the radioactive isotopes within the zeolites or other sorbents. By way of example, heavy metal cations and other ions may be coordinated within the zeolite by an ion exchange process and held by ionic chemical forces. The salt crystal/filler matrix may provide a second layered mechanism and material of containment of the radioactive isotopes. The zeolite or other sorbent may be coated or surrounded, at least partially, by a portion of the salt crystal/filler matrix. In the matrix, the crystals may be bonded with surfaces of the filler. The salt crystals may also be formed in and may tend to close off or block the pores or other openings of the zeolite or other sorbent. The salt crystal/filler within the pores may tend to be relatively dense and impenetrable by the radioactive isotopes and may tend to close or seal the radioactive isotopes therein. In some cases remaining portions of the radioactive isotopes may be incorporated directly into the salt crystals, which may further help to immobilize and contain the radioactive isotopes or hazardous material. The cement or other inorganic binding agent or hardening material may provide a third outer mechanism and material layer of containment of the radioactive isotopes. The cement may tend to coat, surround, and/or encapsulate the crystal/filler matrix. The cement may tend to fill in gaps in the crystal/filler matrix, and may tend to be formed in and close off remaining pores or other openings of the zeolite or other sorbent. This may further tend to close and seal the radioactive isotopes within the cement. In some cases remaining portions of the radioactive isotopes may be incorporated directly into the hardened cement by cementitious reactions, which may further help to immobilize and contain the radioactive isotopes or hazardous material. The use of a layered containment structure including multiple different material layers and mechanisms of containment may help to immobilize and contain the radioactive isotopes. This may help to reduce leaching, migration, and other movement of the radioactive isotopes. A conceptual illustration of the sequential or layered containment process and result for a single sorbent particle have been used for purposes of illustration. It is to be appreciated that when dealing with real materials and processes, perfect layering may not necessarily always be achieved. In addition, such layering may be formed around clumps or other groups of particles, rather than a single sorbent particle. FIG. 2 is a block diagram of a method 200 of treating radioactive materials, according to one or more embodiments of the invention. The method includes treating the radioactive material with a composition including a sorbent and a salt, at block 210. The sorbent may sorb at least a portion of the radioactive material, and the salt may crystallize around the sorbent to help encapsulate the radioactive material within the sorbent and/or the containment structure. Then, the resulting treated radioactive material may be treated with an inorganic binding agent or hardening agent, such as, for example, cement, lime, tricalcium silicate, or other cementitious material, or combination thereof, at block 220. The hardening material or binding material may harden around the sorbent and salt crystals and further help to encapsulate or seal the radioactive material within the sorbent and/or the containment structure. FIG. 3 is a block diagram of another method 300 of treating radioactive materials, according to one or more embodiments of the invention. Examples of suitable radioactive materials that may be treated include, but are not limited to, radioactive isotopes diluted in a filler material to meet criticality objectives. A. Dilution of Radioactive Material in Filler Material In one or more embodiments of the invention, prior to treatment with a radioactive material treatment composition as disclosed elsewhere herein, the radioactive materials may optionally be diluted with a filler material, at block 310. Radioactive isotopes, such as, for example, those of cesium and uranium, are often stored for prolonged periods of time at relatively low concentrations in order to reduce the likelihood that the isotopes can partake in a nuclear reaction. This is sometimes known in the nuclear arts as “criticality”. A method, according to one or more embodiments of the invention, may include dilution of one or more types of radioactive isotopes or materials with a filler material by mixing or otherwise combining the isotopes or radioactive material with the filler material. In one or more embodiments, the isotopes may be diluted to a concentration, such as, for example, in parts-per-million, that complies with criticality standards, such as requirements, regulations, guidelines, and/or preferences, although the scope of the invention is not limited to any known dilution. Examples of suitable filler materials that may be used to dilute radioactive isotopes, according to various embodiments of the invention, include, but are not limited to, silica flour, microcrystalline silica particles, aluminum oxide particles, calcined alumino-silicate or particles, such as, for example, calcined bentonite and kaolin particles, fine sand, flint powder, and combinations thereof. Other examples of suitable filler materials that may be used to dilute radioactive isotopes including cesium may include a powdered conventional lime glass composition. By way of example, the powdered lime glass composition may include varying proportions of powdered silicon dioxide (SiO2) e.g., fused silica, powdered aluminum oxide (Al2O3), and powdered calcium oxide (CaO), which may be mixed together. Another suitable filler material may include a borosilicate glass, such as, for example, those used to fabricate pyrex. Such a filler material may offer a potential advantage of better thermal shock resistance than lime glass. Still other suitable filler materials include, but are not limited to, fly ash, such as, for example, Type C and/or Type F fly ash. In one or more embodiments of the invention, a calcined residual of the spent fuel rods in a conventional spent fuel rod process may also potentially be used as a base material for diluting the cesium or other radioactive isotopes. Other filler materials are also suitable. Various combinations of filler materials are also suitable. One particular example of a filler may include 4 (v/v) % microcrystalline silicon, 23 (v/v) % powdered flint, 22 (v/v) % fine sand, 11 (v/v) % course calcined clay having average diameter of 3-4 millimeters, and 40 (v/v) % calcined bentonite. However, the scope of the invention is not limited to this particular filler material. In one or more embodiments of the invention, the particle sizes of the filler material may optionally be adjusted, such as, for example, by grinding, or sieving, or mixing multiple particle sizes together, to adjust the packing density of the particles. In one particular aspect, the particle sizes may be adjusted to achieve dense close-order packing by approaches that are known in the arts. Such adjustment of the density may potentially allow additional control over isotope concentration and criticality. In one or more embodiments of the invention, the particle sizes of the filler may optionally be adjusted or otherwise controlled so that the filler may have a substantially predetermined fineness modulus, although this is not required. The fineness modulus is an empirical factor that may be determined by adding together percentages of a filler sample retained on each of a specified series of sieves, and dividing the sum by 100. By way of example, in one or more embodiments of the invention, the filler material may have a fineness modulus in the range of from about 3.3 to 3.7, although the scope of the invention is not limited in this respect. Radioactive isotopes may potentially be presented for treatment in a medium including particles or other structures having holes, such as, for example, cavities, pores, or capillaries, of various different sizes and shapes. In such cases, the isotopes may be included within the holes of different sizes and shapes. A filler having particles of different sizes, and in particular a filler having a fineness modulus in the aforementioned range, may be well suited for plugging the holes, which may help to seal the isotopes in the holes. This may help to reduce leaching, isotope migration, or other movement of the isotopes. In one or more embodiments of the invention, superfine particles of the filler material, or a smallest fraction or portion of the filler material, may optionally be removed, such as, for example, by retaining the larger particles over a superfine mesh. For example, superfine particles passing through a 50 mesh, or 100 mesh, or smaller mesh may optionally be discarded, although this is not required. This may help to reduce the superfine particles from clogging or otherwise interfering with the openings and cavities of the zeolites. In one or more embodiments of the invention, an isotope may optionally be mixed or otherwise combined with only a portion of the particles, such as, for example, a particular type of particle, or a particular size of particle (e.g., sieve fraction), although this is not required. For example, an isotope may first be mixed with particles of a first type of material, and then particles of a second different type of material may be mixed in. As another example, an isotope may first be mixed with large particles, and then small particles may be mixed in. By way of example, this may also achieve more sophisticated control over isotope criticality. According to one or more embodiments of the invention, a filler material may optionally include particles of two or more different materials that may be capable of undergoing interfacial reactions with one another, although this is not required. By way of example, in one or more embodiments of the invention, the particles of the different materials may be capable of undergoing cementitious or other binding reactions with one another. As one particular example, a filler material may include particles of silica and alumina, which may undergo interfacial cementitious or other interfacial binding reactions with one another. Other materials capable of participating in such cementitous or other interfacial binding are also suitable, such as, for example, magnesium oxide, lime, lime glass, pozzolanic materials, alumino-silicate materials, and the like. These binding reactions may help to bind the particles together, add mechanical strength and stability to the treated materials disclosed herein, and may further seal radioactive materials within the treated materials. Similarly, in one or more embodiments of the invention, the particles of the two or more different materials may be included in the filler in chemically stable proportions to promote stability in mechanical strength and radioactive material containment properties. In one or more embodiments of the invention, the isotopes and filler material to which the isotopes are to be mixed may be introduced into and mixed in a mixer. Examples of suitable mixers include, but are not limited to, the L-20, KM-1200, or KM-2000 Ploughshare® type plough mixers, which are commercially available from Lodige USA, Inc, of Ronkonkoma, N.Y., or the parent company of Warburg, Germany. However, the scope of the invention is not limited to just these types of mixers. Other types of mixers may also optionally be used. The isotopes and filler material may be mixed until they are sufficiently homogeneous for the intended implementation. As previously stated, the radioactive isotopes may optionally be initially mixed with a portion of the filler material, and then subsequently mixed with another portion of the filler material. In one or more embodiments of the invention, potassium permanganate, ferro-magnetic particles, a dye, or other chemical indicator may be used or referenced as an indicator of the degree of mixing or homogeneity. B. Mixing Radioactive Material with Treatment Composition Then, the radioactive material diluted in the filler material may be contacted and mixed with a radioactive material treatment composition and water, at block 320. In one or more embodiments, shortly prior to use, such as, for example, from several minutes to several hours before use, the radioactive material treatment composition as disclosed elsewhere herein, may be combined with water, to form a thick solid-liquid sludge, “mud”, or slurry. The water of the slurry may be saturated or supersaturated with at least some of the salts of the radioactive material treatment composition. Alternatively, water may be added to the filler material. Then, the solid-liquid mixture may be combined with the radioactive material. In one or more embodiments of the invention, the radioactive material treatment composition and radioactive material may be combined in a volume ratio that ranges from about 1:50 to 1:2, although the scope of the invention is not limited to these particular ratios. In one aspect, the composition added may provide an amount of zeolite that is sufficient to sorb a significant portion or all of the radioactive isotope. In one or more embodiments of the invention, the mixer may include a plough mixer, although this is not required. Non-limiting examples of suitable plough mixers are the L-20, KM-1200, or KM-2000 Ploughshare® mixers, which are commercially available from Lodige USA, Inc, of Ronkonkoma, N.Y., having the parent company of Warburg, Germany. The Ploughshare® mixers may mix the components by utilizing a mechanically induced fluidized bed reportedly created by shovels that rotate close to inner walls of a drum and thrust the components inside the drum. In one or more embodiments of the invention, such mixers may be operated at from about 50 to 300 revolutions per minute (rpm) to achieve a Froude Number ranging from about 6 to 8, such as from 6 to 7, although this is not required. In one or more embodiments of the invention, the radioactive material and treatment composition may be mixed under such conditions for a period of time that is less than about 10 minutes, or less than about 7 minutes, such as, for example, from about 2 to 7 minutes. Often, the period of time is from about 3 to 6 minutes, and may be less than 5 minutes. A potential advantage of using such plough mixers is that they may achieve comparatively good mixing of the contents of the drum in a relatively short period of time, which may tend to be compatible with the treatment processes described herein. The mixing action may also help to avoid separation of the salt crystals from around the zeolites. However, the scope of the invention is not limited to just these types of mixers. Other types of mixers may also optionally be used. For example, Hobart dough mixers have been tested on non-radioactive materials and are believed to be suitable. In one aspect, the mixer may optionally be provided on a mobile platform, such as, for example, a bed of a vehicle, or a trailer, and moved to a remediation site and used there, although this is not required. In one or more embodiments of the invention, the temperature in the mixer during the mixing process may be controlled to be in the range of about 120 to 180° F., although this is not required. Some of the heat may be provided by exothermic processes and additional heating and/or cooling may also optionally be used. In one or more embodiments of the invention, the amount of water in the mixer during this stage of mixing may be adjusted or prescribed to be in the range of 10 to 20 wt %, such as, for example, from 22 to 16 wt %. However, the scope of the invention is not limited in this respect. If desired, a deflocculating agent, such as, for example, sodium silicate or cellulose, may optionally be added, and may potentially be used to reduce the water content. If desired, a solvent, such as, for example, methanol, ethanol, or other short-chain alkanols, may optionally be used to replace a portion of the water. C. Sorbing Radioactive Isotopes with Sorbent Referring again to FIG. 3, during the mixing period, and potentially shortly thereafter, the zeolites or other sorbents of the treatment composition may sorb at least a portion of the radioactive isotopes, at block 330. The zeolites or other sorbents may provide an environment that may sorb and retain or hold the radioactive isotopes. In one or more embodiments of the invention, the zeolites of the treatment composition may initially be dehydrated or dried so that they may sorb more water or other fluid potentially laden with radioactive isotopes into their pores, although this is not required. In one or more embodiments of the invention, the zeolites may be pre-treated to promote binding or retention of the radioactive isotopes, although this is not required. As one example, a surfactant, such as, for example, hexadecyltrimethylammonium bromide, may be used to treat the zeolite to make the internal cavities of the zeolite affinitive for anions instead of cations. Treated and non-treated zeolites may be used in combination to sorb both cations and anions. As another example, a chlorine compound, such as, for example, a perchlorinate, may be used to create a stationary solvent phase in the cavities to customize sorption for organics potentially laden with radioactive isotopes. As another example, a small molecular or ion trap may be included in the zeolites to further retain radioactive isotopes. As yet another example, sodium hydroxide may be used to open up the internal structure, such as to facilitate sorption of larger molecules or more rapid sorption of molecules or water laden with isotopes. D. Growing Salt Crystals Around Sorbent During the mixing period or process, salt of the treatment composition may start to form crystals that may grow in, on, and around, and coat particles, agglomerates, or other portions of the zeolites and filler material, at block 340. Some of the salt materials may react with the surfaces of the filler material and zeolites to provide good contact and adhesion and the salt materials may grow as crystals between the filler material and zeolites. The salts may become occluded into the growing matrix as anion and cation donors. Water having the salts therein may be drawn or sorbed into the pores of the zeolites and thereafter crystals may form in the pores or internal structure to help seal the radioactive isotopes in the zeolites. This may result in an aggregate in which the crystals bound to the filler material and zeolites may form a coating, sheath, or encapsulation layer to help encapsulate the radioactive isotopes within the cavities of the zeolite. This may tend to reduce leaching or removal of the radioactive isotopes from the cavities of the zeolites. In some cases, radioactive isotopes may potentially be incorporated directly in the salt crystals by salt crystal formation reactions or by the salt crystals growing around them, which may further help to contain the radioactive isotopes. It is presently thought that excessive mixing may potentially tend to reduce the effectiveness of the containment of the radioactive isotopes. Without wishing to be bound by this particular theory, one potential explanation is that the chloride salts and other salts may be over mixed or “emulsified” with the filler, which may tend to hinder crystal growth and/or encapsulation of the radioactive materials within the cavities of the zeolites. Another potential explanation is that excessive mixing may potentially break the salt crystals free of the zeolites. E. Mixing Cement with Sorbent and Salt Crystal Mixture Referring again to FIG. 3, after mixing the radioactive materials with the radioactive material treatment composition as described above, the resulting mixture or treated product may optionally be further treated with cement, lime, or one or more other inorganic binding agents. The cement or other inorganic binding agent may be mixed with the aforementioned resulting mixture, at block 350. In various embodiments of the invention, the cement may be mixed in an amount that is from about 0.5 to 10 wt %, or from about 0.5 to 5 wt %, or from about 1 to 3 wt % of the total volume of radioactive material treated, although the scope of the invention is not so limited. More cement may also optionally be used, although this may tend to increase the cost of treatment. The mixing may coat or otherwise provide the cement or other inorganic binding agent or composition around particles, clumps, or other portions of the zeolite, filler material, and growing and/or grown crystals. In one or more embodiments of the invention, the inorganic binding agent may be introduced into the same mixer that already contains the mixture previously described. Alternatively, a different mixer may be used. In one aspect, a first mixer to mix in the treatment composition and a second mixer to mix in the lime and/or cement or other binding agent may be connected in series with one another to provide a continuous mixing process, which may potentially help to reduce downtime needed to load and unload mixers. In one or more embodiments of the invention, the cement and/or lime or other binding agent may be mixed with the treated radioactive material mixture for a period of time ranging from about 30 seconds to 3 minutes, although this is not required. In aspects, the period of time may be less than 2 minutes, or less than 1 minute, although this is not required. It is presently thought that excessive mixing may tend to disrupt the encapsulation or containment of the radioactive materials. When adding the binding agent, or during the mixing period, the water content in the mixer may optionally be adjusted to a value that is appropriate to promote formation of good hardened monolith or other binding material. For example, in one or more embodiments of the invention, an adjustment amount of water may be added to give final water content is in the range of from about 14 to 18 wt %. If less than 14 wt % is desired a deflocculant such as sodium silicate may optionally be added. The wet but hardening final product may be provided to a suitable destination. Examples of suitable destinations include, but are not limited to, a hardening mold or process, or to a packaging process. F. Hardening the Binding Agent Around Sorbent and Salt Crystals Referring again to FIG. 3, the cement or other binding agent may harden, at block 360. As it hardens, the cement may provide a hard coating, sheath, or encapsulation layer around the particles, clumps, or other portions of the zeolite, filler material, and growing and/or grown crystals, which may further help to reduce leaching or other escape of the radioactive isotopes from the internal structures of the zeolites or other sorbent. The cement or other inorganic binding agent may also contribute solidity, mechanical integrity, and/or strength to the treated isotope material. In some cases, radioactive isotopes may be incorporated directly in the cement potentially by cementitious reactions, which may further help to contain these materials. If appropriate, compression and/or leaching tests may be performed for quality control or verification. The treated radioactive isotopes may then be stored for prolonged periods of time. G. Example Optional Variations of the Described Method Various exemplary radioactive material treatment methods have been described in conjunction with FIG. 3, although the scope of the invention is not limited to just these particular methods. Alternate methods are contemplated in which operations are performed in different order. For example, water may be combined after commencement of mixing of the radioactive material treatment composition and the radioactive material. As another example, water may first be combined with the filler and/or the filler with the radioactive material instead of the treatment composition. Still alternate methods are contemplated in which operations are added to the methods. For example, the unhardened cementitious mixture may be shaped or molded. As another example, analysis of the radioactive material may be performed and a treatment composition may be tailored based on the analysis. Many further modifications and adaptations may be made to the methods and are contemplated. As described above, in one or more embodiments of the invention, a radioactive material, such as, for example, radioactive isotopes diluted in a filler material, may be treated with a radioactive material treatment composition containing salt and sorbent. A radioactive material treatment composition, according to one or more embodiments of the invention, may include in relatively larger proportion or mostly a salt mixture and in relatively smaller proportion one or more sorbents to sorb one or more radioactive isotopes or compounds thereof. As used herein, mostly salt means more than 50% salt. By way of example, in one or more embodiments of the invention, the radioactive material treatment composition may include at least 75 wt % salt, such as, for example, from about 75 to 99.5 wt % salt, and at least 0.5 wt % sorbent, such as, for example, from 0.5 to 25 wt % sorbent, although the scope of the invention is not so limited. The inventor has considered numerous different possible treatment compositions, including compositions with varying amounts of various different types of salts, and different types of sorbents. This section describes various embodiments of radioactive material treatment compositions that may be used. However, the scope of the invention is not limited to these particular treatment compositions. Many further modifications and variations of these treatment compositions are contemplated and will be apparent to those skilled in the art and having the benefit of the present disclosure. A suitable salt mixture, according to one or more embodiments of the invention, may optionally include one or more halide salts, such as, for example, one or more chloride salts, together with one or more sulfate salts, although this is not required. The inclusion of both sulfate and halide salts may allow encapsulating the zeolites within a matrix of different crystals formed integrally with one another which may tend to improve encapsulation of the radioactive isotopes. The halide and sulfate salts may crystallize to form different crystal structures that may interlock and thereby help to contain radioactive isotopes within a matrix. The sulfate salts may tend to promote formation of monoclinic crystals or other differently shaped crystals, which may tend to add integrity to the salt crystals and promote good encapsulation. One particular example of a suitable sulfate salt is magnesium sulfate, although others are also suitable. Chloride salts tend to promote good and rapid crystal growth. In one or more embodiments of the invention, the one or more halide salts may optionally include one or more monovalent cation salts, and one or more polyvalent cation salts, although this is not required. Suitable monovalent halide salts include, but are not limited to, sodium chloride, ammonium chloride, potassium chloride, and the like. Suitable polyvalent halide salts include, but are not limited to, calcium chloride, magnesium chloride, magnesium fluoride, and the like. In one aspect, this may create different crystal structures that may add diversity and interlock to aid containment. This may further promote containment of the radioactive isotopes within the confines of a matrix of inter-grown crystals and potentially help to reduce leaching of radioactive isotopes. One type of suitable sorbent is a zeolite. A zeolite may include a natural or synthetic hydrous silicate or aluminosilicate microporous solid that may have a highly organized or structured open three-dimensional crystal structure of openings and cavities in a lattice. The zeolite may act as a molecular sieve, adsorbent, and/or ion exchanger to selectively sorb radioactive isotopes into the internal structure based, at least in part, on a size exclusion process and/or the chemical environment inside the cavities. Other suitable sorbents include, but are not limited to, chelating agents, and other materials known to have binding or bonding properties with respect to radioactive isotopes. Calcined clays, activated carbons, and like sorbents may also potentially be used in some embodiments depending upon the particular radioactive isotopes and implementation. For example, bentonite, illite and kaolin (all potentially calcined) may potentially be used, depending upon the particular implementation. Bentonite, illite and kaolin are also alumino-silicates. Combinations of different types of sorbents may optionally be included in the composition, such as, for example, to each sorb different types of radioactive isotopes and potentially other hazardous materials included along with the radioactive isotopes. The total sorbent included in the composition may be based, at least in part, on the amount of radioactive isotopes to be sorbed. In various embodiments of the invention, the total sorbent may be less than 15 wt %, less than 10 wt %, less than 5 wt %, and/or more than 0.5 wt %, although this is not required. Other components may optionally be included in the radioactive material treatment composition. For example, magnesium may optionally be included in order to help react with filler material, or the like to help promote strong and integral attachment of the crystals to the filler, although this is not required. A relatively higher soluble form of magnesium, such as, for example, magnesium oxide, may optionally be included to increase the amount of soluble magnesium when the composition is combined with water. As another example, the composition may optionally include one or more pH adjustment chemicals, such as, for example, sodium bicarbonate, sodium carbonate, potash, or another base, or a combination of bases, to promote a basic pH, although this is not required. A basic pH, such as, for example, a pH greater than 10, or greater than 11, or greater than 12, may potentially help to avoid build up of hydrogen gas during long-term storage. The high pH may also help to protect against unexpected exposure to acid. As yet another example, the composition may optionally include one or more anti-corrosion chemicals or salts, such as, for example, aluminum chloride, although this is not required. The aluminum chloride may help to reduce corrosion of steel and certain other metals. Aluminum chloride may be included in various proportions in the composition depending upon whether or not the potential for corrosion is of concern. As a still further example, an indicator chemical or material may optionally be included in the composition to provide an aid for visually or otherwise assessing the degree of mixing of the mixture or the homogeneity, although this is not required. This may help to ensure criticality standards are met or exceeded. Suitable indictor chemicals or materials include, but are not limited to, potassium permanganate, dyes, ferromagnetic particles, or other materials whose relative concentration in the mixture may readily be assessed. These are just a few examples. An example of a suitable radioactive material treatment composition, according to one or more embodiments of the invention, is disclosed in Table 1. Components and concentrations are listed. TABLE 1ComponentConcentration (wt %)Salt 75-99.5%Sorbent0.05-25% Another example of a suitable radioactive material treatment composition, according to one or more embodiments of the invention, is disclosed in Table 2. Components and concentrations are listed. TABLE 2ComponentConcentration (wt %)Monovalent Halide SaltRemainderPolyvalent Halide Salt 0-35%Sulfate Salt 0-5%Anti-Corrosion Agent 0-5%Magnesium Oxide 0-5%BaseSufficient to give pH > 10Sorbent0.5-25% In various embodiments of the invention, from all to a small amount of one or more, or various combinations, of the polyvalent halide salt, the sulfate salt, the anti-corrosion agent, and magnesium oxide, may optionally be omitted from the composition. The remaining percentage or bulk of the mixture may be made up of a monovalent halide salt, such as, for example, sodium chloride, which is widely available and relatively inexpensive. Yet another example of a suitable radioactive material treatment composition, according to one or more embodiments of the invention, is disclosed in Table 3. Components, concentrations, and optional particle sizes are listed. TABLE 3ComponentConcentration (wt %)Particle SizeSodium ChlorideRemainder0-1 mmAmmonium Chloride0-2% or about 1%0-1 mmAluminum Chloride0-5% or about 3%0-1 mmPotassium Chloride0-20% or about 15%0-1 mm or flocksCalcium Chloride0-20% or about 15%0-5 mmMagnesium Chloride0-20% or about 15%fine grain or 50 meshMagnesium Oxide0-4% or about 2%−200 meshMagnesium Sulfate0-4% or about 3%0-1 mmSodium Carbonate0-4% or about 3%0-1 mmZeolite (ASM A4)1-10% or about 2-6%−325 meshPotassium Permanganate0-2% or about 0-1%0-1 mm In various embodiments of the invention, from all to a small amount of one or more, or various combinations, of ammonium chloride, aluminum chloride, potassium chloride, calcium chloride, magnesium chloride, magnesium oxide, magnesium sulfate, sodium bicarbonate, and potassium permanganate, may optionally be omitted from the composition. The remaining percentage or bulk of the mixture may be made up of another component, such as, for example, sodium chloride. A still further example of a suitable radioactive material treatment composition, according to one or more embodiments of the invention, is disclosed in Table 4. Components, concentrations, and optional particle sizes are listed. In this composition, the term “about” means±50% of the indicated amount for indicated amounts that are less than 5% and ±20% of the indicated amount for indicated amounts that are over 5%. In this composition, the remaining percentage or bulk of the mixture may be made up of sodium chloride. Sodium chloride may be present in the highest concentration. TABLE 4ComponentConcentration (wt %)Particle SizeSodium Chlorideabout 35%0-1 mmAmmonium Chlorideabout 1%0-1 mmAluminum Chlorideabout 3%0-1 mmPotassium Chlorideabout 15%0-1 mm or flocksCalcium Chlorideabout 15%0-5 mmMagnesium Chlorideabout 15%fine grain or 50 meshMagnesium Oxideabout 2%−200 meshMagnesium Sulfateabout 3%0-1 mmSodium Carbonateabout 3%0-1 mmZeolite (ASM A4)about 1-5%−325 meshPotassium Permanganateabout 0-1%0-1 mm Many other variations of the compositions included in Tables 1-4 are contemplated and will be apparent to those skilled in the art and having the benefit of the present disclosure. For example, compositions are contemplated that include additional components, omit one or more of the listed components, and/or have the components in different proportions. The scope of the invention is not limited to any known composition. Compositions like those shown above, or variations of these compositions, may include solid particulate materials or powders that may be mixed or otherwise combined together in the indicated proportions. The resulting mixtures may optionally be sealed in a container or otherwise packaged and optionally stored until an intended time of use. The radioactive material treatment compositions disclosed above are suitable for treating a wide variety of radioactive materials. However, in one or more embodiments of the invention, a radioactive material treatment composition may optionally be altered and/or tailored for a particular radioactive material based, at least in part, on analysis of the radioactive material. Initially, a radioactive material sample may be collected and analyzed. By way of example, the radioactive material may be analyzed for water content, organic content, radioactive material type, radioactive material content, or hazardous materials included in the radioactive material, electrical conductivity, compressive strength, pH, salt content, and optionally other parameters. The radioactive material treatment composition may be formed or modified based at least in part on the analysis. By way of example, the type of zeolite or other sorbent may be determined based on the type of radioactive and other potential hazardous materials, the amount of zeolite or other sorbent may be determined based on the amount of radioactive and potentially other existing hazardous materials, the amount of water needed for addition to the radioactive material treatment composition may be determined based on the amount of water in the radioactive material. Such adaptations may potentially improve treatment, but are not required. Leaching tests may also optionally be formed on treated samples prior to large-scale remediation treatment. However, such tailoring of the treatment composition is optional and not required. Pre-packaged general-purpose treatment compositions may also optionally be used. Having been generally described, the following examples are given as particular embodiments of the invention, to illustrate some of the properties and demonstrate the practical advantages thereof, and to allow one skilled in the art to utilize the invention. It is understood that these examples are to be construed as merely illustrative. Experiments have been performed to demonstrate the significant increase in compressive strength of a surrogate steam reformed waste product diluted in a filler material and treated as disclosed herein. 2 volumes of the surrogate steam reformed waste product were diluted in 9.2 volumes of filler material. The particular filler material included about 4% microcrystalline silicon, about 23% powdered flint, about 22% fine sand, about 11% coarse calcined clay having average diameter of 3-4 millimeters, and about 40% calcined bentonite (where the percents are expressed in v/v %). This diluted material was treated with about 0.2 volumes of a radioactive material treatment composition similar to that shown in Table 4. The resulting material was treated with about 1.1 volumes of Portland cement as binding agent. Compressive strength was monitored over several weeks using ASTM C-39. The results are presented in Table 5. TABLE 5Hardening TimeCOMPRESSIVE STRENGTH (PSI)Less than two weeksLess than 500At least three weeksGreater than 500Several MonthsApproaching 1000 or greater These results indicate that the compressive strength tends to increase over time. After about three weeks, the compressive strength is greater than 500 psi and still increasing with time. After several months, it is expected that the compressive strength may approach or even exceed 1000 psi. This example demonstrates the effectiveness of treating various soils contaminated with heavy metals with treatment compositions and methods as disclosed herein. The samples were treated using compositions similar to those disclosed in Table 4 using methods similar to those disclosed herein. Leaching was assessed by method EPA 1311. Results are listed in Table 6. TABLE 6HARDNESS(COMPRESSIVELEACHINGLEACHINGSTRENGTH)DESCRIPTIONBEFOREAFTERAFTEROF SAMPLEContaminantTREATMENTTREATMENTTREATMENTTar clay soilAntimony42.5ug/g0.09ug/l 1.8 MPaArsenic115ug/g0.03ug/lLead625ug/gNon DetectableMod. TPH130000Non DetectableAlkaline clayBenzene0.072mg/l0.021mg/l1.65 MPasoilToluene0.1420.030Xylene0.009<0.002Ethylbenzene0.0880.018Method: EPA 1311 As shown the leaching after the treatment was in all cases reduced compared to leaching before treatment. Also, significant increase in compressive strength is obtained after treatment. Similar reductions in leaching and increases in compressive strength may also be observed and are also expected when treating radioactive isotopes diluted in filler. In the following description and claims, the terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements are not in direct contact with each other, but yet still co-operate or interact with each other. In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments of the invention. It will be apparent however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. The particular embodiments described are not provided to limit the invention but to illustrate it. The scope of the invention is not to be determined by the specific examples provided above but only by the claims below. In other instances, well-known structures, devices, and operations have been shown in block diagram form or without detail in order to avoid obscuring the understanding of the description. It will also be appreciated, by one skilled in the art, that modifications may be made to the embodiments disclosed herein, such as, for example, to the sizes, configurations, functions, materials, and manner of operation of the components of the embodiments. All equivalent relationships to those illustrated in the drawings and described in the specification are encompassed within embodiments of the invention. Various operations and methods have been described. Some of the methods have been described in a basic form, but operations may optionally be added to and/or removed from the methods. The operations of the methods may also often optionally be performed in different order. Many modifications and adaptations may be made to the methods and are contemplated. For clarity, in the claims, any element that does not explicitly state “means for” performing a specified function, or “step for” performing a specified function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. Section 112, Paragraph 6. In particular, any potential use of “step of” in the claims herein is not intended to invoke the provisions of 35 U.S.C. Section 112, Paragraph 6. It should also be appreciated that reference throughout this specification to “one embodiment”, “an embodiment”, or “one or more embodiments”, for example, means that a particular feature may be included in the practice of the invention. Similarly, it should be appreciated that in the description various features are sometimes grouped together in a single embodiment, Figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects may lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of the invention. Accordingly, while the invention has been thoroughly described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the particular embodiments described, but may be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting. |
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048213063 | abstract | A system comprising an X-ray source (1), an elongate detector tube (3) including at least one cathode (5) extending in the longitudinal direction of the tube and at least one anode (6) located opposite to the cathode, a slit diaphragm (2), and a filter (4) mounted in the path between the X-ray source (1) and the detector tube (3). The filter (4) blocks relatively low energy X-radiation in a portion of the beam emitted by the source (1). The cathode (5) is provided with an X-ray detection layer consisting of two strips (8',8") extending in the longitudinal direction of the tube (3). One strip (8") receives the radiation passed by the filter (4) and the other strip (8') receives the unfiltered radiation. The one strip (8") is of considerably greater thickness than the other strip (8'). |
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abstract | A system and method for irradiating a product in order to obtain an irradiated product that has a suitable irradiation dose uniformity ratio and is less expensive and faster than existing systems. The system includes a controller that varies the speed at which a product to be irradiated is moved across a radiation beam. The speed is varied in accordance with a speed function that can be a quadratic function of the distance between a position at which the surface of the product is irradiated and a reference position. |
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039492320 | description | DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, it will be observed that this block-diagram showing illustrates a typical circuit as it is employed with a logging tool of the type to which this invention applies. There is a logging cable 11 that carries the electrical circuit connection from the logging tool, or sonde, to the surface when the tool is in a borehole. The system shown includes elements located in the logging sonde. There is a neutron generator tube 12 that has an evacuated interior. This might be one of several commercially available neutron generator tubes, e.g., one designated by the trade name Amperex Model 18603, which is manufactured by Philips Research Laboratories, Eindhoven, Netherlands. This generator develops the required neutrons, and it consists of a gas-replenisher element 15 that keeps a constant gas pressure within the tube 12. Also, there is a Penning ion source which is indicated by box 16 captioned "ION SOURCE". Associated with the ion source, there is a high-voltage section, and a high-voltage pulsing section. These are indicated as being connected to the ion source 16 located between replenisher element 15 and the target 23. This arrangement is indicated by boxes 19 and 20 with the captions "ION SOURCE H.V. SECTION" and "ION SOURCE H.V. PULSING SECTION", respectively, thereon. The target 23 has a steady DC high-voltage negative potential applied for producing the desired neutrons caused by bombarding the target. Target 23 is constructed of titanium that retains tritium in a self-replenishing manner so that it does not limit the lifetime of the tube 12. The high voltage applied to the target 23 is obtained by a Cockroft-Walton type of high-voltage supply. It includes a "VOLTAGE-MULTIPLYING SECTION" 28, and a "DRIVER SECTION" 29 as well as a "TARGET CURRENT DETECTOR & CONTROL SECTION" 30. As already indicated, this supply system develops a high-voltage DC potential of minus 125 KV. It will be observed that the control section 30 is also connected to the replenisher element 15 via a "REPLENISHER POWER SUPPLY" 33. The manner of operating a logging sonde such as that illustrated in FIG. 1, need not be discussed since it is not directly relevant to the invention. It is sufficient to note that under certain conditions the neutron tube 12 will experience a high-voltage discharge or breakdown. During this unstable condition the resulting arcs are frequent and large in amplitude. Consequently, if such arcing is not controlled, the glass envelope of the neutron tube 12 can be punctured. But this can be avoided by the invention. There is a capacitor 34, one plate of which is schematically indicated in FIGS. 1 and 3. It is physically constructed as illustrated in FIG. 2. There is a thin metallic strip 37 that is associated with a high-voltage connector 38. This connector 38 is a good electrical conductor, and it connects the output voltage of the multiplying section 28 with the target 23 of the tube 12. A connector of this type is like a similar one that is illustrated and described in U.S. Pat. No. 3,657,536 where related mechanical details are described more fully. The physical construction of the capacitor 34 is illustrated in FIG. 2 which shows a cross-sectional view of a connector portion 41 of a logging sonde 40 (partially shown). Again, this is similar to the aforementioned U.S. Pat. No. 3,657,536, and it shows that portion of a sonde which joins the neutron tube 12 with the voltage-multiplying section 28 of the sonde. It may be noted that the neutron tube 12 has an end wall 42 through which extends a portion 43 of an insulating material 47 that has the high-voltage connector 38 axially located therein. The connector 38 carries the high voltage from the multiplying section 28 to the target 23, and it is axially located within the portion 41 of the sonde 40 which has a cylindrical housing as indicated. In order to detect any arcing conditions (such as those indicated above), the capacitor 34 is constructed with the metal strip 37 located in a groove 50 that circumferentially surrounds the connector 38 and is spaced radially therefrom. This arrangement forms the capacitor 34. The strip 37 acts as one plate, and the connector 38 acts as the other. The insulating material 47 acts as the dielectric for the capacitor 34. The arcing conditions described above will create variations in the high-voltage potential, and this will be reflected by the capacitor 34. The capacitor 34 is connected in an electrical circuit which includes a resistor 55 that is connected between the strip 37 and a circuit ground 56. In this way the voltage change that is reflected by the charge on the capacitor 34, is applied via a circuit connection 59 to an amplifier 60 (see FIG. 1). Then the amplifier-output signals are carried over a circuit connection 63 to the solenoid of a relay 64 which has a normally closed switch 67 that is actuated to the open position illustrated when the relay is energized. Switch 67 is connected in series with a circuit connection 68 that supplies the driver section 29 of the sonde 40. Consequently, whenever an arc develops, it will cause the indicated change in the charge on capacitor 34, and this is amplified and applied to the relay 64 so as to deenergize the power supply (driver section 29) that controls the 125 KV target voltage. In this manner, the other parts of the system will automatically be turned off also so as to make the tube 12 inoperative. Consequently, any destructive breakdown of the tube envelope is avoided. It will be appreciated that FIG. 3 indicates, in a schematic manner, the electrical circuit relationships of the elements of capacitor 34. One skilled in the art will understand that a capacitor for a system in accordance with the invention might take different forms from that illustrated in FIG. 2. While an embodiment of the invention has been set forth above in considerable detail and in accordance with the applicable statutes, this is not to be taken as in any way limiting the invention but merely as being descriptive thereof. |
055984509 | abstract | In a fuel bundle for a boiling water nuclear reactor comprising a plurality of fuel rods (20) secured within an array and extending between upper and lower tie plates (24, 22), and including at least one additional partial length fuel rod (20') extending from said lower tie plate (22) but terminating short of said upper tie plate (24), an improvement in the form of a removable extension rod (32) secured to said at least one additional fuel rod (20') and extending substantially to said upper tie plate (24). The removable extension rod (32) also permits variance in the reactivity of the partial length fuel rod (20') particularly in the two phase region of the bundle (10). |
050911201 | summary | TECHNICAL FIELD The invention relates to a process for the production of fritted UO.sub.2 nuclear fuel pellets from metallic uranium, resulting more particularly from laser isotopic enrichment, whereby said process leads to no liquid effluents. STATE OF THE ART Conventionally, when starting with metallic uranium, nuclear quality UO.sub.2 fritted fuel pellets are obtained by using a so-called wet route process involving a series of long, expensive chemical operations giving rise to effluents, which it is necessary to process and dispose of. They generally consist of firstly dissolving the metal in a nitric medium and then, on the basis of the solution obtained, precipitating the uranium in diuranate form, or treating said solution by a selective precipitation, e.g. with the aid of hydrogen peroxide, followed by the filtration of the pastes obtained, drying, calcining and then reducing the oxide obtained into UO.sub.2 with the aid of hydrogen or cracked ammonia. Each of these operations gives rise to uraniferous residues which it is necessary to recycle and also effluents are obtained, particularly during purification and precipitation, from which it is necessary to remove the uranium, followed by disposal. In the case of the thus obtained UO.sub.2 powder, it is generally necessary to carry out granulation prior to the pressing of the pellets and the fritting thereof for the purpose of obtaining fuel pellets. OBJECT OF THE INVENTION The object of the invention is to provide a process for obtaining fritted fuel pellets from metallic uranium, which is simple and direct, produces no liquid effluents, leading to directly pourable uranium oxide intermediate powders, i.e. requiring no conditioning operation such as granulation, and having an adequate frittability under standard conditions to obtain fuel pellets. Another object is to obtain a uranium oxide intermediate powder usable for producing after mixing with other metal oxides, e.g. Pu, Th, Ce or neutrophage elements, fritted, mixed fuel pellets, once again without prior granulation. Obviously, the fuel pellets obtained according to the inventive process comply with the specifications for nuclear fuels, more particularly a fritted density above 95% of the theoretical density and an excellent thermal stability. Moreover, in the crude state, they have an equivalent or improved strength. DESCRIPTION OF THE INVENTION The invention relates to a process for obtaining fritted nuclear fuel pellets based on UO.sub.2 from metallic uranium, which does not produce any liquid effluent and which leads to intermediate uranium oxide powders which are dense and directly pourable without requiring any conditioning operation such as granulation, characterized in that the metallic uranium is oxidized by an oxidizing gas at high temperature in order to obtain an oxide of type U.sub.3 O.sub.8, which is crushed or milled to bring about an average grain size of approximately 10 to 30 .mu.m and then either it is reduced chemically to the state of uranium oxide of type UO.sub.2+x followed by the activation thereof by at least one fine gas jet milling operation (jet mill) or vice versa, or it is activated by a heat treatment constituted by a reduction and at least one an oxidation-reduction cycle, the thus obtained activated UO.sub.2+x powder being directly shaped by compression and then fritted. The starting metallic uranium is in solid or divided form and is generally oxidized by air or a gaseous mixture containing oxygen and optionally steam. In a dry atmosphere, normally a temperature of 600.degree. C. is not exceeded. Above this temperature, hard oxide blocks are obtained, which are difficult to use. Preference is given to 400.degree. to 550.degree. C. The oxidation rate increases in the presence of steam and the temperature can be raised to 800.degree. C., but preference is given to 600.degree. to 750.degree. C. This is followed by coarse grinding of the U.sub.3 O.sub.8 powder obtained to an average grain size of approximately 10 to 30 .mu.m. In the case where the powder obtained is activated by gas jet milling, the reduction takes place with the aid of pure or diluted hydrogen, e.g. 50% hydrogen and 50% nitrogen or cracked NH.sub.3, at a temperature above 550.degree. C. and preferably between 600.degree. and 700.degree. C. Gas jet milling activation can take place on the reduced and stabilized powder without having any significant influence on the O/U ratio. This activation corresponds to an increase in the specific surface of the powder, which passes from approximately 0.5 m.sup.2 /g on coarsely ground U.sub.3 O.sub.8 to at least 1.5 m.sup.2 /g and preferably between 1.7 and 3.5 m.sup.2 /g. Alternatively, it is possible to limit the gas jet milling and complete the activation by at least one an oxidation-reduction cycle performed on the reduced oxide. It is important to point out that it is standard practice to use fluidized bed gas jet milling, the apparatus having no impact plate or the like which is struck by the powder jet. For carrying out such milling use is made of an enclosure at the bottom of which are located several jets for supplying at high speed a clean gas (not containing powder). The jets produced by these nozzles converge at a single point. The enclosure is also supplied with powder to be milled, which is put into movement by the gas jets and which is milled at the convergence point by auto-attrition or inter-particular impacts. The gas-particle suspension or aerosol is discharged from the enclosure and separated by all known means (e.g. cycloning), whilst recovering on the one hand the milled powder and on the other the dust-removed gas, which is recycled to the nozzles after having been recompressed. This fluidized bed gas jet milling consequently makes it possible to avoid any pollution of the powder during milling, which could be brought about when using impact plates or nozzles traversed by a mixture of gas and powder, avoids the production of gaseous effluents, the vector gas being recycled and leads to a directly milled powder, which is completely usable for fritting, no powder recycling being required. It is remarkable to note that this process is so efficient, that it makes it possible to unexpectedly obtain an oxide of the type UO.sub.2+x in a direct and very easily frittable manner, because the density of the fritted end product generally exceeds 96% of the theoretical density. It should be noted that this result is also obtained when milling is carried out prior to reduction, which is not generally obtained with the prior art processes. In the case where the said coarsely milled U.sub.3 O.sub.8 powder is activated with the aid of at least one an oxidation-reduction cycle, it is reduced at a temperature above 550.degree. C. and preferably between 600.degree. and 700.degree. C., under a hydrogen-containing atmosphere. It is then oxidized in the presence of an oxygen-containing gas at below 600.degree. C. and preferably between 400.degree. and 500.degree. C. and is then reduced again as hereinbefore. If the oxidation temperature is too high, it is not possible to obtain a final powder with an adequate activity, having a good fritting aptitude, the latter being e.g. evaluatable by the BET specific surface. The choice of the oxidation and reduction temperatures makes it possible to modulate the specific surface of the final uranium oxide powder and it is possible to increase the number of redox cycles until a uranium oxide powder is obtained having the desired specific surface. It generally exceeds 1.5 m.sup.2 /g and is preferably between 1.7 and 3.5 m.sup.2 /g. In both the aforementioned powder activation cases, dense, pourable uranium oxide powders are obtained, which can be pelletized under conventional conditions, without any prior granulation, in order to obtain crude or raw pellets having a density generally between 5.50 and 6.90 g/cm.sup.3. Said crude pellets can be fritted under conventional conditions, e.g. 3 to 4 h at 1700.degree. to 1750.degree. C. under a hydrogen or hydrogen and nitrogen atmosphere, or at 1100.degree. to 1300.degree. C. under an oxidizing atmosphere, followed by a reduction at the same temperature in the presence of a hydrogen-containing gas. The final pellets obtained have a density of at least 95% of the theoretical density and normally above 96% of the theoretical density. By comparison, uranium oxide powders obtained by metallic uranium oxidation (with or without the presence of steam) and which have not been activated have a specific surface generally not exceeding approximately 1 m.sup.2 /g and lead to final fritted pellets, whose density is generally below 90% of the theoretical density, no matter what the crude density of the pellets used, which is inadequate and unacceptable for the use of such fritted fuel pellets in nuclear reactors. When it is wished to produce mixed oxide fuel pellets, prior to pelleting mixing takes place of the activated uranium oxide powder with at least one metallic oxide powder having an appropriate grain size, e.g. an oxide of Pu, Th, Ce, Gd, Hf, etc. Thus, the invention makes it possible to obtain fritted nuclear fuel pellets starting from metallic uranium using a very direct process and which does not generate liquid or in general even gaseous effluents, whose storage, treatment and disposal are normally difficult and costly. It also requires no stage of granulating the intermediate uranium oxide powders obtained prior to pelleting. Such a so-called dry route process is also particularly interesting in nuclear fuel production, because the criticality problems are simplified through the absence of water. Redox activation offers the advantage compared with gas jet milling of simplifying the problems of treating the gaseous effluents, the gas volume to be treated being much lower and the scrubbing of the gases being easier, because they contain very little or no suspended uranium oxide powder. The crude pellets obtained by this process also have an improved strength. |
abstract | A method and a device depressurize a nuclear power plant. A depressurization flow is conducted out of a containment shell into the atmosphere via a depressurization line having a filter system. The filter system contains a filter chamber having an inlet, an outlet, and a sorbent filter. The depressurization flow is first conducted in a high-pressure section, then is depressurized by expansion at a throttle device, then conducted through the filter chamber having the sorbent filter, and finally blown out. To enable an effective retention of activity carriers contained in the depressurization flow, including organic compounds containing iodine, the depressurization flow depressurized by the throttle device is conducted through a superheating section before the depressurization flow enters the filter chamber, in which superheating section the depressurization flow is heated from the not yet depressurized depressurization flow to a temperature that is at least 10° C. above the dew point temperature. |
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claims | 1. A method of sealing a ceramic-containing cladding of a nuclear fuel rod in a core of a nuclear water reactor, comprising:providing the ceramic-containing cladding which comprises:a tubular wall having an interior surface and an exterior surface;a cavity formed by the tubular wall;an end;an opening formed in the end; andan interior diameter;providing an end plug, comprising:a top surface;an opposing bottom surface;an exterior surface extending between the top and bottom surfaces; andan exterior diameter,wherein, the interior diameter of the ceramic-containing cladding is greater than the exterior diameter of the end plug, such as to form a gap there between;inserting the end plug into the opening in the end of the ceramic-containing cladding,wherein, the bottom surface of the end plug is positioned within a portion of the cavity and the top surface forms an end face of the ceramic-containing cladding;positioning a brazing material in at least a portion of the gap;heating the brazing material to a temperature at or above a thermal cure temperature of the brazing material to form a thermally cured brazing material;forming a first seal with the thermally cured brazing material between the exterior surface of the end plug and the interior surface of the ceramic-containing cladding; andforming a second seal on the end of the ceramic-containing cladding, comprising:depositing a coating which comprises SiC on the top surface of the end plug and on at least a portion of the exterior surface of the ceramic-containing cladding;enclosing entirely the top surface of the end plug with the coating; andenclosing completely the end of the cladding with the coating. 2. The method of claim 1, further comprising inserting the brazing material into a fill hole formed in the end plug. 3. The method of claim 1, wherein the positioning the brazing material comprises:depositing the brazing material onto at least one of a portion of the exterior surface of the end plug and a portion of the interior surface of the ceramic-containing cladding; andinserting the end plug into the opening in the end of the ceramic containing cladding, such that the brazing material forms an interface between the exterior surface of the end plug and the interior surface of the ceramic-containing cladding. 4. A method of sealing an open end of a nuclear fuel rod cladding for a nuclear water reactor, comprising:providing the cladding, comprising:a material including silicon carbide;an end;an opening in the end;a tubular wall having an internal surface and an external surface;an internal cavity formed by the tubular wall; andan internal diameter,wherein, nuclear fuel is disposed within the internal cavity;providing an end plug, comprising:a top surface;an opposing bottom surface;an external surface extending between the top and bottom surfaces; andan external diameter,wherein, the internal diameter of the ceramic-containing cladding is greater than the external diameter of the end plug;providing a brazing composition;applying the brazing composition to contact at least a portion of both of the external surface of the end plug and the internal surface of the cladding;applying heat to the brazing composition to form a thermally cured brazing composition;inserting the end plug with the thermally cured brazing composition into the opening in the end, the bottom surface being positioned inside a portion of the internal cavity and the top surface forming a closed end of the cladding, wherein the thermally cured brazing composition forms a first seal between the internal surface of the cladding and the external surface of the end plug; andforming a second seal on the end of the ceramic-containing cladding, comprising:applying a SiC-containing coating to the top surface of the end plug inserted in the opening in the end, and at least a portion of the external surface of the cladding;enclosing entirely the top surface of the end plug with the coating; andenclosing completely the end of the cladding with the coating. 5. A tubular ceramic composite cladding for a nuclear water reactor, comprising:the cladding, comprising:a material including silicon carbide;an end;a tubular wall having an internal surface and an external surface;an internal cavity formed by the tubular wall; andan internal diameter,wherein, nuclear fuel is disposed within the internal cavity;an end plug, comprising:a top surface;an opposing bottom surface;an external surface extending between the top and bottom surfaces; andan external diameter,wherein, the internal diameter of the ceramic-containing cladding is greater than the external diameter of the end plug;a space formed between the external surface of the end plug and the internal surface of the cladding;a first seal, comprising:a thermally cured brazing composition applied to at least one of a portion of the external surface of the end plug and a portion of the internal surface of the ceramic-containing cladding,wherein the thermally cured brazing composition fills the space; anda second seal, comprising:a SiC-containing coating on the top surface of the end plug that is inserted in the end of the cladding and forms an end face, and extends over at least a portion of the external surface of the cladding, to enclose entirely the top surface of the end plug and enclose completely the end of the cladding with the coating. |
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abstract | A computed tomography apparatus includes a scanning unit which is rotatable relative to an examination zone, about an axis of rotation extending through the examination zone, a radiation source for generating a radiation beam, a diaphragm arrangement which is arranged between the radiation source and the examination zone in order to form a fan beam traversing the examination zone from the radiation beam, and a two-dimensional detector arrangement including a plurality of detector elements and a part of the measuring surface of which detects primary radiation from the fan beam whereas another part of its measuring surface detects scattered radiation produced in the examination zone. Perfect acquisition of the momentum transfer spectrum is achieved in that a collimator arrangement with a plurality of lamellas is arranged between the examination zone and the detector arrangement. The lamellas preferably are situated in planes that intersect each other at the focus of the radiation source and subdivide the fan beam into a number of segments so that the detector elements present in a column extending parallel to the axis of rotation are struck by primary radiation or scattered radiation from the same segment. |
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claims | 1. A focused ion beam apparatus comprising:a scanning electron microscope having an objective lens that generates a magnetic field on a sample;an ion source;a focused ion beam optics to focus the ion beam emitted from the ion source on the sample;a canceling magnetic field generator to generate a canceling magnetic field on the optical axis of the ion beam; anda canceling magnetic field control unit for controlling the canceling magnetic field generated from the canceling magnetic field generator with the exciting current of the objective lens of the scanning electron microscope as an input signal thereto,wherein the deflection of the ion beam under the effect of a magnetic field external to the focused ion beam optics is canceled. 2. A focused ion beam apparatus according to claim 1, wherein the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other at a substantially single point on the sample. 3. A focused ion beam apparatus according to claim 2, wherein the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other substantially at 90 degrees to each other, andwherein an electron detector is arranged on the optical axis of the scanning electron microscope on the side of the crossing point far from the objective lens of the scanning electron microscope. 4. A focused ion beam apparatus according to claim 1, further comprising a magnetic shield for covering the focused ion beam optics. 5. A focused ion beam apparatus according to claim 4, wherein the magnetic shield covers the part of the focused ion beam optics far from the sample, andwherein the distance L from the end surface of the magnetic shield nearer to the sample to the sample is given as ∫ L 0 ⅆ z 1 V acc - ϕ ( z 1 ) ∫ L z 1 B y ( z ) ⅆ z ≅ 0where Vacc is the ion beam accelerating voltage, Φ (z1) the electric potential at the coordinate z1 on the optical axis of the ion beam, By(z) a magnetic field component perpendicular to the optical axis of the ion beam at the coordinate z on the optical axis, and the origin of the coordinate is located at the ion beam spot position on the sample in the absence of a magnetic field. 6. A focused ion beam apparatus according to claim 1, wherein the ion beam includes a plurality of ion beams having a plurality of different mass-to-charge ratios. 7. A focused ion beam apparatus according to claim 6, wherein the ion beams having the plurality of different mass-to-charge ratios are a plurality of types of isotope ion beams. 8. A focused ion beam apparatus according to claim 1, wherein the ion source is a Ga liquid metal ion source. 9. A focused ion beam apparatus according to claim 1,wherein the canceling magnetic field generator is arranged on the optical axis of the ion beam and has a beam path allowing the entire the ion beam to pass therethrough. 10. A focused ion beam apparatus according to claim 9,wherein the canceling magnetic field generator is covered by a magnetic shield. 11. A focused ion beam apparats comprising:a scanning electron microscope having an objective lens that generates a magnetic field on a sample;an ion source;a focused ion beam optics to focus the ion beam emitted form the ion source on a sample; anda canceling magnetic field generator to generate a canceling magnetic field on the optical axis of the ion beam;at least one magnetic field sensor in the neighborhood of the focused ion beam optics; anda canceling magnetic field control unit for controlling the canceling magnetic field generated from the canceling magnetic field generator with the output of the magnetic field sensor as an input signal thereto,wherein the deflection of the ion beam under the effect of a magnetic field external to the focused ion beam optics is canceled. 12. A focused ion beam apparatus according to claim 11, wherein the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other at a substantially single point on the sample. 13. A focused ion beam apparatus according to claim 12, wherein the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other substantially at 90 degrees to each other, andwherein an electron detector is arranged on the optical axis of the scanning electron microscope on the side of the crossing point far from the objective lens of the scanning electron microscope. 14. A focused ion beam apparatus according to claim 11, further comprising a magnetic shield for covering the focused ion beam optics. 15. A focused ion beam apparatus according to claim 14, wherein the magnetic shield covers the part of the focused ion beam optics far from the sample, andwherein the distance L from the end surface of the magnetic shield nearer to the sample to the sample is given as ∫ L 0 ⅆ z 1 V acc - ϕ ( z 1 ) ∫ L z 1 B y ( z ) ⅆ z ≅ 0where Vacc is the ion beam accelerating voltage, Φ(z1) the electric potential at the coordinate z1 on the optical axis of the ion beam, By(z) a magnetic field component perpendicular to the optical axis of the ion beam at the coordinate z on the optical axis, and the origin of the coordinate is located at the ion beam spot position on the sample in the absence of a magnetic field. 16. A focused ion beam apparatus according to claim 11, wherein the ion beam includes a plurality of ion beams having a plurality of different mass-to-charge ratios. 17. A focused ion beam apparatus according to claim 16, wherein the ion beams having the plurality of different mass-to-charge ratios are a plurality of types of isotope ion beams. 18. A focused ion beam apparatus according to claim 11, wherein the ion source is a Ga liquid metal ion source. 19. A focused ion beam apparatus according to claim 11,wherein the canceling magnetic field generator is arranged on the optical axis of the ion beam and has a beam path allowing the entire the ion beam to pass therethrough. 20. A focused ion beam apparatus according to claim 19,wherein the canceling magnetic field generator is covered by a magnetic shield. 21. A focused ion beam apparatus comprising:a scanning electron microscope having an objective lens that generates a magnetic field on a sample;an ion source;a focused ion beam optics to focus the ion beam emitted from the ion source on a sample; anda canceling magnetic field generator to generate a canceling magnetic field on the optical axis of the ion beam; wherein:the deflection of the ion beam under the effect of a magnetic field external to the focused ion beam optics is canceled,the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other at a substantially single point on the sample,the optical axis of the scanning electron microscope and the optical axis of the ion beam cross each other substantially at 90 degrees to each other, andthe apparatus further comprises an electron detector arranged on the optical axis of the scanning electron microscope on the side of the crossing point far from the objective lens of the scanning electron microscope. 22. A focused ion beam apparatus according to claim 21, further comprising a magnetic shield for covering the focused ion beam optics. 23. A focused ion beam apparatus according to claim 22, wherein the magnetic shield covers the part of the focused ion beam optics far from the sample, and wherein the distance L from the end surface of the magnetic shield nearer to the sample to the sample is given as ∫ L 0 ⅆ z 1 V acc - ϕ ( z 1 ) ∫ L z 1 B y ( z ) ⅆ z ≅ 0where Vacc is the ion beam accelerating voltage, Φ(z1) the electric potential at the coordinate z1 on the optical axis of the ion beam, By(z) a magnetic field component perpendicular to the optical axis of the ion beam at the coordinate z on the optical axis, and the origin of the coordinate is located at the ion beam spot position on the sample in the absence of a magnetic field. 24. A focused ion beam apparatus according to claim 21, wherein the ion beam includes a plurality of ion beams having a plurality of different mass-to-charge ratios. 25. A focused ion beam apparatus according to claim 24, wherein the ion beams having the plurality of different mass-to-charge ratios are a plurality of types of isotope ion beams. 26. A focused ion beam apparatus according to claim 21, wherein the ion source is a Ga liquid metal ion source. 27. A focused ion beam apparatus according to claim 21,wherein the canceling magnetic field generator is arranged on the optical axis of the ion beam and has a beam path allowing the entire the ion beam to pass therethrough. 28. A focused ion beam apparatus according to claim 27,wherein the canceling magnetic field generator is covered by a magnetic shield. |
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abstract | According to an embodiment, a core catcher has: a main body including: a distributor arranged on a part of a base mat in the lower dry well, a basin arranged on the distributor, cooling channels arranged on a lower surface of the basin connected to the distributor and extending in radial directions, and a riser connected to the cooling channels and extending upward; a lid connected to an upper end of the riser and covering the main body; a cooling water injection pipe open, at one end, to the suppression pool, connected at another end to the distributor; and chimney pipes connected, at one end, to the riser, another end being located above the upper end of the riser and submerged and open in the pool water. |
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045089698 | summary | The invention relates to a container for storing radioactive materials such as burned-out reactor fuel elements. THE PRIOR ART Containers having a cylindrical shape and which hold several fuel elements and can be closed with a cover have been previously disclosed. The loaded containers are put individually or several collectively into boreholes (vertical, horizontal or slanting boreholes) which are provided in the final storage place for instance, a salt mine. To facilitate transporting and handling the containers, they must be limited in size and weight. Special problems result during the production, transporting and final storing of such containers regarding corrosion, shielding against gamma radiation and neutron radiation, sealing, and strength of the connection between the container and the container cover, as well as regarding the reusability of the container and parts thereof. In order to prevent corrosion it has been proposed, depending on environment conditions, to fabricate the container from carbon-steel, high-grade steel or spheroidal graphite iron (GGG). For shielding against gamma radiation it has been known to use lead or other materials, which shield against rays and have a low melting point. For shielding against neutron radiation, hydrocarbons, for instance, polyethylene, have been used. THE INVENTION The object of the present invention is to provide a container of the type described having an absolutely tight, firm and secure connection between the container and the container cover. According to the invention this object is achieved by casting the cover from metal around connector elements projecting from the outer edge of the container wall surrounding the open end. The outer end of said elements are enlarged so that the connection to the cast cover is gastight and mechanically secure. The construction of the invention makes possible the casting of the cover after the loading of the container whereby an intimate connection on the sealing surfaces between cover and container is achieved so that perfect shielding also is obtained in the area of the sealing surface. The strength of the connection is sufficient to enable lifting the container by the cover. |
054421861 | abstract | A source capsule containing a radioactive isotope is put inside of an outer protective jacket or capsule and the outer jacket is sealed. The outer protective jacket is so designed that it can be reopened without disturbing the radioactive isotope source capsule contained therein. Thus, when it becomes necessary to recondition the source capsule, the inner source capsule can be removed from the outer protective jacket and it can be re-encapsulated in another outer protective jacket. There is provided an encapsulated radioactive isotope source which comprises an outer protective jacket in the form of a can having a side wall, an integrally formed bottom wall and an open upper end. A source capsule containing a radioactive isotope is received within this outer protective jacket. A jacket cap is received within the outer protective jacket so as to close the open end of the jacket, with the source capsule thus located within the outer protective jacket. The jacket cap has its outer peripheral edge positioned in close fitting relation to the inner peripheral surface of the jacket side wall to form a narrow gap between the jacket cap and the jacket side wall. A weld extends along the narrow gap to join the jacket cap to the jacket and to seal the source capsule within the protective jacket. |
claims | 1. A side-slotted nozzle type double sheet spacer grid for nuclear fuel assemblies, comprising a plurality of inner strips intersecting each other at a predetermined angle to form a plurality of four-walled cells to receive and support a plurality of fuel rods in the spacer grid, each of said inner strips comprising a plurality of unit strip parts each of which is fabricated by integrating two unit sheet parts together into a single structure, such that the two unit sheet parts face each other and a nozzle type coolant channel with an inlet and an outlet is defined between the two unit sheet parts, wherein each of said unit sheet parts is provided with a slot longitudinally formed on each side surface of a spring which is projected from the unit sheet part to support a fuel rod within a four-walled cell. 2. The side-slotted nozzle type double sheet spacer grid according to claim 1 , wherein the slot has a width in the range from 0.35 mm to 0.8 mm, and a length in the range from 12 mm to 16 mm. claim 1 3. The side-slotted nozzle type double sheet spacer grid according to claim 1 , wherein each of the inner strips is fabricated by integrating two thin sheets, each comprising an alternating arrangement of a plurality of first unit sheet parts not having any coolant channel outlet and a plurality of second unit sheet parts each having a coolant channel outlet, into a single structure such that one first unit sheet part and one second unit sheet part form one unit strip part. claim 1 4. The side-slotted nozzle type double sheet spacer grid according to claim 3 , wherein the intersecting inner strips are encircled with a plurality of perimeter strips, each of said perimeter strips being fabricated by integrating an inner thin sheet comprising the second unit sheet parts each having the coolant channel outlet, and a flat outer thin sheet into a single structure. claim 3 5. The side-slotted nozzle type double sheet spacer grid according to claim 4 , wherein each of said inner and perimeter strips has a thickness in the range from 0.25 mm to 0.40 mm, and each of the springs of said unit sheet parts has a width in the range from 7 mm to 10 mm between both side edges thereof. claim 4 6. The side-slotted nozzle type double sheet spacer grid according to claim 4 , wherein each of the first unit sheet parts not having any coolant channel outlet comprises one or more coolant mixing blades extending from an upper end thereof. claim 4 7. The side-slotted nozzle type double sheet spacer grid according to claim 6 , wherein the coolant mixing blades of the first unit sheet parts comprise lateral flow blades for creating a lateral flow of coolants between neighboring four-walled cells, swirl flow blades for creating a swirl flow of coolants at intersections of the strips in the spacer grid, or a combination of the swirl flow blades and the lateral flow blades. claim 6 |
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claims | 1. A method for making an absorber liquid available in an injection system, which comprises:providing an injection system with a reservoir vessel storing the absorber liquid under an operating pressure, a pressure vessel partially filled with a propelling fluid and an overflow line, the reservoir vessel for the absorber liquid being connected, via the overflow line, to the pressure vessel, the overflow line equalizing a pressure in the pressure vessel with the operating pressure in the reservoir vessel; andheating the propelling fluid such that the propelling fluid is stored during a storage period in a lower region of the pressure vessel in liquid form and in that, above it, a vapor cushion which forms by way of evaporation of the propelling fluid is maintained, thereby generating the operating pressure in the pressure vessel;wherein the pressure vessel is separate from and does not include a primary cooling circuit that cools a boiling-water nuclear power plant. 2. A method for injecting an absorber liquid into a component of a plant connected downstream of a reservoir vessel, the method which comprises:providing an injection system including the reservoir vessel storing the absorber liquid under an operating pressure, a pressure vessel partially filled with a propelling fluid and an overflow line, the reservoir vessel connected via the overflow line to the pressure vessel, the overflow line equalizing a pressure in the pressure vessel with the operating pressure in the reservoir vessel;heating the propelling fluid such that the propelling fluid is stored during a storage period in a lower region of the pressure vessel in liquid form and in that, above it, a vapor cushion which forms by way of evaporation of the propelling fluid is maintained; andintroducing first the propelling fluid in a liquid form and then second the propelling fluid in a vaporous form from the pressure vessel into the reservoir vessel, with the absorber liquid in the reservoir vessel being displaced;wherein the pressure vessel is separate from and does not include a primary cooling circuit that cools the plant and the plant is a boiling-water nuclear power plant. 3. The method according to claim 2, which further comprises setting an overflow speed of the propelling fluid during an injection process such that mixing with the absorber liquid is substantially prevented. 4. The method according to claim 2, which further comprises adjusting the operating pressure by regulating a heater. 5. The method according to claim 1, which further comprises adjusting the operating pressure by regulating a heater. |
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abstract | The present invention relates to a dosimetry device for verification of quality of a radiation beam in standard and conformal radiation therapy, and for IMRT (Intensity Modulated Radiation Therapy) applications. The device includes an active area comprising individual radiation detectors. The active area comprises a limited number of lines of radiation detectors, and a number of extra radiation detectors dedicated to the energy measurement of electrons or photons. It also comprises a build-up plate with energy degraders. The energy degraders are located upstream from the extra radiation detectors in the path of the radiation beam. |
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claims | 1. A system comprising:a charge carrier configured to carry a desired charge distribution, wherein the desired charge distribution has a first electromagnetic field; anda radiation source configured to generate a second electromagnetic field that interacts with the first electromagnetic field so as to produce a force on the charge carrier, wherein the desired charge distribution is not a consequence of the radiation source illuminating the charge carrier. 2. The system of claim 1, wherein the first electromagnetic field is time-variant. 3. The system of claim 2, wherein the first electromagnetic field is phase modulated. 4. The system of claim 2, wherein the time variation of the first electromagnetic field is a square wave. 5. The system of claim 1, wherein the radiation source is a coherent radiation source. 6. The system of claim 5, wherein the second electromagnetic field is phase modulated. 7. The system of claim 5, wherein the second electromagnetic field is directed through a first medium across a first distance, wherein the first medium has a first index of refraction, wherein the second electromagnetic field is directed through a second medium across a second distance, and wherein the first index of refraction is different from the second index of refraction. 8. The system of claim 5, wherein the second electromagnetic field is directed through an aperture in a conductive medium, the conductive medium partially screening the first electromagnetic field. 9. The system of claim 5, further comprising a mirror positioned in the path of the coherent radiation source and configured to retro-reflect the laser light. 10. The system of claim 1, wherein the radiation source is a laser. 11. The system of claim 1, wherein the charge carrier is a plurality of charge carriers configured so as to induce a force at each or any element in the plurality. 12. The system of claim 1, further comprising a platform including both the radiation source and the charge carrier. 13. The system of claim 1, wherein the first electromagnetic field is characterized by a first temporal period and the second electromagnetic field is characterized by a second temporal period, wherein the system further comprises:a phase control system configured to ensure a desired phase relationship between the first and second electromagnetic fields; anda force sensing and feedback system configured to monitor the net force and, in response, to control the radiation source, the charge carrier and the phase control system to adjust the interaction between the first and second electromagnetic fields to realize a desired net force on the charge carrier. 14. The system of claim 1, wherein the system comprises a phase modulation system, and wherein the phase modulation system comprises one of:an electro-optic member configured to receive the laser light from the radiation source and transmit the light after introducing a phase altering delay selected to adjust the interaction between the first and second electromagnetic fields to realize the desired net force; ora translator configured to move the radiation source relative to the first electromagnetic field in a direction selected to adjust the interaction between the first and second electromagnetic fields to realize the desired net force. 15. The system of claim 1, wherein the radiation source comprises a multi-mode laser operable in a plurality of operating modes, and wherein the charge carrier comprises a plurality of charge carriers each configured to interact and produce the desired force during an operating mode of the multi-mode laser. 16. A method of generating a net force in a system using electromagnetic field interaction, comprising the steps of:(a) generating a first electromagnetic field carried by a member of the system; and(b) generating a second electromagnetic field with a radiating device, wherein the second electromagnetic field interacts with the first electromagnetic field to produce the net force on the member and on the system, and wherein the radiating device is not used to generate the first electromagnetic field. 17. The method of claim 16, wherein the generating of the first electromagnetic field in (a) comprises configuring a time-variant charge distribution in the member that produces the first electromagnetic field. 18. The method of claim 16, wherein the radiating device is a laser, wherein the generating of the second electromagnetic field in (b) comprises using the laser to generate a laser beam that carries the second electromagnetic field, and wherein the laser beam is not directed at the member of the system. 19. A system comprising:a set of field carriers that carry electrical signals, wherein the electrical signals generate a first electromagnetic field; anda means for generating an electromagnetic field, wherein the means generates a second electromagnetic field, and wherein the second electromagnetic field interacts with the first electromagnetic field thereby generating a force on the set of field carriers. 20. The system of claim 19, wherein the means for generating an electromagnetic field is a laser that outputs a laser beam, and wherein the laser beam propogates substantially near the set of field carriers. |
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056299710 | description | DETAILED DESCRIPTION OF THE INVENTION In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions relating to functions of the computer system of the present invention utilizing terms such as "processing" or "computing" or "calculating" or "determining" or "displaying" or the like, refer to the action of a computer system, or similar electronic computing device, that is executing a program to manipulate and transform data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers. I. NONUNIFORM ATTENUATION CORRECTION SYSTEM Embodiments of the present invention relate to the collection, generation and usage of nonuniform attenuation correction factors used to improve gamma camera imaging. Since each patient that is imaged by a nuclear medicine gamma camera is different (e.g., differently shaped with different sizes, etc.) the tissue and bone structure that surround an organ of interest is different for each patient. This surrounding tissue and bone structure attenuates the radiation emitted from a radiopharmaceutical distributed within the imaged organ. The attenuation of the radiation is nonuniform because the attenuation coefficients of the different tissues and bone are different. Radiation attenuation non-uniformly reduces the count density in the image. This attenuation can lead to falsely identifying an artifact when in fact healthy tissue is imaged and vice-versa. If an artifact is improperbly diagnosed as a lesion, this can lead to invasive measures which are painful and potentially dangerous (e.g., involves a health risk) for the patient. Nonuniform attenuation caused by the body can be compensated for if the attenuation map of the body is known. Transmission scanning allows a gamma camera and a processing computer system to generate a nonuniform attenuation map of a particular object. This nonuniform attenuation map can be obtained for each rotation angle of a scintillation detector so that a reconstruction algorithm can use the attenuation map for each angle. Generally, during transmission scanning, a source of known radiation is emitted through the patent to be scanned and then the radiation is detected by a scintillation detector. By measuring the intensity of the radiation emitted from the source, and by measuring the intensity of radiation emitted through the object at different ECT angles, the gamma camera's computer system can determine the extent of nonuniform radiation attenuation over different parts of the body. From this, a nonuniform attenuation correction map of the body can be determined using well known methods and procedures. The nonuniform attenuation correction map is used to correct emission image data collected during emission studies. Embodiments of the present invention are directed at improving the collection and use of transmission data for use in generating nonuniform attenuation correction factors to improve emission image data. A. DETECTOR SYSTEM The detector system of an embodiment of the present invention is illustrated in FIG. 1. This is a dual head implementation, however, embodiments of the present invention can operate equally effective within a single or other multi- head embodiment. Regarding the dual head implementation, two scintillation detectors 10 and 12 are installed within the gantry (e.g., between gantry rings 50) and are rotatable about the center of the gantry ring 50. Each scintillation detector contains a crystal layer and an array of photomultiplier tubes, each PMT generates a separate channel signal responsive to light energy released by the crystal layer in response to a gamma interaction therein. As shown in FIG. 1, the detectors are at a 90 degree angle with respect to each other. A table (not shown) is placed into the gantry ring 50 and a patient rests on top of the table for imaging. Channel signals from the detectors are then sent to signal processing hardware 120 and to a computer system 112 for image processing (including corrections), see FIG. 4. The detectors 10 and 12 of FIG. 1 can contain event detection circuitry that transforms the signals from the photomultipliers to digital signals representative of the spatial coordinate of each detected event and the event energy. This logic can also be externally located (for instance, see FIG. 4, signal processing hardware 120). An event, or "count" is reported by the detectors to a computer system 112 for correction, analysis, storage and image generation. The detectors can collect and report radiation that is emitted from a patient (e.g., emission image data) and can also collect and report radiation emitted form a line source (transmission image data). Transmission data is utilized, among other things, for generation of attenuation correction distributions to compensate for nonuniform attenuation attributable to the patient (e.g., the chest region in cardiac studies). A separate radiation emitting line source (with collimator) is mounted and associated with each scintillation detector. For instance, line source assembly 22 is associated with detector 12 and line source assembly 20 is associated with detector 10. Also, line source assembly 22 is mounted on rail 24 and the base of line source assembly 22 can move along the long axis of rail 24, as shown in order to displace ("scan") across the field of view of the associated detector. Likewise, line source assembly 20 is mounted on rail 26 and the base of line source assembly 20 can move along the long axis of rail 26, as shown in order to displace ("scan") across the field of view of the associated detector. It is appreciated that when the detectors rotate about the center of gantry ring 50, the associated emission line source will rotate in like form and degree. The line sources are utilized by the present invention for irradiating a patient in order to gather transmission data for the generation of the nonuniform attenuation correction map that is stored in memory for the patient. The nonuniform attenuation correction map is used to compensate for nonuniform attenuation of the emission data which is also collected by the present invention detectors 10 and 12. The scintillation detectors 10 and 12 can rotate about the gantry 50 so such that they are in the positions (180 degree orientation) shown in FIG. 2. FIG. 2 also illustrates the line source assemblies 22 and 20 in their storage position. FIG. 3 illustrates a cross section of an exemplary design of the scanning line source assembly 22 of one embodiment of the present invention in more detail (line source assembly 20 is similar). The radiation emitting source or rod is shown as 22b. The radiation is emitted through a small aperture and then through a shutter 22a that rotates to vary the amount of radiation emitted. Radiation is allowed pass through small openings places on either side of the shutter 22a. The radiation then exists via slot (collimator) 22c. Different types of line sources 22b can be utilized within the scope of the present invention, including a Tc-99m filled line source or a line source using Gd-153, Am241, or Co-57. Generally, the transmission source should be of a different photo peak energy than the emission source. As will be discussed in further herein, various embodiments of the present invention include special radiation filters within the line source assemblies. B. COMPUTER PROCESSOR SYSTEM Refer to FIG. 4 which illustrates components of a general purpose computer system 112 used by the present invention for processing image information supplied from gamma camera detectors 10 and 12. The general purpose computer system 112 is capable of performing image processing functions (e.g., processing emission and transmission data). The computer system 112 comprises an address/data bus 100 for communicating information within the system, a central processor 101 coupled with the bus 100 for executing instructions and processing information, a random access memory 102 coupled with the bus 100 for storing information and instructions for the central processor 101, a read only memory 103 coupled with the bus 100 for storing static information and instructions for the processor 101, a data storage device 104 such as, a magnetic or optical disk and disk drive coupled with the bus 100 for storing image information and instructions, a display device 105 coupled to the bus 100 for displaying information to the computer user, an alphanumeric input device 106 including alphanumeric and function keys coupled to the bus 100 for communicating information and command selections to the central processor 101, a cursor control device (part of the data input device 106) coupled to the bus for communicating user input information and command selections to the central processor 101, and a communication device 108 coupled to the bus 100 for communicating command selections to the processor 101. A hardcopy device (e.g., printer) may also be coupled to bus 100. The display device 105 of FIG. 4 utilized with the computer system 112 of the present invention may be a liquid crystal device, cathode ray tube, or other display device suitable for creating graphic images and alphanumeric characters recognizable to the user. The cursor control device allows the computer user to dynamically signal the two dimensional movement of a visible symbol (pointer) on a display screen of the display device 105. Many implementations of the cursor control device are known in the art including a trackball, finger pad, mouse, joystick or special keys on the alphanumeric input device 105 capable of signaling movement of a given direction or manner of displacement. The computer system 112 of FIG. 4 interfaces with the gamma detector 10 and 12 via signal processing hardware circuits 120 over bus 122. The signal processing hardware 120 can compose amplification circuitry and analog to digital conversion circuits for converting channel signals from the detectors to digital data for transmission to the computer system 112. It is appreciated that channel signal information from the detectors 10 and 12 are converted into count density information by the computer system 112 and stored within the computer's memory 102 in matrix form, depending on the type of imaging session performed. This is true for both transmission and emission data. Nonuniform attenuation correction maps are stored in the computer memory 102 as well. The signal processing hardware 120 converts photomultiplier output into spatial coordinate data and event energy for detected events. Events within similar spatial coordinates are "binned" together in the memory 102 of the computer system in order to generate image information and form count density information. This image information is collected in the form of a matrix of N rows by N columns. The size of the detector's effective field of view and the number of rows and columns of a particular matrix define the "size" of a pixel of the matrix. A pixel corresponds to one cell or "bin" of the matrix. Image matrices are generally collected at different ECT angles and then a reconstruction is done, using tomographic reconstruction to generate a three-dimensional image of an organ. It is appreciated that the computer system 112 also controls movement of the detectors on the gantry ring 50 and also controls line source motion controllers for controlling the movement of the line sources 20 and 22. Although the scintillation detectors 10, 12 are capable of reporting scintillation events with great accuracy, the computer system 112 collects and interprets the data depending on reference matrix sizes (e.g., 64.times.64, 128.times.128, 512.times.512 and 1024.times.1024). These sizes are programmable. Further, a given matrix size can be allocated to certain portions of the field of view of the detector. For instance, in a cardiac study, a 512.times.512 matrix can be allocated to only the region of the detectors' field of view that covers the heart. Therefore, when data is received from the scintillation detector regarding the energy and location of a detected interaction, this information is "binned" (e.g., placed) into the appropriate matrix entry that corresponds to the location of the interaction as reported by the detector. Count information reported by the detectors is binned into memory 102 and image data is taken from there. This is true for both transmission and emission data. II. SCAN SPEED EMBODIMENT The present invention includes a scan speed embodiment wherein a minimum exposure time period is determined, patient by patient, for exposure to radiation emitted from the scanning line sources 20 and 22 for collection of transmission data. Patients vary by thickness and the more thick the patient, the more the patient needs to be irradiated to gather sufficient transmission data due to transmission radiation attenuation. However, the present invention determines the minimum dosage of radiation required to obtain sufficient transmission image data in order to reduce exposure time on a patient by patient basis. To promote the health of the patient, it is advantageous to reduce this exposure time to the patient. In order for the nonuniform attenuation correction map to be properly determined by the present invention (and there are a number of different ways in which this data can be computed) each elemental bin ("cell" or "pixel") of the image matrix for the patient must collect a certain minimum number of counts as a result of the transmission exposure. Using the reported transmission data, from a pre-scan phase, and knowing the original exposure of radiation, the gamma camera system of the present invention can determine a nonuniform attenuation correction map of the patient. Mechanisms for computing attenuation correction maps based on the above information are well known. If there are more than the minimum number of counts per bin as a result of the transmission exposure, then the nonuniform attenuation correction distribution can still be properly computed, but the patient will be exposed to an unnecessary dosage of radiation to collect the transmission data. This embodiment of the present invention determines the particular dosage of radiation (e.g., exposure time) required in order to not unnecessarily expose the patient to transmission radiation. Generally, in order to perform the above, the present invention provides two different transmission scan sequences or "phases." A first or "prescan" phase is performed and is a rapid single pass scan of low radiation dosage performed to obtain the minimum count density measured as a result of the prescan. Although a single pass scan is utilized for the prescan phase under the preferred embodiment of the present invention, multiple scan passes can also be contemplated within the scope of the present invention. The information of the prescan phase determines the duration of the normal transmission scan, which is the second transmission scan phase. The normal transmission scan is used to generate a nonuniform attenuation correction map. This process is explained in further detail below. It is appreciated that under the preferred embodiment of the present invention, the second or "normal" transmission scan phase is a multi-pass scan phase. The processing 200 of this embodiment of the present invention is performed via general purpose computer system 112 and is illustrated with respect to FIG. 6. Within the discussions of process 200, reference is also made to FIG. 5 and FIG. 7. The process 200 enters and at the first block 205, the known values are input from the data input device 106. The known values input at block 205 consist of (1) the minimum required counts per bin, Co, (which is also called the minimum count density) and (2) the time period for the prescan transmission radiation, Tp. The minimum required counts per bin, Co, in order to compute the nonuniform attenuation correction distribution will vary depending on the type of nonuniform attenuation correction map desired and the type of scan performed. Since this embodiment of the present invention can effectively operate to generate a number of different types of attenuation distributions based on a number of different types of scans, there is no specific minimum count number. However, with respect to the disclosed embodiment herein, the exemplary minimum number of counts is 20 to 30 (e.g., Co=20-30). Also, in one embodiment, the prescan exposure duration is programmable depending on the type of prescan performed and the type of nonuniform attenuation correction map desired. With respect to the disclosed exemplary embodiment, this value is in the range of 5-15 seconds (Tp=5-15). It is appreciated that at block 205, the values Co and Tp can be programmed into computer system 112 as default values and therefore no data input is required unless these defaults are to be altered. At block 205, the patient is placed onto a table and placed with the gantry ring 50. The scintillation detectors are arranged such that they will image different views of the patient. This can be performed a number of different ways and an exemplary arrangement is illustrated in FIG. 5. As shown in FIG. 5, the view is looking into the gantry ring structure 50 of the detector system (at the head of the patient 5 located along the Z axis). Scintillation detectors 10 and 12 are oriented at right angles to each other with detector 10 on top and detector 12 at the side. Line source assembly 20 scans the patient down the Z axis while the surface of detector 10 is above and within the XZ plane. Transmission detection region 10a receives collimated radiation as the scanning line source assembly 20 travels down axis Z. Region 10a scans in synchronization with assembly 20. Similarly, line source assembly 22 scans the patient down the Z axis while the detector 12 is within the YZ plane. Transmission region 12a receives collimated radiation as the scanning line source assembly 22 travels down axis Z. Region 12a moves in synchronization with assembly 22. During the prescan operation, the time period required for the scanning line sources to move completely down the Z axis to radiate the patient 5 is the prescan time Tp. Therefore, the shorter the period selected, the faster the line sources move during the prescan and the longer the period, the slower the line sources move during the prescan. After the prescan, the two line sources 20 and 22 will again scan the patient 5 during the normal transmission scan. Referring back to FIG. 6, with the patient 5 placed in the gantry and the time period Tp selected, the present invention at block 210 prescans the patient, under computer system 112 control, by having both line sources 22 and 20 radiate the patient 5 while moving along the Z axis. Detectors 12 and 10 collect and report the transmission data via regions 10a and 12a to the computer system 112. When the prescan of block 210 is complete, the computer system stores a first count density information and then computer system 112 analyzes this transmission data at block 215 to determine the minimum count density measured, Cm, as a result of the transmission exposure. FIG. 7 illustrates an exemplary transmission map and shows count density versus the patient profile. Computer system 112 stores the "prescan" transmission map in memory 102. The map can also be stored in disk 104. Typically the side portion of the patient is the most attenuated portion and therefore contains the smallest count density measured, Cm. As the patient is thinner at the edges, the count density is larger at these parts of the profile. As the patient is thicker at the center, the count density is smaller at these regions. The result of the patient profile 241 is a U shaped "prescan" transmission map as shown in FIG. 7. It is appreciated that a number of different methods and mechanisms are known in the art to compute a transmission map and different maps can be generated. For instance, a transmission map can be computed in three dimensional space (by gathering transmission data with ECT motion), the transmission map can be presented based on patient profile. Regardless of the way in which the "prescan" transmission data is presented, the present invention at block 215 determines the portion of the patient that most attenuates the transmission exposure. The count density of the "prescan" transmission map at this portion is the minimum count density measured by the prescan, Cm. At block 215, therefore, the "prescan" transmission map 241 is measured and the lowest measured count density, Cm, is recorded to memory 102. At block 220, the present invention computes the minimum scan time required to perform the normal transmission scan that is used to generate the nonuniform attenuation correction factors of the present invention. The following relationship is utilized: ##EQU2## Wherein the ratio between the time period of exposure for the prescan transmission exposure period (Tp) over the measured minimum count density for this prescan transmission (Cm) should be equal to the ration between the minimum time required to perform the normal transmission exposure (Ts) over the minimum count density required to perform the normal transmission exposure (Co). The values Co and Tp are input at block 205. The value Cm is computed at block 215. The above relationship can be rewritten as: ##EQU3## The computer system 112 at block 220 performs the above procedure in order to compute Ts, the minimum exposure time period required to insure that the minimum required count density, Co, is acquired by the normal transmission exposure scan time, Ts. At this time period, Ts, the part of the patient causing the most attenuation in the transmission data will still allow the collection of the minimum counts required of transmission data for generating a sufficient nonuniform attenuation correction map. At block 225, the present invention computer system 112 directs the detector system to perform a normal transmission scan phase of the patient (similar to the scan performed in block 210), however, the normal transmission scan is performed using an exposure period of Ts. As discussed above, the second or "normal" transmission scan phase is a multi-pass scan phase across various angles of rotation around the patient. As a result of this normal transmission exposure, transmission data is collected by both detectors 10 and 12 at different projection angles and a normal transmission map (a second count density) is generated at block 230. From this, a nonuniform attenuation correction distribution is utilized for the correction of emission image data collected from the patient. The nonuniform attenuation correction map is stored in memory 102. Utilizing the above procedure, the patient 5, is exposed to the minimum required radiation dosage during the normal transmission exposure. This is advantageous because of the varying thickness of different patients. If one exposure dosage was developed for all patients, smaller patients would be subjected to an unnecessarily long transmission exposure or larger patients would not have enough exposure to provide a usable attenuation correction map. Therefore, since the present invention determines the optimum exposure dosage on a patient by patient basis, each patient receives the minimum transmission dosage required to generate a sufficient nonuniform attenuation correction distribution. The above procedure can be utilized with respect to a single transmission scan across a patient at a known or given angle relationship with respect to the detector and the patient. However, this technique is extended to include determining the minimum transmission exposure durations required (for each angle) within a transmission session involving ECT motion. This aspect of the present invention involves performing the prescan (of duration Tp) with both line source 20 and line source 22 in order to determine a minimum measured count value for both the anterior and lateral dimensions for a given object during the prescan. In other words, a Cm(anterior) and a Cm(lateral) can be obtained, one from each scintillation detector, during the prescan. By dividing the above count densities by the prescan Tp duration, a count rate can be determined for both the anterior and lateral dimensions. These count rates are expressed as Cr(anterior) and Cr(lateral). EQU Cr(anterior)=Cm(anterior)/Tp EQU Cr(lateral)=Cm(lateral)/Tp Based on a body contour of the object (e.g., cross sectional profile of object) in conjunction with the lateral and anterior measured count rates, a particular transmission count rate can be determined by the computer system 112, using geometry, for each angle of rotation for an ECT scan. This count rate for a given angle is the transmission rate through the body for that given angle. This count rate value can be expressed as Cr(i), where (i) is the angle of rotation for a given ECT scan angle. The count rate for each angle, Cr(i), depends on the width and length of the object which can be supplied via the contour information. A number of well known techniques can be utilized to obtain body contour data. Given the count rate, Cr(i), for each ECT angle, i, and given the minimum number of counts required for a given transmission scan, Co, the computer system 112 can compute the minimum time required for transmission at each ECT angle, Ts(i), according to the below relationship: EQU Ts(i)=Co/Cr(i) The transmission duration, Ts(i), for each angle is then stored in the memory 102 of the computer system 112, for a given angle of rotation, i, the present invention will only expose the patient to the transmission radiation for a duration of Ts(i). This process continues until each angle of rotation is complete. Based on this information, a reconstruction is performed by the computer system 112 (using well known methods and procedures) in order to generate a three dimensional transmission map of the object. From this, a complete nonuniform attenuation correction map is derived for each angle of rotation. This embodiment of the present invention is particularly advantageous in reducing the transmission exposure of the patient because of the number of different transmission exposures (e.g., one for each angle or rotation). It is appreciated that within the scope of the present invention, as an alternative operation to the above, a uniform scan rate can also be applied across all angles of ECT rotation. One embodiment of the present invention using the above ECT technique, utilizes two detectors 10, 12 oriented at 90 degrees and uses both line sources 20, 22 to scan along the patient so that anterior and lateral transmission information are simultaneously gathered by each detector, respectively. In such case, the line sources radiate the patient simultaneously during prescan along the patient's long axis. Then the values Cm(lateral) and Cm(anterior) can be computed from the transmission maps of the detectors. As discussed above, using this information the minimum transmission duration can be obtained for each angle of rotation of the detector pair. The normal transmission scan for each angle can then be performed. III. VARIABLE LINE FILTER EMBODIMENT The present invention includes a variable line filter embodiment that is utilized within the line source assemblies 20 and 22. For discussion, details of line source assembly 22 are illustrated herein, however, it is appreciated that this discussion applies equally to the line source 20. This embodiment of the present invention involves the use of several differently shaped filters that are rotatably mounted to surround the line source within a line source assembly. A control unit provides for the selection of a particular line source filter that can be rotated into place for different projection angles of a scintillation camera during transmission scanning. Reference is made to FIG. 8A where a simplified diagram (cross section) is made of the line source assembly 22. The line source 22b is shown in the center of the assembly and this source 22b is surrounded by different filters 302 and 301. The collimator slit 22c is shown for orientation. Filters 302 and 301 are mounted such that they can rotate around the line source 22b to an installed position. FIG. 8A shows filter 302 in the installed position. A control unit controls the rotation of the filters around the line source such that different filters can be installed. As shown, filter 302 is larger in dimension than filter 301 and would therefore provide more filter capacity when installed. Although the filters 301 and 302 are shown as wedged shaped (e.g., thicker on the ends and thinner in the mid section), the filter can adopt any general shape or profile within the scope of the present invention. The purpose of the installed filter is to reduce the high detected count rates which occurs when portions of the transmission line source 22b extend beyond the object 5 being imaged and directly irradiate the scintillation detector 12. As shown in FIG. 8A, the filter is a wedge which is thickest towards the outside of the detector's field of view and reduces thickness toward the inside (center portion) where it is necessary to have maximum flux to penetrate the object 5 being imaged. For different emission projections, where different portions of the line source are directly exposed to the detector, a filter of different shape is rotated into the installed position, as needed. This reduces the count rate toward the outside of the object 5 while allowing the required penetration through the inside of the object. Therefore, the present invention recognizes that a constant filter is not advantageous during an ECT scan where different portions (unobstructed portions) of the detector are directly exposed to the line source as different projection angles are traversed during the scan. FIG. 8B illustrates another cross sectional view of the simplified line source assembly 22 of the present invention. As shown, the line source 22b is surrounded by four filters 301, 302, 304, 307 of varying size and shape. The filters are rotatably attached together so that they can rotate (clockwise or counter-clockwise) around the line source 22b. During use, at any one time, only one filter is installed. A collimating slit 22c is also shown in the line source assembly 22. The filter 302 is shown in the installed position. FIG. 8C illustrates a more detailed cross sectional view of a scanning line source assembly 22 of this embodiment of the present invention. As shown, the assembly 22 is composed of an upper cover 22d and a lower cover 22f. There is a lead shielding 22e that is surrounded by the upper cover 22d. The shutter assembly is composed of a lead shielding 22ab that surrounds a steel tubing 22ac. The tubing 22ac is rotatably attached such that it may turn with respect to the lead shielding 22ab. The same is true for the shielding 22ab, it may turn with respect to the tubing 22ac. The collimating slit 22c of the line source assembly 22 is also shown. FIG. 8C further illustrates a cross sectional view of the line source 22b that is surrounded by a control junction 310. Different types of line sources 22b can be utilized within the scope of this embodiment of the present invention, including a Tc-99m filled line source or a line source using Gd-153, Am241, or Co-57. The multiple filters 301, 304, 302 and 307 are attached to the control junction 310 and in this way are rotatably attached to surround the line source 22b. It is appreciated that there are a number of well known methods for rotatably attaching the filters to surround the line source 22b and the implementation illustrated in FIG. 8C is exemplary. The lower cover 22f surrounds a leading shielding 22g. The filters can be composed of a number of different materials for providing attenuation of the radiation emitted from the line source 22b, and one such material is copper. It is further appreciated that the number of filters that can be attached to the control junction 310 is variable and constrained only due to the size and physical characteristics of the filters and that the illustration of two and four filters herein is exemplary only. Filter 301 is shown in the installed position and the control junction rotates to install the other filters as needed. The installed filter 301 acts to attenuate the radiation from the line source 22b before the radiation passes through the shutter assembly 22ab and 22ac. Then, the radiation is collimated by slit 22C before exiting the line source assembly 22. It is appreciated that in an alternate embodiment of the present invention, the filter configuration can be placed after the shutter assembly 22ab and 22ac between the shutter assembly and the collimating slit 22c. In such alternate embodiment, the installed filter would be exposed to the radiation while the remainder of the filters are positioned such that they do not interfere with the transmission. FIG. 8D is an outside view of the line source assembly 22 viewing from the lower cover 22f (which appears on top). The upper cover 22d is also shown. Coupled to the side of the assembly 22 are two control rods 314 and 312. Control rod 312 is coupled to the shutter assembly 22ab and 22ac and controls the opening and closing (or partial opening) of the shutter assembly. Control rod 314 is coupled to the control junction 310 of the filter assembly and controls the rotation of the control junction 310. Control rod 314 therefore controls the rotation of the filters about the line source 22b and will control the installation of a particular filter. Both of the control rods 312 and 314 are actuated by a separate computer controlled motor 312a and 314a, respectively. While many different embodiments of actuators can be used consistent within the scope of the present invention, an exemplary motor of 312a and 314a is capable of rotating both clockwise and counter-clockwise and can be of a stepper motor design that is digitally controlled from a computer system such as computer system 112. Under the above implementation, both the shutter assembly 22ab and 22ac and the filter assembly are computer controlled. Operational Use of Variable Filter The following describes the operational use of this embodiment of the present invention variable filter configuration. This embodiment can be used in ECT studies or within studies that do not require ECT motion (such as total body scans). For transmission studies that do not involve ECT movement, the object being scanned can be evaluated to determine the amount of the detector that is unobstructed by the object, and based on this amount, an appropriately sized and shaped filter can be selected to compensate for this unobstructed portion of the detector. Then, either manually or under computer control the proper filter is installed within the line source assembly 22 or assemblies 20 and 22. The amount of the detector that is unobstructed by the object can be determined by using body contour information (see below). FIG. 9 illustrates a process flow 350 representing an operational use for the variable filter embodiment of the present invention for ECT scans. The process starts at block 354 where the shape of the object is determined to arrive at body contour information. A method and apparatus are disclosed in U.S. patent application Ser. No. 07/981,833, entitled "PROXIMITY DETECTOR FOR BODY CONTOURING SYSTEM OF A MEDICAL CAMERA", and issued on Dec. 27, 1994 as U.S. Pat. No. 5,376,796, for obtaining the profile of an object being scanned. This patent is hereby incorporated by reference. This patent discloses that body contour information is constructed for an object to be scanned by integrating the body profile data. However, it is appreciated that the methodology and apparatus described above for obtaining a body contour is exemplary only and that there are a number of different well known ways to collect such data. In any case, the body contour information is typically stored in a computer system's memory (e.g., RAM 102) so that it can be recalled for future use. At block 356 of FIG. 9, during an ECT scanning session, for a particular angle of rotation of the scintillation detector the computer system 112 determines the amount of the detector (e.g., 12) unobstructed by the object during a transmission scan wherein the line source assembly (e.g., assembly 22) is radiating. This determination is performed using well known geometric procedures given the known body contour of the object and the angle of rotation of the scintillation detector. The result can be expressed as: EQU Xunobstructed=F(theta, contour) where Xunobstructed is the amount of the detector's field of view that is unobstructed by the object during transmission, theta is the current rotation angle and contour is the body contour database information for the portion of the body being scanned. The above procedure assumes the configuration of the gantry structure (e.g., the position of the detector with respect to the object) and the location of the object being scanned are known. The process required to compute the unobstructed portion of a detector, given the angle of rotation and the contour data, is well known and a number of different geometric relationships can be used consistent within the scope of the present invention. At block 358 of FIG. 9, the present invention then directs the computer system 112 to select the most appropriate filter to install in order to compensate for the unobstructed portion of the detector and minimize the count density over the unobstructed portions of the detector's field of view. For example, if the ECT rotation angle is low and the side of the object is being scanned, then the unobstructed portion of the detector is large and a filter having relatively thick sides will be used. Or, if the ECT rotation angle is high and the forward portion of the object is being scanned, then a filter is selected having relatively thin and smaller sides. A database can be constructed in memory 104 having a column for the obstructed portion and a column for a corresponding filter type. The computer system 112 then compares the computed unobstructed portions of the detector's field of view (Xunobstructed) to the proper database column until a close match occurs. Then, the computer system selects the proper filter for use based on the corresponding entry of the database. The filter selected is that filter that minimizes the count rate to the unobstructed portion of the detector but allows sufficient count rate through the object for the obstructed detector portion. At block 360 of FIG. 9, the computer system 112 directs motor 314a to turn control rod 314 to actuate the control junction 310 so that the proper (e.g., selected) filter is installed. It is appreciated that the computer system 112 stores a filter order database indicating the order that the filters are installed on the control junction 310 for selection of the proper filter. Further, an identification circuit can also be employed to automatically report the type of filters and their location within the control junction. Such identification circuits using binary coding are well known in the art. At block 362, the present invention then instructs the line source assembly (e.g., 22) to perform the required transmission using the proper filter. The shutter assembly 22ab 22ac is then allowed to open. At block 364, the present invention then determines if more ECT rotation angles are required. If not, the process 350 ends at block 368. If so, the process flows to block 366 where the gantry 50 is actuated so that the detector (e.g., 12) is positioned to a new angle of rotation and block 356 is then entered again with a new theta value. At the completion of the process 350, a transmission map of the object is gathered using variable filters. This transmission map can then be utilized by the present invention to construct a set of nonuniform attenuation correction factors that can be used to correct image data gathered by the scintillation detectors during an emission study. The set of attenuation correction factors is used to compensating for the nonuniform attenuation of the emission radiation caused by the object's body. IV. DUAL LINE SOURCE AND SLIDING DUAL TRANSMISSION DETECTION WINDOWS An embodiment of the present invention is described for reducing the effects of side scatter (or cross-talk) during transmission and emission scanning in a dual scintillation detector environment. The present invention embodiment also operates within the above system wherein transmission and emission data are collected simultaneously. In this embodiment of the present invention, two sliding transmission detection windows are utilized and move across the detector surfaces in conjunction with the two scanning line source assemblies 20 and 22 (also called scanning line sources). The transmission detection windows are electronically generated and are used to define a particular area within the field of view of a detector. In all discussions to follow within this embodiment of the present invention, it is assumed that each detector (e.g., detector 10 and 12) is collimated and further that this embodiment of the present invention can be implemented within a gamma camera performing (1) simultaneous or (2) sequential emission/transmission scanning. The two line sources and the two sliding windows move in synchronization to scan the field of view of the detectors and at any given position are all located within a single spatial plane (e.g., the long axis of the two line sources and the length of the two transmission detection windows are aligned within a single spatial plane). This spatial plane is transverse to the long axis of the object or patient being scanned. Particularly, if considering the gamma camera arrangement of FIG. 1, this spatial plane is perpendicular to the axis that runs through the two gantry rings 50. Using this dual line source scanning configuration, cross talk or scattered radiation originating from a given line source is not detected as emission data by the detector that is not associated with the given line source (e.g., the detector that is not directly facing the line source). Secondarily, the effect of emission crosstalk is also reduced. The sliding transmission detection windows defined within the field of view of the detectors 10 and 12 are programmed by the computer system 112 to detect only photons within the energy level of the transmission radiation; photons of the emission radiation level detected within the window are ignored by the camera system. The ability to electronically define a window region within a scintillation detector is well known in the art, for instance reference is made to U.S. Pat. No. 5,304,806, entitled, "Apparatus and Method for Automatic Tracking of a Zoomed Scan Area in a Medical Camera System," issued Apr. 19, 1994, and assigned to the assignee of the present invention, which discusses tracking or "roving" zoom regions. As is known, a window region can be defined within and moved across the field of view of the detector and by acquisition processing, certain data detected by the scintillation detector within the window can be collected or ignored. An illustration of the transverse orientation dual line source scanning and dual transmission detection window configuration of the present invention are shown in FIG. 10A, FIG. 10C and FIG. 10C. Detectors 10 and 12 are shown in a 90 degree configuration. Detector 10 is said to be associated with line source assembly 20 and detector 12 is said to be associated with line source assembly 22. There is a separate collimator 425 located in front of each detector 10 and 12 in order to collimate incoming photon radiation to the detector surface. As discussed above, each line source assembly 20 and 22 has its own collimating slit. A transmission window region 412 is defined within the field of view (FOV) of detector 12 and corresponds to line source assembly 22. This window region 412 spans the length of the field of view of detector 412 along the Y axis and in width (along X axis) is large enough to detect (and contain) the collimated transmission radiation emitted from line source assembly 22. The long axis of line source assembly 22, as discussed above, extends along the Y axis. Additionally, a second transmission window region 410 is defined within the field of view (FOV) of detector 10 and corresponds to line source assembly 20. This window region 410 spans the length of the field of view of detector 410 along the Z axis and in width (along X axis) is large enough to detect (and contain) the collimated transmission radiation emitted from line source assembly 20. The long axis of line source assembly 20, as discussed above, extends along the Z axis. To gather transmission radiation, the line source assemblies 20 and 22 move along the X axis and scan an object with transmission radiation which is detected within the transmission detection windows of the detectors 10 and 12. As the line source assemblies move along the X axis in synchronization with each other, the associated transmission window regions 410 and 412 also move in synchronization along the X axis with their associated line source assembly. The progression of the line source assemblies and transmission detection windows along the X axis is shown in FIG. 10A, FIG. 10B and FIG. 10C. In FIG. 10A, the two line sources 20, 22 and the two transmission detection windows 410, 412 are shown at a given (start) position along the X axis. In FIG. 10B, the two line sources and transmission detection windows are shown in a further (mid) x axis position. In FIG. 10C, the two line sources and transmission detection windows are shown in another (end) X axis position. Effectively, in electronics, the transmission detection windows 410, 412 are scanned across the detectors FOVs in synchronization with the line sources and create two spatial acceptance windows for acceptance of transmission data and rejection of photon radiation within the emission energy level. It is appreciated that a transmission scanning operation can also occur in the reverse direction across the detector's FOVs, e.g., from FIG. 10C to FIG. 10B to FIG. 10A. It is appreciated that the computer system 112 (of FIG. 4) is programmed according to the present invention to define the transmission detection windows 410, 412 and to displace or scan them along the surface of the detectors 10 and 12 during a transmission scan. Defining a transmission detection window and scanning such along the field of view of a detector is well known in the art, for instance, as taught by Tan et al. in the reference entitled "A Scanning Line Source for Simultaneous Emission and Transmission Measurements in SPECT," cited above. In this configuration, at any point along the x axis, the two line sources 20 and 22 and the two transmission detection windows 410 and 412 are located within the spatial YZ plane. Regardless of the position along the X axis of the transverse line source assembly of the present invention, the long axis of two scanning line sources 20 and 22 and the long axis of two transmission detection windows 410 and 412 remain within a single YZ spatial plane, for instance, refer to FIG. 10A, FIG. 10B, and FIG. 10C. Under this configuration, the long axis of the dual line source assemblies and the dual transmission detection windows are within a spatial YZ plane that is perpendicular (transverse) to long axis of the object being scanned (e.g., assumed to along the X axis). Therefore, the configuration of the present invention is called a "transverse" transmission configuration. During the scan session, transmission radiation is emitted from the line sources and this transmission radiation is detected by the scintillation detectors after passing through an object of interest. Simultaneously, emission radiation is emitted from the object and is detected by the detector. Within the present invention, the transmission radiation is utilized to create a nonuniform attenuation correction map of the object being scanned. Only transmission photons (e.g., photons detected within the transmission energy range) are detected within transmission detection windows 410 and 412. Emission photons (e.g., photons detected within the emission energy range) are ignored within the transmission detection window. Due to source and detector collimation and the configuration of the present invention, transmission photons are not detected outside the transmission detection windows 410 and 412. While transmission information is detected and collected within transmission detection windows 410 and 412, the remainder of the FOVs of the detectors detect and collect emission data. FIG. 11A illustrates in manner in which the transverse configuration of the present invention effectively eliminates the effects of cross-scattering of transmission photons from contaminating the emission image. Essentially, line source collimation and detector collimation (within the configuration of the present invention) ensure that: (1) nonscattered transmission photons fall into the transmission detection window of an associated detector to the radiating line source; and (2) cross scattered transmission photons are detected in the transmission detection window of a detector that is not associated with the radiating line source (e.g., the orthogonal line source). For example, nonscattered transmission photons radiated by line source 20 are detected within transmission detection window 410 and scattered transmission photons radiated by line source 20 are detected within transmission detection window 412 and vice-versa for transmission photons emitted from line source 22. The above is true irrespective of the position of the line sources and transmission detection windows as they are scanned in synchronization along the X axis (which is into and out of the plane of FIG. 11A). Various photon radiation sources can be used within the scope of the present invention. An exemplary implementation is the use of TI-201 (at 100-keV) for the emission radiation source and Gd-153 (at 72-keV) as the transmission radiation source. FIG. 11A illustrates a cross section (in the YZ plane) of the detector configuration of the present invention. The present invention configuration rejects Gd scatter within the TI transmission detection windows 410 and 412. A side view of sources 22 and 20 are shown and detectors 10 and 12 are shown in a 90 degree configuration. As discussed above, the detectors are each collimated. A 100-keV transmission photon (Gd-153) is emitted from line source 22 along path 436, cross-scatters within the object 5 and is detected within nonassociated transmission detection window 410 (e.g., within the orthogonal detector). The detection window 410 is not associated with source 22 because transmission radiation emitted from line source 22 should be detected by the associated detection window 412 (e.g., absent photon scatter). Due to the configuration of the present invention, this cross scattering photon is forced to be detected within window 410, otherwise, it would have been absorbed by the detector's collimator and not detected at all by detector 10. There is no other location within detector 10 or detector 12 where the cross-scattering transmission photon can end up (assuming only one scatter event occurs) besides a transmission detection window. The scattering within object 5 reduces the energy of the transmission photon, so it is detected within transmission detection window 410 at 72-keV (which is within the emission energy level). However, within the present invention, window 412 only responds to photons within the transmission energy level (e.g., within 100-keV). Therefore, this scattered transmission photon (having a TI-201 count) is vetoed by the transmission detection window and not recorded by detector 10. TI-201 photons detected elsewhere within the FOV of detector 10 are recorded as proper emission photons. It is appreciated that if the photon had cross scattered out of the (YZ) plane of FIG. 11A, due to collimation of the detector 10, the photon would not have been detected by detector 10. In addition, since the source 22 is collimated, nonscattered transmission photons are not detected outside window 412 of detector 12. Shown by FIG. 11A, cross-scattering can also occur as a result of transmission radiation (100-keV) emitted from line source 20 shown by path 438. The transmission photon cross-scatters through object 5 and due to the configuration of the present invention the cross scattering photon is detected within nonassociated detection window 412. However, after scattering, the photon loses energy and becomes a 72-keV energy photon. Window 412 responds only to photons within the transmission energy level (100-keV in this example), therefore, this cross scattered photon is ignored. If the cross scattered photon did not remain within the YZ plane of FIG. 11A, it would have been stopped by the collimator of detector 12 and not detected at all. The remainder of the FOV of detector 12 is free to detect and collect emission radiation within the 72-keV energy level. Since the source 22 and detector 12 are collimated, nonscattered transmission photons are not detected outside window 412. Since the source 20 is collimated, nonscattered transmission photons are not detected outside window 410. As a result of the present invention, cross scattering transmission photons are not allowed to fall within regions of any detector that are gathering emission data. This effectively eliminates the contamination of cross scattering transmission data within the emission image of a dual detector gamma camera. When a scattered transmission photon falls within a transmission detection window, the scatter photons are effectively eliminated by the data acquisition electronics of the present invention (e.g., circuit 120 or computer system 112) due to energy discrimination. The emission image is free of cross-scatter contamination because the cross-scatter photons emitted from line sources 20 and 22 either: (1) fall within nonassociated detector regions 412 and 410 (respectively) and are therefore ignored; or (2) are absorbed by the detectors' collimators and are not detected at all. Further, transmission radiation (emitted by the line sources) that does not scatter is detected within associated transmission detection windows 410 and 412 and does not contaminate the emission image. Within the present invention, there is no need to perform additional measurements to estimate and subtract effects due to cross-scatter. FIG. 11B illustrates the energy distribution of detected photons resultant from the configuration of FIG. 11A. As shown in FIG. 11B, the shaded area 404 represents the emission Gd photopeak plus the scatter radiation. As shown, the scatter distribution tails into the transmission TI window. Also shown is the TI photopeak distribution 403. An implementation of the present invention is shown in FIG. 12A wherein the scanning line source system effectively rejects TI scatter in a Gd window within a system using TI-201 (at 167-keV) as emission photons and Gd-153 (at 100-keV) is used for transmission. In this configuration, the effects of emission cross-talk are reduced. The resultant Gd transmission image is effectively free of emission cross-scatter contamination. For example, an emission photon may scatter of off the object 5 following in path 434 and lose energy. The emission photon will then fall within transmission detection window 410 and will have an energy level of a transmission 100-keV photon. However, the transmission Gd count rate inside the detection window 410 overwhelms the downscatter from the 167-keV emission cross scatter. The line source collimation and detector collimation ensure that nonscattered transmission radiation falls into the transmission detection window of its associated detector. Gd counts occurring inside the transmission detection windows are accepted and Gd counts outside moving transmission detection windows are rejected because source collimation means no valid Gd photons are outside of the transmission detection windows. FIG. 12B illustrates the energy distribution of detected photons resultant from the configuration of FIG. 12A. As shown in FIG. 12B, the shaded area 402 represents the TI photopeak plus the scatter radiation. As shown, the scatter distribution tails into the transmission Gd window. Also shown in the Gd photopeak distribution 401. FIG. 13A and FIG. 13B illustrate that the transverse transmission detection window configuration of the present invention is advantageous for reducing cross-scatter wherein an axial orientation of the transmission detection windows is not. Axial orientation means that the long axis of the detection window and the long axis of the patient are along the X axis. FIG. 13A illustrates the present invention configuration wherein the long axis of the line sources and transmission detection windows are oriented transverse with respect to the patient. FIG. 13B illustrates the present invention configuration wherein the long axis of the line sources and transmission detection windows are oriented axial with respect to the patient. For both configurations, an exemplary implementation is the use of TI-201 for the emission radiation source and Gd-153 as the transmission radiation source. For example, FIG. 13A illustrates a cross section of the present invention transmission configuration that is sliced along the transmission detection windows in the YZ plane. Within the present invention, the long axis of the line sources and transmission detection windows are oriented transverse to the object (e.g., perpendicular to the X axis). As before, detectors 12 and 10 are at right angles and windows 412 and 410 extend along the detector surfaces due to the orientation of the cross section. A transmission photon is emitted from line source 22, along path 482 and is scattered within the plane of FIG. 13A and is detected within window 410 of detector 10. As discussed previously, it will be excluded from the transmission data because transmission detection window 410 only responds to transmission energy level photons. However, assume the long axis of the transmission detection windows were oriented axial to the object (e.g., along the X axis). The cross sectional view of FIG. 13B (within the YZ plane) illustrates once again that detectors 10 and 12 are at 90 degree orientation, but the cross section of transmission detection window associated with detector 12 is displayed as area 472. The cross section of transmission detection window associated with detector 10 is shown as 474. The long axis of these transmission detection windows 472 and 474 extends out of and into the page of FIG. 13B. Also, in this cross section, line source 22 appears as a circle as shown. Assuming a transmission photon taking path 484 cross scatters from object 5, it can land on detector 10, but it is not guaranteed to land within transmission detection window 474. As shown in FIG. 13B, the cross scatter photon lands within the emission recording portion of the FOV of detector 10. In this case, the transmission photon will be improperly detected as a emission photon because the energy loss from the photon due to the scatter will reduce its energy level to that of the emission energy level. Therefore, using traverse scanning line sources and traverse oriented moving transmission detection windows, the cross scatter contamination is not eliminated within the axial oriented transmission detection windows. This is one reason why the present invention utilizes transverse scanning line sources and transverse oriented moving transmission detection windows, in this way, the cross scatter contamination is effectively eliminated. The present invention transverse dual transmission line source scanning configuration is used to collect uncontaminated transmission data (e.g., free from cross-scatter contamination). The transmission data or "counts" collected by the present invention are stored in a computer memory, such as memory 102 of computer system 112 (see FIG. 4). Using well known procedures, the transmission data is converted into nonuniform attenuation correction factors by the computer processor 101. These nonuniform attenuation correction factors are also stored in a computer memory, such as memory 102 or even 104 of FIG. 4. As discussed previously, the nonuniform attenuation correction factors are used by the computer processor 101 in well known procedures to correct collected emission data from the gamma camera system to compensate for nonuniform attenuation from the scanned object. A. Combination With Zoom Tracking An embodiment of the transverse dual sliding detection window and dual transmission line source system of the present invention is advantageously utilized in conjunction with zoom tracking windows that allow detailed images of a particular organ of interest (e.g., such as in cardiac studies). The details of the zoom tracking implementation within a dual detector system are described in U.S. Pat. No. 5,304,806, entitled, "Apparatus and Method for Automatic Tracking of a Zoomed Scan Area in a Medical Camera System," issued Apr. 19, 1994, and assigned to the assignee of the present invention. According to this disclosure, a special zoom window (or region) is defined within the FOV for each detector within the detector electronics and/or computer system's data acquisition processes. This window is defined to cover the field of view of the detector which coincides with a particular organ of an imaged patient, e.g., the heart. The detector electronics provide for an image magnification for emission radiation that are detected within the zoom windows. As the detectors traverse about the object under ECT movement, the zoom windows displace ("rove") relative to the surface of the detectors so that the heart (or other organ of interest) remains centered and within the FOV of each zoom window. In effect, the zoom windows track the heart for each ECT rotation angle. These zoom windows detect emission radiation from the tracked organ (e.g., heart). Since the zoom windows are smaller than the entire FOV of a detector, the image rendering capacity of the gamma camera can be focused on the zoom window and the resultant image generation quality is increased (e.g., resolution is increased). In effect, the size of the pixels defined within the zoom window can be decreased relative to their full FOV size. FIG. 14A illustrates that zoom tracking is implemented in conjunction with the transverse dual detector transmission window and dual line source scanning configuration of the present invention. In such case, the zoom windows 452 and 454 are defined on the surface of the detectors and move up and down as shown by the arrows 452a and 454a in order to track an object of interest as the detectors 10 and 12 undergo ECT rotation about the object. It is appreciated that for any given angle, the zoom windows remain fixed and rove only between angles. In an exemplary configuration, the FOV of a particular detector is roughly 51.times.31 cm in area and a particular zoom window can be 30.times.30 cm or 38.times.38 cm in area. The detectors 10 and 12 electronically collect emission data (e.g., counts) only within the zoom regions 452 and 454 for each angle of rotation. It is appreciated that the entire FOV of the detector 10 and 12 may be detecting emission radiation, however, only that emission radiation that is detected within the zoom windows (regions) is stored and used for image reconstruction. According to this aspect of the present invention, simultaneous with the collection of emission data within the two roving zoom windows, transmission data is also collected within the scanning transmission detection windows 410 and 412. An exemplary implementation is the use of TI-201 for the emission radiation source and Gd-153 as the transmission radiation source. Although not shown in FIG. 14A, two scanning line sources are also present and move in synchronization with the two transmission detection windows, as discussed previously. For each angle of rotation, the transverse transmission detection windows scan across the FOV of the detector according to arrows 412a and 410a as discussed above in order to collect transmission data. For each angle or rotation, the zoom windows 452 and 454 assume a new spatial position (rove) to track the object of interest. However, unlike the scanning transmission detection windows 412 and 410, at any given angle of rotation the zoom windows 452 and 44 remain fixed until the next angle of rotation is entered. The transmission detection windows 410 and 412 of the present invention report only photons within the transmission energy level and reject other detected photons, e.g., emission energy level photons which result from: (1) scattered transmission photons; and (2) nonscattered emission photons. The zoom regions 452 and 454 report emission photons because the collimation of the line sources and the detector provides that no valid transmission data should fall outside the two transmission detection windows 410 and 412. It is possible, as shown in FIG. 14A, for the transmission detection windows 410 and 412, as they scan across the FOV of their associated detectors, to partially coincide with the zoom windows 452 and 454. When this happens, an area of the zoom windows (e.g., 452b and 454b) that overlaps with the transmission detection windows act as a transmission detection window and acts to reject photons of the emission energy level. In effect, regions 452b and region 454b collect transmission energy level photons and rejects emission photons. However, this state is temporary as the transmission detection windows are moving across the FOV of the scintillation detectors. FIG. 14B and FIG. 14C illustrates a flow diagram of the processing tasks 510 performed by the scanning transmission line source embodiment of the present invention used in conjunction with roving zoom tracking windows. An exemplary implementation is the use of TI-201 for the emission radiation source and Gd-153 as the transmission radiation source. The processing starts at block 512 in which the patient is placed in the gamma camera system (e.g., such as the one shown in FIG. 1) and the gamma camera is initially configured and initialized. The two detectors 10 and 12 are oriented at a 90 degree angle about the patient. At block 512, the zoom regions are initially defined in terms of size and initial placement in order to locate the object of interest (e.g., the heart); this can be accomplished according to the procedure and mechanisms described in U.S. Pat. No. 5,304,806 (cited above). At block 514, if not already at the starting angle, the detectors are rotated by the gantry structure to the first angle for the ECT study. At block 516, the positions of the zoom windows (regions) are computed for each detector for the initial rotation angle; this can be accomplished according to the procedure and mechanisms described in U.S. Pat. No. 5,304,806. At block 518, the two scanning line source assemblies 20 and 22 scan across the FOV of each detector to irradiate the patient and their associated transmission detection windows scan in synchronization with the associated line source; this process is accomplishing using the configuration shown in FIG. 10A, FIG. 10B, and FIG. 10C. Although programmable, an exemplary scan speed is 1 cm/sec for the configuration. It is appreciated that a scan speed computation (as described above based on a prescan duration) can be performed during this step in order to reduce the radiation exposure amount for the patient. Block 520 occurs simultaneously with block 518. At block 520, the present invention detects and reports transmission energy level photons (100-keV) within the transmission detection windows (e.g., 410 and 412). Simultaneously, emission energy level photons (e.g., 72-keV) are detected and reported within the roving zoom regions (e.g., 452 and 454), but transmission energy level photons are rejected (or not detected at all due to collimation of the source and the detectors) within the roving zoom regions. At block 522, emission energy level photons (e.g., 72-keV) are rejected within the transmission detection windows 410 and 412 (e.g., they can be a result of cross-scatter). Cross-scatter transmission photons are eliminated during this step. It is appreciated that block 522 is performed simultaneously with block 520. At block 524, the present invention checks if any part of the transmission detection windows overlap with a zoom window as the transmission detection windows scan across the FOV of the detector surfaces. If so, then at block 526, transmission energy level photons are detected and reported in the overlap area and emission energy level photons are rejected within the overlap area; in effect, the overlap is treated as purely a part of the transmission detection windows. Processing then flows to block 528. At block 524, if no overlap, then processing flows to block 528. It is appreciated that block 524 and block 526 can effectively occur simultaneously with block 522. At block 528, the transmission and emission scanning operations for a given ECT angle are completed and the proper transmission and emission image data is stored in computer system 112. If another ECT angle of rotation is required (e.g., the ECT session is not complete), then at block 530, the gantry structure rotates the detectors 10 and 12 to a new angle of rotation and block 516 is once again entered. If the ECT rotation angles are complete, then the ECT data acquisition session is over. At block 532, a nonuniform attenuation correction map is generated based on the transmission data collected for each ECT angle of rotation. At block 534, the emission data is corrected utilizing well known correction factors (e.g., for linearity and energy) including the nonuniform attenuation correction map generated by block 534 to correct for nonuniform attenuation of the patient's body. At block 536, the present invention then reconstructs the corrected emission data (using well known reconstruction procedures) and displays the reconstructed data as required for diagnosis. V. LFOV TRANSMISSION ACQUISITION USING SFOV EMISSION ACQUISITION The present invention also includes an embodiment directed at eliminating truncation associated with the reconstruction of transmission information as done in prior art systems. Transmission image information truncation results when a field of view smaller than that required to image the entire body is used to collect transmission information. Emission information is also collected with this small field of view (SFOV). The result is higher resolution for the emission data but transmission data truncation results because not all of the body was scanned during the transmission scan. Special algorithms are then needed to anticipate the transmission data in those areas of the body that were not directly scanned. Body contour information is used to supplement the transmission data. The present invention utilizes a large field of view scintillation detector (e.g., 20".times.15") and therefore is able to collect transmission data using a large field of view acquisition (LFOV) scan. However, in order to increase image quality when imaging a small organ (e.g., the heart), the present invention allows emission data to be collected using a roving zoom region having a small field of view. Use of a roving zoom region for emission data acquisition within the present invention analogous to the technique described in detail within U.S. Pat. No. 5,304,806, entitled, "Apparatus and Method for Automatic Tracking of a Zoomed Scan Area in a Medical Camera System," issued Apr. 19, 1994. Since a LFOV transmission scan is acquired the whole body is scanned and no transmission truncation is required. Further, since a SFOV emission window is used, the image quality of the resulting emission image is high. Further, since no transmission truncation is performed, the approximations and corrections performed by the prior art (e.g. in order to anticipate the transmission data not gathered by a SFOV transmission scan) are not required. As such, the present invention offers the advantageous results of providing a complete and accurate transmission map (e.g., nonuniform attenuation correction map) in addition to a high quality emission image. Because the present invention utilizes a LFOV transmission acquisition scan and a SFOV emission acquisition scan, the pixel sizes between the transmission and the emission data are different since, generally, the number of pixels with a transmission or emission scan is constant. Namely, the pixel sizes with respect to the transmission scan are larger than the pixel sizes with respect to the emissions scan. Although many different matrix sizes can be utilized within the scope and spirit of the present invention, a particular and exemplary size is illustrated for discussion only. For example, the present invention can utilize a 64.times.64 imaging matrix to cover the full field of view of the detector (e.g., 20".times.15") for the transmission scanning. The present invention an also utilize a 64.times.64 imaging matrix to cover a small field of view (e.g. for cardiac studies) to encompass a particular organ of interest (e.g., the heart) over an imaging area of 10".times.10" or 15".times.15", for instance. As can be seen, the pixel sizes for the emission data acquisition are smaller than the transmission acquisition and therefore offer more resolution for imaging the organ of interest. The SFOV emission window is the roving zoom window or region referred to in U.S. Pat. No. 5,304,806. Because the pixels are smaller within the zoom region the emission image appears larger and therefore "zoomed." Further, the zoom region is called a "roving" zoom region because (as discussed above) during ECT rotation, the region or window will displace relative to the surface of the scintillation detector in order to track the organ of interest within its field of view as the scintillation detector rotates about the body. It is appreciated that for a tomographic reconstruction, the images taken at each rotation or ECT angle assume a fixed center of rotation and further that the objects imaged are steady with respect to that center of rotation. However, with respect to a roving zoom region, since the emission acquisition window displaces relative to the detector surface to track an organ, effectively the emission tomogram has a "virtual" center of rotation in that the imaged organ becomes the "virtual" center of rotation. Further, this "virtual" center of rotation may not be the same as the actual physical center of rotation defined by the gantry 50 and detector mechanisms. It is appreciated that if the imaged organ is at the physical center of rotation then the zoom region would not need to rove at all. Within the present invention embodiment that allows the emission acquisition to be imaged through a roving zoom region of the detector surface, the transmission acquisition data (which is taken simultaneously) is spatially shifted or corrected to account for the "virtual" center of rotation defined by the imaged organ. Namely, as the roving zoom region displaces a given amount for a given angle of rotation (e.g., to track the "virtual" center of rotation), the transmission data collected for that angle will too be shifted or corrected to account for this displacement from the true center of rotation. In this way, during reconstruction of both the transmission and the emission data, like parts of the same body are reconstructed in the same position. The attenuation correction map will spatially match up or align with the emission reconstruction map. In addition, since the transmission pixels are larger than the emission pixels, generally, the present invention also performs a form of linear interpolation of the transmission data in order to generate transmission pixels of the same size as the emission pixels. Again, this is done so that the resolution between the transmission map and emission data are the same in order to properly correct the emission data. FIG. 15A and FIG. 15B illustrate a processing diagram of the procedure 610 utilized by the present invention to perform LFOV transmission scan with a SFOV emission scan. It is appreciated that the computer system 112 performs many of the tasks as will be described hereinafter apart from the actual data acquisition steps performed by the scintillation detectors 10 and 12 and the signal processing acquisition hardware 120. It is appreciated that the process 610 can be employed for both a single detector and for a dual detector configuration. However, the implementation with a dual detector is described below for illustration. The implementation within a single detector system is a readily determinable from the dual detector process. To perform LFOV transmission with SFOV emission, the process 610 is entered and at block 615 the gantry is positioned at an initial ECT angle for the scan. Detectors 10 and 12 are oriented at 90 degrees to each other. For this ECT angle, a transmission scan is performed on the entire body using the line sources 20 and 22 and imaging information is recorded (e.g., transmission data) across the full field of view of the detectors 10 and 12. It is appreciated that a scan speed determination can be performed at this step 615 to reduce the transmission radiation exposure to the patient. Namely, a prescan can be performed followed by the transmission scan of a minimum duration of time. The transmission acquisition is then based on a LFOV because is it based on the entire field of view of the detector (for example 20".times.15"). An exemplary matrix size acquisition for transmission information is 64.times.64 pixels (per detector), but is programmable. It is appreciated that the transmission scan of block 615 may also include the use of a variable filter assembly 22 as shown in FIG. 8C in order to reduce count density in unobstructed portions of the detector. In such case, the computer system obtains a body contour of the object and then selects the proper filter for use. At block 617 of FIG. 15A, at the initial ECT angle, the proper initial positions of the roving zoom windows (e.g., with respect to the scintillation detectors) are determined as described in U.S. Pat. No. 5,304,806. Also, the proper sizes of the roving zoom windows are selected which determines the amount of magnification utilized. Exemplary sizes for the emission windows are 10".times.10" and 15".times.15", but these are programmable. At block 617, emission data is obtained from the detectors 10, 12 only from the roving zoom window regions of each detector. It is appreciated that the transmission data acquisition of block 615 and the emission data acquisition of block 617 can occur simultaneously. Further, it is appreciated that the above described method for eliminating cross-talk can be utilized by this implementation of the present invention at blocks 615 and 617, as appropriate. The emission acquisition is then based on a SFOV since is relates only to the data acquired via the roving zoom regions. An exemplary matrix size acquisition for emission information is 64.times.64 (per detector) pixels but is programmable. FIG. 16C and FIG. 16D illustrate movement of the roving zoom window during emission scanning. FIG. 16C illustrates the roving zoom window 641 located at an initial position (i, k) within the field of view 635 of a detector 10. An image of the heart 643 is displayed. FIG. 16D illustrates the same configuration at a different ECT angle. The roving zoom region 641 has displaced by some di and dk value. As shown, the roving zoom region 641 has been displaced upward to track the heart so that the image of the heart 643 remains in the field of view of the zoom region for each ECT rotation angle. At the completion of the ECT scan, there is a different displacement (di and dk) for each roving zoom region for each angle of rotation (theta). For example, there is a di(theta) and dk(theta) for the roving zoom region for detector 10 and for detector 12. These are stored in memory. The following dataset can be generated in memory: ______________________________________ ECT Angle Detector 10 Detector 12 ______________________________________ theta0 di(theta0), dk(theta0) di(theta0), dk(theta0) theta1 di(theta1), dk(theta1) di(theta1), dk(theta1) theta2 di(theta2), dk(theta2) di(theta2), dk(theta2) theta3 di(theta3), dk(theta3) di(theta3), dk(theta3) theta4 di(theta4), dk(theta4) di(theta4), dk(theta4) . . . . . . . . . thetan di(thetan), dk(thetan) di(thetan), dk(thetan) ______________________________________ Refer to FIG. 15A. After a predetermined imaging duration wherein sufficient transmission and emission counts are obtained, at block 619, the present invention then stores the emission image counts as a dataset matrix into a memory device and references this information by the current ECT angle. Also at block 619, the present invention stores the transmission image counts as a dataset matrix into a memory device and references this information by the current ECT angle. At block 621, the present invention then determines if the ECT scan is complete. If not, then a new angle of rotation is determined and the gantry structure 50 is positioned such that the detectors 10 and 12 are properly rotated to the new angle. Processing then returns to block 615 so that the image dataset matrices for the transmission scan and for the emission scan can be completed and stored for this new ECT angle. At block 621, if the scanning for the last ECT angle is complete, then processing continues to block 623 where the transmission dataset matrices for each detector and for each ECT angle are spatially corrected to account for the virtual center of rotation defined by the roving zoom region. In effect, the roving zoom regions, by moving across the field of view of the detectors to track the organ of interest, act to create virtual center of rotation that is different from the actual center of rotation of the gantry structure. The virtual center of rotation centers the target of interest (e.g., the heart in cardiac scans). However, the transmission information LFOV does not adjust spatially (e.g., rove) during the ECT scan because this is a full field of view scan which is based on the actual center of rotation of the gantry. Therefore, the transmission information is acquired based on the center of rotation of the gantry and the emission data is gathered based on the virtual center of rotation. For proper application of the transmission image data to the emission image data, after tomographic reconstruction, their data must be based on a similar center of rotation. At block 621 of FIG. 15A, the transmission data is adjusted or compensated so that it is becomes based on the virtual center of rotation so that the transmission data and the emission data match for a proper reconstruction. Therefore, for each angle (theta) the transmission dataset matrix is offset spatially according to the amount of displacement of the roving zoom region for that angle theta (e.g., by di(theta), dk(theta)). An illustration of this effect is shown in FIG. 16A and FIG. 16B for a given angle of rotation. FIG. 16A illustrates the dataset matrix for a transmission scan of detector 10 without compensation. FIG. 16B illustrates that this raw transmission data is offset by di and dk for a given angle theta to account for the roving zoom motion. By performing the compensation for each dataset matrix of each detector for each angle, theta, the present invention effectively translates the transmission data so that it becomes based on the virtual center of rotation (e.g., the organ) and not the gantry center of rotation. After the repositioning of block 621 is complete, the transmission data matrices are stored in memory. At block 625 of FIG. 15A, the present invention adjusts the pixel size of the transmission dataset matrices to match the zoom region pixel size. For instance, assuming the zoom region pixels are at some magnification M, larger than 1.0 (e.g., 1.5 magnification), then the pixels of the transmission scans are reduced in size and increased in number by the present invention and interpolated using a linear interpolation procedure until the pixel sizes between the transmission and the emission scan match. For example, assume the transmission dataset matrices are acquired in matrix sizes of 64.times.64. The present invention transforms this matrix onto a larger matrix dataset (e.g., 128.times.128) which effectively reduces the size of the individual pixels of the transmission dataset matrices but increases their number. However, since the magnification of the emission data (e.g., M) can be less than twice magnification, the new transmission matrix may not be completely filled. Refer to FIG. 17 for an illustration. An exemplary transmission dataset matrix is shown as 705 (composed of a matrix of 64.times.64 pixels). The image data of the matrix 705 is transformed by the present invention into a larger matrix 706 (composed of a matrix of 128.times.128 for example). The transmission image data from matrix 705 is recorded into area 715 and the boarder 710 is empty for cases where the zoom magnification (M) is less than 2. If, for example, the zoom magnification was 1.5, then each pixel within matrix 705 would be linear interpolated with its neighboring pixels wherein two adjacent pixels would create additional pixels in between, and so on, until the region 715 was complete and the transmission data was adjusted to the 1.5 zoom magnification. However, since matrix 706 is twice as large in both dimensions as matrix 705 and since M=1.5, the border 710 of the matrix 706 remains empty of valid data. It is appreciated that there are a number of different techniques for transforming one image from a first matrix 705 to a second matrix 706 that contains more pixels of a smaller size, individually, as compared to the first matrix. Any of a number of well known methods may be performed consistent within the scope of the present invention. The important aspect of step 625 of FIG. 15A is that the pixel size of the transmission image information is adjusted to match that of the emission image information. Step 625 is done for each transmission dataset matrix for each detector for each angle of rotation and the results are stored in memory. Process 610 of the present invention continues with FIG. 15B step 627 where the present invention performs a reconstruction of the transmission information to create a nonuniform attenuation correction map. It is appreciated that the processes and procedures for reconstructing image information taken at different ECT angles is well known. Any of a number of different and well known reconstruction procedures can be used consistent within the scope of the present invention. As an exemplary procedure, the present invention performs the maximum likelihood and expectation maximization iterative (MLEM) process for transmission reconstruction. According to the MLEM process, each transmission data set from each detector for each angle of rotation (e.g., after being corrected for roving zoom displacement and pixel size) is recalled from memory and used to generate the transmission reconstruction map. The transmission reconstruction can be performed a number of different ways, however, under an embodiment of the present invention it is performed across slices and the reconstruction image is stored in memory. During step 627, the reconstruction of the transmission data does not involve truncation, as done in the prior art, since a transmission scan using a LFOV was acquired. The present invention then at step 629 performs the emission reconstruction using the emission data collected via the roving zoom region for each detector for each angle of rotation. During the reconstruction (e.g., using the MLEM process), the transmission reconstruction map is applied using well known procedures to correct for nonuniform attenuation of the body. Since the transmission reconstruction was corrected for roving zoom displacement (e.g., step 623) and corrected for pixel size (e.g., step 625), the transmission reconstruction can be directly applied to the emission reconstruction as a nonuniform attenuation correction map. The emission reconstruction/correction can be performed a number of different ways, however, under one embodiment of the present invention it is perform on a slice by slice basis and the reconstruction image is stored in memory. At the completion of step 629, a reconstruction of the organ of interest is generated that has been corrected for nonuniform attenuation of the body. This reconstruction is of a high image quality because a SFOV emission scan was acquired using relatively small pixel sizes. Further, since the transmission scan was acquired under a LFOV, no errors due to transmission truncation were contributed by the present invention. Therefore, the emission image is of a higher quality because it was more accurately corrected for nonuniform attenuation by the complete transmission reconstruction of step 627. At block 631 of FIG. 15B, the present invention then allows different "slices" through the emission reconstruction image to be displayed (e.g., on a CRT, printer, hardcopy device, disk drive storage, etc.) for diagnostic purposes and different slices may be selected and displayed via user input, as is well known. It is appreciated that the flow 610 of the present invention has been described herein as providing the various fields of view (e.g., LFOV and SFOV) during the acquisition or "front end" processing of the present invention. Then, after acquisition, this information is processed by the computer system as described above. However, in certain circumstances, the application of certain fields of view can be performed at the processing side or "back end." In other words, some detector systems provide a maximum resolution of 4096.times.4096 for each detector head. The present invention can also be implemented by acquiring the totality of the possible data from each detector (assuming adequate storage and processing speed are provided) and then applying the field of views to this data electronically by discarding some of the acquired data for processing and binning the remainder of the image data to fit the particular pixel size desired. It is appreciated that implementations of the present invention that apply the fields of view as described in the present invention (LFOV for transmission and SFOV for emission) at the "back end" processing stages are within the scope of the present invention as described herein. The preferred embodiment of the present invention, a gamma camera system providing improved image generation via nonuniform attenuation correction maps, is thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims. |
summary | ||
abstract | An aqueous assembly has a negative coefficient of reactivity with a magnitude. The aqueous assembly includes a vessel and an aqueous solution, with a fissile solute, supported in the vessel. A reactivity stabilizer is disposed within the aqueous solution to reduce the magnitude of the negative coefficient of reactivity of the aqueous assembly during operation of the aqueous assembly. |
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042008034 | claims | 1. A multiple collimator comprising: a thick shield plate member opaque to penetrative radiation (1); a plurality of bores traversing said shield plate and so shaped as to provide a frustoconically shaped cavity surface; a rotatable body (9) of material opaque to penetrative radiation rotatably mounted in each of said bores having a frustoconical surface fitting the frustoconical surface of the bore, and having also a cavity for mounting, at an adjustable angle to the axis of said rotary body, a collimator tube body, said cavity being open through the end faces of said rotary body, and a collimator tube body (4a, 4b) of material opaque to penetrative radiation having a collimator tube bore therethrough and movably mounted in said cavity of each of sair rotary bodies so as to permit variation of the angular position of said collimator tube bore with respect to said shield plate (1), the collimator tube bore of each of said collimator tube bodies (4a, 4b) having a portion shaped for removably seating a radiation detector (6). 2. A multiple collimator as defined in claim 1, in which said cavity in said rotatable body (9) has a portion of prismatic shape and said collimator tube body (4a) seated therein has a portion shaped with two parallel sides fitting parallel plane walls of said prismatic portion of said cavity and is mounted so as to be able to swing about an axis perpendicular to the axis of rotation of said rotary body. 3. A multiple collimator as defined in claim 1, in which said cavity in said rotatable body (9) is of conical shape having an axis running eccentrically in said body with respect to the axis of symmetry and rotation of said body and in which the said collimator tube (4b) is rotatably mounted in said cavity and has a collimator bore therethrough which is disposed eccentrically with respect to the axis of rotation of said collimator tube (4b). 4. A multiple collimator as defined in any one of claims 1 to 3, in which a scale is provided on said shield plate (1) for determining the angular position of said rotatable body (9). 5. A multiple collimator as defined in any one of claims 1 to 3, in which a scale is provided on said rotatable body (9) for determining the angular position of said rotatable body. 6. A multiple collimator as defined in any one of claims 1 to 3, in which a scale is provided for determining the position of each collimator tube relative to said rotatable body. 7. A multiple collimator as defined in any one of claims 1 to 3, in which a fixed collimator bore is also provided, said fixed collimator bore having a position into which a radiation detector can be removably seated. |
description | This is a divisional application of U.S. patent application Ser. No. 13/389,246 filed Feb. 6, 2012, the entire disclosure of which is hereby incorporated by reference herein. The present invention relates to a method for operating a nuclear reactor. The reprocessing of spent nuclear fuel assemblies has made large amounts of plutonium available. Using this plutonium in order to mix it with uranium and thereby form nuclear fuels has been suggested for a long time. These fuels, which contain, before irradiation, a mixture of uranium and plutonium oxides are generally called MOX (Mixed Oxide) fuels. The assemblies containing such MOX fuels, called MOX assemblies in the following, have been loaded into the cores of nuclear reactors where they coexist with assemblies, the nuclear fuel of which before irradiation does not contain any plutonium oxide but only uranium oxide. Such assemblies will be called UO2 assemblies in the following, and the fuel which they contain UO2 fuel. A nuclear reactor core loaded with assemblies of MOX fuel and with assemblies of UO2 fuel will be called a mixed core in the following. The isotopes of plutonium and those of uranium have very different neutron properties and in particular differences in cross section. Considering that these differences in neutron properties made it impossible to purely and simply substitute UO2 fuel with MOX fuel in order to produce MOX assemblies, document FR-2 693 023 described a zoned MOX assembly, i.e. for which the nuclear fuel rods have a same plutonium isotope composition (or vector), i.e. the same composition in terms of percentages of the respective mass fractions of each isotope making up the plutonium, and nominal total plutonium mass contents different from one zone to the other of the assembly. Thus, the nominal total plutonium mass content is lower on the faces than at the centre of the assembly, and even lower in the corners of the assembly. With this, it is possible to obtain a radial distribution of the linear power density in the core of the nuclear reactor, in particular in the peripheral rods of the MOX assemblies adjacent to UO2 assemblies, which is acceptable. Moreover, as recalled in this document, the plutonium stemming from reprocessing has an isotope composition which strongly varies in particular depending on the initial uranium 235 enrichment, on the burn-up rate and on the storage duration of the fuel before reprocessing. In order to compensate for the neutron behavior differences which such differences in isotope compositions might induce, energy equivalence relationships were established in order to determine the nominal total plutonium mass contents for different isotope compositions corresponding to a reference uranium 235 content. With these equivalent nominal total mass contents it is possible to compensate for the differences in isotope compositions and to reach the same burn-up rates in the same type of fuel management. The equivalence relationships use equivalence coefficients which depend on the isotope composition of the relevant MOX fuel, i.e. on the isotope composition of the plutonium and the uranium 235 content of the uranium associated with the plutonium. These equivalence relationships are for example mentioned in pages 41 to 43 of the document entitled Status and Advances in MOX Fuel Technology, Technical Reports Series No. 415 and published by the International Atomic Energy Agency in 2003. As an example, the table below specifies in its first portion the typical compositions of the plutonium stemming from the reprocessing of UO2 fuel assemblies for a pressurized water reactor as a function of the initial uranium 235 enrichment of the fuel and of the burn-up rate attained by the fuel, the storage duration before reprocessing being the same for all the examples mentioned in the table. The table specifies in its second portion (last line) the nominal total plutonium mass contents with which it is possible to attain the same burn-up rate as a UO2 assembly enriched to 3.70% by mass of uranium 235 and therefore to compensate for the decrease in the quality of the plutonium: reduction in the amount of fissile isotopes (plutonium 239 and plutonium 241) and increase in the amount of absorbent fertile isotopes (plutonium 238, plutonium 240, plutonium 242 and americium 241). UO2 assembly for a pressurized water reactorEnriched toEnriched toEnriched toEnriched to3.25% by mass3.70% by mass4.00% by mass4.95% by massof 235U andof 235U andof 235U andof 235U andirradiated at 30irradiated at 40irradiated at 50irradiated at 70Origin of the Pu usedGWd/tHMGWd/tHMGWd/tHMGWd/tHMIsotope238 Pu1.22.13.15.3compsition239 Pu62.257.753.648.9(% by mass)240 Pu23.024.124.924.9241 Pu8.08.79.19.3242 Pu4.46.17.910.2241 Am1.21.31.41.4Nominal total Pu6.77.68.710.7mass content (%)equivalent to anenrichment to3.70% by mass of235U Taking into account the more and more substantial available amounts of plutonium, certain producers of electricity have desired that the newly built nuclear reactors may be loaded with up to 50% of MOX assemblies. Document U.S. Pat. No. 6,233,302 describes a nuclear reactor in which all the nuclear fuel assemblies loaded into the core contain MOX fuel. In order to ensure a homogeneous radial distribution of the linear power density, these assemblies always have a zoned configuration and further comprise nuclear fuel rods which do not contain any plutonium oxide and for which the nuclear fuel in addition to the inevitable impurities resulting from the manufacturing, contains consumable neutron poisons such as erbium oxide. However, this reactor does not allow optimum use of the plutonium and these assemblies are complex and costly to produce. An object of the invention is to provide a method for operating a pressurized water nuclear reactor, the method being useful in order to be able to use more plutonium with increased efficiency and more reduced costs. A method is provided for operating a pressurized water nuclear reactor comprising a core containing nuclear fuel assemblies comprising nuclear fuel rods, the method comprising steps consisting of operating the nuclear reactor during successive cycles with between each cycle, steps for replacing spent nuclear fuel assemblies with fresh nuclear fuel assemblies, a method wherein: the reactor is operated at least one plutonium-equilibrium cycle during which the core contains plutonium-equilibrium nuclear fuel assemblies, the plutonium-equilibrium nuclear fuel assemblies comprising, before irradiation, nuclear fuel rods exclusively based on uranium and plutonium mixed oxide, and for each plutonium-equilibrium nuclear fuel assembly the nuclear fuel rods having a same isotope composition of nuclear fuel and a same nominal total plutonium mass content, and then the reactor is operated for transition cycles, at least some of the plutonium-equilibrium nuclear fuel assemblies being progressively replaced, during the replacement steps preceding transition cycles, with: zoned transition nuclear fuel assemblies, the zoned transition nuclear fuel assemblies each comprising: a central zone comprising nuclear fuel rods, which, before irradiation, contain uranium oxide but do not contain any plutonium oxide, and a peripheral zone extending along outer faces of the zoned transition nuclear fuel assembly, the peripheral zone only comprising before irradiation nuclear fuel rods exclusively based on mixed uranium oxide and plutonium oxide, and then uranium-equilibrium nuclear fuel assemblies, the uranium-equilibrium nuclear fuel assemblies only comprising before irradiation nuclear fuel rods which contain uranium oxide but do not contain any plutonium oxide, the nuclear reactor is operated for at least one uranium-equilibrium cycle in which the core contains uranium-equilibrium nuclear fuel assemblies, the uranium-equilibrium nuclear fuel assemblies only comprising before irradiation nuclear fuel rods which contain uranium oxide but do not contain any plutonium oxide. According to particular embodiments, the method may comprise one or more of the following features, taken individually or according to all technically possible combinations: during the uranium-equilibrium cycle, the core only contains uranium-equilibrium nuclear fuel assemblies which only comprise before irradiation nuclear fuel rods which contain uranium oxide but do not contain any plutonium oxide; during the plutonium-equilibrium cycle, the core only contains plutonium-equilibrium nuclear fuel assemblies; during the plutonium-equilibrium cycle, the plutonium-equilibrium nuclear fuel assemblies only comprise before irradiation nuclear fuel rods exclusively based on uranium and plutonium mixed oxide; during the plutonium-equilibrium cycle, the nuclear fuel rods of all the plutonium-equilibrium nuclear fuel assemblies have a same isotope composition of nuclear fuel and a same nominal total plutonium mass content; at least some of the zoned transition nuclear fuel assemblies comprise in their central zone poisoned nuclear fuel rods, the poisoned nuclear fuel rods containing before irradiation, at least one consumable neutron poison; in at least some of the zoned transition nuclear fuel assemblies, the nuclear fuel rods of the peripheral zones have nominal plutonium fissile isotope contents of less than those of nuclear fuel rods of plutonium-equilibrium nuclear fuel assemblies; during the replacement step preceding a first transition cycle, first zoned transition nuclear fuel assemblies are loaded into the core, and during the replacement step preceding a second transition cycle, second zoned transition nuclear fuel assemblies, for which the nuclear fuel rods of the central zones have, except for the possible poisoned nuclear fuel rods, uranium 235 enrichments different from those of the nuclear fuel rods of the central zones of the first zoned transition nuclear fuel assemblies, are loaded into the core; except for the possible poisoned nuclear fuel rods, the nuclear fuel rods of the central zones of the second zoned transition nuclear fuel assemblies have substantially the same uranium 235 enrichment as the nuclear fuel rods of the uranium-equilibrium nuclear fuel assemblies; the zoned transition nuclear fuel assemblies are not loaded into the outer peripheral layer of the core and at least some of the zoned transition nuclear fuel assemblies are loaded in the layer immediately adjacent to the outer peripheral layer of the core. A nuclear fuel assembly a pressurized water nuclear reactor is also provided. The nuclear fuel assembly includes a central zone comprising nuclear fuel rods which, before irradiation, contain uranium oxide but do not contain any plutonium oxide, and a peripheral zone extending along outer faces of the nuclear fuel assembly, the peripheral zone only comprising before irradiation nuclear fuel rods exclusively based on uranium and plutonium mixed oxide. According to particular embodiments, the nuclear fuel assembly may comprise one or more of the following features, taken individually or according to all the technically possible combinations: at least some of the nuclear fuel rods of the central zone are poisoned nuclear fuel rods which contain before irradiation a consumable neutron poison; in addition to the nuclear fuel rods, guide tubes for receiving rods of a control cluster and optionally an instrumentation tube, the nuclear fuel rods, the guide tubes and the optional instrumentation tube occupying all the nodes of a regular network; the assembly does not comprise any outer casing; the nuclear fuel is formed as solid pellets contained in the nuclear fuel rods. FIG. 1 schematically illustrates a pressurized water nuclear reactor 1 which conventionally comprises a core 2, and one or more of each of the elements below, only one of each of these elements being illustrated in FIG. 1: a steam generator 3, a turbine 4 coupled with an electric power generator 5 and a condenser 6. The nuclear reactor 1 comprises a heavy reflector. The nuclear reactor 1 further comprises a primary circuit 7 equipped with pumps 8 and in which pressurized water flows along the path materialized by the arrows in FIG. 1. This water in particular flows upward through the core 2 in order to be heated therein while ensuring cooling and moderation in the core 2. The primary circuit 7 further comprises a pressurizer 9 with which the pressure of the water flowing in the primary circuit 7 may be regulated. A circuit 10, a so-called water makeup network or further WMN is connected to the primary circuit 7, for example via the pump 8, in order to supply water to the primary circuit 7. The circuit 10 for example comprises reservoirs 11 containing soluble boron, for example in the form of boric acid H3BO3. The WMN circuit 10 thus allows introduction of the boron into the water of the primary circuit 7 and therefore a decrease in the reactivity in the core 2. Preferably, the boron contained in the reservoir 11 is enriched with boron 10, for example so that its atomic content of this isotope is greater than 40% and is for example of about 50%. It is recalled that the isotope 10 atomic content of natural boron is about 20%. The water of the primary circuit 7 also feeds the steam generator 3 where it is cooled by ensuring vaporization of water flowing in a secondary circuit 12. The steam produced by the steam generator 3 is channeled through the secondary circuit 12 towards the turbine 4 and then towards the condenser 6 where this steam is condensed by indirect heat exchange with the water coolant flowing in the condenser 6. The secondary circuit 12 comprises downstream from the condenser 6, a pump 13 and a heater 14. Also conventionally, the core 2 comprises nuclear fuel assemblies 16 which are loaded in a vessel 18. A single assembly 16 is illustrated in FIG. 1, but the core 2 for example comprises 241 assemblies 16. FIG. 2 shows a top view of an example of distribution of these different assemblies 16 within the core 2. Each square materializes an assembly 16. Conventionally, during the operation of the reactor 1, the latter operates during successive cycles which are separated by steps for replacement during which the spent assemblies 16 are replaced with fresh assemblies 16 and the assemblies 16 remaining in the core 2 may change position. The reactor 1 comprises control clusters 20 (FIG. 1) which are arranged in the vessel 18 above certain assemblies 16. A single cluster 20 is illustrated in FIG. 1, but the core 2 may for example comprise 89 clusters 20. The clusters 20 may be moved by mechanisms 22 so as to be inserted into the assemblies 16 which they overhang, or be extracted therefrom. Conventionally, each control cluster 20 comprises absorbent rods which include one or more materials absorbing neutrons and optionally inert rods, i.e. which do not have any specific capability of absorbing neutrons. Thus, by the vertical displacement of the clusters 20 it is possible to adjust the reactivity in the core 2 and it allows variations in the overall power P provided by the core 2 from zero power up to the nominal power NP, depending on the depth of introduction of the control clusters 20 in the assemblies 16. Some of these control clusters 20 are intended to ensure the regulation of the operation of the core 2, for example in power or in temperature and are called regulating clusters. Other ones are only intended for stopping the reactor 1 and are called stopping clusters. In the illustrated example, the nuclear reactor 1 comprises 40 regulating clusters and 49 stopping clusters. The assemblies 16 surmounted with a regulating cluster are located by hatchings and those surmounted with a stopping cluster by dots in FIG. 2. As illustrated by FIGS. 3 and 4, each assembly 16 conventionally comprises a bundle of nuclear fuel rods 24 and a frame 26 for supporting the rods 24. The frame 26 conventionally comprises a lower end piece 28, an upper end piece 30, guide tubes 31 connecting both end pieces 30 and 28 and intended to receive rods of the control clusters 20, and spacer grids 32. FIG. 4 shows the distribution of the nuclear fuel rods 24 in an assembly 16 according to the described example. The nuclear fuel rods 24 and the guide tubes 31 form therein a network with a square base with a side of 17 rods. The assembly 16 thus comprises for example 24 guide tubes 31 and 265 nuclear fuel rods 24. The nodes of the network are preferably each occupied by a nuclear fuel rod 24, by a guide tube 31, and optionally by an instrumentation tube 29 which replaces a nuclear fuel rod 24 at the center of the assembly 16. Thus, all the nodes of the network are occupied by a nuclear fuel rod 24, an instrumentation tube 29 or a guide tube 31 and the assemblies 16 therefore do not include any water hole in their network. FIG. 19 shows an assembly without an instrumentation tube. Thus, all the nodes of the network are occupied by a nuclear fuel rod 24 or a guide tube 31 and the assemblies 16 therefore do not include any water hole in their network. As the assembly 16 is intended for a pressurized water reactor, it does not comprise any outer casing surrounding the nuclear fuel rods 24, like the assemblies 16 for a boiling water reactor where this casing channels the moderation water and steam. In such an assembly 16 for a pressurized water reactor, the zone with the strongest moderation is not located between two adjacent assemblies but around the guide tubes. As illustrated in FIG. 5, each nuclear fuel rod 24 conventionally comprises a cladding 33 in the form of a circular tube closed by a lower plug 34 and an upper plug 35. The rod 24 contains the nuclear fuel formed for example in the form of a series of pellets 36 stacked in the cladding 33 and bearing against the lower plug 34. A holding spring 39 is positioned in the upper segment of the cladding 33 in order to bear upon the upper plug 35 and upon the upper pellet 36. The pellets 36 may include recesses 37 in the form of spherical caps. Preferably, these pellets 36 are nevertheless solid and therefore do not for example include any through-passage giving them an annular shape. Conventionally, the cladding 33 is in a zirconium alloy. According to a preferred operation mode of the reactor of FIG. 1, the nuclear fuel used in all the rods 24 of the assemblies 16 is a same MOX fuel. The reactor 1 then operates according to successive cycles called plutonium-equilibrium cycles in the following and the assemblies 16 used during such a cycle will be called plutonium-equilibrium assemblies 16. For a same isotope composition, all the rods 24 have a same nominal total Pu mass content. Thus, except for the differences necessarily resulting from the manufacturing, all the rods 24 have exactly the same total Pu mass content. This total mass content is defined as being the ratio between the total mass (Pu+Am) in the nuclear fuel and the total mass of the heavy isotopes (U+Pu+Am), in percent. Conventionally, this total mass content is presently less than a threshold content of 13% and for example equal to about 7% or 10%. For a given isotope composition, the uncertainties resulting from the manufacturing may lead to relative differences in the contents D varying in a range of + or −5%, the relative difference D being defined by: D ( % ) = ( actual content - nominal content ) nominal content Preferably, no rod 24 contains any consumable neutron poison, such as rare earth oxides for example, except for the inevitable impurities resulting from the manufacturing. A space 38 for expansion of the gases produced during the irradiation of the nuclear fuel is delimited inside the cladding 33 by the nuclear fuel, the lower plug 34, the upper plug 35 and the spring 39. The expansion spaces 38 preferably have volumes V which are adjusted in order to take into account the greater release of fission gases during irradiation of the MOX fuel as compared with a UO2 fuel which would be irradiated under the same conditions. Moreover, specific steps may be taken in order to increase the volume of the expansion spaces 38 such as by the use of shims or the presence of lower plugs 34 with a specific shape as described in FR-2 864 322. The core 2 preferably has a nominal linear power density NPlin of less than 175 W/cm and still preferably less than 170 W/cm. The nominal linear power density is defined by: NPlin = NP N * H wherein NP is the nominal power of the core 2, N is the number of nuclear fuel rods 24 present in the core 2 and H is the height of nuclear fuel (further called fissile column height), i.e. the height of the stack of pellets 36 (see FIG. 5). FIG. 6 illustrates the structure of a control cluster 20. This control cluster 20 includes absorbent rods 40 and a spider 42 ensuring the supporting and holding of the absorbent rods 40 in the form of a bundle in which the absorbent rods 40 are parallel with each other and laterally positioned according to the same network as that of the guide tubes 31 of the corresponding assembly 16. The spider 42 for example includes a knob 44 with which the control cluster 20 may be connected to the corresponding displacement mechanism 22 and fins 45 firmly attached to the knob 44 on each of which one or more absorbent rods 40 are attached. The absorbent rod 40 illustrated in FIG. 6 includes a tube 46 containing a stack of pellets 48 in boron carbide B4C. The tube 46 is closed at its upper end with a plug 50 and at its lower end with a dome-shaped plug 52. The tube 46 and the plugs 50 and 52 are for example made in steel and were optionally subject to a treatment against wear such as nitridation, oxidation. The stack of pellets 48 in B4C is held inside the tube 46 by a spring or any other blocking device 54. In the illustrated example, the lower end of the column of pellets 48 bears upon the lower plug 52 via a spacer 56. The spacer 56 may for example be made in a silver-indium-cadmium (SIC) alloy. The upper 50 and lower 52 plugs were welded to the tube 46, for example with laser beam, electron beam, TIG, friction or resistance welding. In a preferred alternative, the boron contained in the pellets 48 is enriched with boron 10, for example at an atomic content of more than 30%, still preferably more than 40% and for example 50%. In a preferred alternative, the reactor 1 comprises an auxiliary cooling facility 62 illustrated by FIG. 7. This auxiliary facility 62 is in particular used for cooling the core 2 when it is shut down, for cooling pools for laying or storing the fuel . . . . With the cooling facility 62 it is possible to thermally connect the elements 64 of the reactor 1, a single one of which is illustrated in FIG. 7, to a cold source 66. The element 64 may for example be the pool of a fuel building, a pump, a heat barrier. The cold source 66 may for example be formed by a stream of water or the sea or a dry air coolant. The element 64 and the cold source 66 are thermally connected in the example illustrated by an outer circuit 68 which for example is the so-called backup raw water circuit or further BWC, an intermediate circuit 70 which for example is the so-called intermediate cooling circuit or ICC and an inner circuit 72, the circuit 68, 70 and 72 are put into a thermal relationship with each other through heat exchangers 74 which preferably are plate exchangers. During a plutonium-equilibrium cycle mentioned earlier, the core 2 is loaded with 100% of MOX fuel, so that the amount of plutonium consumed by the core 2 is greater than that consumed by the reactors of the prior state of the art. The use of a single total plutonium mass content gives the possibility when it is compared with zoned MOX assemblies according to the state of the art, of loading more plutonium into the core 2 and/or guaranteeing an additional margin relatively to the allowed threshold content. Because of the homogeneous plutonium content within the core 2 and of the slow decrease in the reactivity of the MOX fuel depending on the irradiation, the radial dispersion of the linear power density around the average value is low and the power is therefore actually radially homogeneous within the core 2. A larger stability of the axial distribution of the power in the core 2 is also noted due to the lower efficiency of the xenon in a 100% MOX environment. By the low nominal linear power density NPlin, the reactor gives the possibility of having increased safety margins which may be used for increasing the flexibility of the management of the core 2 as well as of having margins on the pressure inside the rods 24 thereby allowing higher burn-up rates and longer irradiation cycle durations. This allows a further increase in the performances of the MOX fuel used and allows them to be brought to the level of those of UO2 fuel in a 100% UO2 core. Thus, the burn-up rate for the assemblies 16 of the core 2 described above may attain 50 GWd/tHM (GigaWatt·day per ton of Heavy Metal) or even 60 GWd/tHM and more. These performances may be obtained while controlling the internal pressure in the rods by adjusting the volumes V of the expansion spaces 38 in order to take into account the characteristics of the MOX fuel, present alone in the core 2, without it being necessary to take into account the characteristics of another fuel like in the state of the art. It should be noted that this low linear power density goes against the present trend of increasing this power. The absence of water rods in the plutonium-equilibrium assemblies 16 also gives the possibility of having a structure totally similar to that of UO2 assemblies, which further allows reduction in the costs associated with the core 2 by greater standardization. This absence of water rods further allows low nominal linear power density to be retained and therefore increased safety margins to be retained. The use of a single total plutonium mass content also gives the possibility of reducing the costs by a greater standardization and the absence of neutron poisons avoids specific and costly manufacturing and reprocessing steps as well as perturbation of the power shape of the reactor due to progressive depletion of these neutron poisons during irradiation. In certain alternatives, the nominal total plutonium mass content and/or the isotope compositions of the nuclear fuel may vary between the assemblies 16 present in the core 2 during a same plutonium-equilibrium cycle. In these alternatives, these different contents are not necessarily equivalent to each other and the isotope composition of the fuel of the relevant assemblies 16 may even be identical. By boron 10 enrichment of the boron contained in the control clusters 20 and of the soluble boron introduced into the primary circuit 7, the stopping margins and the safety criteria may be more easily observed in spite of the specific neutron behavior of the MOX fuel, in particular its neutron spectrum. The total boron concentration in the primary circuit 7 thus remains acceptable with regard to the specifications of the chemistry of the primary circuit and there is no risk of crystallization. The use of plate exchangers 74 in the auxiliary cooling facility 62 further allows if necessary compensation for the larger residual heat from the MOX fuel. Thus, the core 2 has increased operating and safety margins in normal and accidental situations in the operation of the reactor 1 and allows consumption of more plutonium with increased efficiency. In order to attain better performances, it is appropriate to use a low nominal linear power density of the core, as well as not to have any water hole in the plutonium-equilibrium assemblies, not to have any neutron poison in the nuclear fuel rods used, to use enriched boron in the control clusters 20 and in the boron supply circuits of the primary circuit 7, to have expansion spaces 38 with an optimized volume and to use, if necessary plate exchangers 74 as described above. However, in certain alternatives, all these features or either one or the other may be absent. As an example, rather than a boron 10 enrichment of the B4C contained in the clusters, it is possible to use for example a larger number of control clusters 20 or to keep the number of control clusters 20 and change their distribution between regulating clusters and stopping clusters. Also, the assemblies 16 may have structures and/or features different from those described above and in particular comprise a different number of nuclear fuel rods 24. The control clusters 20 described earlier may also be used in reactors for which the cores are loaded conventionally, i.e. with conventional MOX assemblies and/or with UO2 assemblies. The reactor 1 described above may start with a core 2, 100% loaded with MOX assemblies. This being said, economically, it seems to be more advantageous to start the reactor 1 with a core 2 partly or even 100% loaded with UO2 assemblies and to then pass during a subsequent cycle to a core 2, 100% loaded with MOX assemblies. Thus, and only as an example, the steps of an operating method for starting the reactor 1 with a core 2 partly loaded with UO2 assemblies and reaching a plutonium-equilibrium cycle, are described with reference to FIGS. 8 to 12. This method for operating the reactor 1 thus comprises an initial cycle and several transition cycles with which it is possible to reach a plutonium-equilibrium cycle which may be followed by any number of such plutonium-equilibrium cycles. FIGS. 8 to 12 respectively illustrate the configurations of the core 2 corresponding to the initial cycle, to the transition cycles, three in the described example, and to the plutonium-equilibrium cycle. Between each cycle, the method comprises steps for replacing spent nuclear fuel assemblies with fresh nuclear fuel assemblies. Because of the symmetry of the core 2 relatively to horizontal axes X and Y, only a quarter of the structure of the core 2 has been illustrated in FIGS. 8 to 13. As an example, in the initial cycle, the core 2 is loaded with so-called initial nuclear fuel assemblies which may for example be distributed in four categories: initial nuclear fuel assemblies 16A, these assemblies being UO2 assemblies which contain uranium enriched for example to 2.1% by mass of uranium 235, the number of assemblies 16A being for example 97, initial nuclear fuel assemblies 16B, these assemblies being UO2 assemblies which contain uranium enriched to a value by mass greater than that of the assemblies 16A, for example to 3.2% of uranium 235, the number of assemblies 16B being for example 72, initial nuclear fuel assemblies 16C, these assemblies being UO2 assemblies which contain uranium enriched to a value by mass greater than that of the assemblies 16B, for example to 4.2% of uranium 235, the number of assemblies 16C being for example 32, and initial nuclear fuel assemblies 16D which are MOX assemblies, the number of assemblies 16D being for example 40. The nuclear fuel rods 24 of the assemblies 16A to 16C therefore before irradiation do not contain any plutonium. Some of the initial nuclear fuel assemblies 16A to 16C may comprise nuclear fuel rods 24 containing before irradiation a consumable neutron poison, such as gadolinium oxide. The structure of the initial assemblies 16D is illustrated by FIG. 13. This structure is zoned so that the MOX nuclear fuel used varies between different zones of the assembly 16D. In order to distinguish these different MOX nuclear fuels, in the following of the description, the plutonium fissile isotope content t which is defined as the ratio in % between the total mass of fissile isotopes (Pu239 and Pu241) and the total mass of heavy isotopes (U+Pu+Am). The reactivity of a MOX fuel assembly depends on the content t and on the isotope composition of the plutonium used. Other parameters such as for example the total plutonium mass content may be used. In the described example, the plutonium used has the same isotope composition and the assembly 16D comprises: a first central zone 80 consisting of nuclear fuel rods 24 having a first nominal fissile isotope content t1, for example of 4.6% which corresponds to a nominal total plutonium mass content of 6.3% in the example considered, and a second zone 82 extending along the outer faces of the nuclear fuel assembly 16D and consisting of nuclear fuel rods 24 having a second nominal fissile isotope content t2 strictly less than the first content t1, the content t2 for example having the value of 3.4% which corresponds to a nominal total plutonium mass content of 4.6% in the example considered, a third zone 84 positioned at the corners of the nuclear fuel assembly 16D and consisting of nuclear fuel rods 24 having a third nominal fissile isotope content t3 strictly less than the second content t2, the content t3 for example having the value of 2% which corresponds to a nominal total plutonium mass content of 2.7% in the example considered. In the described example, the third zone 84 comprises 12 rods 24. In an alternative not shown, the assembly 16D may only comprise two zones, one corresponding to the zone 80 described earlier and the second to the union of the zones 82 and 84 described earlier where, in this alternative, the rods 24 are identical. The nominal average plutonium mass content of the assemblies 16D in the considered example is 5.7%. More generally it is substantially larger than the uranium 235 enrichment of the nuclear fuel assemblies 16C, which is 4.2% in the example described, in order to obtain the energy equivalence. As illustrated by FIG. 8, the assemblies 16D are positioned in the outer peripheral layer of assemblies 86 of the core 2. The presence of the assemblies 16D in the outer peripheral layer 86 allows limitation of the number of interfaces between the MOX fuel and the UO2 fuel and therefore a limitation of the values reached by the enthalpy rise factor FΔH of the hottest rod of the core 2. In the replacement step preceding the first transition cycle, are introduced: for example 24 transition nuclear fuel assemblies 16E which only contain UO2 fuel, possibly with certain rods 24 containing a consumable neutron poison, such as gadolinium oxide, and for example 56 nuclear fuel assemblies 16F which contain MOX fuel. The assemblies 16E for example contain uranium enriched to 4.8% by mass of uranium 235. Although these solutions are less advantageous economically, it is possible to load in place of the assemblies 16E, transition nuclear fuel assemblies only containing MOX fuel with a nominal fissile isotope content t less than that of the assemblies 16F or of the zoned MOX assemblies. The assemblies 16F are for example assemblies only containing MOX fuel, and for example are identical with the assemblies which will be used in the subsequent plutonium-equilibrium cycles. In the considered example, the nominal fissile isotope content of their MOX fuel is for example 5.4% which corresponds, still in the considered example, to a nominal total plutonium mass content of 7.3%. It will be observed that in this example, the nominal fissile isotope content t of the nuclear fuel assemblies 16F is greater than that of the central zones 80 of the nuclear fuel assemblies 16D. In order to load the assemblies 16E and 16F into the core 2 during the replacement step preceding the first transition cycle, nuclear fuel assemblies 16A are unloaded. As illustrated by FIG. 9, the assemblies 16F are loaded into the layer of assemblies 88 of the core 2 immediately adjacent to the peripheral layer 86. The assemblies 16D are displaced towards the layer of assemblies 90 of the core 2 immediately located in the inside of the core 2 relatively to the layer 88. In the replacement steps preceding the second (FIG. 10) and third (FIG. 11) transition cycles and the equilibrium cycle (FIG. 12), nuclear fuel assemblies 16F are loaded in order to progressively replace the nuclear fuel assemblies 16A to 16D. These assemblies 16F are respectively marked by mixed oblique hatchings, wide oblique hatchings and double oblique hatchings depending on the replacement step during which they were introduced. Thus, during the plutonium-equilibrium cycle (FIG. 12), the core is entirely loaded with assemblies 16F, i.e. plutonium-equilibrium assemblies. With the method described earlier, it is possible to start the reactor 1 and to reach an equilibrium cycle with a core 100% loaded with MOX assemblies and this with reduced costs as compared with the direct starting of a core 100% loaded with MOX assemblies. Indeed, this allows reduction in the number of MOX assemblies unloaded during the transition cycles without having been subject to complete depletion, assemblies, the cost of which is substantially higher than that of the UO2 assemblies. This method was only described as an example and many aspects may change from one alternative of this method to another, in particular as regards the nuclear fuel assemblies which may be introduced during the replacement steps. In particular, nuclear fuel assemblies other than those described earlier may be used. As an example, in the initial cycle, the core 2 may only be loaded with UO2 nuclear fuel assemblies. It is then possible for example by using nuclear fuel assemblies 16D, 16E and 16F to attain an equilibrium cycle with a core 100% loaded with MOX assemblies. FIG. 14 illustrates such an initial cycle where the core is loaded with initial assemblies, 16A, 16B and 16C such as those described earlier with reference to FIG. 8. Unlike the case of FIG. 8, the number of assemblies 16C is 72. The rise to equilibrium with 100% MOX may be achieved in a similar way to what was described earlier with, before a first transition cycle, the replacement of most of the nuclear fuel assemblies 16A and of a few nuclear fuel assemblies 16B with nuclear fuel assemblies 16D in order to obtain a core 2 similar to that of FIG. 8 with optionally adapted respective localizations of the assemblies in the core and contents and enrichments. The continuation of the rise to equilibrium with 100% MOX is achieved in a similar way to what was described earlier with, if necessary, adjustments of the number and of the positions of the assemblies in the core and adjustments of the enrichments and contents. It may also be desirable to pass from an operation of the core 2 100% loaded with MOX assemblies, to an operation where the core 2 is 100% loaded with UO2 assemblies or with a mixture of UO2 assemblies and MOX assemblies. For this, advantageously, it is possible to use transition nuclear fuel assemblies 16G as the one illustrated by FIG. 15. This assembly 16G has a zoned configuration and comprises: in its central zone 80 nuclear fuel rods 24 only containing before irradiation uranium oxide and no plutonium oxide, i.e. UO2 fuel, and in its peripheral zone 81 extending along the outer faces 82 of the assembly 16G and in particular in its corners 84, nuclear fuel rods 24 containing before irradiation fuel based on uranium and plutonium mixed oxide, i.e. MOX fuel. The peripheral zone 81 corresponds to the outer layer of nuclear fuel rods 24. Typically, the nuclear fuel rods 24 of the peripheral zone 81 have a nominal fissile isotope content t less than that of the plutonium-equilibrium nuclear fuel assemblies 16 present in the core 2 before the transition, for example the assemblies 16F. In certain alternatives, the central zone 80 may contain rods 24 containing before irradiation a consumable neutron poison. A method for operating the nuclear reactor will thus be described in the following, giving the possibility of passing from a plutonium-equilibrium cycle, in which the core 2 is for example 100% loaded with MOX nuclear fuel assemblies such as the assemblies 16F described earlier, to a uranium-equilibrium cycle, in which the core 2 is, as an example, 100% loaded with UO2 assemblies 16H (FIG. 16). In a replacement step preceding a first transition cycle, plutonium-equilibrium MOX assemblies 16F are removed and for example 81 transition nuclear fuel assemblies 16G are introduced. The rods 24 of the central zone 80 of the assemblies 16G may have an enrichment of 4.0% by mass of uranium 235 and some rods 24 of the zone may contain a consumable neutron poison, for example gadolinium oxide generally with a uranium 235 enrichment of the supporting uranium oxide less than that of the non-poisoned rods 24, for example an enrichment of 2% by mass. The uranium 235 enrichment of the rods of the central zone 80 is generally different and lower than that of the rods of the assemblies 16H. The rods of the peripheral zone 81 preferably have a nominal total plutonium mass content of less than half of that of the plutonium-equilibrium MOX assemblies 16F. The transition assemblies 16G are for example introduced into the layer of assemblies 88 immediately adjacent to the outer peripheral layer of assemblies 86 of the core 2, and more at the centre of the core 2, but not in the outer peripheral layer 86. After having operated the reactor 1 for the first transition cycle, 80 plutonium-equilibrium MOX assemblies 16F are removed. 80 transition nuclear fuel assemblies 16G are then loaded, which are distinguished from those described earlier by the fact that the uranium 235 enrichment of the rods of the central zone 80 is not necessarily identical with that of the preceding assemblies 16G nor with that of the rods 24 of the assemblies 16H. This enrichment may for example be adjusted in order to meet the needs of the operator in particular as regards cycle length. However in order to reach equilibrium more rapidly, the assemblies 16G loaded before the second transition cycle preferably have a uranium 235 enrichment similar to that of the assemblies 16H. These transition assemblies 16G are for example loaded while avoiding the outer peripheral layer of assemblies 86 of the core 2. The reactor 1 is then operated for a second transition cycle. Next, in the replacement step preceding a third transition cycle, the remaining last plutonium-equilibrium MOX assemblies 16F will be replaced with 80 UO2 assemblies 16H with a uranium 235 enrichment of for example 4.95% by mass. These last assemblies 16H which are uranium-equilibrium nuclear fuel assemblies only contain UO2 rods, some of which possibly contain a consumable neutron poison. The method will further comprise if necessary two transition cycles in which the residual transition assemblies 16G introduced earlier, are replaced with uranium-equilibrium nuclear fuel assemblies in order to attain a core 2, 100% loaded with UO2 assemblies, with for example a uranium 235 enrichment of 4.95% by mass. The operating method described earlier for passing from a plutonium-equilibrium cycle to a uranium-equilibrium cycle gives the possibility of ensuring such a transition in an economical way, while limiting the risks of damaging the nuclear fuel assemblies, and in particular the risks of damaging nuclear fuel rods 24 containing MOX fuel. Thus, FIG. 17 illustrates the maximum linear power density as seen by the nuclear fuel rods 24 of the MOX assemblies respectively unloaded in the replacement step preceding the first transition cycle (curve 90), in the replacement step preceding the second transition cycle (curve 92) and in the replacement step preceding the third transition cycle (curve 94). FIG. 18 illustrates the same curves, obtained not by using transition assemblies 16G such as those of FIG. 15, but by simply replacing the MOX assemblies with UO2 assemblies. It may be seen, in particular for curves 92 and 94, that the lower linear power density values are in particular reached for large burn-up rates, in particular beyond 50 GWd/tHM when transition assemblies 16G are used. Thus, the release of fission gases out of the pellets at the end of their lifetime is substantially reduced because the linear power densities of the fuel rods 24 are strongly reduced for large burn-up rates. The transition may therefore be ensured safely. This operating method with which it is possible to pass from a plutonium-equilibrium cycle to a uranium-equilibrium cycle, does not allow per se the use of more plutonium in a reactor, but it is useful for this purpose. Indeed, a nuclear power plant operator may desire, in order to use a core loaded 100% with MOX assemblies and therefore consume more plutonium, to have a solution allowing return to a more conventional operation with UO2 assemblies. This method was only described as an example and many aspects may vary from one alternative of this method to another. Thus, transition nuclear fuel assemblies 16G may only be introduced in one replacement step, their central zones 80 may comprise rods 24 containing MOX fuel . . . . The operating methods described earlier and giving the possibility to pass from a plutonium-equilibrium cycle to a uranium-equilibrium cycle on the one hand, and to reach a plutonium-equilibrium cycle on the other hand may be used independently of each other and independently of the features described above for the reactor 1. They may further be used with plutonium-equilibrium cycles in which the core is not 100% loaded with MOX assemblies and with uranium-equilibrium cycles in which the core 2 is not 100% loaded with UO2 assemblies. |
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claims | 1. A differential phase contrast X-ray imaging system, comprising:an X-ray illumination system;a beam splitter arranged in an optical path of said X-ray illumination system; anda detection system arranged in an optical path to detect X-rays after passing through said beam splitter, said detection system comprising an X-ray detection component,wherein said beam splitter comprises a splitter grating arranged to intercept an incident X-ray beam and provide an interference pattern of X-rays,wherein said detection system comprises an analyzer grating arranged to intercept and block at least portions of said interference pattern of X-rays prior to reaching said X-ray detection component,wherein said analyzer grating has a longitudinal dimension, a lateral dimension that is orthogonal to said longitudinal dimension and a transverse dimension that is orthogonal to said longitudinal and lateral dimensions, said analyzer grating comprising a pattern of optically dense regions each having a longest dimension along said longitudinal dimension and being spaced substantially parallel to each other in said lateral dimension such that there are optically rare regions between adjacent optically dense regions,wherein each optically dense region has a depth in said transverse dimension that is smaller than a length in said longitudinal dimension,wherein said analyzer grating is arranged with said longitudinal dimension at a shallow angle relative to incident X-rays, andwherein said shallow angle is less than 30 degrees. 2. A differential phase contrast X-ray imaging system according to claim 1, wherein each optically dense region has a depth in said transverse dimension that is smaller than a length in said longitudinal dimension by at least a factor of two. 3. A differential phase contrast X-ray imaging system according to claim 1, wherein each optically dense region has a depth in said transverse dimension that is smaller than a length in said longitudinal dimension by at least a factor of ten. 4. A differential phase contrast X-ray imaging system according to claim 1, wherein each optically dense region has a depth in said transverse dimension that is smaller than a length in said longitudinal dimension by at least a factor of one hundred. 5. A differential phase contrast X-ray imaging system according to claim 1, wherein said shallow angle is less than 25 degrees and greater than 3 degrees. 6. A differential phase contrast X-ray imaging system according to claim 1, wherein said shallow angle is less than 15 degrees and greater than 5 degrees. 7. A differential phase contrast X-ray imaging system according to claim 1, wherein said splitter grating is a reflection grating. 8. A differential phase contrast X-ray imaging system according to claim 1, wherein said splitter grating is a transmission grating. 9. A differential phase contrast X-ray imaging system according to claim 8, wherein said splitter grating has a longitudinal dimension, a lateral dimension that is orthogonal to said longitudinal dimension and a transverse dimension that is orthogonal to said longitudinal and lateral dimensions, said splitter grating comprising a pattern of optically dense regions each having a longest dimension along said longitudinal dimension and being spaced substantially parallel to each other in said lateral dimension such that there are optically rare regions between adjacent optically dense regions,wherein each optically dense region has a depth in said transverse dimension that is smaller than a length in said longitudinal dimension,wherein said splitter grating is arranged with said longitudinal dimension at a shallow angle relative to incident X-rays, andwherein said shallow angle is less than 30 degrees. 10. A differential phase contrast X-ray imaging system according to claim 1, wherein said X-ray illumination system comprises:an X-ray source, anda source grating arranged in an optical path between said X-ray source and said beam splitter,wherein said source grating provides a plurality of substantially coherent X-ray beams. 11. A differential phase contrast X-ray imaging system according to claim 1, wherein said X-ray illumination system comprises:a poly-energetic X-ray source, anda band-pass filter arranged in an optical path of X-rays from said poly-energetic X-ray source,wherein said band-pass filter allows X-rays within a band of energies to pass more strongly than X-rays outside said band of energies. 12. A differential phase contrast X-ray imaging system according to claim 11, wherein said band-pass filter comprises:a high-pass X-ray mirror that reflects a first portion of an incident beam of X-rays that have energies less than a lower pass-band energy and allows a second portion of said incident beam of X-rays to pass therethrough,a first beam stop arranged to intercept and at least attenuate said first portion of said incident beam of X-rays that have energies less than said lower pass-band energy,a low-pass X-ray mirror that reflects a portion of said second portion of said incident beam of X-rays after passing through said high-pass X-ray mirror that have energies less than a upper pass-band energy, anda second beam stop arranged to intercept and at least attenuate X-rays that miss said high-pass X-ray mirror prior to reaching said second beam stop,wherein said first and second beam stops are arranged to allow a beam of X-rays having energies between said upper pass-band energy and said lower pass-band energy to pass therethrough. 13. A differential phase contrast X-ray imaging system according to claim 12, wherein said low-pass X-ray mirror is a membrane X-ray mirror comprising a reflecting layer that comprises a high-Z material on a support layer that comprises a low-Z material,wherein Z is an atomic number,wherein said high-Z material includes atomic elements with Z at least 42, andwherein said low-Z material includes atomic elements with Z less than 14. 14. A differential phase contrast X-ray imaging system according to claim 1, wherein said splitter grating and said analyzer grating are arranged with a separation determined according to Talbot-Lau conditions. 15. A differential phase contrast X-ray imaging system according to claim 1, wherein said splitter grating and said analyzer grating have grating patterns determined according to Talbot-Lau conditions. 16. An X-ray illumination system, comprising:a poly-energetic X-ray source; anda band-pass filter arranged in an optical path of X-rays from said poly-energetic X-ray source,wherein said band-pass filter allows X-rays within a band of energies to pass more strongly than X-rays outside said band of energies,wherein said band-pass filter comprises:a high-pass X-ray mirror that reflects a first portion of an incident beam of X-rays that have energies less than a lower pass-band energy and allows a second portion of said incident beam of X-rays to pass therethrough,a first beam stop arranged to intercept and at least attenuate said first portion of said incident beam of X-rays that have energies less than said lower pass-band energy,a low-pass X-ray mirror that reflects a portion of said second portion of said incident beam of X-rays after passing through said high-pass X-ray mirror that have energies less than a upper pass-band energy, anda second beam stop arranged to intercept and at least attenuate X-rays that miss said high-pass X-ray mirror prior to reaching said second beam stop, andwherein said first and second beam stops are arranged to allow a beam of X-rays having energies between said upper pass-band energy and said lower pass-band energy to pass therethrough. 17. An X-ray illumination system according to claim 16, wherein said low-pass X-ray mirror is a membrane X-ray mirror comprising a reflecting layer that comprises a high-Z material on a support layer that comprises a low-Z material,wherein Z is an atomic number,wherein said high-Z material includes atomic elements with Z at least 42, andwherein said low-Z material includes atomic elements with Z less than 14. |
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052934136 | claims | 1. In a nuclear power plant having a containment, a system for pressure relief of the containment, comprising: a filter disposed inside the containment, said filter having a container, at least part of said container having two walls defining a chamber between said walls, and a heat-conducting fluid at least partly filling said chamber during a heating period and being at least half evaporated after attainment of an operating temperature. 2. The system according to claim 1, wherein said container has double-walled portions with said two walls, said double-walled portions having a heat-conducting resistance when not filled with said heat-conducting fluid being at least ten times higher than when filled with said heat-conducting fluid. 3. The system according to claim 1, wherein said two walls define a double-walled middle part of said container surrounding a vertical axis, and including a single-walled curved base firmly closing said middle part toward the bottom, and a curved cap firmly closing said middle part toward the top. 4. The system according to claim 1, wherein said middle part is cylindrical. 5. The system according to claim 3, wherein said double-walled middle part has a given height, said container includes a first inner chamber, said chamber between said walls is a second chamber having an annular cross section extending over the entire given height, and said second chamber communicates with said first inner chamber through openings formed just below said cap. 6. The system according to claim 5, including washing fluid filling substantially 30 to 80% of said first inner chamber, said second chamber having a lower part being filled with said heat-conducting fluid and an upper part, and filter mats filling at least said upper part. 7. The system according to claim 6, wherein said washing fluid fills substantially 50% of said first inner chamber. 8. The system according to claim 5, wherein said two walls have portions surrounding said heat-conducting fluid, and at least part of said second chamber has heat transfer fins increasing surface area in the vicinity of said wall portions. 9. The system according to claim 6, wherein both said washing fluid and said heating-conducting fluid are water. 10. The system according to claim 6, including a convection barrier in the form of a horizontal partition disposed in said second chamber above said heat-conducting fluid, said horizontal partition having at least one of a perforation and a slit formed therein. 11. The system according to claim 6, including mist separators disposed in said upper part of said inner chamber in front of said openings leading to said second chamber. 12. The system according to claim 5, including a vertical pipe disposed centrally in said first inner chamber for delivering a gas-steam mixture to be filtered, said vertical pipe having upper and lower ends, a horizontally extending segment through which the upper end of said vertical pipe communicates with the interior of the containment, radially disposed horizontal feed pipes into which the lower end of said vertical pipe discharges, and Venturi nozzles communicating with said horizontal feed pipes above said base. 13. The system according to claim 6, wherein said two walls are in the form of an inner wall and an outer wall, said outer wall has another opening formed therein leading to the outside, and including a pipe penetrating the containment and communicating with said other opening for carrying a filtered gas-steam mixture out of said second chamber through said outer wall at approximately half the height of said filter mats. 14. The system according to claim 1, including an overpressure line through which the interior of said filter communicates with a primary loop of the nuclear power plant, said overpressure line having an overpressure valve being closed during normal operation. 15. The system according to claim 1, including an overpressure line through which the interior of said filter communicates with the interior of the containment, said overpressure line having an overpressure valve being closed during normal operation. |
046817328 | summary | CROSS REFERENCE TO RELATED APPLICATION This application is related to the commonly assigned copending application Ser. No. 657,332 filed Oct. 3, 1984, based upon our German application P No. 33 35 888.5 filed Oct. 3, 1983. FIELD OF THE INVENTION Our present invention relates to a method of reducing the reactivity of a nuclear reactor core, even to the point of shutdown of the reactor and to a device for this purpose. More particularly, the invention relates to the reduction of the activity and shutdown of a gas-cooled graphite-moderated nuclear reactor of the type in which a cooling gas passes through a core containing nuclear fuel materials embedded in graphite and in which the fuel elements have a graphite surface. BACKGROUND OF THE INVENTION From U.S. Pat. No. 4,239,697 and the corresponding German Open Application DE-OS No. 27 53 928, it is known to reduce the reactivity and shutdown of a nuclear reactor by coating free surfaces of graphitic bodies containing fissionable fuel materials with a neutron-absorbing substance. Nuclear reactor installations generally comprise a number of control systems for regulating the reactivity of the reactor core and shutting down the chain reaction thereof. It is important that such shutdown systems be of such nature that they allow reactivation of the core in the event an emergency situation has been alleviated. In the aforementioned U.S. patent and the corresponding German application, there is described a gas-cooled nuclear reactor having graphitic fuel elements and in which the nuclear reaction is quenched by depositing a gadolinium-containing substance on the surfaces of the fuel elements by introducing that substance into the circulating cooling gases of the primary coolant contacting the fuel elements. The more gadolinium which is deposited upon the graphite surfaces of the fuel elements, the greater will be the absorption of thermal neutrons and the greater the reduction in reactivity. Since gadolinium in its natural isotropic mixture has the greatest neutron absorption cross-section for thermal neutrons of all naturally occurring elements, it suffices to deposit a comparatively small amount of gadolinium upon the fuel element. If it is desired to restart the reactor, the gadolinium may be desorbed by increasing the temperature of the reactor core and flushing it with gadolinium-free gas, by scrubbing or by a nuclear decomposition. Replacement of the gadolinium-cooled fuel elements by fresh fuel elements can also restore the reactivity. According to this earlier patent, the gadolinium compounds which are used can be applied as sols or solutions or in a gaseous state. Preferably an aqueous gadolinium acetate is used on the substance if applied as a gadolinium compound in a gaseous form, e.g. as gadolinium aluminum isopropoxide --Gd(Alc.sub.3 H.sub.7 O).sub.4).sub.3 -- or gadolinium tricyclopentadienyl --Gd(C.sub.5 H.sub.5).sub.3, the latter sublimating from the solid phase since it has no liquid phase. In their earlier system the substances were introduced through a bypass to the cooling gas circulation and, naturally, this poses no problem when the cooling gas circulation is intact. However, since a frequent cause of breakdown and the need for shutdown of the reactor is failure of the primary cooling system, the reliable introduction of the gadolinium compounds could not be ensured. Consequently, it is necessary to provide additional piping for the reactor to enable the introduction of the reactivity-reducing compounds. OBJECTS OF THE INVENTION It is the principal object of the present invention to provide an improved method of reducing the reactivity of a nuclear reactor of the type described whereby the need for additional piping is eliminated and which enables the reduction of reactivity to be effected reliably. Another object of this invention is to provide an improved method of shutting down a nuclear reactor of the gas-cooled graphite moderated type. Still another object of the invention is to provide an improved device or article for use in the improved method of the invention. It is also an object of the invention to provide a method for the purposes described which can be utilized even when there has been damage to the primary coolant circulation of a gas-cooled graphite moderated nuclear reactor. SUMMARY OF THE INVENTION These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, in a method for reducing the activity and shutting down a gas-cooled graphite moderated nuclear reactor which comprises graphite fuel elements in which the nuclear fuel material is embedded in the graphite and the fuel elements are cooled by direct contact with a primary cooling gas and have a graphitic surface. According to the invention, at least one element is introduced into the reactor core which comprises a graphite body permeable to vapor of a neutron-absorbing substance and the neutron-absorbing substance is incorporated into this body in the form of particles from which the neutron-absorbing substance is released at in a gas form to temperatures above a predetermined temperature so that the gaseous neutron-absorbing material can enter the gas spaces of the reactor core and coat upon the graphitic surfaces of the fuel elements. To reduce the activity or shutdown the nuclear reactor, therefore, we introduce into the latter elements which are comprised of graphite and contain particles with the neutron absorbing substances. This neutron absorbing substances may hereinafter be referred to also as a shutdown substance or as an activity-reducing substance. The particles are so constituted that the neutron absorbing substance is released at a predetermined temperature which corresponds to the desired shutdown temperature in the reactor core to release the substance into the latter. This can be accomplished by providing the particles with a coating which becomes gas-permeable at the predetermined temperature or melts or otherwise is decomposed. The graphite body containing these particles is permeable to the gas phase of the neutron-absorbing substance and the latter substance. The coating and the graphite of the shutdown element are so related to one another that the shutdown substance which is capable of absorbing thermal neutrons can reach the fuel elements around the shutdown elements when the shutdown temperature is reached. The shutdown elements can be introduced into the reactor core together with the usual fuel elements. In that case, no further means for activating the shutdown by this mechanism is required since shutdown will ensue only upon attainment of the threshold temperature. However, we can also introduce the shutdown element into the core utilizing the mechanism for controlling the introduction and removal of fuel elements, as need arises. In the first case the reactor has an inherent shutdown system which is activated when the critical temperature is such so as to liberate the shutdown substance automatically even in the case of complete failure of the primary coolant circulation. The number of particles and their distribution in the reactor core or the number of particles per shutdown element and the distribution of the latter in the reactor core can be comparatively small because of the high neutron absorption cross section of the substance. Hence neutron losses during normal operation can be held relatively low. This is because the shutdown substance is highly concentrated at sparse localities. Indeed, we have found that a ratio of shutdown elements to fuel elements can be 1:1000 with effective shutdown response. The static inherent shutdown system described eliminates the need for any active measures in effecting shutdown and the invention also has the advantage that the simple replacement of a comparatively few fuel elements in existing nuclear reactors under construction and reactors in the planning stages be shutdown elements according to the invention and can suffice to provide the requisite security against failure without modification of the reactor or its design. According to a feature of the invention, the particles have such a size that they are practically self-shielding and thus are subjected to a minimum of decomposition in the neutron flux of the reactor under normal operation. The shutdown elements can have outer dimensions corresponding to those of a fuel element so that direct substitution is possible in the manner described. Furthermore, the particles should contain the neutron absorbing substances in such quantities that, even with decomposition in the neutron flux of the reactor, a sufficient quantity of the substance will remain at the end of the usefulness of the reactor charged to be effective until a few changes occurs. The coating is most advantageously that of a metal from the rare earth group or alloys containing rare earth elements, although coatings of pyrolytic carbons can also be used and the neutron absorbing substance is preferably gadolinium, but also can be samarium or europium, in the form most advantageously of halogen compounds. The arrangement whereby the quenching elements have particles with neutron-absorbing substances in amounts sufficient to survive the thermal neutron flux for at least the duration of the residence times of the fuel elements with which they are associated, enable the invention to be used with particle effectiveness for a nuclear reactor in which the fuel elements may be graphite balls containing the fissionable fuels and traverse the reactor in a single pass, e.g. when the reactor operates on the so-called OTTO-process. When the pure rare earth elements are used as coating materials, they can cover the melting point range between 800.degree. C. for samarium to 1650.degree. C. for lutetium, thereby establishing corresponding thresholds for release of the neutron absorption substance. Consequently, we can make the particles respond to relatively low temperatures as well as relatively high temperatures of potential failure of a particular reactor. Alloys of these metals can also be used and when the coating is constituted of gadolinium, samarium, europium or dysprosium, the coating also acts as a protective coating against nuclear decomposition of the absorbing substances since the coating also acts as a neutron getter or absorber. The use of pyrolytic carbons has been found to be advantageous because it becomes progressively more porous at temperatures above 1000.degree. C. and thus allows proportioning the reactivity reduction release of the absorbing substance at temperature increase. The higher the temperature in the rector core, therefore, the more of the absorbtion substance will be released and cooled onto the surfaces of the fuel element. As the temperature in the reactor core drops, the pyrolytic carbon coating becomes less permeable until eventually it hermetically seals. The preferred halides of the gadolinium, samarium or europium or other rare earth absorbing substances are fluoride, bromide and iodide. These halides are stable at high temperatures and pyrolytic decomposition does not occur prematurely or undesirably when the material is introduced into the particles or distributed in the hot reactor core. Another neutron-absorbing substance can be used as long as its vapor pressure and stability allows it to be utilized in the manner which has and will be described although the ones named have been by far found to be the best. |
summary | ||
abstract | An embodiment of the present invention takes the form of a system that allows for simultaneously assembling or disassembling multiple segmented nuclear fuel rods (hereinafter “segmented rods”). An embodiment of the present invention, may receive, secure, and move the segmented rods into a position that allows for performing the tasks of either assembly or disassembly, allowing for an operator to use a tool to complete the aforementioned tasks. |
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description | Referring now to the drawings generally and to to FIG. 1 in particular, the prior art tube plug (10) comprises a shell (12) and an expander member (14) The shell (12) is a substantially cylindrical member manufactured from a metal such as Inconel. Shell (12) has a conical inner surface (16) that has a larger diameter at the closed end (18) and a smaller diameter at the open end (20). The inner surface (16) is arranged such that expander member (14) is captured within the shell (12) so that movement of the expander member (14) relative to inner surface (16) causes shell (12) to expand without allowing the expander member (14) to be removed from the shell (12). The shell (12) also has a threaded bore (22) near open end (20) which has a diameter larger than the smallest diameter of inner surface (16) which allows the apparatus to be inserted through the hreaded bore (22) and into the interior of shell (12) which also has a substantially uniform wall thickness in the portion of shell (12) that is expanded by expander member (14). In addition, a plurality of lands (24) are formed on the outside surface of the shell (12) in a manner such that the height of each land (24) increases from the closed end (18) to the open end (20) while the outer surfaces of all the lands (24) is maintained at approximately the same external diameter and while the wall thickness of shell (12) remains substantially constant throughout the portion of shell (12) wherein the lands (24) are located. As shown in FIG. 2, the relative movement of expander member (14) with respect to inner surface (16) causes the shell (12) to expand until the lands (24) contact the heat exchange tube (26). As the expander member (14) is moved relative to the shell (12) the metal in the wall of shell (12) tends to flow around the expander member (14) such that inadvertent backward motion of the expander member (14) is lessened. This provides a self-locking feature. Once tube plug (10) has been expanded, the tube plug (10) is in the locked position as shown in FIG. 2. When in this locked position, a plurality of lands (24) are impressed in the wall of heat exchange tube (26). The lands (24) thereby establish a type of labyrinth seal along the inner surface of the heat exchange tube (26) which prevents fluid from flowing therethrough. Moreover, since the shell (12) has a closed end (18) there is no potential leak path through the tube plug (10). Turning next to FIGS. 3 and 4, a ribbed plug integrity testing apparatus (28) is seen to be comprised of a cup-shaped member (30) having a groove (32) at an open end (34) of the member (30) into which is fitted a circular O-ring seal (36). The member (30) is fitted over the ribbed seal plug (10) with the open end (34) of the member (30) resting against the primary face of a tube sheet (38) through which the tube (26) and plug (10) sealably extend. An anchoring rod or member (40) having an end located mandrel (42) is rotatably mounted through a seal (44) into the closed top (46) of the member (30). The mandrel (42) is flexible and is threaded to be complimentary to the internal threads of the expander member (14). Thus, ehen a speed wrench (not shown) which is connected to the anchoring rod (40) turns the attached mandrel (42) into the expander member (14), the anchoring rod is driven down against the saucer-shaped member (30) by the enlarged rod head portion (46) located above the cup-shaped member. The threading action of the mandrel (42) into the expanding member (14) thus draws the o-ring seal (36) against the tube sheet (38). Once this o-ring seal is sufficiently pressed against the tubesheet, the testing apparatus is sealed and the ribbed plug (10) can be in-situ pressure tested. The test is performed by opening a valve (48) which is normally closed and which transports a known high-pressure fluid in line (50) to the testing apparatus (28) through an aperture (52). The known pressure fluid enters the testing apparatus and is transmitted to the inside of the plug (10) above the mandrel (42) which is now in a position sealing the expander member (14). An aperture (54) located in the hollow anchoring rod (40) transmits the known pressure fluid to the bottom (56) of the plug (10) below the member (14) through an opening (58) in the mandrel (42). The pressure of the fluid in the entire plug (10) and in the testing apparatus (28) is monitored by a meter (58) located in line (50) downstream of the valve (48). The valve (48) is again closed, and if the meter indicates the pressure is stable at the known line pressure P the plug (10) is then shown free of defects. If the pressure begins to fall below the known line pressure then it is seen that the plug (10) is leaking and is faulty and must be replaced. Certain additions and modifications to the present disclosure generally and the testing apparatus in particular have been deleted herein for the sake of conciseness and readability but are considered to fall within the scope of the following claims. As an example, there are multiple variations to the design that could be made and still accomplish some or all of the goals of this invention. As described, the testing apparatus rod and mandrel threads into and pulls on the expander member of the ribbed plug. This arrangement could easily change with different types of plugs and conditions. It is possible to thread the rod directly into the head of the plug rather than using the mandrel. Likewise, the seal could be made against the head of the plug instead of on the face of the tubesheet. This would not test all the available leak paths but would check the integrity of the plug itself. Also, the testing apparatus uses the rotation of a threaded rod to apply the force necessary to create the face seal. This seal could also be accomplished by locking into the plug and hydraulically compressing the seal. |
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054266769 | description | SUMMARY OF THE INVENTION Disclosed herein is an extrusion-resistant seal assembly for sealing a gap defined between a first structure spaced-apart from a second structure, which first structure and second structure may be an annular first flange and an annular second flange belonging to an instrumentation column of the kind typically found penetrating nuclear reactor pressure vessels. The seal assembly includes an annular retainer disposed in a gap defined between the first flange and the second flange, the first flange and the second flange capable of being moved into close proximity to reduce the gap therebetween. The retainer includes a first surface thereon having a first recess extending therearound. The retainer also has a second surface thereon having a second recess extending therearound. An annular first gasket is retained in the first recess and an annular second gasket is retained in the second recess. The first gasket intimately engages the first flange and the second gasket intimately engages the second flange to seal the gap as the first and second flanges are brought into close proximity and clamped together. The first and second gaskets resist extrusion as they are compressed between the flanges and as they are subjected to high fluid pressure because the gaskets are constrained within their respective recesses and because the retainer bears the compressive load. In this manner, the seal assembly seals the gap as the first and second flanges are brought into close proximity and clamped together. In its broad form, the invention is an extrusion-resistant seal assembly, comprising a retainer having a recess therein and a gasket retained in the recess. An object of the present invention is to provide an extrusion-resistant seal assembly for sealing a gap defined between a first structure spaced-apart from a second structure, which first structure and second structure may be a first flange and a second flange belonging to an instrumentation column of the kind typically found penetrating nuclear reactor pressure vessels. A feature of the present invention is the provision of an annular stainless steel retainer disposed in a gap defined between two opposing flanges belonging to a nuclear reactor pressure vessel instrumentation column, the retainer having an annular top recess and an annular bottom recess extending therearound. An annular graphite top gasket is retained in the top recess and an annular graphite bottom gasket is retained in the bottom recess. An advantage of the present invention is that the top and bottom graphite gaskets retained in the top and bottom recesses, respectively, resist extrusion even under relatively high pressures so that the gap is suitably sealed thereby. These and other objects, features and advantages of the present invention will become evident to those skilled in the art upon a reading of the following detailed description taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is shown a typical nuclear reactor pressure vessel, generally referred to as 10, for producing heat by the controlled fission of fissile nuclear fuel material (not shown). Pressure vessel 10 includes a vertically-oriented vessel shell open at its top end and having a plurality of inlet nozzles 30 and outlet nozzles 40 attached to the upper portion thereof (only one of each nozzle is shown). A hemispherical closure head is mounted atop vessel shell 20 and is sealingly attached to the open top end of vessel shell 20, so that closure head 50 sealingly caps vessel shell 20. Capping vessel shell 20 in this manner allows for suitable pressurization of the coolant within vessel shell 20 as pressure vessel 10 operates. Still referring to FIG. 1, disposed in pressure vessel 10 is a nuclear reactor core, generally referred to as 60, containing the nuclear fuel which is disposed in a plurality of fuel assemblies 70. A plurality of control rod drive assemblies 80 penetrate the top of closure head 50, each control drive assembly 80 including a plurality of movable control rods (not shown) slidably extending into fuel assemblies 70 for controlling the fission process in fuel assemblies 70 in a manner well known in the art. Disposed inwardly of vessel shell 20 is a horizontal upper support plate 90 that transmits mechanical loads from reactor core 60 and other internal components to the wall of vessel shell 20. A horizontal upper core plate 100 is also disposed inwardly of vessel shell 20 for supporting and locating the top ends of fuel assemblies 70, which upper core plate 100 is spaced below upper support plate 90. Upper core plate 100 has a multiplicity of coolant flow orifices 100 for flow of the pressurized liquid moderator coolant (i.e., demineralized borated water) therethrough. Penetrating closure head 50 and connected to the top of upper support plate is an elongate instrumentation column, generally referred to as 130, for reasons disclosed hereinbelow. Instrumentation column 130 may include a plurality of segments, such as segments 135/137/139, for ease of assembly and servicing. During operation of pressure vessel 10, the coolant, which may be pressurized to approximately 2,500 psia during normal operation or approximately 3,000 psia during off-normal operation (e.g., during "overpressure" transients), enters inlet nozzle 30 and circulates through vessel shell 20 to eventually exit through outlet nozzle 40, whereupon it is piped to a heat exchange device (not shown) for generating steam. In this regard, the coolant circulates through pressure vessel 10 generally in the direction of the arrows shown in FIG. 1. The steam is then piped from the heat exchange device to a turbine-generator set (not shown) for producing electricity in a manner well understood in the art. Referring to FIGS. 1, 2 and 3, instrumentation column 130 has a longitudinal bore 140 therethrough for receiving a hollow elongate shroud, such as an elongate thermocouple column 150. Thermocouple column 150 has wiring 160 extending therethrough, the wiring 160 being connected to a core physics device (not shown), such as a suitable thermocouple, for measuring core physics quantities (e.g., temperature) in reactor core 60. Instrumentation column 130 includes at least one opening 165 in the side thereof for allowing passage of wires 160 therethrough. Moreover, after passing through opening 165, wires 160 may pass through suitably sized openings (not shown) in upper support plate 90. From there the wires 160 may pass into associated ones of a plurality of elongate hollow instrumentation tubes 170. Each instrumentation tube 170 has an upper end thereof connected to upper support plate 90 and also has a lower end thereof connected to upper core plate 100, each instrumentation tube 170 being suitably aligned with its associated fuel assembly 70. An end of each wire 160 is electrical connected to the previously mentioned core physics device, which is itself disposed in its respective fuel assembly 70. As best seen in FIGS. 3 and 4, first segment 135 has an annular positioner member 180 surrounding a proximal end portion 185 thereof, the positioner member 180 engaging an annular groove 186 formed in first segment 135 in order to fix the position of positioner member 180 on first segment 135. Moreover, proximal end portion 185 of first segment 135 also includes an annular outwardly-directed integrally attached flange 187. Spaced-apart and disposed below positioner member 180 is a second segment 137. Second segment 137 has a distal end portion 190 having an annular inwardly-directly flange 200 integrally attached thereto and coaxially disposed opposite and above flange 187. Matingly interposed between positioner 180 and second segment 137 are a plurality of annular wedge-shaped first clamp members 210 for moving flange 200 and 187 into close proximity. Second segment 137 also has a proximal end portion 220 having an outwardly directed integrally attached annular flange 230 extending therearound. Flange 230 has an upwardly facing chamfered surface 235 thereon for reasons disclosed hereinbelow. Moreover, spaced-apart and below second segment 137 is a third segment 139 having a distal end portion 240 that in turn has an annular outwardly-directed integrally attached flange 250 extending therearound, flange 250 defining a downwardly facing chamfered surface 255 thereon. Formed on the bottom surface of flanges 200/230 is an annular generally downwardly sloping or canted groove 256 for reasons disclosed hereinbelow. In addition, formed on the top surface of flanges 187/250 is an annular generally downwardly sloping or canted groove 257 for reasons disclosed hereinbelow. Flange 230 is coaxially disposed opposite and above flange 250. Interconnecting flange 230 and flange 250 are a plurality of annular second clamps 260 for moving flange 230 and flange 250 into close proximity, so that second segment 137 and third segment 139 may be locked or clamped together. Each first clamp member 260 has a downwardly facing chamfered surface 265 thereon for matingly slidably engaging the chamfered surface 235 of flange 230 and also an upwardly facing chamfered surface 267 thereon for matingly slidably engaging the chamfered surface 255 of flange 250. It will be understood from the description hereinabove that spaced-apart flanges 200 and 187 define a first gap 270 therebetween and spaced-apart flanges 230 and 250 define a second gap 280 therebetween. Moreover, it will be understood with reference to the drawings that the terminology "proximal end portion" means that end portion nearer reactor core 60 and the terminology "distal end portion" means that end portion further away from reactor core 60. It will be appreciated from the description hereinabove that the pressurized radioactive coolant is in fluid communication with bore 140. Therefore, for safety reasons, it is prudent to provide suitable seals between flanges 187/200 (i.e., gap 270) and between flanges 230/250 (i.e., gap 280) to seal gaps 270/280 so as to prevent leakage of the pressurized radioactive coolant through gaps 270/280. Therefore, referring now to FIGS. 3, 4, 5, 6, 7 and 8, there is shown an extrusion-resistant seal assembly, generally referred to as 290, for sealing gaps 270/280 defined between a first structure spaced-apart from a second structure, which first structure and second structure may be first flange 200/230 and second flange 187/250, respectively, belonging to the instrumentation column 130 which is shown penetrating closure head 50 of nuclear reactor pressure vessel 10. For purposes of convenience only, seal assembly 290 will be described hereinbelow with reference to sealing second gap 280. However, it will be appreciated that seal assembly 290 may be equally useable for sealing first gap 270 or any similar gap defined between two opposing structures. Still referring to FIGS. 3, 4, 5, 6, 7 and 8, seal assembly 290 comprises a generally annular retainer 300, which may be stainless steel for resisting primary water stress corrosion cracking caused by the borated reactor coolant. Retainer 300 has an exterior top surface 310 and an exterior bottom surface 320 thereon. Moreover, retainer 300 has a centrally disposed opening 325 therethrough for passage of thermocouple column 150. Formed in top surface 310 is a circular or annular top recess 330 extending therearound for reasons disclosed presently. Moreover, formed in bottom surface 320 is a circular or annular bottom recess 340 extending therearound for reasons disclosed presently. Matingly constrained or retained in top recess 330 is an annular top seal or gasket 350, which is preferably graphite for reasons disclosed hereinbelow. In addition, matingly constrained or retained in bottom recess 340 is a bottom seal or gasket 360, which is also preferably graphite. Gaskets 350/360 may be bonded in recesses 330/340, respectively, such as by a suitable adhesive resistant to radiation degradation. Bonding gaskets 350/360 within their respective recesses 330/340 will permanently affix them within the recesses 330/340. Affixing gaskets 350/360 within their respective recesses 330/340 provides additional assurance that gaskets 350/360 will not inadvertently disassociate from recesses 330/340 and become loose parts within pressure vessel 10. The presence of loose parts within pressure vessel 10 is highly undesirable for safety reasons. With particular reference to FIG. 7, seal assembly 290 has a generally canted (i.e., 10.degree. with respect to the horizontal) transverse cross section so that it will matingly fit within grooves 256/257. As best seen in FIGS. 6 and 7, seal assembly 290 is capable of sealing gaps 270/280 so that the pressurized coolant does not flow therethrough. In this regard, first gasket 350 will intimately engage groove 256 formed in first flange 230 and second gasket 360 will intimately engage groove 257 formed in second flange 250 as flanges 230/250 are brought into close proximity and clamped together. As flanges 230/250 are brought into close proximity, gaskets 350/260 will bear a compressive load due to their intimate engagement with their associated flanges 230/250. However, gaskets 350/360 will not substantially radially extrude under such a compressive load because they are constrained within their respective recesses 330/340 and because the compressive load borne by gaskets 350/360 is substantially transferred to retainer 300. Moreover, gaskets 350/360 will not substantially radially extrude when exposed to the relatively high pressure of the coolant in bore 140 because they are constrained within their respective recesses 330/340 and because the pressure borne by gaskets 350/260 is transferred to retainer 300. Therefore, gaskets 350/360, when installed during operation of pressure vessel 10, prevent the pressurized coolant from flowing past seal assembly 290 and through gaps 270/280. OPERATION Extrusion-resistant seal assembly 290 is interposed between flanges 200 and 187 and also between flanges 230 and 250 for sealing first gap 270 and second gap 280, respectively. For purposes of convenience only, use of seal assembly 290 will be described hereinbelow with reference to the method of sealing second gap 280. However, it will be appreciated that seal assembly 290 may be equally useable for sealing first gap 270 or any similar gap defined between two opposing structures. In this regard, seal assembly 290 is interposed between flanges 230/250, such that seal assembly 290 rests in groove 257 formed in flange 250. Next, each second clamp 260 is inwardly moved, generally horizontally, such as illustrated by the straight horizontal arrow shown in FIGS. 5 and 6. In this manner, chamfered surfaces 265/267 of second clamp 260 progressively slidably engage chamfered surfaces 235/255 of flanges 230 and 250, respectively, in order to move flanges 230 and 250 into close proximity. As flanges 230 and 250 are moved into close proximity, in the manner disclosed hereinabove, top gasket 350 and bottom gasket 360 will intimately engage groove 256 and groove 257, respectively (see FIG. 6). As gaskets 350/360 intimately engage grooves 256/257, respectively, the pressurized coolant will not admit past gaskets 350/360 to exit through second gap 280. In this manner, second gap 280 is sealed. First gap 270 is sealed in a similar manner. As pressure is exerted by the coolant against gaskets 350 and 360, this pressure will be transferred to retainer 300 because gaskets 350 and 360 are matingly retained with recesses 330 and 340, respectively, of retainer 300. Thus, graphite gaskets 350 and 360 will coact with retainer 300 so as to maintain their sealing function without "blow-out" because they do not solely bear the relatively high pressure of the coolant. Although the invention is fully illustrated and described herein, it is not intended that the invention as illustrated and described be limited to the details shown, because various modifications may be obtained with respect to the invention without departing from the spirit of the invention or the scope of equivalents thereof. For example, the use of seal assembly 290 need not be limited to sealing the gaps 270/280 of the nuclear pressure vessel instrumentation column 130; rather, seal assembly 290 is usable whenever it is desired to seal any similar gap defined between two opposing structures. Therefore, what is provided is an extrusion-resistant seal assembly for sealing a gap defined between a first structure spaced-apart from a second structure, which first structure and second structure may be a first flange and a second flange belonging to an instrumentation column of the kind typically found penetrating nuclear reactor pressure vessels. |
039719502 | summary | BACKGROUND OF THE INVENTION Breast compression in the mammographic field is necessary to provide images of improved quality since the breast is an object of wide geometric change. For example, compression has reduced the thickness variation of the breast from the nipple to the chest wall. An image of the compressed breast therefore will provide more uniform information in the developed image. Breast compression also inhibits the movement of the breast during exposure thereby reducing image degradation due to breast movement. Prior art x-ray systems, such as that shown in U.S. Pat. No. 3,824,397, generally incorporate integral compression devices. The degree of flexibility in positioning the integral device to accommodate the desired image view or breast size is limited by the associated x-ray system. Further, the non-universality of integral compression devices limit their capability of incorporating significant improvements in breast compression devices and techniques. An independent compression device will serve those users having an existing x-ray facility who are unwilling to dedicate a portion of the equipment to mammography. The breast compression device, in addition to being independent of the x-ray system, should preferably be lightweight and portable and incorporate a compression member which may be adjusted in a plurality of directions, allows breast visualization during compression and provides for optiminum compression over the contacted breast area. SUMMARY OF THE PRESENT INVENTION The present invention provides a mammographic compression and positioning device which is independent of x-ray system utilized to produce images of an object being examined. A slide assembly is movable along a vertical post member which is adjustably secured to a base member. A compression paddle is coupled to the slide assembly and has a curved lower surface which, upon contacting the object, exerts a variable compressive force thereon. The position of the compression paddle is adjustable in a plurality of directions, allowing the paddle to be exactly positioned whereby an image of a selected object view may be obtained. The compression paddle is transparent enabling the user of the device to visualize the object being compressed and to take the steps necessary to provide initial image results which are satisfactory thereby reducing the number of reimages which normally would be required. It is an object of the present invention to provide a compression device for use in mammography which is independent of the x-ray system and imaging member. It is a further object of the present invention to provide a breast compression device which is independent of the x-ray system and imaging member which is adjustable in a plurality of directions enabling the device to be positioned whereby an image of a selected breast view may be obtained. It is still a further object of the present invention to provide a breast compression device which is independent of the x-ray system and imaging member and which includes a compression member having a curved lower portion which exerts optimum compression over the contacted area of the breast, by bringing the breast away from the chest wall and aiding in the separation of structure within the breast. It is still an object of the present invention to provide a breast compression device which is independent of the x-ray system and imaging member and which includes a transparent compression member having a curved lower portion which exerts variable pressure over the contacted area of the breast and which allows a user to visually verify the position of the breast with respect to the imaging member and to verify the absence of skin folds prior to imaging. It is a further object of the present invention to provide a breast compression device which is independent of the x-ray system and imaging member and is lightweight and portable. |
description | The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. HR0011-07-9-0007 awarded by the Defense Advanced Research Projects Agency. The present application is related to U.S. patent application Ser. No. 11/983,069, filed Nov. 7, 2007 by Luca Grella, Leonid Baranov, and Yehiel Gotkis, the disclosure of the aforementioned application is hereby incorporated by reference. 1. Technical Field The present invention relates generally to semiconductor manufacturing and related technologies. More particularly, the present invention relates to electron beam lithography. 2. Description of the Background Art As is well-understood in the art, a lithographic process includes the patterned exposure of a resist so that portions of the resist can be selectively removed to expose underlying areas for selective processing such as by etching, material deposition, implantation and the like. Traditional lithographic processes utilize electromagnetic energy in the form of ultraviolet light for selective exposure of the resist. As an alternative to electromagnetic energy (including x-rays), charged particle beams have been used for high resolution lithographic resist exposure. In particular, electron beams have been used since the low mass of electrons allows relatively accurate control of an electron beam at relatively low power and relatively high speed. Electron beam lithographic systems may be categorized as electron-beam direct write (EBDW) lithography systems and electron beam projection lithography systems. In EBDW lithography, the substrate is sequentially exposed by means of a focused electron beam, wherein the beam either scans in the form of lines over the whole specimen and the desired structure is written on the object by corresponding blanking of the beam, or, as in a vector scan method, the focused electron beam is guided over the regions to be exposed. The beam spot may be shaped by a diaphragm. EBDW is distinguished by high flexibility, since the circuit geometries are stored in the computer and can be optionally varied. Furthermore, very high resolutions can be attained by electron beam writing, since electron foci with small diameters may be attained with electron-optical imaging systems. However, it is disadvantageous that the process is very time-consuming, due to the sequential, point-wise writing. EBDW is therefore at present mainly used for the production of the masks required in projection lithography. In electron beam projection lithography, analogously to optical lithography, a larger portion of a mask is illuminated simultaneously and is imaged on a reduced scale on a wafer by means of projection optics. Since a whole field is imaged simultaneously in electron beam projection lithography, the attainable throughputs can be markedly higher in comparison with electron beam writers. Disadvantages of conventional electron beam projection lithography systems includes that a corresponding mask is necessary for each structure to be exposed. The preparation of customer-specific circuits in small numbers is not economic, because of the high costs associated with mask production. As discussed above, electron-beam direct write (EBDW) lithography has the potential to achieve excellent resolution. However, EBDW has a traditional problem relating to its low throughput. For example, it may take ten to one hundred hours to inscribe an entire wafer using EBDW lithography. One previous approach to attempt to increase the throughput is by increasing the beam current. However, when the current density exceeds a certain threshold, electron-electron interactions cause the beam to blur. This patent application relates to a system and method of electron beam lithography that overcomes the above-discussed disadvantages and problems. Rather than focusing the electron beam into a tiny spot, the approach described herein floods the wafer with the electron beam. This enables use of a high beam current while keeping the beam current density at a level consistent with minimal electron-electron interactions. For example, an area roughly 0.1 millimeters (mm) wide may be illuminated. That area is several orders of magnitude larger than a traditional EBDW system that focuses the beam into a much smaller spot, for example, with a spot size on the order of tens of nanometers (nm) wide. A flood beam 0.1 mm wide would normally not provide a writing resolution sufficiently high for practical use in integrated circuit manufacturing. However, the system and method disclosed herein enables high-resolution writing by partitioning the flood beam into a multitude (for example, four million) of independently controllable beams. While others have tried building multiple columns with multiple sources to achieve multiple beams, that approach has various difficulties, including the difficulty of making the multiple columns behave uniformly. The system and method disclosed herein may be implemented using a single column and a single source. A conventional multi-beam system would require a large array of blankers to achieve a multitude of controllable beams from a single column, each blanker being a small and independently controllable element that can be switched on and off rapidly. However, it is quite problematic to build and control such a large array. For example, a blanker array for a conventional multi-beam system is not normally buildable using integrated circuits because such integrated circuits are opaque to electrons. The system and method disclosed herein re-directs the beam (or array of beamlets) out of the direct line of sight between the electron source and the semiconductor wafer. Independently-controllable voltages are applied to cells of a dynamic pattern generator array that may be implemented using integrated circuit technology. The voltages determine whether each cell reflects electrons onto the wafer or absorbs electrons (preventing them from being reflected onto the wafer). The system and method disclosed herein advantageously breaks through the traditional EBDW speed-versus-resolution tradeoff by illuminating a large area and simultaneously exposing a multitude of pixels on the wafer. For example, four million pixels may be exposed using a 4000×1000 array of individually addressable elements. This may be achieved using a single column and a conventional electron source. FIG. 1 is a schematic diagram of a maskless reflection electron beam projection lithography system 100 in accordance with an embodiment of the invention. The name may be abbreviated to a reflection electron beam lithography or REBL system. As depicted, the system 100 includes an electron source 102, illumination electron-optics 104, a magnetic prism 106, an objective electron lens 110, a dynamic pattern generator (DPG) 112, projection electron-optics 114, and a stage 116 for holding a wafer or other target to be lithographically patterned. In accordance with an embodiment of the invention, the various components of the system 100 may be implemented as follows. The electron source 102 may be implemented so as to supply a large current at low brightness (current per unit area per solid angle) over a large area. The large current is to achieve a high throughput rate. Preferably, the material of the source 102 will be capable of providing a brightness of about 104 or 105 A/cm2 sr (Amperes per cm2 steradian). One implementation uses LaB6, a conventional electron emitter, which typically have a brightness capability of about 106 A/cm2 sr, as the source material. Another implementation uses tungsten dispenser emitters, which typically have a brightness capability of about 105 A/cm2 sr when operating at 50 kilovolts, as the source material. Other possible emitter implementations include a tungsten Schottky cathode, or heated refractory metal disks (i.e. Ta). The electron source 102 may be further implemented so as to have a low energy spread. The REBL system 100 should preferably control the energy of the electrons so that their turning points (the distance above the DPG 112 at which they reflect) are relatively constant, for example, to within about 100 nanometers. To keep the turning points to within about 100 nanometers, the electron source 102 would preferably have an energy spread no greater than 0.5 electron volts (eV). LaB6 emitters have typical energy spreads of 1 to 2 eV, and tungsten dispenser emitters have typical energy spreads of 0.2-0.5 eV. In accordance with one embodiment of the invention, the source 102 comprises a LaB6 source or tungsten Schottky emitter that is operated a few hundred degrees C. below its normal operating temperature to reduce the energy spread of the emitted electrons. However, cooler operating temperatures can destabilize the source 102, for example, due to impurities settling on the source surface and thereby diminishing its reliabilty and stability. Therefore, the source material may be preferably selected to be a material in which impurities are unlikely to migrate to the surface and choke off emission. Moreover, the vacuum on the system may be made stronger to overcome the impurity problem. Conventional lithography systems operate at a vacuum of 10−6 Torr. A scanning electron microscope (SEM) with a LaB6 source typically operates at 10−7 Torr. A SEM with a Schottky emitter typically operates at 10−9 Torr or better in the gun region. In accordance with one implementation, the REBL operates with a gun region vacuum of 10−9 Torr or lower to protect the stability of the source 102. The illumination electron-optics 104 is configured to receive and collimate the electron beam from the source 102. The illumination optics 104 allows the setting of the current illuminating the pattern generator structure 112 and therefore determines the electron dose used to expose the substrate. The illumination optics 104 may comprise an arrangement of magnetic and/or electrostatic lenses configured to focus the electrons from the source 102. The specific details of the arrangement of lenses depend on specific parameters of the apparatus and may be determined by one of skill in the pertinent art. In accordance with an embodiment of the invention, a shadow mask 120 is configured at an image plane (120A) in the illumination optics 104 or a conjugate image plane (120B) in the magnetic prism/separator 106. A top down schematic view of a portion of the shadow mask 120 is shown in FIG. 10, which is discussed below. The shadow mask 120 includes an array of holes which may be illuminated on one side using a flood beam 118 of electrons from the source. This creates an array of beamlets 105 on the other side of the shadow mask. The array of beamlets is preferably configured such that each well in the pattern generator 112 is illuminated by a corresponding beamlet. Herein, the array of beamlets created by the shadow mask 120 may be referred to in the aggregate as “the incident beam.” The magnetic prism 106 is configured to receive the incident beam 105 from the illumination optics 104. When the incident beam traverses the magnetic fields of the prism, a force proportional to the magnetic field strengths acts on the electrons in a direction perpendicular to their trajectory (i.e. perpendicular to their velocity vectors). In particular, the trajectory of the incident beam 105 is bent towards the objective lens 110 and the dynamic pattern generator 112. In one embodiment, the magnetic prism 106 is configured with a non-uniform magnetic field so as to provide stigmatic focusing, for example, as disclosed in U.S. Pat. No. 6,878,937 to Marion Mankos, entitled “Prism Array for Electron Beam Inspection and Defect Review.” A uniform magnetic field provides astigmatic focusing wherein focusing occurs in only one direction (for example, so as to image a point as a line). In contrast, the magnetic prism 106 configuration should focus in both directions (so as to image a point as a point) because the prism 106 is also utilized for imaging. The stigmatic focusing of the prism 106 may be implemented by dividing it into smaller sub-regions with different but uniform magnetic fields. Furthermore, the lens elements in the prism 106 may be of a relatively longer length and width so as to provide for a low distortion image over a large field size. However, increasing the length of the prism 106 involves a trade-off of more electron-electron interactions causing more blur. Hence, the reduced image distortion should be balanced against the increased blur when increasing the prism length. Below the magnetic prism 106, the electron-optical components of the objective optics are common to the illumination and projection subsystems. The objective optics may be configured to include the objective lens 110 and one or more transfer lenses (not shown). The objective optics receives the incident beam from the prism 106 and decelerates and focuses the incident electrons as they approach the DPG 112. The objective optics is preferably configured (in cooperation with the gun 102, illumination optics 104, and prism 106) as an immersion cathode lens and is utilized to deliver an effectively uniform current density (i.e. a relatively homogeneous flood beam) over a large area in a plane above the surface of the DPG 112. In one specific implementation, the objective lens 110 may be implemented to operate with a system operating voltage of 50 kilovolts. Other operating voltages may be used in other implementations. The dynamic pattern generator 112 comprises an array of pixels. Each pixel may comprise a metal contact to which a voltage level is controllably applied. The principle of operation of the DPG 112 is described further below in relation to FIGS. 3A and 3B. The extraction part of the objective lens 110 provides an extraction field in front of the DPG 112. As the reflected electrons 113 leave the DPG 112, the objective optics is configured to accelerate the reflected electrons 113 toward their second pass through the prism 106. The prism 106 is configured to receive the reflected electrons 113 from the transfer lens 108 and to bend the trajectories of the reflected electrons towards the projection optics 114. The projection electron-optics 114 reside between the prism 106 and the wafer stage 116. The projection optics 114 is configured to focus the electron beam and demagnify the beam onto photoresist on a wafer or onto another target. The demagnification may range, for example, from 1× to 20× demagnification (i.e. 1× to 0.05× magnification). The blur and distortion due to the projection optics 114 is preferably a fraction of the pixel size. In one implementation, the pixel size may be, for example, 22.5 nanometers (nm). In such a case, the projection optics 114 preferably has aberrations and distortions of less than 10-20 nm. The wafer stage 116 holds the target wafer. In one embodiment, the stage 116 is stationary during the lithographic projection. In another embodiment, the stage 116 is in linear motion during the lithographic projection. In the case where the stage 116 is moving, the pattern on the DPG 112 may be dynamically adjusted to compensate for the motion such that the projected pattern moves in correspondence with the wafer movement. In other embodiments, the REBL system 100 may be applied to other targets besides semiconductor wafers. For example, the system 100 may be applied to reticles. The reticle manufacturing process is similar to the process by which a single integrated circuit layer is manufactured. FIG. 2 is a schematic diagram of a maskless reflection electron beam projection lithography system 200 showing further components in accordance with an embodiment of the invention. The additional components illustrated include a high voltage source 202, a parallel datapath 204, an interferometer 206, a height sensor 208, feedback circuitry 210, and beam deflectors 212. The high voltage source 202 is shown as providing a high voltage to the source 102 and to the DPG 112. The voltage provided may be, for example, 50 kilovolts. The parallel data path 204 is configured to carry control signals to the DPG 112 for controlling the voltage on each pixel (so that it either absorbs electrons or reflects them). In one embodiment, the control signals are adjusted so that the pattern moves electronically across the DPG pixel array in a manner that is substantially the same as the way signals move through a shift register and at a rate so as to match the linear movement of the wafer. In this embodiment, each exposed point on the wafer may receive reflected electrons from an entire column (or row) of DPG pixels, integrated over time. In one implementation of this embodiment, the DPG 112 is configured to resemble a static random access memory (SRAM) circuit. In another embodiment, the control signals are such that the DPG 112 exposes one complete frame at a time. In this embodiment, each pixel on the DPG 112 exposes a corresponding pixel on the wafer. The pattern on the DPG 112 remains constant during the exposure of each frame. In one implementation of this embodiment, the DPG 112 is configured to resemble a dynamic random access memory (DRAM) circuit. The interferometer 206 may be included to provide tight coupling and positional feedback between the electron beam location and the target on the wafer. In one embodiment, the optical beams are reflected off mirrors on the stage. The resulting interference pattern depends on the difference of the individual beam paths and allows accurate measurement of the stage and wafer position. Vertical positional information may be provided by a height sensor 208. The positional information may be fed back via feedback circuitry 210 so as to control beam deflectors 212. The deflectors 212 are configured to deflect the projected beam so as to compensate for vibrations and positional drift of the wafer. FIGS. 3A and 3B are diagrams illustrating the basic operation of a dynamic pattern generator. FIG. 3A shows a cross-section of a DPG substrate 302 showing a column (or row) of pixels. Each pixel includes a conductive area 304. A controlled voltage level is applied to each pixel. In the example illustrated in FIG. 3A, four of the pixels 304 are “on” (reflective mode) and are grounded (have 0 volts applied thereto), while one pixel (with conductive area labeled 304x) is “off” (absorptive mode) and has a positive voltage (1 volt) applied thereto. The specific voltages will vary depending on the parameters of the system. The resultant local electrostatic equipotential lines 306 are shown, with distortions 306x relating to “off” pixel shown. In this example, the incident electrons 308 approaching the DPG 112 come to a halt in front of and are reflected by each of the “on” pixels, but the incident electrons 308x are drawn into and absorbed by the “off” pixel. The resultant reflected current (in arbitrary units) is shown in FIG. 3B. As seen from FIG. 3B, the reflected current is “0” for the “off” pixel and “1” for the “on” pixels. FIG. 4 is a schematic diagram showing a cross-sectional view of a well-based pixel structure of a dynamic pattern generator in accordance with an embodiment of the invention. The well-based pixel structure includes multiple stacked electrodes configured to collect, focus, and extract electrons in accordance with an embodiment of the invention. As shown, the sidewalls surrounding each well (cup) opening 402 comprises a stack with multiple conductive layers (for example, 411, 412, 413, and 414) separated by insulating layers 410. In addition, each well includes a base electrode 420 at the bottom of each well. The stacked electrode well structure may be fabricated on a silicon substrate (with an oxide layer on the substrate). As shown in FIG. 4, a preferred embodiment may include the base electrode and four stacked electrodes (five electrodes total) in the well structure. Other embodiments may include a total number of electrodes in a range from three to ten electrodes in the well structure (i.e. a base electrode and from two to nine stacked electrodes). Each stacked electrode layer is, in effect, a microlens array fabricated on a silicon substrate. In accordance with an embodiment of the invention, the base electrode 420 is formed to have an advantageously curved (cupped) surface. In an “off” mode or state, instead of absorbing electrons into the base electrode, the electrons may be reflected at a large angle so that they are not accepted by the numerical aperture of the projection optics. This advantageously lowers the voltage required to turn off the pixel. The particular implementation shown in FIG. 4 is further described as follows. Other specific dimensions and voltages may be utilized in other implementations, depending on the particular system being implemented. As shown in FIG. 4, each well may be about 1.4 microns across. The first conductive stack layer (Metal 1) 411 (about 1 micron above and closest to the curved base electrode at the bottom of the well) and the second conductive stack layer (Metal 2) 412 (about 1 micron above the first conductive stack layer) may both have an applied voltage of negative 2.5 V. The negative voltages applied to these lower electrodes in the stack may be used to focus the electrons. The third conductive stack layer (Metal 3) 413 (about 1 micron above the second conductive stack layer) may have an applied voltage of positive 15V. This relatively strong positive voltage is applied to this electrode (which is just beneath the uppermost electrode) so as to both focus the incoming electrons by drawing them into the well and extracting the reflected electrons by drawing them out of the well. The fourth conductive layer (Metal 4) 414 (about 1 micron above the third conductive stack layer) may have an applied voltage of positive 0.5 volts. This relatively weak positive voltage applied to the uppermost conductive electrode to both screen the insulator from the incoming electron current and to deflect the incoming electrons with lower energy towards the inside of a nearest well. Finally, the base electrode 420 may have an applied voltage that is switched between 0 volts and negative 5 volts, for example, in order to achieve the “off” and “on” states, respectively. FIG. 5 is a schematic diagram showing a top down view of a portion of a dynamic pattern generator in accordance with an embodiment of the invention. This embodiment comprises well openings or cavities 402 that are round-shaped in a square grid. For example, the well openings may be 1.4 microns in diameter, and the pitch of the square grid may be 1.5 microns in diameter. As discussed above in relation to FIG. 4, the interstitial regions 502 including the sidewalls of the wells may comprise a metal-insulator-metal-insulator-metal-insulator-metal-insulator stack (tetrode or four electrode lens), and the bottom of each well 402 may comprise a curved base electrode. The voltage applied to each base electrode is individually controllable. FIG. 6 is a schematic diagram depicting computer simulations of electron trajectories 624 for a well with a flat bottom electrode 620. As shown, for this simulation, the sidewall stack has three electrodes: Metal 1 611 (above the flat base electrode 620); Metal 2 612 (above Metal 1 611); and Metal 3 613 (above Metal 2 612). As further shown, the voltage applied to the flat base electrode 620 is negative 2 volts (−2 V), and the voltages applied to Metal 1 611, Metal 2 612, and Metal 3 613 are positive 5 volts (+5 V), positive 23.5 volts (+23.5 V), and positive 5 volts (+5 V), respectively. In FIG. 6, an incident electron direction 626 is highlighted by an arrow for an electron closer to the sidewall of the well, and a corresponding reflected electron direction 628 is shown for such an electron. Note that the trajectory of the reflected electron is at an angle (rather than vertical or perpendicular) as it exits the well. In other words, the reflected electron exits the well at a much higher angle (relative to perpendicular) compared to the near perpendicular angle that the incident electron enters the well. The angular trajectory of the reflected electron is indicative of an undesirable lenslet aberration. FIG. 7 is a schematic diagram depicting computer simulations of electron trajectories 724 a well with a curved base electrode 720 in accordance with an embodiment of the invention. As shown, for this simulation, the sidewall stack has three electrodes: Metal 1 711 (above the curved base electrode 720); Metal 2 712 (above Metal 1 711); and Metal 3 713 (above Metal 2 712). As further shown, the voltage applied to the curved base electrode 720 is negative 2 volts (−2 V), and the voltages applied to Metal 1 711, Metal 2 712, and Metal 3 713 are positive 5 volts (+5 V), positive 16.7 volts (+16.7 V), and positive 5 volts (+5 V), respectively. For this simulation, the curved base electrode 720 was approximated by a series of six stepped ringlets which provided a reasonably smooth equipotential from which the electrons were mirrored or reflected. Based on trial and error, it was determined by the applicants that a concave curvature was much superior to a convex curvature for the base electrode 720. In FIG. 7, an incident electron direction 726 is highlighted by an arrow for an electron closer to the sidewall of the well, and a corresponding reflected electron direction 728 is shown for such an electron. Note that the trajectory of the reflected electron is vertical or perpendicular (not at an angle) as it exits the well. In other words, the reflected electron exits the well at a similar angle (near perpendicular) as the angle that the incident electron enters the well. This is indicative of substantially reduced lenslet aberration. FIG. 8 is a schematic diagram showing a cross-sectional view showing the bottom and sidewalls of the wells 402 as covered by a conformal coating 802 in accordance with an embodiment of the invention. Applicants believe that such a conformal coating may advantageously serve to reduce or drain charge that otherwise builds up on the insulator layers (in between the electrodes) while being of sufficiently high resistance so as not to substantially perturb the electromagnetic field produced by the electrodes. In accordance with an embodiment of the invention, the conformal coating may be applied using atomic layer deposition (ALD), and the materials deposited may be ZnO and Al2O3. Advantageously, the resistivity of the ZnO/Al2O3 material may be varied over a large range because the resistivity of Zn is on the order of 1E-3 (10−3) Ohm-cm, and the resistivity of Al2O3 is on the order of 1E16 (1016) Ohm-cm. In a specific example, a conformal coating with a sheet resistance on the order of 100 giga ohms per square (GOhms/Sq) may be formed by atomic layer deposition (ALD) using six pulses of ZnO, then one pulse of Al2O3, and repeating this forty times (such that in the end the coating includes forty super-layers, each super-layer including six layers of ZnO and one layer of Al2O3). The coating is then terminated with a film of ZnO used as the last deposited layer. Such a conformal coating is believed to be practical for a dynamic pattern generator with approximately one million wells (an array on the order of 1,000 by 1,000). More generally, applicants believe that a preferred range of sheet resistance for the conformal coating is from 10 to 100 GOhms/Sq. Other conformal coatings that may be used include an ALD coating of ZnO and ZrO, a carbon coating, and a diamond like carbon (DLC) coating. FIG. 9 is an electron micrograph showing a conformal coating 930 which was deposited in wells of a dynamic pattern generator in accordance with an embodiment of the invention. The electron micrograph provides a cross-sectional view (after etching to provide the cut-away view) which shows two stacked electrodes (Metal 1 911 and Metal 2 912) and a curved base electrode 920. Insulator layers 910 are shown between the electrodes. Further shown is the conformal coating 930 which was deposited by atomic layer deposition (ALD) within the wells. FIG. 10 is a schematic diagram showing a top down schematic view of a portion of a shadow mask 120 in accordance with an embodiment of the invention. As seen, the shadow mask 120 includes an array of holes 1002 which may be illuminated on one side using a flood beam 118 of electrons from the source to create an array of beamlets on the other side. The array of holes 1002 is preferably configured so as to correspond to (match one hole to one well with) the array of wells of the dynamic pattern generator 112. Hence, for each well of the DPG 112, there is a corresponding hole in the shadow mask 120. The shadow mask 120 is configured to advantageously reduce the electron-beam illumination of the interstitial regions 502 between the wells 402 of the DPG 112. Applicants believe that reducing the electron-beam illumination of the interstitial regions has several advantages. First, the amount of electrons emitted from the interstitial regions is reduced so as to decrease background noise or loss of contrast in the signals detected by the DPG 112. Second, the removal of electrons from the incident beam by the shadow mask 120 causes decreased electron-electron interactions within the beam and so results in a decrease in energy spread or blur in the beam. Third, there is a reduction in the amount of electrons illuminating the periphery of the wells of the DPG 112 because each illumination beamlet may be created so as to be somewhat smaller than the well size when it reaches the DPG 112. Such electrons illuminating the periphery of the wells are frequently reflected outside of the acceptance angle of the projection optics. Hence, reducing such peripheral electrons reduces the background noise and increases the contrast of the system. The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. In the above description, numerous specific details are given to provide a thorough understanding of embodiments of the invention. However, the above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the invention. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined by the following claims, which are to be construed in accordance with established doctrines of claim interpretation. |
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062460528 | description | DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention generally discloses a micro-positioning motion transducer in the form of a flexure device. The flexure device includes a rigid frame or support structure securely carrying a flexure carriage assembly. The flexure carriage assembly includes a carriage having a plurality of structures which permit high precision translational movement in an X and a Y direction defining a substantially flat plane of movement. The structure precisely transmits forces at least partially applied in the X direction that are converted to translational movement of a translational section only in the X direction. The structure also transmits forces at least partially applied in the Y direction into translational movement of the translational section only in the Y direction. The structure essentially prevents any substantial movement of the translational section of the carriage in a Z direction perpendicular to the X-Y plane. The flexure carriage assembly includes a pair of piezoelectric assemblies that drive the translating section of the flexure carriage. One piezoelectric element drives the translating element in the X direction and the other piezoelectric element assembly drives the translating element in the Y direction. The piezoelectric assemblies are oriented substantially parallel to the Z axis, though they impart precision movement in the X-Y plane perpendicular to the Z axis. Referring now to the drawings, FIG. 1 illustrates generally a flexure device 20 having a frame or support structure 22 and a flexure carriage assembly 24 rigidly affixed to and supported by the frame. The carriage assembly 24 includes a carriage 25 and also includes a pair of piezoelectric assemblies 26 each having opposed distal end couplers 28 fixed to the frame 22. The piezoelectric assemblies 26 have a central coupler 30 fixed to a translating section 29 of the flexure carriage 25. In general, the frame or support structure 22 can be a separate frame element as is illustrated in FIG. 1 that is further attached to a suitable instrument or device. Alternatively, the frame 22 can be an integral portion of the instrument or device (not shown). The piezoelectric elements 26 are energized from a source of electric energy (also not shown) and, in accordance with known principles of such elements, the piezoelectric assemblies 26 move according to the applied energy. Since the elements have a central coupler 30 coupled to the translating section 29 of the flexure carriage 25, the translating section as described in detail below, moves in accordance with the motion of the piezoelectric assemblies 26. As described and shown herein, the movement of the piezoelectric assemblies 26 and the translating section 29 of the flexure carriage 25 is highly precise and has a relatively large range of motion. However, as discussed above, the typical and desirable range of motion for such a device is small in reality, for example, on the order of one .ANG. to about a few hundred .mu.. FIG. 2A illustrates the flexure carriage assembly 24 in perspective view. FIG. 2B illustrates the carriage 25 in perspective view. FIGS. 3 and 4 illustrate two sides in plan view of the carriage 25 which have been arbitrarily selected for illustration. The carriage need not have a front, back and designated sides. However, for illustrative purposes, FIG. 3 illustrates a view arbitrarily shown as a back surface of the carriage 25, and FIG. 4 illustrates a side surface of the carriage which can be either side of the carriage when the carriage is rotated 90 degrees about a vertical axis relative to the views in FIGS. 3 and 4. Turning again to FIGS. 2-4, the flexure carriage 25 of the carriage assembly 24 is in the form of a rectangular three-dimensional structure. The carriage 25 is preferably made from a substantially rigid material such as stainless steel or the like wherein the material is not too brittle, soft or flexible so that it may perform the intended functions of the invention. The carriage 25 is comprised of a substantially symmetrical structure and is described herein including a top and bottom end as well as front, rear and side surfaces. However, these designations are arbitrarily selected and utilized only for simplicity of description. It will be obvious to one of ordinary skill in the art that the carriage as well as the flexure device 20 can be oriented in any manner and manipulated to any orientation without departing from the scope of the invention. With that in mind, FIG. 2A illustrates the flexure carriage assembly 24 and FIG. 2B illustrates the carriage 25. The carriage 25 includes four elongate vertical columns disposed parallel to one another and spaced equal distance from one another. Each of the elongate columns includes a first end, herein designated as a top end and a second end, herein designated as a bottom end. The four elongate columns are identified herein for simplicity as 32A, 32B, 32C and 32D. The respective top ends are identified as 34A, 34B, 34C and 34D. The respective bottom ends 36 are represented by 36A, 36B, 36C and 36D. Each of the elongate columns is essentially the same length and oriented so that each of the top ends terminate in the same plane relative to one another and each of the bottom ends terminate in the same plane relative to one another. Each of the top ends of the carriage 25 are interconnected to adjacent top ends of corresponding elongate columns by first cross members 38 A-D. For example, the cross member 38A extends between the top ends 34A and 34B of the adjacent elongate columns 32A and 32B. Similarly, the cross member 38B extends between the top ends 34B and 34C, the cross member 38C extends between the top ends 34C and 34D, and the cross member 38D extends between the top ends 34D and 34A. The first cross members 38 A-D combine to define an arbitrary top 39 of the carriage 25. Similarly, four second cross members 40 A-D extend between the bottom ends 36 A-D of the elongate columns 32 A-D in an identical manner. The four second cross members 40 A-D combine to define an arbitrary bottom 41 of the carriage 25. Each of the cross members 38 A-D and 40 A-D are arranged at right angles relative to one another when viewed from either the top 39 or the bottom 41 of the carriage 25. Thus, the combination of the cross members 38 A-D and 40 A-D along with the elongate columns 32 A-D define a right angle three dimensional parallelogram. In the present embodiment, all of the cross members are of equal length so that the top 39 and bottom 41 are square. A symmetrical shape is preferred for the carriage but the overall cross section need not be a square shape in order to fall within the scope of the invention. The elongate columns 32 A-D and the cross members 38 A-D and 40 A-D are each preferably integrally formed with one another and therefore, without more, would form a rigid frame structure. However, the carriage 25 of the flexure device 20 must allow for certain flexible movements as described below in detail. The flexible nature of the carriage 25 is provided by adding a plurality of flexures 50 to the structure of the carriage 25. The construction of one flexure 50 is now described in detail below. Subsequently, the placement of the flexures 50 on the carriage 25 is described along with the function and flexible nature of the carriage. In order to simplify the description of the carriage 25, a coordinate system is arbitrarily chosen and utilized in conjunction with the discussion herein. Referring to FIG. 2B, an X axis or X coordinate is defined along one axis perpendicular to the four elongate columns 32 A-D and perpendicular to arbitrary side surfaces 52 and surface 54. A Y axis as illustrated in FIG. 2B is perpendicular to the X axis and also perpendicular to an opposed front 56 and back 58 of the carriage 25. The front and back 56 and 58, respectively, are perpendicular to the sides 52 and 54. A Z axis is also illustrated in FIG. 2B disposed parallel to and between to the four elongate columns 32 A-D and perpendicular to the X-Y plane. The arbitrary back 58 is illustrated in FIG. 4 and the arbitrary side 52 is illustrated in FIG. 3. FIG. 5 illustrates the construction of one flexure 50 taken at the juncture between the elongate column 32C at its top end 34C and the cross member 38B. FIG. 6 illustrates the same flexure 50 viewed 90 degrees relative to the flexure shown in FIG. 5. Each flexure 50 includes an interior first material web 60 nearer the X and Y plane and an exterior second material web 62 nearer either the top 39 or bottom 41 of the carriage and essentially perpendicular relative to the first material web 60. Each material web is formed by creating a pair of opposed slots 64 perpendicularly or transversely into opposed surfaces of the appropriate elongate column 32. Thus, each material web 60 and 62 is a thin web or membrane of material between the slots 64 and extends the entire width of the appropriate elongate column 32 when viewed into one of the slots 64. Therefore, the view of the flexure 50 in FIG. 5 shows the interior material web 60 on an end view so that the thin-walled construction is visible. The exterior material web 62 is illustrated lengthwise. The same flexure 50 is illustrated in FIG. 6 where the interior material web 60 is lengthwise and the exterior material web 62 is in an end view. Each flexure 50 permits linear movement in the X direction and the Y direction but not in the Z direction. The web 60 will permit slight lateral movement of the elongate column 32C relative to the cross member 38B when a force is applied in the X direction. The web 62, because it is oriented lengthwise in the X direction and rigidly connected to both the cross member 38B and the elongate column 32C, prevents movement in the X direction. However, when viewed at a 90 degree angle as shown in FIG. 6, the web 62 permits movement in the Y direction upon an applied Y direction force. Each flexure 50 therefore permits movement in the X direction and the Y direction upon an applied force, respectively, in the X or the Y direction. Each flexure 50 also prevents any movement in the Z direction based on the rigid connections between each structural element connected to each flexure 50. The construction of each flexure 50 also enhances direct movement only in the direction of the applied force in that one web is oriented to permit movement only in one linear direction wherein the other web is oriented to permit movement in only one linear direction perpendicular to the linear direction of movement for the other web. Each web is also constructed to prevent any movement at that web other than in its intended direction of movement. Therefore, each flexure 50 provides a precise X or Y flexure according to the applied force and prevents any other movement and particularly prevents movement in the Z direction. As best illustrated in FIG. 2A, a flexure 50 is disposed at each top end 34 A-D and each bottom end 36 A-D between the respective elongate columns 32 A-D and cross members 38 A-D and 40 A-D. Each flexure 50 disposed at the top ends 34 of the elongate columns 32 is oriented so that all interior webs 60 are oriented in the same direction relative to one another and all exterior webs 62 are oriented in the direction relative to one another. Each of the flexures 50 disposed at the bottom ends 36 of the elongate columns 32 is also oriented identically relative to one another. Each flexure 50 disposed at opposite ends of each of the elongate columns 32 A-D are preferably oriented as mirror images of one another to provide symmetry in the construction of the carriage 25. For example, the flexures 50 on ends 34A and 36A of the elongate column 32A each have the exterior material webs 62 oriented parallel relative to one another and have the interior material webs 60 oriented parallel relative to one another. Each of the elongate columns 32 A-D also has at least one, and preferably, a pair of flexures 50 disposed near the center defined by the X axis and Y axis noted in FIG. 2A with one flexure 50 being disposed on each side of the mid- line or X-Y plane. Again, each of these interior flexures 50 are disposed so that they are mirror images relative to one another. Therefore, the interior material webs 60 are oriented parallel relative to one another and the exterior material webs 62 are also oriented parallel relative to one another. Additionally, each of the flexures disposed near the mid-line 50 is oriented identically on each of the elongate columns 32 A-D to provide uniform flexure. The translating section 29 is connected to each of the mid-line flexures 50 of the carriage. The translating section 29 is disposed corresponding to the X-Y plane of the carriage 25 so that the carriage is essentially symmetrical on either the top portion or the bottom portion of the carriage 25 relative to the translating section 29. A force F applied to a back surface 68 of the translating member in the X direction will cause all of the flexures 50 to flex at the appropriate material web to permit movement in the X direction as seen in phantom lines in FIG. 7. Because the carriage 25 is constructed symmetrically, any small movement in a Z direction of any particular flexure 50 on one side of the X-Y plane is negated by mirror image movement of the corresponding flexure on the other side of the X-Y plane. This mirror image movement also offsets emperical strain on the carriage during microactuator actuation. Thus, the translating section 29 moves in a very flat movement along the X-Y plane at the center axis of the carriage. A force applied to a side surface 70 of the translating section 29 in the Y direction causes each flexure 50 to bend slightly about the appropriate material web oriented to permit movement in the Y direction. Again, because of the symmetry of the structure, movement in the Y direction of the translating section 29 will be a very flat planar movement along the X-Y plane. Because of the construction of the flexures 50 and the carriage 25, any load applied along the Y axis is transmitted as movement only in the Y direction and yields no movement in the X or the Z direction. Loads applied in both the X direction and the Y direction simultaneously will move the translating section 29 in both the X direction and the Y direction but only for a distance according to the force vectors in each direction respectively. An X direction force produces no substantial movement in the Y direction, and a Y direction force produces no substantial movement in the X direction. Therefore, extremely accurate results are produced by utilizing the carriage assembly 24 of the invention. As illustrated in FIGS. 3 and 4, the carriage 25 includes a plurality of stiffening beams 80 spanning each adjacent pair of elongate columns 32 A-D and running essentially parallel to the top and bottom cross members 38 A-D and 40 A-D. Each stiffening beam 80 is connected to an elongate column 32 A-D at its opposite ends 82 and 84 by a material web 86. Each material web 86 is formed similar to any one of the material webs 60 or 62 described above in that a pair of opposed notches or slots 88 are cut into the carriage material adjacent to each of the ends 82 and 84 to form a thin web of material interconnecting the stiffening beams 80 to the elongate columns 32 A-D. Each stiffening beam 80 essentially locks the adjacent elongate columns 32 A-D laterally relative to one another so that if they move in either the X or the Y direction, they will move in tandem and not move closer to or further away from one another. However, the web 86 at each end of each stiffening beam permits the stiffening beams to pivot slightly relative to the respective one of the elongate columns 32 A-D so that the carriage 25 can perform its intended flexure function by allowing the translating section 29 to move in the X-Y plane. As illustrated in FIGS. 3 and 4, the front 56, back 58, and sides 52 and 54 can include a stiffening beam 80 adjacent to each of the flexures 50 to provide lateral support to the carriage structure. As illustrated in FIGS. 1 and 2B, one side, such as the front 56, can be devoid of a stiffening beam to permit access to the interior of the carriage 25. Access may be necessary in order to activate or install or replace a sensor probe (not shown) or other apparatus attached to or carried by the translating section 29 of the flexure device. The number of stiffening beams 80 as well as the position or location of the stiffening beams can vary considerably without departing from the scope of the present invention. The addition and strength of the stiffening beams is determined by the particular application for which the flexure device 20 is intended. Some applications may require a stiffer carriage 25 while other applications may require a more flexible structure. As illustrated in FIGS. 1 and 2A, the back 58 and one side 52 are coupled to the piezoelectric assemblies 26. In the present embodiment, each piezoelectric assembly 26 has a pair of piezoelectric elements 90 extending symmetrically outward from a central block coupler 30 as illustrated in FIGS. 2A and 8. The coupler 30 is rigidly affixed to the back surface 68 of the translating section 29 for movement therewith. The coupler 30 includes a pair of symmetrically opposed flexures 50 essentially identical in construction to those described above for the carriage 25. Each of the flexures 50 is attached to one of the piezoelectric elements 90. Each piezoelectric element 90 is attached at their opposite distal ends to a corresponding end coupler 28, which is rigidly affixed to the frame or support structure 22 and retained thereby. Each of the end couplers 28 also includes a flexure 50 for coupling the piezoelectric elements 90 to the end couplers 28. Each piezoelectric element 90 is electrically connected to a power supply (not shown) wherein the power supply is utilized to energize each piezoelectric element and to move each element and hence the translating section 29. The flexures at each coupler 30 and 28 permit the piezoelectric elements 90 to drive the central coupler 30 and hence the translating section 29 as described above in either the X direction or the Y direction or both depending on how the piezoelectric assemblies 26 are energized. The piezoelectric elements 90 are intended to be identical in nature for each piezoelectric assembly 26 so that each piezoelectric element 90 of a particular assembly produces an equivalent movement. This insures that no out of balance force is applied to the translating section 29. Additionally, the movement produced by each piezoelectric assembly 26 is essentially only in the X or the Y direction because of the symmetrical construction of the piezoelectric assemblies 26 and because each end coupler 28 is rigidly affixed to the frame 22. Any movement which would otherwise be created in the Z direction at one end of the piezoelectric assembly is cancelled by an opposite and equal reaction at the other end of the assembly 26. As illustrated in FIG. 2A, the central couplers 30 of each piezoelectric assembly 26 are different in construction. However, the only difference is in the size of the rigid central portion of the couplers 30 affixed to the translating section 29. The size of this central portion of the central couplers is merely adapted to coincide or correspond to the size and shape of the particular surface 68 or 70 of the translating section 29 to which the coupler is attached. The shape and construction of the end couplers 28 as well as the central couplers 30 may vary considerably without departing from the scope and spirit of the invention. Additionally, the particular size, type and configuration of the piezoelectric elements may also vary considerably. The invention is not intended to be limited to any particular piezoelectric element construction. To summarize the invention, the structure of the flexure carriage 25 transmits an applied force in the X direction into an X direction movement of the translating section 29 without producing any movement in the Y direction or the Z direction. Similarly, an applied force in the Y direction produces movement of the translating section 29 only in the Y direction without producing any movement in the X direction or the Z direction. An applied force by both of the piezoelectric assemblies 26 produces corresponding movement in both the X and the Y direction wherein the movement in the X direction corresponds only to the applied X direction force and movement in the Y direction corresponds only to the applied Y direction force. The construction of the flexure device of the invention produces a highly accurate X-Y coordinate movement and produces such movement in a very flat X-Y plane virtually over a relatively large area while eliminating any significant movement of the translating section in the Z direction. Many modifications and changes to the invention as described may be made without departing from the spirit and scope of the invention. For example, the size, shape and construction of each of the elongate columns 32 A-D, cross members 38 A-D and 40 A-D, flexures 50, material webs 60, 62, and 84, slots 64 and 86, and translating sections 29 may vary considerably without departing from the invention. The size, shape and construction as well as the materials utilized to produce the flexible carriage 25 may be selected and determined according to a particular application for which the device 20 is intended. The compact nature of the overall carriage assembly 24 including the piezoelectric elements 26 permits utilizing the invention in application environments smaller than previously possible. This is accomplished by the novel construction of the invention wherein the piezoelectric assemblies 26 are oriented in the Z direction relative to the X-Y plane of movement of the translating section produced by the piezoelectric assemblies. Other variations and modifications to the specifically described embodiments may be made without departing from the spirit and scope of the present invention. With that in mind, the invention is intended to be limited only by the scope of the appended claims. |
claims | 1. A control rod assembly for a nuclear reactor having a reactor core and a pressurized fluid source, comprising:a control rod disposed within a control rod sleeve;a piston disposed on the control rod;a lead screw that is selectively secured to the control rod;a pneumatically-operated trip latch that is secured to a bottom end of the lead screw, the trip latch being selectively securable to a top end of the control rod and movable between a closed position and an open position, wherein when the trip latch is in the open position the control rod is not secured to the lead screw;a control rod drive motor that is operably connected to the lead screw; anda gas valve that is configured to be in fluid communication with the pressurized fluid source of the nuclear reactor andwherein with the gas valve in fluid communication with the pressurized fluid source, the gas valve is movable betweena first position in which the pressurized fluid source is isolated from the control rod sleeve anda second position in which the pressurized fluid source is in fluid communication with the control rod sleeve;whereinin the first position of the gas valve the pressurized fluid source is isolated from the trip latch and the trip latch is in the closed position andin the second position of the gas valve the pressurized fluid source is in fluid communication with both the piston and the trip latch to trip the trip latch into the open position,wherein with the trip latch in the open position, pressurized fluid from the pressurized fluid source acts against the piston to drive the control rod. 2. The control rod assembly of claim 1, wherein when the trip latch is in the open position the control rod is released from the lead screw. 3. The control rod assembly of claim 2, wherein when the gas valve is in the second position the control rod is driven into the reactor core by the pressurized fluid source. 4. The control rod assembly of claim 1, wherein the pressurized fluid source is a pneumatic source. 5. The control rod assembly of claim 1, wherein the piston forms a gas-tight seal with the inner surface of the control rod sleeve. 6. A nuclear reactor comprising:a reactor core;a pressurized fluid source; anda control rod assembly comprising:a control rod disposed within a control rod sleeve;a piston disposed on the control rod;a lead screw that is selectively secured to the control rod;a pneumatically-operated trip latch that is secured to a bottom end of the lead screw, the trip latch being selectively securable to a top end of the control rod and movable between a closed position and an open position, wherein when the trip latch is in the open position the control rod is not secured to the lead screw;a control rod drive motor that is operably connected to the lead screw; anda gas valve that is in fluid communication with the pressurized fluid source of the nuclear reactor andwherein with the gas valve in fluid communication with the pressurized fluid source, the gas valve is movable betweena first position in which the pressurized fluid source is isolated from the control rod sleeve anda second position in which the pressurized fluid source is in fluid communication with the control rod sleeve;whereinin the first position of the gas valve the pressurized fluid source is isolated from the trip latch and the trip latch is in the closed position andin the second position of the gas valve the pressurized fluid source is in fluid communication with both the piston and the trip latch to trip the trip latch into the open position,wherein with the trip latch in the open position, pressurized fluid from the pressurized fluid source acts against the piston to drive the control rod. 7. The control rod assembly of claim 6, wherein when the trip latch is in the open position the control rod is released from the lead screw. 8. The control rod assembly of claim 7, wherein when the gas valve is in the second position the control rod is driven into the reactor core by the pressurized fluid source. 9. The control rod assembly of claim 6, wherein the nuclear reactor further comprises a nuclear thermal propulsion space reactor. 10. The control rod assembly of claim 6, wherein the pressurized fluid source is a pneumatic source. 11. The control rod assembly of claim 6, wherein the piston forms a gas-tight seal with the inner surface of the control rod sleeve. |
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039490262 | description | One embodiment of the method according to the invention will now be given hereinafter by way of example without thereby implying any limitation of the invention. a. Fabrication of the jacket: a graphite powder (particle size within the range of 80 to 125 .mu.) is mixed in the dry state with 20 % by weight of a phenolic resin, PA1 an aluminium mould which has previously been coated with a lubricant is filled with this mixture while being subjected to vibrations in the cold state, PA1 the mixture is heated in a hot-air oven at 300.degree.C, PA1 the jacket is removed from the mould and then has a density of approximately 0.72, PA1 coking is carried out in a nitrogen atmosphere at 900.degree.C, PA1 consolidation is carried out by means of a treatment at 900.degree.C with methane over a period of 5 hours (velocity of the methane stream: 6 cm/sec.). PA1 the jacket is filled while being subjected to vibrations with a mixture of 60 % of fuel particles (particle diameter of 1100 .mu.) and of 40 % graphite (in which a proportion of 20 % has a particle size within the range of 0 to 80 .mu. and in which a proportion of 80 % has a particle size within the range of 80 to 125 .mu.), PA1 the complete assembly is closed by a plug fabricated from a graphite paste (having a particle size within the range of 80 to 125 .mu.) to which are added 2 % polysaccharides, PA1 the fuel element is impregnated with propane at 940.degree.C over a period of 10 hours (velocity of the propane stream: 1 cm/sec.), PA1 there is thus obtained a fuel element in which the carbonaceous matrix has a density of 1.6. The jacket finally obtained then has a density of approximately 0.70 and a total open pore volume of 70 %. b. Fabrication of the fuel element: The fuel elements obtained by means of the method according to the invention can also be employed in water-cooled reactors of the pressurized-water or boiling water type. The powdered resins are preferably thermosetting resins such as urea formaldehyde, epoxydes, melamine formaldehyde, polyurethanes, phenol-formaldehyde and polyesters. The resin used in the example is a phenol-formaldehyde resin of the type sold under the trademarks "Resophene PB 105", "Resophene PL 353", "Resophene PL 149", by the Promedo Co and "Resin TPR" by Borden Chemical Co. The mixture of resin and graphite is heated to thermoset the resin which holds the mass together until the subsequent coking operation. The deposition of carbon by cracking of the gaseous hydrocarbon is disclosed in report CEA-R 2535 "Fabrication et proprietes de corps carbones prepares par craquage de gaz naturel" 1964 available at Documentation Francaise, Secretariat General, Direction de la Documentation, which is incorporated herein in its entirety. Said treatment is effected in a furnace, for example in an electrical furnace, preferably a high frequence induction furnace. Gaseous hydrocarbon or mixture of gaseous hydrocarbons such as methane, butane, propane, propene, etc.. are injected into said furnace. The element situated in the furnace is consequently in contact with the gaseous hydrocarbon stream. The velocity of said stream is a few cm/sec and the temperature is between 700.degree. and 1000.degree.C. |
044787863 | abstract | A boiling water reactor fuel assembly is provided with an elongated, vertical stiffening device having four stiffening wings which are each attached to a wall of the fuel box. Each stiffening wing has at least one vertically directed passageway for water. |
054815860 | description | DETAILED DESCRIPTION OF THE INVENTION A mammography machine (200) in accordance with an illustrative embodiment of the present invention is shown in FIG. 4. The system (200) has an x-ray source (3) and an x-ray sensor (1) which is associated with a support platform (6). The support platform (6) supports an object to be examined, e.g., a patient's breast (2). The foregoing elements are all attached to a column (25). The mammography system (200) also includes a CCD detector (5) for forming an image of the x-rayed breast. Alternatively, a film may be used to form the image instead of the CCD detector. The x-ray source (3) has a beam limiting device (4) which defines a narrow beam of x-ray energy (8) covering the sensor (1). It is desirable in such a system to keep the x-ray sensor (1) centered under the x-ray beam (8) as the beam is scanned across the object (2) to be imaged. Thus, the x-ray beam (8) and sensor (1) travel synchronously in the direction indicated by the arrows (111). The x-ray imaging system (200) has a microprocessor (11) which receives predetermined system requirements, such as a sweep time, from a system operator. Based on these inputs, the microprocessor (11) provides a first control signal (12) to a motor controller (13). In turn, the motor controller (13) provides a second control signal (14) to a servo-positioning motor (10). The servo-positioning motor (10) moves the beam limiting device (4) thus scanning the narrow x-ray beam (8). The programming and operation of the microprocessor to perform this function is well known. The microprocessor (11) also provides third control signal (15) to a sensor positioning motor (16). The sensor positioning motor (16) moves the sensor (1) and detector (5). The sensor (1) senses the position of the x-ray beam (8) and provides a fourth control signal (17) containing position information to a feedback amplifier (18). The fourth control signal (17) is dependent on the position of the x-ray beam (8) relative to the sensor (1). This fourth control signal (17) is part of a feedback loop (21) which controls the position of the x-ray beam (8) to keep it centered over the sensor (1). The feedback amplifier (18) compares the fourth control signal (17) to a reference signal (20) and outputs a fifth control signal (19) which represents the difference between the fourth control signal (17) and the reference signal (20). The fifth control signal (19) is input into the servo-positioning motor controller (13) and completes a feedback loop (21). The closed loop feedback (21) comprises the positioning sensor (1), the amplifier (18), the motor controller (13), and the servo-positioning motor (10). The motor controller (13) controls the servo-positioning motor (10) which in turn moves the beam limiting device (4) so that the x-ray beam (8) is centered over the sensor (1). An x-ray exposure is taken by starting the x-ray imaging system (200) with the narrow x-ray beam (8) at one side of the breast support platform (6). The beam limiting device (4) collimates the beam (8) to expose the CCD detector (5). As the exposure begins, the beam limiting device (4) and the sensor (1) transverse the breast (2) in the direction of arrows (111) at a constant velocity. The sensor (1) and detector (5) move synchronously with the beam (8) as driven by the sensor positioning motor (16). The positioning sensor (1) is used in conjunction with a feedback loop (21) to ensure that the x-ray beam (8) is aligned with a specified location such as the center of the sensor (1). The sensor (1) can be implemented by various technologies, including but not limited to an ionization chamber, a photodiode array, a CCD array or a photocell with phosphor. According to one embodiment of the present invention, the sensor (1) is an ionization chamber (300) shown in FIGS. 7(a), 7(b) and 7 (c). The ionization chamber (300) comprises an element 1 and an element 2 located in a conductive plastic housing (80) as shown in FIG. 7 (c) . The housing (80) is filled with a gas and the elements 1 and 2 are spaced apart by 13 millimeters. As shown in FIG. 7(a), element 1 comprises a series of 15 sensing strips (30) separated by resistors (31) of R ohms. The sensing strips are made of copper and have dimensions of 4mm.times.25mm. There are N resistors (31) of R ohms. Element 1 has two output terminals (32) and (33), each terminated by a terminating resistor (34) of R.sub.2 ohms. Resistor (34) is defined by equation (1) EQU R.sub.2 =N.multidot.R/2 (1) For example, R is 100,000 ohms and R.sub.2 is 700,000 ohms. There are 15 strips and N=14 series resistors. The element 2 of FIG. 7(b) is made of aluminum and has dimensions of 35mm.times.75mm.times.0.1mm thick. One of the fundamental properties of x-rays is that they can ionize gases; that is, remove electrons from atoms to form ions, which can be used for measuring and controlling exposure. Gas-filled detectors, regardless of the shape or size, generally use ionization of the gas by the incoming radiation to produce a signal with a corresponding current and voltage. The important characteristic is that the current is directly proportional to or otherwise represents the intensity of the incoming radiation. The invention uses this principle to generate current in the resistors (31) and (34) and produce a voltage which indicates position as well as intensity. When an x-ray photon ionizes the air or gas separating two sensing strips (30), free electrons are generated. A bias voltage for example 300 volts, is applied between terminal 32 of element 1 and terminal 37 of element 2 so that these free electrons generate a current I. The current I flows through the resistors (31) and generates voltages V.sub.1 and V.sub.2 at each output terminal (32) and (33) of element 1 respectively. The voltage levels V.sub.1 and V.sub.2 are proportional to the number of resistors transversed by the ionization current I. For example, if the x-ray energy was all impinging on the left sensing strip, then the voltage at the left output terminal (32) would be ##EQU1## where I is the ionization current. The voltage at the right output terminal (33) would be EQU V.sub.2 =V/3 (3) The voltages V.sub.1 and V.sub.2 go to a differential amplifier circuit (not shown) whose output is the difference between the two voltages. This difference voltage is given by equation (4). ##EQU2## The difference voltage V.sub.1 -V.sub.2 is proportional to the position of the x-ray energy. When the x-ray beam (8) is centered on the ionization chamber (300), the voltage out of the differential amplifier will be 0 volts. If the beam is off-center to the left or to the right, the voltage out of the differential amplifier will be positive or negative, corresponding to the distance from the center of the chamber (300). It can, therefore, be used in a control feedback loop (21) shown in FIG. 4, to keep the x-ray beam (8) centered over the detector (5). According to another embodiment of the present invention, the sensor (1) is a photodiode (400) shown in FIG. 8. The photodiode (400) is similar to the ionization chamber, but instead of using the ionization of gas, the x-ray photons impinge on junction diodes. The photodiode (400) comprises junction diodes (40), series resistors (31) of R ohms and terminating resistors (34) of R.sub.2 ohms. The value R.sub.2 of the terminating resistors (34) is given by equation (1). These junction diodes (40) generate a current I proportional to the x-ray energy. This current I through the series resistors (31) generates the voltages V.sub.1 and V.sub.2 at the output terminals (42) and (43). The difference between the voltage represents the position of the x-ray beam (8) as described above in the ionization chamber implementation. As in the ionization chamber implementation, voltages V.sub.1 and V.sub.2 are used in a control feedback loop (21) shown in FIG. 4, to keep the x-ray beam (8) aligned to the detector (5). Finally, the above described embodiments of the invention are intended to be illustrative only. Numerous alternative embodiments and equivalent structures may be devised by those skilled in the art without departing from the spirit and scope of the following claims. |
abstract | One embodiment of the present disclosure is directed to a mobile X-ray unit. The mobile X-ray unit may include a base having a control unit and a power supply. The mobile X-ray unit may further include an articulated arm associated with the base. The articulated arm may be coupled to an X-ray applicator having an X-ray tube. The X-ray tube may include a target for generating an X-ray beam, a collimator for shaping the X-ray beam, and an exit surface through which the X-ray beam is configured to exit the X-ray tube. The mobile X-ray unit may have at least one light source configured to illuminate at least a portion of the X-ray beam emitted from the exit surface. |
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claims | 1. A pressure-relief system for a containment of a nuclear plant, the pressure-relief system comprising:a shutoff device;a pressure-relief line being led through the containment and being closed by said shutoff device;a wet scrubber in a section of said pressure-relief line lying outside the containment, for a pressure-relief gas flow developing in a pressure-relief operating mode with said shutoff device being open;a reservoir disposed in the containment in such a way that an overpressure, as compared with an outer environment, present in the containment, is transferred completely to said reservoir;a further shutoff device; anda feeding line, separate from and in addition to said pressure-relief line, leading from said reservoir to said wet scrubber and being closed by said further shutoff device, said feeding line configured to feed a liquid active as a scrubbing liquid from said reservoir to said wet scrubber; andsaid wet scrubber containing a washing liquid tank which is sealed with respect to a rest of an inner chamber of the containment; andsaid pressure-relief line having a throttle portion disposed upstream of a point at which said pressure-relief line opens inside said washing liquid tank. 2. The pressure-relief system according to claim 1, wherein said reservoir has a condensation chamber. 3. The pressure-relief system according to claim 1, wherein said reservoir is a pool, a pressure of said pool is equalized with respect to a rest of the containment. 4. The pressure-relief system according to claim 1, wherein said throttle portion is configured such that, at a start of a pressure-relief process, said shut-off device in said pressure-relief line being open and a pressure inside the containment being at least 2 bar absolute, a pressure inside said washing liquid tank is reduced by comparison by at least 0.3 bar. 5. The pressure-relief system according to claim 1, further comprising a fill-level control, formed by a float valve inside said washing liquid tank, for the scrubbing liquid fed in via said feeding line. 6. The pressure-relief system according to claim 1 wherein said wet scrubber is a Venturi scrubber having a Venturi tube disposed in said washing liquid tank and at a neck portion of said Venturi tube there is an intake opening formed therein for Venturi injection of the scrubbing liquid into the pressure-relief gas flow. 7. The pressure-relief system according to claim 6, further comprising an inner liquid tank for the scrubbing liquid disposed inside said washing liquid tank and encloses said intake opening in said Venturi tube, a capacity of said inner liquid tank is less than 1/10 of a total volume capacity, relative to a design filling level, of said washing liquid tank. 8. The pressure-relief system according to claim 1, further comprising a throttle device disposed in said pressure-relief line downstream of said wet scrubber when viewed in a flow direction of the pressure-relief gas flow, said throttle device configured for critical pressure relief of the pressure-relief gas flow. 9. The pressure-relief system according to claim 1, further comprising a drainage and residual heat removal system connected to said reservoir. 10. The pressure-relief system according to claim 3, wherein said a pool is an open pool. 11. The pressure-relief system according to claim 1, wherein said throttle portion is configured such that, at a start of a pressure-relief process, said shut-off device in said pressure-relief line being open and a pressure inside the containment being at least 4 bar absolute, a pressure inside said washing liquid tank is reduced by comparison by at least 1 bar. 12. A nuclear plant, comprising:a containment; anda pressure-relief system for said containment, said pressure-relief system containing:a shutoff device;a pressure-relief line being led through said containment and being closed by said shutoff device;a wet scrubber in a section of said pressure-relief line lying outside said containment, for a pressure-relief gas flow developing in a pressure-relief operating mode with said shutoff device being open;a reservoir disposed in said containment in such a way that an overpressure, as compared with an outer environment, present in said containment, is transferred completely to said reservoir;a further shutoff device; anda feeding line, separate from and in addition to said pressure-relief line, leading from said reservoir to said wet scrubber and being closed by said further shutoff device, said feeding line configured to feed a liquid active as a scrubbing liquid from said reservoir to said wet scrubber; andsaid wet scrubber containing a washing liquid tank which is sealed with respect to a rest of an inner chamber of the containment; andsaid pressure-relief line having a throttle portion disposed upstream of a point at which said pressure-relief line opens inside said washing liquid tank. 13. The nuclear plant according to claim 12, wherein said nuclear plant is a boiling water reactor nuclear plant. 14. A method for operating a pressure-relief system, the pressure-relief system containing:a shutoff device;a pressure-relief line being led through a containment and being closed by the shutoff device;a wet scrubber being switched into a section of the pressure-relief line lying outside the containment, for a pressure-relief gas flow developing in a pressure-relief operating mode with the shutoff device being open;a reservoir disposed in the containment in such a way that an overpressure, as compared with an outer environment, present in the containment, is transferred completely to the reservoir;a further shutoff device; anda feeding line, separate from and in addition to said pressure-relief line, leading from the reservoir to the wet scrubber and being closed by the further shutoff device, said feeding line configured to feed a liquid active as a scrubbing liquid from the reservoir to the wet scrubber;which method comprises the steps of:prior to or during a pressure-relief process, conveying the liquid from the reservoir into the wet scrubber, by means of the feeding line, as a result of a pressure difference between the containment and the wet scrubber. 15. The method according to claim 14, wherein, during an activation phase preceding the pressure-relief process, filling the wet scrubber for a first time by the further shut-off device in the feeding line being opened while the shut-off device in the pressure-relief line is closed. 16. The method according to claim 14, wherein, during the pressure-relief process, the shut-off device in the pressure-relief line being open and the shut-off device in the feeding line being open, used scrubbing liquid is fed back into the wet scrubber. |
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summary | ||
summary | ||
claims | 1. A particle arrangement providing a primary beam path for a primary beam of charged particles to an object, and providing a secondary beam path for charged-particles extending from the object to a detector arrangement, the particle-optical arrangement comprising:at least one charged-particle source for generating at least one beam of charged particles, the primary beam path extending from the at least one charged-particle source to the object;a first focusing lens providing a focusing magnetic field;an objective lens; anda beam splitter, wherein the beam splitter is disposed in the primary beam path between the at least one charged-particle source and the objective lens and in the secondary beam path between the objective lens and the detector arrangement;wherein the objective lens provides a focusing magnetic field for the charged-particles of the primary beam and for the charged particles of the secondary beam; andwherein the at least one charged-particle source is arranged within the magnetic field provided by the first focusing lens. 2. The particle-optical arrangement of claim 1, wherein the magnetic field where the at least one charged-particle source is arranged is a homogeneous magnetic field. 3. A particle-optical arrangement, comprising:at least one charged-particle source for generating at least one beam of charged particles,at least one multi-aperture plate having a plurality of apertures formed in the plate, wherein the plurality of apertures is arranged in a first pattern, wherein a plurality of charged-particle beamlets is formed from the beam of charged particles downstream of the aperture plate; anda first focusing lens providing a magnetic field having a focusing field portion in a region between the charged-particle source and the multi-aperture plate;wherein the at least one charged-particle source is arranged within the magnetic field provided by the first focusing lens. 4. The particle-optical arrangement of claim 3, wherein the magnetic field where the at least one charged-particle source is arranged is a homogeneous magnetic field. 5. The particle-optical arrangement of claim 3, wherein the first focusing lens is further configured to provide a focusing field throughout a first region adjacent to the multi-aperture plate in a direction of the at least one beam; andwherein the particle-optical arrangement further comprises an energy changing electrode providing an electrical field for changing a kinetic energy of charged particles of the at least one beam throughout a second region adjacent to the multi-aperture plate in the direction of the at least one beam, and wherein the first region of the focusing field and the second region of the electrical field overlap in an overlapping region. 6. The particle-optical arrangement according to claim 5, wherein the overlapping region is located upstream of the multi-aperture plate. 7. The particle-optical arrangement according to claim 5, wherein the overlapping region is located downstream of the multi-aperture plate. 8. The particle-optical arrangement according to claim 5, wherein the electrical field is a decelerating electrical field for reducing the kinetic energy of the charged particles of the beam. 9. The particle-optical arrangement according to claim 5, wherein the electrical field is an accelerating electrical field for increasing the kinetic energy of the charged particles of the beam. 10. The particle-optical arrangement according to claim 5, wherein an overlap between the energy changing field and the focusing field is more than 1%. 11. A particle-optical arrangement, comprising:at least one charged-particle source for generating at least one beam of charged particles;at least one multi-aperture plate having a plurality of apertures formed in the plate, wherein a plurality of charged-particle beamlets is formed from the at least one beam of charged particles downstream of the aperture plate;a first focusing lens providing a focusing field throughout a first region adjacent to the multi-aperture plate in a direction of the at least one beam; andan energy changing electrode providing an electrical field for changing a kinetic energy of charged particles of the beam throughout a second region adjacent to the multi-aperture plate in the direction of the at least one beam, and wherein the first region of the focusing field and the second region of the electrical field overlap in an overlapping region. 12. The particle-optical arrangement according to claim 11, wherein the overlapping region is located upstream of the multi-aperture plate. 13. The particle-optical arrangement according to claim 11, wherein the overlapping region is located downstream of the multi-aperture plate. 14. The particle-optical arrangement according to claim 11, wherein the electrical field is a decelerating electrical field for reducing the kinetic energy of the charged particles of the beam. 15. The particle-optical arrangement according to claim 11, wherein the electrical field is an accelerating electrical field for increasing the kinetic energy of the charged particles of the beam. 16. The particle-optical arrangement according to claim 11, wherein an overlap between the energy changing field and the focusing field is more than 1%. 17. The particle-optical arrangement according to claim 11, further comprising an objective lens and a detector arrangement. 18. A charged-particle beam manipulation method, the method comprising:generating at least one beam of charged particles with at least one charged-particle source;forming a plurality of charged-particle beamlets from the at least one beam of charged particles with at least one multi-aperture plate having a plurality of apertures formed in the plate;generating a magnetic field, wherein the at least one charged-particle source is positioned within the magnetic field; andfocusing the at least one beam of charged particles with the magnetic field. 19. The method of claim 18, further comprisingfocusing the charged-particle beamlets to form an array of charged-particle beamlet foci on an object; andimaging the array of charged-particle beamlet foci. 20. A charged-particle beam manipulation method, the method comprising:generating at least one beam of charged particles, the particles forming a plurality of charged-particle beamlets from the at least one beam of charged particles with at least one multi-aperture plate having a plurality of apertures formed in the plate;providing a focusing field and a kinetic energy changing field overlapping the focusing field, wherein the kinetic energy changing field changes a kinetic energy of the at least one beam of charged particles upstream of the at least one multi-aperture plate. 21. The method of claim 20, further comprisingfocusing the charged-particle beamlets to form an array of charged-particle beamlet foci on an object; andimaging the array of charged-particle beamlet foci. 22. A charged-particle beam manipulation method, the method comprising:generating at least one beam of charged particles, the particles forming a plurality of charged-particle beamlets from the at least one beam of charged particles with at least one multi-aperture plate having a plurality of apertures formed in the plate;providing a focusing field and a kinetic energy changing field overlapping the focusing field, wherein the kinetic energy changing field changes a kinetic energy of the plurality of charged-particle beamlets downstream of the at least one multi-aperture plate. 23. The method of claim 22, further comprisingfocusing the charged-particle beamlets to form an array of charged-particle beamlet foci on an object; andimaging the array of charged-particle beamlet foci. |
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041487451 | abstract | Admixtures of radioactive material contaminated phosphoric acid ester and polyvinyl chloride from essentially nonvolatile masses which do not flow. |
045338329 | claims | 1. A radiation attenuation system, comprising: a plurality of radiation attenuation module means, each having a substantially dimensionally stable preformed body shaped to stack against and on top of adjacent module means around a radiation emitting object, each said substantially dimensionally stable body having sufficient flexiblity to be molded against one or more irregular surfaces of said radiation emitting object, when said module means are stacked into an assembly around at least a portion of said radiation emitting object for substantially eliminating radiation exposure from said radiation emitting object. each said preformed body includes skin means for retaining a radiation attenuation medium within said body in said preformed shape. said radiation attenuation medium includes a plurality of lead particles. said lead particles are lead shot. each said skin means include substantially flexible inner skin means for retaining said radiation attenuation medium and substantially rigid outer skin means for maintaining said preformed body shape and to assist in preventing ruptures of said inner skin means. said radiation attenuation medium further includes a binding medium for substantially retaining said lead particles from free movement if said skin means are ruptured. said binding medium also self seals said skin means if said skin means are ruptured. said radiation medium includes compressed lead wool. said compressed lead wool includes a plurality of sheets of lead wool. each said skin means include substantially flexible inner skin means for retaining said radiation attenuation medium and substantially rigid outer skin means for maintaining said preformed body shape and to assist in preventing ruptures of said inner skin means. frame means for defining said assembled module means around said radiation emitting object. each said preformed body is substantially rectangularly shaped. a substantially dimensionally stable preformed body shaped and adapted to stack against another of said bodies; and said body including skin means for retaining a radiation attenuation medium within said body in said preformed shape. said radiation attenuation medium includes a plurality of lead particles. said lead particles are lead shot. each said skin means include substantially flexible inner skin means for retaining said radiation attenuation medium and substantially rigid outer skin means for maintaining said preformed body shape and to assist in preventing ruptures of said inner skin means. said radiation attenuation medium further includes a binding medium for substantially retaining said lead particles from free movement if said skin means are ruptured. said binding medium also self seals said skin means if said skin means are ruptured. said radiation medium includes compressed lead wool. said compressed lead wool includes a plurality of sheets of lead wool. each said skin means include substantially flexible inner skin means for retaining said radiation attenuation medium and substantially rigid outer skin means for maintaining said preformed body shape and to assist in preventing ruptures of said inner skin means. each said preformed body is substantially rectangularly shaped. forming a substantially rigid preformed module skin; and substantially filling said skin with a radiation attenuation medium to form a dimensionally stable unit. forming a first substantially flexible inner skin; filling said inner skin with said radiation attenuation medium; and forming said preformed skin around said inner skin and said radiation attenuation medium. forming said outer preformed skin by coating said inner skin with a plastic material. filling said inner skin with lead particles to form said attenuation medium. filling said inner skin with said particles and a binding medium to prevent free movement of said particles if said skin is ruptured. filling said inner skin with a binding medium which will self seal said skin if it is ruptured. filling said skin with compressed lead wool to form said attenuation medium. forming said preformed skin in a substantially rectangular shape. 2. The radiation attenuation system according to claim 1, wherein: 3. The radiation attenuation system according to claim 2, wherein: 4. The radiation attenuation system according to claim 3, wherein: 5. The radiation attenuation system according to claim 3, wherein: 6. The radiation attenuation system according to claim 3, wherein: 7. The radiation attenuation system according to claim 6, wherein: 8. The radiation attenuation system according to claim 2, wherein: 9. The radiation attenuation system according to claim 8, wherein: 10. The radiation attenuation system according to claim 8, wherein: 11. The radiation attenuation system according to claim 1, further including: 12. The radiation attenuation system according to claim 1, wherein: 13. A radiation attenuation module, comprising: 14. The module according to claim 13, wherein: 15. The module according to claim 14 wherein: 16. The module according to claim 14, wherein: 17. The module according to claim 14, wherein: 18. The module according to claim 17, wherein: 19. The module according to claim 13, wherein: 20. The module according to claim 19, wherein: 21. The module according to claim 19, wherein: 22. The module according to claim 13, wherein: 23. A method of making a radiation attenuation module, comprising: 24. The method according to claim 23, including: 25. The method according to claim 24, including: 26. The method according to claim 24, including: 27. The method according to claim 26, including: 28. The method according to claim 26, including: 29. The method according to claim 23, including: 30. The method according to claim 23, including: |
047132094 | summary | BACKGROUND OF THE INVENTION The present invention relates to a drain recovery system for the condensate feedwater system of a nuclear power plant and, more particularly, to a drain recovery system in which drains from feedwater heaters of the condensate feedwater system is collected and recovered to the condensate feedwater system. Hitherto, a drain recovery system for the condensate feedwater system of a nuclear power plant has been known in which the feedwater heater drains stored in a drain tank are pumped-up by drain pumps to be injected into the condensate feedwater system at a predetermined portion thereof for recovery of the drains. For instance, "Technical Report of Mitsubishi Heavy Industries", page 16, vol. 17 (published March, 1980) discloses such a system adapted to recover a high-pressure feedwater heater drain in a pressurized water reactor (PWR) plant. On the other hand, boiling water reactor (BWR) plants operating in the U.S.A., such as BRUNSWICK Nos. 1 and 2 and Grand Gulf No. 1 each incorporate therein also such a systems adapted to recover a high-pressure feedwater drain. In these known drain pumping-up recovery systems, the pressure of the portion of the condensate feedwater system at which the feedwater heater drain is injected thereinto varies significantly depending on the level of the load of the plant. More specifically, when the plant load is comparatively in a low level, the flow rate of the feedwater is correspondingly small, which in turn reduces the pressure drop in the condensate feedwater system and also serves to increase the delivery head of the condensate pumps, so that the pressure in the portion of the condensate feedwater system at which the collected drain is injected thereinto is increased. This means that the required delivery head of the drain pumps is largely changed depending on the level of the load. Namely, delivery head required for the drain pumps is large at the low load level and small at the high load level, so that the drain pumps must satisfy a wide range of the required delivery head. This in turn requires drain pumps of a large capacity, and complicates the specification and design of the drain pumps. On the other hand, in case of the BWR plant, it is necessary to maintain the purity of the water in the condensate feedwater system at a degree as high as possible. This requires that the drain pumping-up recovery system incorporates a water purifier through which the drain is purified before it is injected into the condensate feedwater system. Unfortunately, however, the provision of the water purifier increases the pressure drop, which necessitates a further increase in the drain pump capacity and further complication of the design of the drain pumps. It might be desired that not only the high-pressure feedwater heater drain but also the low-pressure feedwater heater drain, are pumped-up and injected into the condensate feedwater system. In such a case, however, the flow rate of the drain treated by the drain pumps is increased and, in addition, the use of the water purifier becomes essential. This further increases the drain pump capacity so that the design of the pump is extremely complicated. For these reasons, the conventional system is not provided with a water purifier and is not designed for recovery of the low-pressure feedwater heater drain. The known drain pumping-up recovery system encounters a problem that, when the drain pumps malfunction and trip, the flow rate of the drain injected into the condensate feedwater system and, hence, the flow rate of the feedwater supplied to the nuclear reactor becomes insufficient, and thus the plant might be scramed as a whole. In addition, the drain level in the drain tank is raised so that the flow of the drain from the high-pressure feedwater heater into the drain tank is impeded to cause a risk of a rise in the drain level in the high-pressure feedwater heater, which in turn may cause a reverse flow of the drains into the high-pressure turbine, with a result that the turbine is damaged. In order to obviate this problem, it has been necessary to install a spare drain pump and to start this spare drain pump in the event of a trip of one of the drain pumps. This also complicates the design of the drain pumps. SUMMARY OF THE INVENTION Accordingly, an object of the invention is to provide a drain recovery system for the condensate feedwater system of a nuclear power plant in which the drain pumps of the drain pumping-up recovery system for pumping-up and injecting the feedwater heater drain into the condensate feedwater system at a predetermined portion thereof can be of a small capacity, and the necessity of a spare drain pump is eliminated, thus simplifying and facilitating the design of the drain pump system. To this end, according to the invention, there is provided a drain recovery system for the condensate feedwater system of a nuclear power plant, said condensate feedwater system including condensate pumps for boosting the condensate from a condenser, and feedwater heaters for heating the condensate from said condensate pumps, said drain recovery system comprising: drain pumping-up recovery means including a drain tank for storing a feedwater heater drain, and drain pump means connected to said drain tank for pumping-up the drain therein to inject the drain into said condensate feedwater system at a predetermined portion thereof; and drain level control means including conduit means connected between a portion of said drain pumping-up recovery means upstream of said drain pump means and a portion of said condensate feedwater system upstream of said condensate pumps for causing the drain in said drain tank to be returned to said portion upstream of said condensate pumps by a pressure differential therebetween so as to maintain drain level in said drain tank at a predetermined position when the plant operates at a low load level or said drain pump means malfunctions. |
051030950 | description | DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is depicted in its most basic form in FIG. 4 where it is incorporated into a scanning probe microscope 10'. As depicted therein, according to the improvement of this invention all three of the supports are moveable supports 16 which are independently movable by motor drives 24 which are controlled by the control computer 26. In tested versions of this embodiment, the supports 16 were designed to have much longer travel, higher speed, and finer resolution than the single motorized support of the prior art microscope 10 of FIG. 1. These features allow for substantial increase in the utility of SPMs. It should also be noted initially that while the primary illustrations contained hereinafter have the sample fixed, within the scope of the invention the probe can also be fixed with the sample being mounted on a device wherein the orientation between the sample and the probe is determined by three legs on the device. In the basic embodiment of FIG. 4, the movable supports 16 are mounted in the base 18 with the scanner 12 resting on the supports 16. A more versatile (and preferred) configuration is shown in FIG. 5. This embodiment is functionally equivalent to the embodiment of FIG. 4; but, has the advantage that the head design can be used in many SPM configurations, as will be illustrated. This embodiment is a free-standing head generally indicated as 28. The piezoelectric tube scanner 12 is mounted perpendicularly downward in the center of a support structure 30 (which may be, for example a cylindrical or triangular plate) which has three hollow legs 32 attached thereto and extending perpendicularly downward therefrom. While not completely necessary, it is preferred that the legs 32 be spaced radially at 120.degree. intervals about the scanner 12. Threadedly disposed within each leg 32 is an inner leg 34 having a ball 36 on the bottom end thereof. The legs 32 could also be replaced by a solid structure such as a cylinder with threaded holes to receive the three inner legs 34 and a central bore for the scanner as depicted in FIG. 7. The inner legs 34 are fine threaded screws (1/4-80 having been used in tested embodiments) which are rotated by individual small DC motors 38 that drive individual 1000:1 transmissions 40 which, in turn, rotate the screws 34. The motors 38 can have optical encoders on them to monitor their rotation, if desired; but, this is not considered as necessary and is, therefore, not preferred. The motors 38 are connected through an appropriate interface for the particular implementation (not shown and as will be readily determined by those skilled in the art without undue experimentation) to tilt control logic 42 which is most likely contained within the control computer 26 which controls the entire microscope. A separate tilt controller could, of course, be employed if desired and more applicable in certain applications. The head 28 in this particular illustration rests on a base 44 which holds the sample 22. The base 44 could be flat so that the head 28 could be moved around on it; or could have indexing marks (e.g., hole, groove, flat) to position the balls 36 to place the probe 20 over the sample 22 as shown in FIG. 5. The inventors herein have found that it may be useful to use magnetic balls or magnets behind ferromagnetic balls to hold the head 28 down snugly on the base 44. The DC motors 38 are energized by the tilt control logic 42 to rotate the threaded inner legs 34 and thereby move the legs 32 up and down which, in turn, moves the support structure 30 and scanner 12 up and down. When all of the legs 32 are driven simultaneously, the support structure 30 and scanner 12 move up and down without tilting. This type of motion would be used for approaching the tip of the probe 20 to the surface of a sample 22. The motion can be quite large (several millimeters) so that the tip would not need to be placed near the sample 22 by an operator before automatic approach is started. The tilt of the head 28 is varied by not energizing the motors 38 equally. Given the configuration depicted in FIG. 5 (i.e. one leg 32 in front of the probe 20 on the left side as the figure is viewed and two legs 32 spaced equally on either side of and behind the probe 20 on the right side as the figure is viewed), the scanner 12 can be tilted in the Y direction by raising/lowering the two right legs 32 an equal amount and/or lowering/raising the left leg 32. The scanner 12 can be tilted in X by a similar process, i.e., by raising/lowering the left leg 32 and one of the two right legs 32 an equal amount and/or lowering/raising the other right leg 32. The tilt can be monitored by the data taken from the scanning probe 20 and this data can be taken while the legs 32 are being raised and lowered so that the tilt can be set by the system even though the motorized screws do not have encoders. In this preferred approach, the feedback for the tilting comes from the scanning system itself by fitting to the plane of the vertical data instead of from positional readout devices on the motors 38. This preferred approach makes the scanning head 28 simpler and less expensive. After the tilt of the head 28 is set to a particular value, the head 28 can then be raised and lowered for changing the sample 22 by driving all three legs 32 at the same rate and in the same direction. As thus described, the improved scan head 28 of FIG. 5 allows for long distance probe approach or removal without operator participation. At the same time, it also allows for compensation for probe/sample tilt, or for the addition of controlled tilt. These abilities allow for several new SPM configurations that will be capable of automatic operation with accuracy and high throughput for large samples, multiple samples, and special applications such as integrated circuits which have steep cliffs or trenches. These various uses for the free-standing, tiltable scan head 28 of FIG. 5 will now be described in detail. FIG. 6 shows the scan head 28 resting directly on a large sample 22. The scan head 28 may be placed on an reasonably flat surface with the probe 20 withdrawn above the bottom of the supports. The approach and leveling operations can be accomplished automatically, making this configuration extremely convenient to use for suitable applications. This configuration would be useful for verifying surface structure or finish on large objects that would not be damaged by supporting the scan head 28. FIG. 7 shows an extremely useful SPM configuration employing the free-standing, tiltable scan head 28. The legs 32 of the scan head 28 rest on a rigid structure 46. The structure 46 has an opening 48 in the top thereof located under the scan head 28 allowing the scan head 28 to lower the probe 22 into the structure 46. Within the structure 46 is a sample positioning system 50 that can translate a large sample 22 (or several separate samples) attached thereon in two horizontal axes on perpendicular shafts 52 by drive 54 under the control of sample positioning logic 56, allowing for rapid and automatic probing of any part of the sample 22. The positioning also could be done with a rotary stage. This would be useful for multiple samples which could be rotated into position under the scan head. Standard commercial computer-controlled positioning products, for optical and other applications, can be employed for the system 50 and have several inches of travel as well as resolution and repeatability of 1 micron or less. Given a typical large scan head 28 that can cover up to 100 microns square or more scan size, this system can probe any section of a large sample automatically. The inventors herein have tested this configuration with structures 46 made of aluminum, and also of ceramics. The structure 46 must be rigid and isolated from vibration to maintain the stability required between probe and sample. The inventors herein have demonstrated adequate stability for sample sizes of up to eight inches, which is adequate for integrated circuit wafers and most magnetic or optical storage media. The translation stage of the system 50 can be either x, y or r, .theta. oriented, depending on the application. The scan head 28 of this invention is critical to making a large sample system accurate and versatile as it provides the abilities to compensate for local sample tilt, or to tilt the probe 22 relative to the sample 20, allowing for accurate mapping of steep structures. In this regard, the tilt can be determined from the data gathered by fitting the vertical scan information to a plane and then calculating the tilt required to level the plane relative to the scanner axes. Another potentially useful SPM configuration as depicted in FIGS. 8 and 9 employs the scan head 28 in an inverted orientation. A sample holding disk 58 is disposed horizontally for indexed rotation around a support shaft 60 by an indexing mechanism 62. The sample holding disk 58 has a plurality of shouldered bores 64 therein at sampling stations of the disk 58. This configuration facilitates the rapid changing of samples as an operator may attach the samples 22 to inserts 66 that may be dropped into the bores 64 from above to rest on the shoulders 68 thereof supported by gravity without conflict with the scan head 28. This system could support continuous sample cycling as the samples 22 in the sample holding disk 58 could be quickly changed without stopping the system. Preferably, the head 28 is mounted on a raise and lower mechanism 70 that works in combination with the indexing mechanism 62 under the joint control of the control computer 26. The raise and lower mechanism 70, when engaged, pushes the legs against the sample holder, thus maintaining the tilting capability. To index the sample holding disk 58 to a new sample scanning position, the head 28 is dropped slightly by the raise and lower mechanism 70 and the sample holding disk 58 is rotated to the next position with a bore 64 positioned under the probe 22. The head 28 is then raised by the raise and lower mechanism 70 until the balls 36 contact the bottom of the sample holding disk 58. The head 28 is then raised, lowered and tilted in the manner described above, as required to accomplish the scanning of the sample. As those skilled in the art will readily recognize and appreciate, this approach could also work well rotated 180.degree. to a "right side up" configuration and, in fact, such an orientation might be preferred in some instances as there would be no necessity of the positive upward force of the scanner mechanism against the sample mount. |
description | This application is a divisional application claiming priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 15/729,745 filed on Oct. 11, 2017, and entitled “Method and Apparatus for Planting and Harvesting Radioisotopes on a Mass Production Basis,” all of which is hereby incorporated herein by reference in its entirety. This invention pertains generally to methods and devices for the insertion and removal of radioactive isotopes into and out of a nuclear core and, more particularly, to the insertion and removal of such isotopes into and out of a commercial nuclear reactor on a mass production basis without reducing the reactor's facilities ability to generate electricity. The commercial production of radioactive isotopes for medical and other commercial enterprises, such as Radioisotope Thermal Generators (RTG), is a process which is limited by the very high costs associated with developing the neutron source infrastructure required to create commercial quantities of the useful isotopes. This makes the useful applications of these radioactive isotopes very expensive and subject to extreme supply and cost fluctuations due to actual or perceived potential interruptions at the very limited number of isotope production facilities available. The human cost associated with this situation is that most people are not able to afford the cost of the medical benefits that can be provided by the large number of available radioactive isotope diagnostic and treatment modalities. Furthermore, the reactors that are currently used to produce the radioisotopes that are processed to produce radio-pharmaceuticals are very old, and continued operation requires very expensive upgrades that appear to provide poor return on investment. Consequently, the reactor resources required to maintain existing production capability is disappearing. The fundamental issue to be addressed is the loss of medical radioisotope production capability due to obsolescence issues in the existing medical radioisotope production infrastructure that will lead to a shortage of the radioisotopes needed to diagnose and treat serious medical issues. Accordingly, a need exists for an alternative, and preferably less expensive, way of producing radioisotopes. A number of operating nuclear reactors used in commercial electrical generation facilities employ a moveable in-core detector system such as the one described in U.S. Pat. No. 3,932,211, to periodically measure the axial and radial power distribution within the core. The moveable detector system generally comprises four, five or six detector/drive assemblies, depending upon the size of the plant (two, three or four loops), which are interconnected in such a fashion that they can assess various combinations of in-core flux thimbles. To obtain the thimble interconnection capability, each detector has associated with it a five or six-path and ten or fifteen-path rotary mechanical transfer device. A core map is made by selecting, by way of the transfer devices, particular thimbles through which the detectors are driven. To minimize mapping time, each detector is capable of being run at high speed (72 feet per minute) from its withdrawn position to a point just below the core. At this point, the detector speed is reduced to 12 feet per minute and the detector traversed to the top of the core, direction reversed, and the detector traversed to the bottom of the core. The detector speed is then increased to 72 feet per minute and the detector is moved to its withdrawn position. A new flux thimble is selected for mapping by rotating the transfer devices and the above procedure repeated. FIG. 1 shows the basic system for the insertion of the movable miniature detectors. Retractable thimbles 10, into which the miniature detectors 12 are driven, take the routes approximately as shown. The thimbles are inserted into the reactor core 14 through conduits extending from the bottom of the reactor vessel 16 through the concrete shield area 18 and then up to a thimble seal table 20. Since the movable detector thimbles are closed at the leading (reactor) end, they are dry inside. The thimbles, thus, serve as a pressure barrier between the reactor water pressure (2500 psig design) and the atmosphere. Mechanical seals between the retractable thimbles and the conduits are provided at the seal table 20. The conduits 22 are essentially extensions of the reactor vessel 16, with the thimbles allowing the insertion of the in-core instrumentation movable miniature detectors. During operation, the thimbles 10 are stationary and will be retracted only under depressurized conditions during refueling or maintenance operations. Withdrawal of a thimble to the bottom of the reactor vessel is also possible if work is required on the vessel internals. The drive system for insertion of the miniature detectors includes, basically, drive units 24, limit switch assemblies 26, five-path rotary transfer devices 28, 10-path rotary transfer devices 30, and isolation valves 32, as shown. Each drive unit pushes a hollow helical wrap drive cable into the core with a miniature detector attached to the leading end of the cable and a small diameter coaxial cable, which communicates the detector output, threaded through the hollow center back to the trailing end of the drive cable. The use of the moveable in-core detector system flux thimbles 10 for the production of irradiation desired neutron activation and transmutation products, such as isotopes used in medical procedures, requires a means to insert and withdraw the material to be irradiated from inside the flux thimbles located in the reactor core 14. Preferably, the means used minimizes the potential for radiation exposure to personnel during the production process and also minimizes the amount of radioactive waste generated during this process. In order to precisely monitor the neutron exposure received by the target material to ensure the amount of activation or transmutation product being produced is adequate, it is necessary for the device to allow an indication of neutron flux in the vicinity of the target material to be continuously measured. Ideally, the means used would be compatible with systems currently used to insert and withdraw sensors within the core of commercial nuclear reactors. Co-pending U.S. patent application Ser. No. 15/210,231, entitled Irradiation Target Handling Device, filed. Jul. 14, 2016, describes an Isotope Production Cable Assembly that satisfies all the important considerations described above for the production of medical isotopes that need core exposure for less than a full fuel cycle. There are other commercially valuable radioisotopes that are produced via neutron transmutation that require multiple neuron induced transmutation reactions to occur in order to produce the desired radioisotope product, or are derived from materials having a very low neutron interaction cross section, such as Co-60, W-188, Ni-63, Bi-213 and Ac-225. These isotopes require a core residence time of a fuel cycle or more. Commercial power reactors have an abundance of neutrons that do not significantly contribute to the heat output from the reactor used to generate electrical power. This invention describes a process and associated hardware that may be used to utilize the neutron environment in a commercial nuclear reactor to produce commercially valuable quantities of radioisotopes that require long-term neutron exposure, i.e., a fuel cycle or longer, or short term exposure, i.e., less than one fuel cycle, with minimal impact on reactor operations and operating costs. The hardware and methodology described in U.S. patent application Ser. No. 15/341,478, filed Nov. 2, 2016, will enable the production of radioisotopes that require relatively long residence times in the core, currently produced in outdated isotope production reactors, using the foregoing moveable in-core detector system equipment without interfering with the functionality of the moveable in-core detector system power distribution measurement process. There is still a further need for a more efficient radioisotope production process that can produce radioisotopes in commercial nuclear reactors on a mass production scale, without negatively impacting the electrical power output of those commercial facilities. It is an object of this invention to satisfy that need. This and other objects are achieved, in accordance with this invention, with an irradiation target handling system having an isotope production cable assembly comprising a target holder drive cable constructed to be compatible with conduits of an existing nuclear reactor moveable in-core detector system that convey in-core detectors from a detector drive system to and through instrument thimbles within a reactor core. The target holder drive cable has a remotely controlled one of a male or female coupling on a leading end of the drive cable. A target holder drive cable drive motor unit is provided separate from and independent of the detector drive unit on the existing nuclear reactor moveable in-core detector system. The target holder drive cable drive motor unit is configured to drive the target holder drive cable into and out of the core and is structured to drive the target holder drive cable into and through the conduits, a first multipath selector and a second multipath selector on the existing nuclear reactor moveable in-core detector system. A specimen target holder is provided having another of the male or female coupling on a trailing end of the specimen target holder with the another of the male or female coupling configured to mate with the one of the male or female coupling on the leading end of the target holder drive cable. A third multipath selector is connected to and structured to receive an input from an outlet path on the second multipath selector and provides a first output to a new specimen attachment location, a second output to an irradiated specimen offloading location and a third output to the core. In one embodiment the specimen target holder has a radial projection extending from or through an outside wall of the specimen target holder into contact with an interior wall of an instrument thimble in the reactor core, into which the specimen target holder is driven by the target holder drive cable, which maintains an axial position of the specimen holder within the instrument thimble, when the specimen holder is detached from the drive cable. Preferably, the one of the male or female coupling is configured to move the radial projection away from the interior wall of the instrument thimble when coupled to another of the male or female coupling on the specimen target holder. In still another embodiment the irradiation target handling system includes an axial positioning device attached to the specimen target holder for determining when the specimen target holder achieves a preselected axial position within an instrument thimble within the core, which the specimen target holder is driven into by the drive cable. Preferably, the instrument thimbles have a closed upper end and a leading end of the specimen target holder has an axial projection that is sized to contact an interior of the closed upper end of the instrument thimble into which the specimen target holder is driven. In one such embodiment the length of the axial projection is a wire having an adjustable length. Desirably, the target holder drive cable enters the conduits through a “Y” connection with one leg of the “Y” connected to the target holder drive cable drive motor unit and a second leg of the “Y” connected to the detector drive unit. The invention also contemplates a method of irradiating multiple specimens within a core of a nuclear reactor that has a moveable in-core, radiation detector flux mapping system, wherein the core comprises a plurality of fuel assemblies respectively having instrument thimbles into which a radiation detector of the flux mapping system can be inserted and travel through. The method comprises the step of inserting a first specimen holder containing a first specimen at a lead end of a first drive cable driven by a first drive unit, into a first instrument thimble in the core. Next, the method remotely detaches the first drive cable from the first specimen holder and fixes an axial position of the first specimen holder within the first instrument thimble. Then the first drive cable is withdrawn from the reactor. Next, a second specimen holder containing a second specimen is attached to the lead end of the first drive cable driven by the first drive unit. The second specimen holder containing the second specimen is then inserted into a second instrument thimble in the core. The first drive cable is next remotely detached from the second specimen holder and the second specimen holder is fixed at an axial position that it was driven to within the second instrument thimble. The next step withdraws the first drive cable from the reactor. In between the withdrawing step and the second inserting step, the method inserts a moveable in-core radiation detector from the moveable in-core detector radiation flux mapping system, attached to a second drive cable driven by a second drive unit, into and through a third instrument thimble and withdraws the moveable in-core radiation detector from the reactor after performing a flux mapping exercise. In one embodiment of the method, the inserting steps insert specimen holders into as many as half the instrument thimbles accessible by the flux mapping system for simultaneous irradiation at a time when a flux map is to be conducted. Preferably, the steps of fixing the axial position of the specimen holders within the respective instrument thimbles includes the steps of determining when the respective specimen holders are at a preselected axial position within the corresponding instrument thimbles. In one such embodiment, the step of withdrawing the first drive cable from the reactor comprises withdrawing the first drive cable out of the moveable in-core, radiation detector flux mapping system prior to the running of a flux map. To accomplish the foregoing objectives, this invention modifies the traditional flux mapping system described above with respect to FIG. 1, as shown in FIG. 2. FIG. 2 shows a portion of the moveable in-core detector flux mapping system containing the detector drive unit 24, the five-path transfer device 28, the ten-path transfer device 30 and the seal table 20 in schematic form with the incidental components, like the limit switches, safety switches and isolation valves omitted. Also shown in FIG. 2 are the core components of the modifications introduced by this invention to the moveable in-core detector flux mapping system, to convert the moveable in-core detector flux mapping system into a radioisotope mass production facility, without compromising the flux mapping function. In accordance with this invention a specimen holder cable drive unit 34 is provided that is distinct and independent of the detector drive unit 24. The specimen holder cable drive unit 34 drives a specimen holder drive cable 36 that has a specimen holder 48 detachably attached to the lead end of the specimen holder drive cable 36. The specimen holder 48 is shown in and will be described in more detail with regard to FIG. 3. It should also be appreciated that the specimen holder cable drive unit 34 and the specimen holder cable 36 may be configured the same as the detector motor drive unit 24 and the detector drive cable 50, though other configurations are also compatible with this invention. The specimen holder drive cable 36 is fed into the conduits of the moveable detector in-core flux mapping system through a “Y” connection 38 that communicates with the input to the five-path transfer device 28. One of the outputs of the five-path transfer device similarly feeds the input to the ten-path transfer device 30, one of the outputs 52 of which feeds a new three-path transfer device 40. One output of the three-path transfer device feeds a new specimen attachment point 42, at which a new specimen holder and specimen can be attached to the specimen holder drive cable; a second output of the three-path transfer device feeds a specimen holder catcher 44 in which the specimen holder can be offloaded; and a third output of the three-path transfer device provides a path to the core 54. It should be appreciated that while five-path, ten-path and three-path transfer devices are disclosed these devices may have as many paths as necessary to access the desired locations within the core and currently five-path and six-path devices 28 and ten-path and fifteen-path devices 30 are in use or planned for use, depending on the size of the core. FIG. 3 shows the lead end of the specimen holder drive cable 36 and the specimen holder 48. The specimen holder drive cable 36 has a spiral wire wrap 56 that mates with drive gears in the specimen holder drive motor unit 34 to advance and withdraw the specimen holder drive cable 36 through the conduits of the flux mapping system. At the lead end of the specimen holder drive cable 36 is a remotely operated male coupling component 58 that fits within a female coupling component 60 on the specimen holder 48. The male coupling component 58 has a remotely operated pneumatic latch plug 62 that when fully activated in its extended position fits within an annular groove 64 in the female coupling component 60. The latch plug 62 is shown in more detail in FIG. 3A, in the activated position, and includes an unlatching spring 66 that retracts the latch plug 62 when the pneumatic pressure supplied through the pneumatic fluid supply channel 70 is released. The pneumatic fluid is supplied from a pneumatic fluid supply reservoir 46, shown in FIG. 2, through the pneumatic fluid supply channel 70 that runs through the center of the specimen holder drive cable 36. A retaining clip 68 prevents the latch plug 62 from leaving the channel in which it travels. The specimen holder 48 has a payload chamber 72 that houses the specimen to be irradiated and two or more positioning tabs 76 that extend from an interior of the specimen holder housing 74, through the specimen holder housing and up against an interior surface of a fuel assembly instrument thimble in which the specimen holder 48 is to be inserted, to hold the specimen holder in position, by friction, when it is remotely disconnected from the specimen holder drive cable 36. The positioning tabs 76 are biased in a fully extended position and are rotated out of contact with the side walls of the instrument thimble by the male coupling component 58 when the male coupling component is fully inserted into the female coupling component 60. An end view of the positioning tabs 76 is shown in FIG. 3B. The specimen holder 48 also has an adjustable positioning cable 78 which extends out the lead end of the specimen holder 48. The desired axial position of the specimen within the instrument thimble is determined in advance of inserting the specimen into the moveable in-core detector flux mapping system and the length of the positioning cable 78 is adjusted so its lead end abuts the closed upper end of the instrument thimble when the specimen is at the desired position. Thus, in between flux map runs, which are typically conducted once a quarter, the moveable in-core detector flux mapping system is available to insert isotopes into and harvest isotopes from all of the instrument thimbles in a reactor core accessible to the flux mapping system, so long as at least fifty percent of those thimbles are unoccupied at the time a flux map is to be run. Prior to a flux mapping run, the specimen holder drive cable 36 has to be withdrawn above the “Y’ connection 38 to provide the miniature detector access to the five-path transfer device 28. Similarly, once a flux mapping run is completed, the miniature detector needs to be withdrawn above the “Y” connection to provide the specimen holder drive cable 36 access to the five-path transfer device 28. It should be appreciated that a typical reactor facility employing a moveable in-core flux mapping system has four, five or six parallel, interconnected trains of detectors whose detector drive cables can be run simultaneously so long as they are routed through different conduits to the core. In accordance with this invention each one of the detector trains can be provided with its own specimen holder cable drive unit that are individually programmed to plant isotopes at different desired locations within the core. Thus, this invention provides modifications to an existing moveable in-core detector system and a method to perform the following functions that: (i) enables the insertion of specially configured specimens through a specially configured access to the existing multi-path transfer devices from one or more detector drive trains that enable the specimen to be inserted into a desired radial reactor core location that can be reached through the existing multi-path routing options; (ii) enables the specimen to be inserted into the desired available core location at a predetermined axial position inside the moveable in-core detector system instrument thimble relative to the top of the active fuel in the desired fuel assembly; (iii) enables the specimen holder drive cable to be disconnected from the specimen holder and withdrawn from the reactor above the multipath transfer devices with the axial position of the specimen in the reactor fixed by mechanical features on the specimen holder side of the specimen holder drive cable connector; (iv) enables the specimen holder drive cable to be inserted through a specific existing multi-path transfer device position selection to another specially configured transfer device, located downstream of the existing multi-path transfer devices (hereafter referred to as the lower path selector), that has a position that enables the specimen holder drive cable end to reach a location that enables the specimen holder drive cable to have another specimen holder payload attached; (v) enables a new specimen to be withdrawn above the existing multi-path transfer devices and then repeat the above steps 1 through 4 until all the desired specimens are “planted” in the reactor core as planned; (vi) enables the specimen holder drive cable to be inserted into a planted specimen location so that the mating portions of the specimen holder drive cable connector are brought together to enable the latching plugs on the drive cable side of the connector to be activated using a pneumatic fluid, such as nitrogen, to pressurize the pneumatic fluid supply channel so the latch plugs insert into the latch channel located on the specimen side of the connector so that the specimen holder can be withdrawn, or “harvested,” following completion of the desired irradiation levels; (vii) enables the harvested specimen to be withdrawn through the ten-path selector device where the specimen holder latch to the drive cable is released by reducing the applied pneumatic fluid pressure, and then it is inserted through the lower path selector position that enables insertion of the specimen holder until it is captured by a device designed to coil the specimen holder with the specimen payload, to fit within the payload bay of a radioactive material transfer cask used for transportation of the specimen to a processing facility; (viii) enables the specimen holder cable to be positioned as described in step 4, above, and repeat steps 1 through 5 as desired; and (ix) enables steps 1 through 8 to be repeated as desired. While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof. |
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description | Field of the Invention The present disclosure relates to a radiation tube applicable to non-destructive X-ray inspection apparatuses in an industrial equipment field or a medical equipment field and a radiation inspection apparatus using the radiation tube. Description of the Related Art Radiation tubes produce radiation, such as an X-ray, by applying a high voltage between a cathode and an anode and emitting electrons from an electron source to a target. For example, a radiation tube is applied to an inspection apparatus for inspecting a foreign substance in an article as an X-ray source. Japanese Patent Laid-Open No. 2013-88199 describes an X-ray inspection apparatus including an X-ray source that emits an X-ray beam to an article, a slit forming member that controls the irradiation area of the X-ray beam, and a conveyance unit that conveys an article. FIG. 10 illustrates an existing X-ray inspection apparatus 301. The X-ray inspection apparatus 301 conveys an article to be inspected 307 using a conveyance unit 304, emits an X-ray beam from an X-ray tube 302 to the article 307, and detects the X-ray beam passing through the article 307 using an X-ray line sensor 305. The X-ray inspection apparatus 301 controls the irradiation area of an X-ray beam 308 in the shape of a cone emitted from an X-ray tube 302 using a slit forming member 306 having a slit extending in a direction perpendicular to a direction in which the article 307 is conveyed. The dashed arrows indicate X-ray beams scattered from the slit forming member 306. To block the X-ray beams from being emitted to the outside of an inspection space, an X-ray shielding wall 309 is provided. In the X-ray inspection apparatus 301, a distance between an X-ray focal position (a target) and the slit is large and, thus, the X-ray is scattered into a wide area between the target and the slit. Accordingly, an area in which the X-ray shielding wall 309 needs to be provided increases. As a result, the size of the apparatus is disadvantageously increased. As disclosed herein, a radiation tube includes an enclosure having an opening portion, an electron source disposed inside the enclosure, a target unit configured to generate radiation by being bombarded with electrons emitted from the electron source, and a front shield disposed on the opening portion and joined to the target unit. The front shield has a slit-shaped opening that shields some of the radiation radiated from the target unit. Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. Exemplary embodiments of the present disclosure are described below with reference to the accompanying drawings. As disclosed herein, an X-ray is suitably used as radiation. Alternatively, the radiation such as a neutron ray or a proton beam may be used. First Exemplary Embodiment FIG. 1 is a schematic illustration of the radiation source according to the present exemplary embodiment. A radiation source 81 includes a radiation tube 88 and a high voltage generation unit 82 disposed in a container. A void space of the container is filled with insulating oil 87. The radiation tube 88 includes an enclosure having a cylindrical insulating tube 83. One end portion of a cylindrical insulating tube 83 is joined to a cathode 84, and the other end portion is joined to an anode 85. The high voltage generation unit 82 applies a desired voltage to each of the cathode 84 and the anode 85. Electrons emitted from an electron source 86 that constitutes the cathode 84 are accelerated by an accelerating voltage (a voltage between the cathode and the anode) and strike a target unit 12. Among radiation generated when the electrons strike the target unit 12, the radiation radiated from a surface of the target unit 12 opposite to the surface which the electrons strike are emitted to the outside of the enclosure. That is, according to the present exemplary embodiment, the radiation tube 88 is of a transmission type. A front shield 21 is connected to an opening portion of the enclosure (an anode flange portion) and blocks some of the radiation emitted from the target unit 12. That is, the radiation produced by the radiation tube 88 are emitted in the form of fan beam by the front shield 21 that has a slit-shaped (rectangular) opening 25 and that is connected to the target unit 12. The insulating tube 83 is made of an electrically insulating material, such as a ceramic material (e.g., alumina) or glass. The flange portion of each of the cathode 84 and the anode 85 is made of an alloy of a low coefficient of linear expansion, such as MONEL® (Ni—Cu based alloy), INCONEL® (Ni-based superalloy), or KOVAR® (Fe—Ni—Co based alloy), or a metal, such as a stainless steel. The electron source 86 is disposed in the enclosure so as to face the target unit 12 that constitutes the anode 85. The electron source 86 includes a hot cathode, such as a tungsten filament or an impregnated cathode, or a cold cathode, such as a carbon nano-tube. The electron source 86 has a lead electrode and a lens electrode disposed therein used for performing control so that the electrons reach a desired position and region of the target unit 12. FIG. 2A is a schematic illustration of the front shield 21. FIG. 2A includes a front view, a cross-sectional view taken along a line A-A, and a cross-sectional view taken along a line B-B. The front shield 21 has the slit-shaped opening 25 (a radiation passage hole). The ratio of a longitudinal width (L1) to a transverse width (L2) of the opening 25 is about 2:1 to about 50:1 and is, more preferably, about 4:1 to about 20:1. As illustrated in FIG. 2B, the target unit 12 includes a disk-shaped base member 18 and a circular target film 19 formed on a surface of the base member 18 adjacent to the electron source (a surface opposite to a connection surface with the front shield 21). It is desirable that the base member 18 have a strength to support the circular target film 19 and retain vacuum in the enclosure. In addition, it is desirable that the base member 18 have low absorption of the radiation generated by the target film 19 and a high thermal conductivity so that heat generated by the target film 19 is promptly dissipated. For example, diamond, silicon carbide, or aluminum nitride can be used for the base member 18. It is desirable that the material used for the target film 19 have a high melting point and a high radiation generation efficiency. For example, tungsten, tantalum, or molybdenum can be used as the material. To reduce absorption of the generated radiation when the radiation passing through the target film 19, it is desirable that the target film 19 is about 1 μm to about 100 μm in thickness. For the same reason, it is desirable that the base member 18 is 500 μm to 5 mm in thickness. It is desirable that the front shield 21 have a high shielding capability against radiation. It is further desirable that the front shield 21 have a high thermal conductivity to dissipate heat generated by the target unit 12 to the outside. The front shield 21 is made of a metal, such as copper, iron, nickel, tungsten, or lead, an alloy containing such a metal as a main component, or a composite material of such materials. In addition, since the front shield 21 is disposed such that part of the front shield 21 protrudes from the inside to the outside of the enclosure, the heat generated by the target unit 12 is promptly dissipated to the outside via the front shield 21. FIG. 4A is a schematic illustration of the slit-shaped opening 25 of the front shield 21 and the diameter of an electron beam emitted onto the target film 19. That is, FIG. 4A illustrates a positional relationship between the opening 25 and a focal point 23 of the electron beam. A diameter d1 of the focal point 23, a diameter D1 of the target film 19, and a transverse width L2 of the opening 25 satisfy the following expression:d1<L2≦D1.That is, the transverse width is greater than the diameter of the focal point and is less than the diameter of the target film. By setting such a relationship, the radiation emitted in the shape of a cone at the focal point 23 can be reformed into fan-beam shaped radiation. In addition, the radiation emitted in an unnecessary direction can be efficiently blocked.Second Exemplary Embodiment FIG. 3 is a schematic illustration of the front shield 21. FIG. 3 includes a front view, a cross-sectional view taken along a line E-E, and a cross-sectional view taken along a line F-F. The slit-shaped opening 25 has a taper so that the longitudinal width increases from the target unit side to the outside. By increasing the thickness of the front shield 21 in a region around the target unit where the dosage to be shield is large, the size of the front shield 21 required for blocking unnecessary radiation can be reduced. In addition, the taper need not be a linear taper if a portion of the opening 25 adjacent to the target unit in the longitudinal direction is narrower than a portion on the emission side. For example, the taper may be a stepped taper. It is desirable that the longitudinal width of the end portion adjacent to the target unit be wider than the diameter of the focal point and be the same as the diameter of the opening of a rear shield 64 (described in more detail below). Furthermore, it is desirable that the longitudinal width be the same as the transverse width (L3 =L2, that is, the end portion adjacent to the target unit is square in shape). FIG. 4B is a schematic illustration of a positional relationship between the opening 25 of the front shield 21 and the focal point 23. The diameter d1 of the focal point 23, the diameter D1 of the target film 19, and the transverse width L2 of the opening 25 satisfy the following expression:d1<L2≦D1.By setting such a relationship, the radiation emitted in the shape of a cone at the focal point 23 can be reformed into fan-beam shaped radiation. In addition, the radiation emitted in an unnecessary direction can be efficiently blocked.Third Exemplary Embodiment FIG. 5 is a schematic illustration of the radiation source according to the present exemplary embodiment. The configuration is similar to those of the first or second exemplary embodiments except that the rear shield 64 is additionally disposed. Radiation and reflected electrons generated on the cathode side of the target unit 12 are blocked by the rear shield 64. The material of the rear shield 64 is the same as that of the front shield 21. In addition, each of the front shield 21 and the rear shield 64 may have a double-layered structure in which a material having a high shielding effect (e.g., tungsten) is disposed inside and a material having a high thermal conductivity (e.g., copper) is disposed outside. FIGS. 6A and 6B are schematic illustrations of the rear shield 64. FIG. 6A includes a front view, a cross-sectional view taken along a line L-L, and a cross-sectional view taken along a line K-K. As illustrated in FIG. 6A, the rear shield 64 has a cylindrical opening (an electron passage hole) 66. The rear shield 64 is connected to the target unit 12. The target unit 12 is fitted into a notch formed in the end portion of the rear shield 64 and is joined to the rear shield 64. The front shield 21, the target unit 12, and the rear shield 64 are joined to the opening portion of an anode flange portion in an integrated manner. In addition, as illustrated in FIG. 6B, the opening 66 may be tapered. Such a structure effectively blocks the radiation around the target unit where unnecessary dosage increases. In addition, such a structure prevents the electrons from striking a side surface of the rear shield adjacent to the cathode and, thus, prevents generation of unnecessary radiation. Furthermore, if the size of the opening 66 of the rear shield on the target unit side is smaller than that on the cathode side, the taper needs not be a linear taper. For example, a stepped taper may be employed. Fourth Exemplary Embodiment The radiation inspection apparatus according to the present exemplary embodiment is described below with reference to FIG. 7. A system control unit 502 controls the radiation tube 88, a radiation detecting unit 501, and a conveyance drive unit 505 so that the radiation tube 88, the radiation detecting unit 501, and the conveyance drive unit 505 cooperatively operate. The radiation tube described in one of the first to third exemplary embodiments is used as the radiation tube 88. Under the control of the system control unit 502, a radiation tube control unit 504 outputs a variety of control signals to a radiation source 81. The radiation emitted from the radiation tube 88 is controlled by the control signals. The conveyance drive unit 505 drives an article placing unit 506 so that an article to be inspected passes between the radiation tube 88 and a detector 507. The radiation emitted from the radiation tube 88 penetrates an article 509 and is detected by the detector 507. The detector 507 converts the detected radiation into an electric signal and outputs the electric signal to a signal processing circuit 508. Under the control of the system control unit 502, the signal processing circuit 508 performs predetermined signal processing on the electric signal and outputs the processed electric signal to the system control unit 502. The system control unit 502 generates an image signal on the basis of the processed electric signal and instructs a display unit 503 to display a video image of the inside of the article on the basis of the image signal. In addition, the system control unit 502 determines whether a foreign substance is included in the article. The result of the determination is displayed on the display unit 503. The article 509 that has been already inspected is conveyed to one of different predetermined locations by the article placing unit 506 in accordance with the result of the determination. The article 509 is continuously conveyed at predetermined intervals, and radiation is emitted from the radiation tube 88 in synchronization with the points in time at which the article 509 enters the irradiation area of the radiation tube 88 and at which the article 509 moves out of the irradiation area. An example of the radiation tube is described with reference to FIGS. 4A and 4B, FIG. 5, and FIGS. 6A and 6B. In the radiation tube 88, the cathode 84 is joined to one end portion of the insulating tube 83 made of alumina, and the anode 85 is joined to the other end portion. In this manner, the enclosure is formed. The materials of the flange portions of the cathode and the anode are KOVAR. The anode 85 includes the target unit 12, the front shield 21, and the rear shield 64. The target unit 12 is formed by depositing tungsten having a size of φ3 mm×t5 μm onto a surface of a diamond substrate adjacent to the cathode. The diamond substrate has a size of φ5 mm×t2 mm. The front shield 21 is made of copper and is substantially cylindrical in shape. The front shield 21 has a size of φ20 mm×t10 mm. A longitudinal width L1 of the slit-shaped opening 25 on the radiation side is 10 mm, and a longitudinal width L3 on the target unit side is 2.5 mm. The transverse width L2 is 2.5 mm. Thus, the opening 25 is tapered. The diameter D1 of the target is 3 mm, and the diameter d1 of the focal point is 2 mm. Thus, the condition d1 < L2 ≦ D1 is satisfied. The rear shield 64 is made of copper and is substantially cylindrical in shape. The size of the rear shield 64 is φ20 mm×t10 mm. The rear shield 64 has the cylindrical opening 66 of φ2 mm. A depression having a size that is substantially the same as the size of the target unit 12 is formed in the rear shield 64. The target unit 12 is fitted into the depression and is brazed with silver alloy solder. In addition, the surface of the front shield 21 having the smaller opening 25 is brazed to a connection surface of the rear shield 64 with silver alloy solder. The high voltage generation unit 82 includes a Cockcroft circuit. The high voltage generation unit 82 applies a voltage of about 40 kV to about 120 kV in accordance with the usage of the radiation. The electron source 86 is the impregnated cathode. The generated radiation is converted into a fan beam having a desired shape by the front shield 21 and is emitted to the outside. In addition, the radiation produced on the cathode side is effectively blocked by the rear shield 64. An example of the radiation inspection apparatus of the present invention is described below. FIG. 8 is a schematic cross-sectional front view and a schematic cross-sectional side view of the configuration of the radiation inspection apparatus of the present example. A radiation inspection apparatus 101 conducts inspection of a foreign substance using radiation emitted from the radiation tube 88 while an article 107 is being conveyed by a conveyance unit 104. The conveyance unit 104 is formed as a belt conveyer. By using drive motors disposed at both ends of the belt conveyer, the conveyance unit 104 conveys the article 107 to the right or left. The opening 25 of the front shield 21 is formed so that the longitudinal direction thereof is a direction that crosses the conveyance direction of the conveyance unit 104 and, more preferably, the longitudinal direction thereof is a direction that is perpendicular to the conveyance direction of the conveyance unit 104. As a result, the radiation emitted from the radiation tube 88 has a shape of a fan beam having a fan angle that provides an irradiation area larger than the size of the article 107 in a direction perpendicular to the conveyance direction and a radiation angle that provides the irradiation area sufficiently smaller than the size of the article in the conveyance direction. The radiation that has passed through the article 107 is detected by a line sensor 105 serving as the detector. The radiation inspection apparatus 101 of this example blocks unnecessary radiation using the front shield 21. Accordingly, the radiation inspection apparatus 101 does not have scattered radiation that occur from the slit forming member 306 in the existing radiation inspection apparatus illustrated in FIG. 10. As a result, even when a radiation shielding wall 109 is simplified, scattered radiation can be sufficiently blocked. Another example of the radiation inspection apparatus of the present invention is described below. FIG. 9 is a schematic cross-sectional front view and a schematic cross-sectional side view of the configuration of the radiation inspection apparatus of the present example. The configuration is similar to that of EXAMPLE 2 except that a slit portion 206 is provided between the front shield 21 and the article 107. The slit portion 206 is made of tungsten. A slit-shaped opening (a slit) is formed so as to extend in a direction perpendicular to the conveyance direction of the conveyance unit 104. The longitudinal direction of the slit is the same as the longitudinal direction of the opening 25 of the front shield 21. The radiation in the form of a fan beam emitted from the radiation tube 88 passes through the slit portion 206. Thus, the irradiation area is maintained in the direction perpendicular to the conveyance direction. In contrast, a fan beam having a smaller irradiation area is formed in the conveyance direction. According to the present example, the resolution in the conveyance direction is increased and, thus, inspection can be conducted more accurately. In addition, the amount of radiation scattered by the slit portion 206 can be made significantly smaller than that in an existing radiation inspection apparatus. As a result, the radiation shielding wall 109 can be simplified and, thus, the size of the apparatus is reduced. According to the present invention, by using the radiation tube including the front shield having a slit-shaped opening formed therein, radiation can be emitted in the form of a fan beam suitable for an inspection apparatus. In addition, since unnecessary radiation in a region around the target unit can be effectively blocked, scattering of the radiation between the target unit and a slit portion can be prevented. As a result, scattering of the radiation into a space other than an inspection space can be prevented and, thus, a safe and compact radiation inspection apparatus can be provided. While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. This application claims the benefit of Japanese Patent Application No. 2013-254542 filed Dec. 9, 2013, which is hereby incorporated by reference herein in its entirety. |
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047175109 | summary | BACKGROUND OF THE INVENTION According to the present invention, a method of encapsulating waste material, enabling filling of encapsulent to be carried out less restrictively, comprises charging the material into a container through an opening in the container in an environmentally retricted area, closing the openign with a cover having temporarily closed filling and vent port means to isolate the material, transporting the thus closed container to an encapsulating station, there establishing sealed filling and vent connections respectively with the filling and vent port means and causing simultaneously or thereafter the removal of the temporary sealing of the filling and vent port means while otherwise maintaining isolation of the material, and feeding a solidifiable encapsulating medium into the container through the filling connection to fill space in the container unoccupied by the waste material whilst allowing the atmosphere in that space to be displaced from the container through the vent connection exclusively. The closing of the container opening with the temporarily sealed cover implies total closure of the container such that it may then, in accordance with the invention, be transferred to a less restricted area for the filling with encapsulating medium; the temporary sealing in the cover is not removed until the connections for carrying out the filling and venting have been established so that the isolation of the container contents is maintained. Advantageously the operation of a connector means to establish the filling and vent connections is effective to remove the temporary sealing so that such removal is not only accomplished in one single operation but is also prevented from occurring prematurely. It is also advantageous that the connector means should be sacrificial, that is to say, left in place to become an integral part of the resulting monolithic block. To this end, the connector means is conveniently a unitary nozzle having separate passages connectable on the one hand to the filling and vent port means and on the other hand to feed and vent lines; from the latter, the unitary nozzle should be readily detachable so as to be retainable sacrifically in the ports. The filling of the container with the encapsulating medium is continued to the point of ensuring that the nozzle is internally occupied substantially fully by the medium so that on solidification the interior of the nozzle is plugged. In terms of apparatus, the invention provides a lid or cover by which to close a container holding waste material, such lid having filling and vent port means adapted for the making thereto of a filling connection for the introduction of encapsulating medium and a vent connection for the release of displaced atmosphere, and temporary sealing of the filling and vent port means which is removable simultaneously with or subsequent to the making of the connections. The temporary sealing may be diaphragm arranged to be ruptured by filling and venting nozzle means or similarly may be a disc which such nozzle means can dislodge and eject in being brought into its connected position. |
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abstract | A condensation chamber cooling system of a condensation chamber for a boiling water reactor has a heat exchanger outside the condensation chamber. An elongate cooling module is provided in the condensation chamber with an evaporation space in its upper region. The cooling module is configured such that the evaporation space is located above a maximum filling level of the condensation chamber. The cooling module includes at least one riser pipe and one downpipe that issue with their upper ends into the evaporation space and with their lower ends in the condensation chamber. A first pressure line leads from the evaporation space to the heat exchanger and, from there, a second pressure line which issues in the condensation chamber below the minimum filling level. Thus, the condensation chamber, the pressure lines, the cooling module and the heat exchanger form a passive closed cooling circuit. |
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claims | 1. A quantum entangled photon pair generating device comprising:a polarization maintaining loop path;a loop input unit for receiving excitation light from a first optical path, separating the excitation light into a first excitation light component and a second excitation light component, the first excitation light component and the second excitation light component having mutually orthogonal polarization, feeding the first excitation light component into the polarization maintaining loop path in a clockwise direction, and feeding the second excitation light component into the polarization maintaining loop path in a counterclockwise direction;an optical conversion generation unit including a first second-order nonlinear optical medium disposed in the polarization maintaining loop path for generating first up-converted light from the first excitation light component by second harmonic generation, a second second-order nonlinear optical medium disposed in the polarization maintaining loop path for generating second up-converted light from the second excitation light component by second harmonic generation, and a first half-wave plate disposed in the polarization maintaining loop path between the first and second nonlinear optical media for rotating planes of polarization of the first and second excitation light components by ninety degrees, the second second-order nonlinear optical medium also generating first down-converted light from the first up-converted light by spontaneous parametric down-conversion, the first second-order nonlinear optical medium also generating second down-converted light from the second up-converted light by spontaneous parametric down-conversion, the first up-converted light and the first down-converted light propagating on the polarization maintaining loop path in the clockwise direction, the second up-converted light and the second down-converted light propagating on the polarization maintaining loop path in the counterclockwise direction;a loop output unit for receiving the first down-converted light and the second down-converted light from the polarization maintaining loop path, combining the first down-converted light and the second down-converted light to obtain combined light, and outputting the combined light on a second optical path; anda polarization manipulation unit for manipulating a polarization direction of at least one of the first excitation light component, the second excitation light component, the first down-converted light, and the second down-converted light, thereby causing the loop output unit to output the combined light as polarization entangled light. 2. The quantum entangled photon pair generating device of claim 1, wherein the first and second up-converted light and the first and second down-converted light are generated entirely by the optical conversion generation unit. 3. The quantum entangled photon pair generating device of claim 1, wherein a single polarization splitting-combining module is used as both the loop input unit and the loop output unit. 4. The quantum entangled photon pair generating device of claim 3, wherein the polarization manipulation unit is a second half-wave plate disposed in the polarization maintaining loop path to rotate a plane of polarization of the first down-converted light by ninety degrees. 5. The quantum entangled photon pair generating device of claim 4, wherein the first optical path and the second optical path share a common section, further comprising an optical circulator disposed at one end of the common section. 6. The quantum entangled photon pair generating device of claim 4, wherein the first optical path and the second optical path share a common section, further comprising a first wavelength selective filter disposed at an end of the common section to reflect one of the excitation light and the combined light and transmit another one of the excitation light and the combined light. 7. The quantum entangled photon pair generating device of claim 3, wherein the polarization manipulation unit further comprises:a first nonreciprocal polarization converter preceding the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path, the first nonreciprocal polarization converter including a first Faraday rotator and a third half-wave plate, the first Faraday rotator preceding the third half-wave plate in the counterclockwise direction on the polarization maintaining loop path; anda second nonreciprocal polarization converter following the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path, the second nonreciprocal polarization converter including a second Faraday rotator and a fourth half-wave plate, the second Faraday rotator preceding the fourth half-wave plate in the counterclockwise direction on the polarization maintaining loop path. 8. The quantum entangled photon pair generating device of claim 3, wherein the polarization manipulation unit further comprises:a second half-wave plate following the optical conversion generation unit in the clockwise direction on the polarization maintaining loop path, for rotating a plane of polarization of the first down-converted light by ninety degrees; anda polarization converter following the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path to rotate a plane of polarization of the second up-converted light by ninety degrees, the polarization converter including a fifth half-wave plate, a quarter-wave plate following the fifth half-wave plate in the clockwise direction on the polarization maintaining loop path, and a sixth half-wave plate following the quarter-wave plate in the clockwise direction on the polarization maintaining loop path. 9. The quantum entangled photon pair generating device of claim 8, wherein the first optical path and the second optical path share a common section, further comprising an optical circulator disposed at one end of the common section. 10. The quantum entangled photon pair generating device of claim 8, wherein the first optical path and the second optical path share a common section, further comprising a first wavelength selective filter disposed at an end of the common section to reflect one of the excitation light and the combined light and transmit another one of the excitation light and the combined light. 11. The quantum entangled photon pair generating device of claim 3, wherein the polarization manipulation unit further comprises:a first nonreciprocal polarization converter preceding the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path, the first nonreciprocal polarization converter including a first Faraday rotator and a third half-wave plate, the first Faraday rotator preceding the third half-wave plate in the counterclockwise direction on the polarization maintaining loop path;a second nonreciprocal polarization converter following the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path, the second nonreciprocal polarization converter including a second Faraday rotator and a fourth half-wave plate, the second Faraday rotator preceding the fourth half-wave plate in the counterclockwise direction on the polarization maintaining loop path; anda polarization converter disposed between the second nonreciprocal polarization converter and the optical conversion generation unit on the polarization maintaining loop path, the polarization converter including a fifth half-wave plate, a quarter-wave plate following the fifth half-wave plate in the clockwise direction on the polarization maintaining loop path, and a sixth half-wave plate following the quarter-wave plate in the clockwise direction on the polarization maintaining loop path. 12. The quantum entangled photon pair generating device of claim 1, further comprising a second wavelength selective filter having two output ports, disposed on the second optical path to reject the excitation light, output a signal wavelength component of the combined light from one of the two output ports, and output an idler wavelength component of the combined light from another one of the two output ports. 13. The quantum entangled photon pair generating device of claim 12, wherein the second wavelength selective filter comprises an arrayed waveguide grating. 14. The quantum entangled photon pair generating device of claim 1, further comprising an optical low-pass filter disposed on the second optical path to reject at least one of the first up-converted light and the second up-converted light. |
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055027547 | claims | 1. A method for stabilizing the core fuel of a light water reactor of the type having a multiplicity of fuel assemblies supported by a core plate attached along its periphery to a core shroud support ring of a shroud concentrically arranged inside a reactor pressure vessel, comprising the steps of installing first, second and third spacers between an outer circumferential surface of the core plate and an inner circumferential surface of the shroud, said first, second and third spacers being wedged with preload between said circumferential surfaces at first, second and third azimuthal positions respectively. 2. The method as defined in claim 1, further comprising the steps of installing a fourth spacer between said circumferential surfaces, said fourth spacer being wedged with preload between said circumferential surfaces at a fourth azimuthal positions, said first through fourth azimuthal positions being spaced at about 90 degrees. 3. The method as defined in claim 1, wherein radial preload is applied by sliding said first spacer relative to said second spacer along a line disposed at an oblique angle relative to a centerline axis of said shroud. 4. The method as defined in claim 3, wherein said sliding occurs in response to rotation of a remotely manipulated tool. |
abstract | A radiation therapy apparatus includes a housing, a radiation source carried by the housing, and at least one aperture assembly carried by the housing. The aperture assembly includes a radiation aperture body having a shaped opening therein to control a radiation dosing profile, and an aperture holder with an aperture-receiving passageway therein receiving the radiation aperture body, and having a recessed end. A cover is received within the recessed end of the aperture holder and retains the radiation aperture body within the aperture holder. The cover has an opening aligned with the shaped opening in the radiation aperture body. The radiation aperture body, the aperture receiving passageway of the aperture holder and the opening of the cover have angled interfaces therebetween. A radiation filter is carried by the housing. |
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051075245 | description | DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a first embodiment of the synchrotron radiation utilizing apparatus according to the present invention. Referring to FIG. 1, synchrotron radiation 4 emitted from an electron beam 2 circulating at a high velocity in an electron accumulation ring 8 of a synchrotron radiation generator (not shown) is radiated in a direction tangential with respect to the orbit of the electron beam 2. The synchrotron radiation 4 is guided by a beam duct 5 whose end is closed so as to maintain a vacuum thereinside. The synchrotron radiation guided through the beam duct 5 penetrates a window (made of a metal, for example, Be) provided at the closed end of the beam duct 5. Objects 9 which are targets to be exposed to the radiated synchrotron radiation 4 are fixedly mounted on supporting members 13 respectively, and these object supporting members 13 are disposed so as to be rotatable relative to a support base 11 supporting the object supporting members 13. Each of the object supporting members 13 is provided with an integral rotary shaft 14, and the support base 11 is combined with an integral rotary body 10 which is provided with an integral rotary shaft 12. In the apparatus having the structure described above, the rotary body 10 is rotated by rotation of the rotary shaft 12, so that the objects 9 can be moved in a direction of 90.degree. relative to the electron orbital plane while maintaining such an angular relation. Therefore, when the rotation velocity of the rotary body 10 is suitably selected, the synchrotron radiation 4 can be directed toward the entire surface of each of the objects 9 in a quantity suitable for exposure. Thus, the apparatus is suitable for semiconductor lithography in which accurate circuit patterning through a mask over a wide area of the objects 9 is especially required. A rotation angle sensor 25 is associated with the rotary shaft 12, and its detection output signal is transmitted to a controlled power supply unit 26. A rotation angle sensor 27 is also associated with each of the rotary shafts 14, and its detection output signal is also transmitted to the controlled power supply unit 26. A rotation drive unit 28 is provided for the rotary shaft 12, and a rotation drive unit 29 is also provided for each of the rotary shafts 14. Controlled power is supplied from the controlled power supply unit 26 to the drive control units 28 and 29 so as to rotate the rotary shafts 12 and 14 at required angular velocities respectively. For example, the rotation drive units 28 and 29 are stepping motors, and the controlled power supply unit 26 supplies controlled power in pulse form. The operation of the drive units 28 and 29 is such that the rotary shafts 12 and 14 are rotated in directions opposite to each other at the same angular velocity. Thus, the object supporting members 13 do not rotate in the horizontal plane but merely make parallel movement in the horizontal plane. On the other hand, the synchrotron radiation 4 is directed to scan the objects 9 at a predetermined angle with respect to the horizontal plane (commonly in a relation parallel to the foundation). Therefore, the objects 9 fixedly mounted on the respective supporting members 13 are uniformly exposed to the synchrotron radiation 4. FIG. 2 shows that the objects 9 to be exposed to the synchrotron radiation 4 are fixedly mounted on the supporting members 13 combined with their rotary shafts 14 respectively. In FIG. 2, the supporting members 13 fixedly mounting the respective objects 9 thereon are thus revolvable relative to the support base 11. Because the object supporting members 13 revolve around the rotary shaft 12 of the support base 11, and the peripheral velocity of the area of the object supporting members 13 remote from the rotary shaft 12 of the support base 11 differs from that of the area near the shaft 12, the area of each of the objects 9 located near the shaft 12 is exposed to a greater quantity of the synchrotron radiation 4 than that of the area located remote from the shaft 12. For example, in the case of semiconductor lithography, the diameter of each wafer 9 is about 10 cm. Therefore, when the distance between the shaft 12 of the support base 11 and the center of each supporting member 13 supporting the wafer 9 is 25 cm, the velocity difference on the wafer 9 is calculated to be 33% at a maximum, as follows: The rotation velocity at a point on the wafer 9 is proportional to the distance between that point and the shaft 12 of the support base 11. Thus, the velocity difference between two points remotest from and nearest to the shaft 12 is expressed as follows: EQU 1-(25-10/2)/(25+10/2)=0.33 However, when, in the arrangement shown in FIG. 2, the object 9 is rotated in one direction around the shaft 14 through the same angle as that of the revolution in the other direction around the shaft 12, the same angle is maintained between an imaginary line 15 drawn on the object 9 and the orbital plane of the electrons. Because the rotation velocity of the object 9 is maintained constant, the exposure quantity of the synchrotron radiation 4 on the object 9 can be maintained constant regardless of the position and time. Further, the synchrotron radiation 4 is very finely converged so that it can accurately scan all over the wafer 9 to ensure uniform exposure. Therefore, while revolving the wafer supporting member 13 around the shaft 12 of the support base 11, the wafer supporting member 13 is rotated around its own shaft 14 in a direction opposite to the revolving direction and at the same angular velocity as that in the revolving movement, so that undesirable non-uniform exposure may not result from the tangential velocity difference between the radially inner and outer areas of the wafer 9 due to the revolution of the wafer supporting member 13 around the shaft 12 of the support base 11. FIG. 3 shows a second embodiment of the present invention, and in FIG. 3, like reference numerals are used to designate like parts appearing in FIG. 1. Referring to FIG. 3, a support block 19 in the form of a frustum of a right cone is mounted at its central axis on a rotary shaft 17. A plurality of object mounting jigs 18 each including a support base 11 and a rotary shaft 10 are perpendicularly mounted on the side surface of the frusto-conical support block 19, and each of the object mounting jigs 18 is rotatable around the axis 12 of the rotary shaft 10. In the apparatus shown in FIG. 3, the combination of the support block 19 and the rotary shaft 10 acts to successively feed the object mounting jigs 18 to a predetermined exposure position, and, when each object mounting jig 18 is fed to the predetermined exposure position, the jig 18 is rotated around the axis 12 of the rotary shaft 10, so that objects 9 can be successively exposed to synchrotron radiation 4. In this case, objects 9 mounted on any one of the object mounting jigs 18 not located at the predetermined exposure position can be replaced as desired. In the apparatus shown in FIG. 3, the side surface of the support block 19 mounted on the rotary shaft 17 is inclined relative to the direction of the synchrotron radiation 4 by an angle equal to the vertical angle of the cone. Therefore, the rotary shaft 17 need not be disposed in parallel to a beam duct 5, and this is desirable from the viewpoint of design in that there is an increased freedom for the layout of the exposure apparatus. FIG. 4 is a partly sectional side elevational view of the support base 11 shown in FIG. 3 to illustrate how the support base 11 and object supporting members 13 are mounted on their rotary shafts. Referring to FIG. 4, the rotary shaft 10 supporting the support base 11 is rotatably journalled in two bearings 22 and 23, while each of the rotary shafts 14 supporting the object supporting members 13 is rotatably journalled in two bearings 20 and 21. When each of the bearings 20, 21, 22 and 23 has the same clearance of 12.5 .mu.m between it and the associated shaft, and the distance between the bearings 20 and 21 and that between the bearings 22 and 23 are more than 10 cm, the ratio between the clearance of the bearings and the distance between the bearings is less than 1/8000. A paper (reported in Applied Physics, Vol. 53, No. 1, 1984, pp. 17-25) describes that, in the case of lithography in which the line width is 0.5 .mu.m, and the spacing between masks and wafers is 20 .mu.m, the dimensional error is to be less than 0.01 .mu.m. when the ratio between the clearance of the bearings and the distance between the bearings is 1/8000 as described above, a maximum inclination of 1/4000=2.5 .times.10.sup.-6 radians is caused due to the clearances of the left and right two bearings. This angle produces, together with the spacing of 20 .mu.m between the masks and the wafers, an error which is calculated as 20 .mu.m .times.2.5.times.10.sup.-6 =0.005 .mu.m. This error may be 0.005.times.2=0.01 .mu.m even when all the clearances of the bearings 20, 21, 22 and 23 are combined. The ratio between the clearance of the bearings and the distance between the bearings can be decreased to less than 1/8000 by employing, for example, angular ball bearings and selecting the distance between the bearings to be more than 10 cm. In this case, the clearance of the angular ball bearings is to be selected to be less than 12.5 .mu.m. In the synchrotron radiation utilizing apparatus of the present invention, the rotary shafts are expected to be rotated at a very low speed lower than one revolution per minute. Therefore, the clearance of the angular ball bearings can be easily set at a value less than 12.5 .mu.m. Even when the clearance of the angular ball bearings cannot be set to be less than 12.5 .mu.m, the distance between the bearings can be easily set at a value more than 10 cm. Displacement of the objects 9 caused by the clearance of the bearings may be parallel movement, besides that attributable to the inclination described above. However, such parallel movement will hardly occur in view of the very low rotation speed of the rotary shafts in the apparatus. Even when such parallel movement may occur, it does not pose any practical problem. This distant is because light emitted from a point light source distant by 5 m from an object makes a very small angle of 12.5.times.10.sup.-6 /5=2.5.times.10.sup.-6 radians with the object making parallel movement of 12.5 .mu.m=12.5.times.10.sup.-6 m. FIG. 5 shows a third embodiment of the present invention, and, in FIG. 5, like reference numerals are used to designate like parts appearing in FIGS. 1 and 3. Referring to FIG. 5, a large-diameter support disc 16 is mounted on a rotary shaft 17 at its central axis. A plurality of object mounting jigs 18 each including a support base 11 and a rotary shaft 10 are mounted on the support disc 16, so that each object mounting jig 18 is rotatable around the axis 12 of the rotary shaft 10 relative to the support disc 16. In the apparatus shown in FIG. 5, the combination of the support disc 16 and the rotary shaft 17 acts to successively feed the object mounting jigs 18 to a predetermined exposure position, and, when each object mounting jig 18 is fed to the predetermined position, the jig 18 is rotated around the axis 12 of the rotary shaft 10, so that objects 9 can be successively exposed to synchrotron radiation. In this case, objects 9 mounted on any one of the object mounting jigs 18 not located at the predetermined exposure position can be replaced as desired. Therefore, the desired replacement operation can be continued without discontinuing the exposure of the objects. It will be understood from the foregoing detailed description of the present invention that a plurality of objects can be efficiently exposed to synchrotron radiation within a short period of time, and a very fine circuit pattern can be uniformly formed on the objects, so that the productivity of semiconductor devices can be greatly improved. |
claims | 1. An apparatus of a dynamic pattern generator for reflection electron beam lithography, the apparatus comprising:a plurality of base electrodes in a two-dimensional array;an insulating border surrounding each base electrode so as to electrically isolate the base electrodes from each other; anda sidewall surrounding each base electrode,wherein the sidewall comprises a plurality of stacked electrodes which are separated by insulating layers; further comprising a conformal coating over the base electrodes and sidewalls; andfurther wherein the base electrodes are shaped so as to be concave. 2. The apparatus of claim 1, wherein the plurality of stacked electrodes includes at least two electrodes. 3. The apparatus of claim 2, wherein the plurality of stacked electrodes includes no more than nine electrodes. 4. The apparatus of claim 1, wherein the conformal coating has a sheet resistance from 10 to 100 giga Ohms per square. 5. The apparatus of claim 1, wherein the conformal coating comprises ZnO and Al2O3. 6. The apparatus of claim 1, wherein the conformal coating comprises ZnO and ZrO. 7. An apparatus for reflection electron beam lithography, the apparatus comprising:an electron source configured to emit electrons;illumination electron-optics configured to receive the emitted electrons and form an array of incident electron beamlets;a shadow mask configured to form the array of incident electron beamlets;an electron reflective patterned structure having a plurality of pixel pads in an array and an insulating border surrounding each pixel pad so as to electrically isolate the pixel pads from each other; anda stage to hold a target substrate,wherein the shadow mask comprises an array of holes which correspond one-to-one with the array of pixel pads of the electron reflective patterned structure. 8. The apparatus of claim 7, wherein the electron reflective patterned structure comprises a dynamic pattern generator which is configured to apply voltages to the pixel pads on an individually controllable basis. 9. The apparatus of claim 8, wherein incident electron beamlets are reflected to the target substrate by pixel pads having a lower or more negative voltage level applied thereto and are deflected or absorbed by pixel pads having a higher or more positive voltage level applied thereto. 10. The apparatus of claim 7, wherein the pixel pads are shaped to be concave. 11. The apparatus of claim 7, further comprising sidewalls surrounding each pixel pad, wherein the sidewalls include at least two metal layers and insulating layers to isolate said metal layers from each other and from the pixel pad. 12. An apparatus of a dynamic pattern generator for reflection electron beam lithography, the apparatus comprising:a plurality of base electrodes in a two-dimensional array;an insulating border surrounding each base electrode so as to electrically isolate the base electrodes from each other;a sidewall surrounding each base electrode, wherein the sidewall comprises a plurality of stacked electrodes which are separated by insulating layers, anda conformal coating over the base electrodes and sidewalls. 13. The apparatus of claim 12, wherein the conformal coating has a sheet resistance from 10 to 100 giga Ohms per square. 14. The apparatus of claim 12, wherein the conformal coating comprises ZnO and Al2O3. 15. The apparatus of claim 12, wherein the conformal coating comprises ZnO and ZrO. |
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abstract | The present invention relates to a nondestructive method or inspecting defects of the cladding of a nuclear fuel rod, which is featured by a wave emitter obliquely discharging an inspection wave to an inspected tube and a receiver arranged at a side of the inspected tube with respect to the wave emitter. If liquid is accumulated inside the tube, the incident inspection wave will be refracted so that the receiver can receive the refracted inspection wave at a specific location. The method can determine whether liquid is accumulated inside the tube and further is able to detect the level of the liquid. |
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043292488 | summary | This invention relates to the treatment and disposal of high level radioactive wastes (HLW) containing high levels of iron, aluminium, nickel, manganese, sodium and uranium, such as those which have been produced by reprocessing of fuel from nuclear reactors used in the United States defence program. In particular, this invention relates to a process for immobilization of such wastes in a product which will safely retain dangerously radioactive isotopes from the waste for periods sufficient to ensure that they do not enter the biosphere prior to their decay. In prior U.S. Patent Application Ser. No. 054,957 filed July 3, 1979, there are described methods for immobilizing high level wastes produced by typical non-military nuclear reactors. According to the disclosure of this prior specification, the high level wastes are incorporated in the form of dilute solid solutions in the crystal lattices of the minerals of a synthetic rock. A typical composition of this synthetic rock is given in Table 1. TABLE 1 ______________________________________ Typical synthetic rock composition according to U.S. Pat. Appl. Ser. No. 054,957. wt. % ______________________________________ TiO.sub.2 60.4 ZrO.sub.2 9.9 Al.sub.2 O.sub.3 11.0 CaO 13.9 BaO 4.2 NiO 0.6 Mineralogy BaAl.sub.2 Ti.sub.6 O.sub.16 "hollandite" CaTiO.sub.3 perovskite CaZrTi.sub.2 O.sub.7 zirconolite ______________________________________ This synthetic rock is composed mainly of the oxides of titanium, zirconium, calcium, aluminium and barium. When a mixture of oxides of this composition is heated (e.g. between 1000.degree. C. and 1400.degree. C.) it crystallizes to form a mixture of titanate minerals including BaAl.sub.2 Ti.sub.6 O.sub.16 possessing the hollandite structure, CaTiO.sub.3 perovskite, and CaZrTi.sub.2 O.sub.7 zirconolite. Up to 30 percent of calcined high level wastes (a typical composition of which is given in Table 2 below) may be intimately mixed with the oxide mixture from which this synthetic rock is prepared. When the mixture of high level wastes plus synthetic rock oxides is heated at an appropriate temperature (e.g. 1000.degree.-1400.degree. C.) the high level waste components enter into solid solutions with the minerals of the synthetic rock. Once the wastes have been incorporated into the synthetic rock in this manner, they are extremely resistant to leaching and alteration when buried in appropriate geological environments. In such a manner, the wastes may be isolated from the biosphere for millions of years. TABLE 2 ______________________________________ Typical composition of calcined high level nuclear reactor wastes derived from reprocessing of fuel rods from civilian light-water reactors. Mol percent ______________________________________ I. Fission Products Rare earths (REE) 26.4 Zr 13.2 Mo 12.2 Ru 7.6 Cs 7.0 Pd 4.1 Sr 3.5 Ba 3.5 Rb 1.3 II. Actinides U + Th 1.4 Am + Cm + Pu + Np 0.2 III. Processing Contaminants Fe 6.4 PO.sub.4 3.2 Na 1.0 IV. Others (mainly Tc, Rh, Te, I and processing contaminants including Ni, Cr) 9.0 ______________________________________ In the United States military reactor program, the high level wastes have been treated differently from wastes generated in civilian nuclear reactor programs. After the fuel rods have been dissolved in nitric acid, the solutions are made alkaline by the addition of large amounts of sodium hydroxide. In addition, large amounts of other elements, particularly iron, aluminium, manganese and nickel are introduced into the wastes. In the tank farms at Hanford and Savannah River, U.S.A., this procedure has caused most of the high level waste fission products and actinides (Table 2) to be precipitated to form a sludge of mixed oxides, hydroxides and other compounds at the bottom of the tanks. Mixed with these active components are large amounts of the hydroxides of aluminium, iron, manganese and nickel and other minor components including phosphorus, silicon, bismuth and mercury. In addition, variable amounts of sodium are adsorbed on, and/or combined with the sludge. It is proposed to treat these sludges by removing them from the tanks, adjusting the pH, washing, filtering and drying. After calcining, the composition of the sludges could be represented by a mixture of the fission products (minus Cs and Rb) and actinides of Table 2 with varying amounts of the oxides of aluminium, iron, manganese, nickel, sodium and silicon. The proportion of high level waste components (i.e. fission products and actinides, but excluding uranium) to the remaining "inert" oxides (of Al, Fe, Mn, Ni, Na, Si, and U) may vary widely from 0.5 to about 5 percent, but is commonly in the range 2 to 3 percent by weight. Likewise, the relative proportions of Al, Fe, Mn, Ni, Na, Si and U in the sludges from different tanks also vary between wide limits, except that (Fe+Al+Mn) are by far the major components and Fe is more abundant than Mn. Typical compositions of some dried and calcined sludges are given in Table 3. The sodium content of the sludge is largely dependent upon the nature of the washing process prior to calcining. If desired, sodium could be reduced below the levels given in Table 3 by a more efficient washing process. TABLE 3 ______________________________________ Estimated mean compositions of calcined sludges from Savannah River HLW tank farm (weight percent). I II III Composite Composite Composite for H area F area entire area ______________________________________ SiO.sub.2 -- 2.2 0.9 UO.sub.2 3.5 3.7 3.4 Al.sub.2 O.sub.3 50.3 5.8 30.9 Fe.sub.2 O.sub.3 26.4 57.7 39.5 MnO 7.9 9.5 8.9 NiO 0.9 10.3 4.9 CaO 3.1 2.9 2.9 Na.sub.2 O 5.0 5.0 5.6 Fission products.sup.1,2 .about.3.0 .about.3.0 .about.3.0 plus actinides ______________________________________ Notes: .sup.1 Uranium has not been included with the remaining actinides. It is more appropriately classed with the `inert` components because of its ver long halflife and correspondingly low alphaactivity Approximate relative proportions of individual fission products (excludin Cs, Rb) and actinides are given in Table 2. The present invention provides a process for the treatment and immobilization of the mixture of high level waste containing large amounts of aluminium, iron, manganese, nickel and sodium compounds as components as described above. The essence of the invention is the incorporation of the radioactive waste component in synthetic titanate minerals as disclosed in the prior patent specification and the crystallization of the excess aluminium, iron, manganese, nickel and sodium oxides in highly refractory and leach-resistant minerals which are compatible thermodynamically with the waste-containing minerals of the previously disclosed synthetic rock. According to one embodiment of the present invention, particularly applicable to sludges low in sodium, there is provided a process for immobilizing high level waste (HLW) sludge containing aluminium and/or iron compounds which comprises the steps of (1) mixing the sludge with a mixture of oxides, the oxides in said mixture and the relative proportions thereof being selected so as to form a mixture which when heated at temperatures between 800.degree. and 1400.degree. C. crystallizes to produce a mineral assemblage containing (i) crystals capable of providing lattice sites in which the fission product and actinide elements of said HLW sludge are securely bound, and (ii) crystals of at least one inert phase containing excess aluminium and/or iron, said crystals belonging to or possessing structures closely related to crystals belonging to mineral classes which are resistant to leaching and alteration in appropriate geologic environments; and (2) heating and then cooling said mixture under reducing conditions so as to cause crystallization of the mixture to a mineral assemblage having the fission product and actinide elements of said HLW sludge incorporated as solid solutions within the crystals thereof, and the excess aluminium and/or iron crystallized in at least one inert phase. As the proportion of fission product and actinide elements in most HLW sludges containing aluminium and/or iron compounds is very small, only a minor proportion, for example from 20 to 40% by weight, of added oxides may be necessary to form the desired mineral assemblage. This embodiment of the present invention, which is particularly applicable to sludges low in sodium, also provides a mineral assemblage containing immobilized HLW sludge containing aluminium and/or iron compounds, said assemblage comprising crystals belonging to mineral classes which are resistant to leaching and alteration in appropriate geologic environments having fission product and actinide elements of said HLW sludge incorporated as solid solutions within the crystals thereof, and the excess aluminium and/or iron crystallized in at least one inert phase. In one aspect, this embodiment of the invention provides a process for the treatment and immobilization of sludges consisting mainly of mixtures of oxides of aluminium and iron with fission products and actinides, as described above, which comprises, in essence, the incorporation of the fission products and actinides (Tables 2 and 3) in synthetic titanate minerals (as disclosed in U.S. Patent Application Ser. No. 054,957) and the crystallization of the excess aluminium and iron oxides in highly refractory and leach resistant minerals which are thermodynamically compatible with the waste-containing minerals of the previously disclosed synthetic rock. According to this aspect of the invention, the excess Al and Fe oxides are immobilized in spinels such as FeAl.sub.2 O.sub.4 (hercynite) and Fe.sub.2 TiO.sub.4 (ulvospinel) and their solid solutions, ilmenite FeTiO.sub.3 pseudobrookite solid solutions (Al.sub.2 TiO.sub.5 --Fe.sub.2 TiO.sub.5), hollandite solid solutions (BaAl.sub.2 Ti.sub.6 O.sub.16 --Ba(FeTi)Ti.sub.6 O.sub.16), a davidite-type mineral BaAl.sub.2 Fe.sub.8 Ti.sub.13 O.sub.38 (approx.) and corrundum Al.sub.2 O.sub.3. It has been demonstrated that all of these minerals, capable of immobilizing Al and Fe oxides, are also thermodynamically compatible with the zirconolite+"hollandite"+perovskite mineral assemblage employed to immobilize the actinide and fission product elements as dilute solid solutions. Where predominantly only the oxides of aluminium and iron are present in the sludge with fission products and actinides, the process may be carried out under a chemically reducing environment such that nearly all iron is maintained in the divalent state. A second, and preferred embodiment of the present invention, however represents an improvement of the first embodiment described above, principally in two areas. Firstly, it can be applied to sludges containing relatively high amounts of sodium (e.g. 3-6 percent Na.sub.2 O), and secondly, it provides a more efficient means of immobilizing sludges very rich in iron such as the composition given in Column 2 of Table 3. In general, in this embodiment of the invention, in order to immobilize sludges rich in sodium (Table 3) sufficient silica and, if necessary, alumina, are added so that on heating to temperatures in the range 800.degree.-1400.degree. C., a nepheline-type mineral (NaAlSiO.sub.4) is formed. In many sludges, there is already sufficient Al.sub.2 O.sub.3 present to combine with sodium in forming nepheline, so that further additions of this component are unnecessary. Furthermore, in order to immobilize sludges which are rich in iron (columns 2 and 3, Table 3), the heat treatment is carried out under conditions which, although generally reducing, are not so strongly reducing as described with reference to the first aspect of the invention above. Preferably, the oxygen fugacity lies near the nickel-nickel oxide buffer. Under these conditions, when the sludges are heated, a substantial proportion of the iron occurs in the ferric state, whilst manganese and nickel are present as divalent species. Accordingly, most of the iron, aluminium, nickel, manganese, together with some of the added titanium crystallize to form a series of spinel-type solid solutions embracing the principal end members ##STR1## An advantage of carrying out the heat treatment under these conditions which are somewhat more oxidizing than described previously is that the amount of additives (e.g. TiO.sub.2) necessary to immobilize ferrous iron, manganese and nickel in the sludge is substantially reduced. According to this preferred embodiment of the present invention, there is provided a process for immobilizing high level waste (HLW) sludge containing high concentrations of Al, Fe, Mn, Ni and Na compounds which comprises the steps of (1) mixing the sludge with a mixture of oxides, the oxides in said mixture and the relative proportions thereof being selected so as to form a mixture which when heated at temperatures between 800.degree. and 1400.degree. C. crystallizes to produce a mineral assemblage containing (i) crystals capable of providing lattice sites in which the fission product and actinide elements of said HLW sludge are securely bound, and (ii) crystals of at least one inert phase containing excess aluminium, iron, manganese, nickel and sodium, said crystals belonging to or possessing crystal structures closely related to crystals belonging to mineral classes which are resistant to leaching and alteration in the appropriate geologic environments, and (2) heating and then cooling said mixture under controlled redox conditions so as to cause crystallization of the mixture to a mineral assemblage having the fission product and actinide elements of said HLW sludge incorporated as solid solutions within the crystals thereof, and the excess aluminium, iron, manganese, nickel and sodium crystallized in at least one inert phase. Again, as the proportion of fission products and actinide elements in most HLW sludges containing Al, Fe, Mn, Ni and Na compounds is very small (e.g. .about.3%--Table 3), only a minor proportion, for example from 20 to 40% by weight of added oxides may be necessary to form the desired mineral assemblage. The present invention also provides in this preferred embodiment, a mineral assemblage containing immobilized HLW sludge containing Al, Fe, Mn, Ni and Na compounds, said assemblage comprising crystals belonging to mineral classes which are resistant to leaching and alteration in appropriate geologic environments and having fission product and actinide elements of said HLW sludge incorporated as solid solutions within the crystals thereof, and the excess Al, Fe, Mn, Ni and Na crystallized in at least one inert phase. Preferably, the mixture of oxides which are added to the sludge in accordance with the present invention to produce the desired mineral assemblage is comprised of at least four members selected from the group TiO.sub.2, ZrO.sub.2, SiO.sub.2, Al.sub.2 O.sub.3, CaO, SrO, BaO, at least one of said members being selected from the subgroup consisting of TiO.sub.2, ZrO.sub.2 and SiO.sub.2. Still more preferably, the mixture of oxides which is added to the sludge in accordance with the present invention produce the desired mineral assemblage is comprised of at least three members selected from the group TiO.sub.2, ZrO.sub.2, SiO.sub.2, Al.sub.2 O.sub.3, CaO, at least two of said members being selected from the subgroup consisting of TiO.sub.2, ZrO.sub.2, and SiO.sub.2. It will be appreciated, however, that where NaO is not present in the sludge, for example, where it has been removed by pretreatment, the formation of nepholine is not required and accordingly the presence of SiO.sub.2 in the mixtures described above is unnecessary. As described above, the process of this aspect of the invention requires the heating stage to be carried out under controlled redox conditions so that manganese and nickel are maintained dominantly in the divalent state, whilst iron is maintained dominantly in the divalent or trivalent state, according to the particular composition of the sludge as described below. There are many methods well known to the art by which this can be achieved. According to one method, the required redox conditions can be achieved by heating in an atmosphere of controlled composition, for example an atmosphere consisting of an appropriate mixture of hydrogen, hydrocarbons, carbon monoxide, water vapour and carbon dioxide. According to another method, the sludge can be heated in the presence of metallic nickel, sufficient in amount to reduce all higher oxides of Mn to the MnO component and some of the ferric iron to the ferrous state. According to yet another method, the sludge can be heated in the presence of metallic iron, or of a mixture of metallic iron and metallic nickel sufficient in amount to reduce all higher oxides of Mn to the MnO component and most or all of the ferric iron to the ferrous state. These processes aimed at achieving preferred redox states may be performed as preliminary steps in the process; however they are preferably performed simultaneously with the heating stage of the process as the heating and cooling operations must be performed under controlled redox conditions in either case. In one preferred embodiment of the invention, particularly applicable to sludges rich in Al.sub.2 O.sub.3 (e.g. Column 1, Table 3), the oxides are selected so as to form a mixture which on heating and cooling in accordance with the invention, will crystallize to form a mineral assemblage containing crystals belonging or closely related to hercynite-rich spinel and at least one of the mineral classes selected from perovskite (CaTiO.sub.3), zirconolite (CaZrTi.sub.2 O.sub.7 --CaUTi.sub.2 O.sub.7 solid solution), and nepheline (NaAlSiO.sub.4). It has been shown in U.S. Patent Application Ser. No. 054,957 that the first two of these minerals are capable of accepting most of the fission products and actinide elements (Table 2) into solid solution in their crystal lattices. It has since been found that zirconolite alone can accept most of these products and elements in the absence of perovskite and that nepheline can accept as much as four percent of caesium (Table 2) into solid solution in its structure. Nepheline is the mineral employed to immobilize most of the sodium in the sludge and if sodium is present in the sludge sufficient silica is added to form this mineral during heat treatment. If sodium is not present, however, formation of nepheline is unnecessary. In this particular embodiment of the invention, most of the excess Al.sub.2 O.sub.3 in the sludge crystallizes to form the mineral hercynite ##STR2## In order to obtain this result, the heat treatment is carried out under conditions wherein nearly all iron, nickel and manganese are maintained in the divalent state. Dependent upon the exact composition of the sludge and the exact proportion of added oxides, additional minerals containing Al, Fe, Mn, Ni and Ba can be formed, thereby immobilizing these elements. These minerals include corundum (Al.sub.2 O.sub.3), pseudobrookite solid solutions (Al.sub.2 TiO.sub.5 --FeTi.sub.2 O.sub.5), and hollandite solid solutions (BaAl.sub.2 Ti.sub.6 O.sub.16 --Ba(FeTi)Ti.sub.6 O.sub.16). All of these minerals have been shown to be thermodynamically compatible with perovskite, zirconolite and nepheline. It will be appreciated by persons skilled in the art that the formulae of these minerals as given above have been simplified for convenience; for example part of the ferrous iron in the above minerals is replaced by Ni.sup.2+ and Mn.sup.2+, whilst some Ti.sup.4+ occurs in the hercynite. Actual measured compositions of individual minerals occurring in a typical high-alumina sludge (Table 3, Column 1) when treated according to the present invention are given in Table 4 hereinafter. In another preferred embodiment of the invention particularly applicable to sludges rich in iron (e.g. Column 2, Table 3), the oxides are selected so as to form a mixture which on heating and cooling in accordance with the invention, will crystallize to form a mineral assemblage containing crystals belonging or closely related to ferrite spinel and at least one of the mineral classes selected from perovskite (CaTiO.sub.3), zirconolite (CaZrTi.sub.2 O.sub.7 --CaUTi.sub.2 O.sub.7 solid solution), and nepheline (NaAlSiO.sub.4). As demonstrated above, these minerals immobilize nearly all of the fission products and actinides. Again, if sodium is not present in the sludge formation of nepheline is unnecessary. Also, zirconolite alone can accept most of the fission products and actinide elements. In this particular embodiment of the invention, most of the excess iron in the sludge crystallizes to form a complex ferrite spinel solid solution composed principally of the end members ##STR3## In order to obtain this objective, the heat treatment is carried out under somewhat more oxidizing conditions than in the previous case, so that a large proportion of iron occurs in the ferric state, whilst manganese and nickel are maintained dominantly in the divalent state. Dependent upon the exact composition of the sludge, and the exact proportions of added oxides, additional minerals containing Al, Fe, Mn, Ni and Ba can be formed, thereby immobilizing these elements. These minerals include ilmenite (FeTiO.sub.3), ulvospinel (Fe.sub.2 Ti.sub.3 O.sub.4), ferropseudobrookite (FeTi.sub.2 O.sub.5), ##STR4## All of these minerals have been shown to be thermodynamically compatible with perovskite, zirconolite and nepheline. It will be appreciated by persons skilled in the art that the formulae of these minerals as given above have been simplified for convenience; for example, part of the ferrous iron in the above minerals is replaced by Ni.sup.2+ and Mn.sup.2+, whilst some Ti.sup.4+ occurs in the ferrite spinel solid solution. Actual measured compositions of individual minerals occurring in a typical high-iron sludge (Table 3, Column 2) when treated according to the present invention are given in Table 5 hereinafter. In this particular embodiment of the invention, most of the sodium present in the sludge is immobilized in the mineral nepheline, NaAlSiO.sub.4. Accordingly, additional silica, and (if not already present) alumina, must be added to the sludge during or prior to heat treatment in such quantities that nepheline is preferentially formed. It has been demonstrated that nepheline is thermodynamically compatible with all of the other minerals and phases described above. In other embodiments of this invention as applied to sludges containing intermediate amounts of excess aluminum and iron oxides (e.g. Table 3, Column 3), various mixtures of the above minerals may be formed when the sludge is heated with the added oxides as disclosed above. In general, the conditions for application of the invention to these intermediate compositions are themselves intermediate between those described separately for the cases of high-aluminium and high-iron sludges. The selected mixture of oxides is preferably mixed directly with the sludge and without any preliminary drying or calcining of the sludge, as the use of a sludge assists in the mixing step. If desired or convenient, however, dried or calcined sludge may also be used in the purpose of the invention. The broad objective of the present invention is to produce a synthetic rock, composed of titanate minerals chosen from the above groups, some of which (e.g. perovskite, zirconolite, hollandite) have the capacity to accept fission products and actinide elements from the sludge into solid solution into their crystal lattices and retain them tightly, whilst the excess Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3, FeO, MnO,NiO and Na.sub.2 O present in the sludge crystallizes to form additional inert phases, which are thermodynamically compatible with the minerals accepting the fission products and actinides. An important characteristic of the minerals chosen to make up the assemblage is that they belong to classes of natural minerals which are known to have been stable in a wide range of geological and geochemical environments for periods ranging from 20 million to 2000 million years. It is this characteristic, combined with existing knowledge in the fields of geochemistry, mineralogy and solid state chemistry which makes it possible to predict with a high degree of confidence, the capacity of the mineral assemblages of this invention to immobilize HLW elements for periods greatly exceeding the one million years interval necessary for decay of radioactive HLW elements to safe levels. It is emphasized that although several of the minerals used in the assemblages of this invention have compositions similar to, or identical with natural minerals, the overall chemical compositions of these assemblages do not resemble those of any known kind of naturally occurring rock. It should also be noted that the crystal structure of zirconolite minerals is very closely related to that of pyrochlore, which possesses an identical stoichiometry. It is thus possible that some of the zirconolite-type phases (essentially CaZrTi.sub.2 O.sub.7 --CaUTi.sub.2 O.sub.7 solid solutions) as described above and also in Tables 4 and 5 hereinafter, may actually have crystal structures more closely resembling those of pyrochlore than of zirconolite. For this reason, it is emphasized that the Ca-Zr-U-Ti phase used as a host for actinide elements in this invention may be either a zirconolite-type mineral or a mineral which is structurally and chemically very similar to natural zirconolite, including minerals with similar stoichiometries but with structures related to those of pyrochlore and defect fluorite. The immobilization of fission products, actinide elements and excess Al, Fe, Mn, Ni and Na oxides in the sludge are accomplished as follows. The sludge is intimately mixed with selected additional components in the proportions necessary to form the desired mineral assemblage. A mixture of sludge and additional components is then heated under controlled redox conditions in order to achieve the desired oxidation states for Fe, Mn and Ni. The temperature of heating may be in the range 800.degree.-1400.degree. C., but is insufficient to cause extensive melting. This heat treatment, which may be carried out by sintering at atmospheric pressure in a controlled atmosphere, or which may be carried out under a confining pressure under controlled redox conditions, causes extensive recrystallization and sintering, mainly in the solid state, and yields a fine grained mineral assemblage in which the fission products and actinide elements of the HLW sludge are incorporated to form dilute solid solutions mainly in perovskite and zirconolite phases, and in which the excess Al, Fe, Mn, Ni and Na oxides are contained in at least one inert phase. The product, containing immobilized HLW elements, can then be safely buried in an appropriate geologic environment. |
claims | 1. Process to simulate the response of a radiation detector (D) in detecting radiation emitted by radioactive objects ( 16 ), each object containing a radioelement or a mix of radioelements, comprising the steps of: memorizing the radioactive emission spectra representative of the radioelements or mixes of radioelements; determining the operating characteristics of received radiation; choosing the radioelements or mixes of radioelements from the memorized radioactive emission spectra corresponding to the content of objects, and processing the radiation emitted for the chosen radioelements or mixes of radioelements using the detection characteristics of the radiation detector and the operating characteristics of the received radiation to individually develop and reproduce a simulated response of the radiation detector. 2. Process according to claim 1 , in which the detection characteristics of the detector comprises data representative of the thickness through which the radiation passes before it is detected. claim 1 3. Process according to claim 1 , in which the operating characteristics of the received radiation includes the operative angle of the radiation detector (D), detected energy bands and electronic amplification characteristics of the radiation detector. claim 1 4. Process according to claim 1 , in which regression straight lines are also built up starting from the simulated response. claim 1 5. Process according to claim 1 , in which the detector (D) is a xcex3 radiation detector. claim 1 |
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description | The present application claims the priority benefit of U.S. Provisional Patent Application Ser. No. 62/007,717, filed Jun. 4, 2014, entitled MICROENCAPSULATION TECHNIQUE AND PRODUCTS THEREOF, incorporated by reference in its entirety herein. Field of the Invention The present invention relates to processes and techniques for preparing non-alginate hydrogel microbeads and the resulting products thereof. Description of Related Art Cell encapsulation in a hydrogel microparticle is a promising technique in regenerative medicine for two key reasons. First, the structure of the hydrogel matrix is such that encapsulated cells can exchange nutrients and therapeutic molecules with the surrounding environment, while other cell types, namely host immune cells, cannot penetrate and mediate immune rejection of the encapsulated cells when transplanted. Second, hydrogel microparticles are well suited for transplantation of encapsulated cells. Their small physical size and spherical shape allows simple and easy delivery via syringe and needle, rather than an invasive surgical procedure. Furthermore, this small physical size generates minimal resistance of molecular diffusion to and from encapsulated cells, compared with larger, “bulk” gel constructs. The current methods used for fabricating cell-containing hydrogel microbeads are highly limited. To date, microbeads can only be fabricated using alginate or agarose polymers owing to their unique and simplistic gelation mechanisms. Unfortunately, neither of these materials is desirable with regard to cellular health or function. Many novel hydrogel materials are available that are far superior to alginate or agarose in this respect. However, due to their specific gelation mechanisms, they cannot be prepared as spherical microbeads containing living cells. The present invention provides a method for producing such constructs, and is applicable for a wide variety of hydrogel forming materials. Methods of encapsulating biological material in a 3-dimensional hydrogel matrix are described herein. The methods generally comprise providing a hydrogel precursor solution that comprises a hydrogel precursor compound, the biological material, and a divalent cation selected from the group consisting of calcium, barium, strontium, and combinations thereof, dispersed or dissolved in a solvent system. The hydrogel precursor solution is combined with alginate to yield core/shell microparticles. Each of the core/shell microparticles comprises an alginate shell and a liquid core comprising the hydrogel precursor solution. The hydrogel precursor compound in the liquid core is crosslinked to yield core/shell crosslinked microparticles. Each of the core/shell crosslinked microparticles comprises the alginate shell and a core comprising a 3-dimensional hydrogel matrix and the biological material. Advantageously, the biological material is suspended, encapsulated, aka entrapped in the hydrogel matrix. The temporary alginate shell is then removed to yield self-sustaining hydrogel microbeads. Each of the hydrogel microbeads comprises the 3-dimensional hydrogel matrix and biological material entrapped therein. 3-dimensional hydrogel microbeads prepared according to the inventive techniques are also described herein. The microbeads are self-supporting bodies that comprise a 3-dimensional hydrogel matrix and biological material entrapped therein. A composition for transplantation in a subject is also described herein. The composition comprises a plurality of 3-dimensional hydrogel microbeads prepared according to the inventive techniques. The microbeads are self-supporting bodies that comprise a 3-dimensional hydrogel matrix and biological material entrapped therein. The invention is concerned with an inside-out gelation process to generate hydrogel microcapsules (aka microbeads). More particularly, methods of encapsulating biological material in the microbead 3-dimensional hydrogel matrix are described herein. The process generally comprises formation of a mixture of a hydrogel precursor compound, an optional biological material, and a divalent cation. The mixture is then combined with alginate, to generate an alginate shell around droplets of the mixture, followed by gelation of the hydrogel precursor core, and removal of the alginate shell to yield self-sustaining microbeads. In one aspect, a hydrogel precursor solution is provided. The hydrogel precursor solution is prepared by dispersing the hydrogel precursor compound in a solvent system to form a solution before mixing with the other components. Preferred solvent systems for the hydrogel precursor solution include water, buffering agents (e.g. histidine, HEPES), density-modifying agents (e.g. iodixanol, ficoll), viscosity-modifying agents (e.g. PEG, carboxymethyl cellulose, xanthan gum), or mixtures thereof. The hydrogel precursor compound will be included in the solution at a level of from about 0.4% to about 4.0% weight/volume (4-40 mg/mL), based upon the total volume of the solution. Suitable hydrogel precursor compounds include hydrogel-forming polymers, oligomers, and/or monomers, and as such are capable of forming a cross-linked or network structure or matrix (i.e., “hydrogel”) through polymerization and/or crosslinking, wherein liquid and biological materials may be retained, suspended, entrapped, and/or encapsulated within the interstitial spaces or pores of the resulting gelled structure or matrix. Hydrogel precursor compounds for use in the invention are preferably non-alginate hydrogel precursor compounds. That is, the hydrogel precursor solution is preferably substantially free of alginate compounds, i.e., compounds based upon alginate, alginic acid, or salts or derivatives thereof. The term “substantially free,” as used herein, means that the ingredient is not intentionally added to the composition, although incidental impurities may occur. In such embodiments, the hydrogel precursor solution compositions comprise less than about 0.05% by weight, preferably less than about 0.01%, and more preferably about 0% by weight of such an ingredient, based upon the total weight of the emulsion taken as 100% by weight. Any crosslinkable hydrogel precursor compounds would be suitable for use with the invention, with preferred compounds being biocompatible non-alginate copolymers, and particularly non-alginate block copolymers, as well as other types of crosslinkable monomers and/or oligomers. Exemplary precursor compounds include, without limitation, non-alginate polysaccharides, modified hyaluronic acid, collagen/gelatin, polyethylene glycol, chitosan, agarose, and the like. A particularly preferred hydrogel precursor compound is hyaluronic acid. In one or more embodiments, the hydrogels are slow-gelling hydrogels. Biocompatible hydrogels are also particularly preferred, depending upon the designated end use of the hydrogel. As used herein, “biocompatible” means that it is not harmful to living tissue, and more specifically that it is not biologically or otherwise undesirable, in that it can be administered to a subject without excessive toxicity, irritation, or allergic or immunogenic response, and does not cause any undesirable biological effects or interact in a deleterious manner with any of the other components of the composition in which it is contained. Biocompatible hydrogels would be selected to minimize any degradation of the biological material and to minimize any adverse side effects in the subject, as would be well known to one of skill in the art. Additional optional ingredients that may be included with the hydrogel precursor include fibronectin, laminin, collagen, other components of the extracellular matrix, and the like, including synthetic versions thereof. The hydrogel precursor solution further comprises a divalent cation. Suitable divalent cations for use in the invention include any ions capable of forming a gel upon interaction with alginate. More particularly, the divalent cations are preferably biocompatible. In one or more embodiments, the divalent cations are selected from the group consisting of calcium, barium, strontium, and combinations thereof. The divalent cations are dispersed or dissolved in the solvent system along with the hydrogel precursor compound. The divalent cations should be included in the solution at a level of from about 0.025 moles/liter to about 0.25 moles/liter, based upon the total volume of the solution taken as 100%. The hydrogel precursor solution further comprises a biological material. Exemplary biological materials include populations of cells, cell clusters, tissues, combinations thereof, and fragments thereof. The biological materials can be naturally derived or isolated, or they can be engineered or genetically-modified cells, clusters, tissues or the like. Non-limiting examples of biological materials for use in the invention include islets, islet clusters, hepatocytes, stem cells, and related cells and tissue, as well as endocrine cells, stem cell clusters, thyroid clusters, adrenal gland clusters, pituitary clusters, and other 3-dimensional cell clusters for tissue engineering or cell-based treatments. Combinations of cell types and/or tissues could also be used in the invention. In one or more embodiments, additional additives, media, nutrients, pH buffers, density modifying agents, viscosity modifying agents, or the like can be included in the hydrogel precursor solution. In some embodiments, the hydrogel precursor solution consists essentially or even consists of the hydrogel precursor compound, divalent cation, and biological materials, dispersed or dissolved in the solvent system. The hydrogel precursor solution remains is in liquid form until gelation as described below, and as such, in one or more embodiments, the solution is preferably essentially free of hydrogel crosslinking agents. In one or more embodiments, the density of the hydrogel precursor solution is “matched” to the density of the biological material to improve the ability maintain the biological material suspended throughout the hydrogel precursor solution (and throughout the resulting droplet, discussed below). Thus, in some embodiments, it may be desirable to increase the viscosity of the hydrogel precursor sufficiently in order to slow or prevent migration or “settling” of the biological material to the edge of the liquid core prior to gelation/crosslinking, as discussed in more detail below. The hydrogel precursor solution is then combined with alginate. In one or more embodiments, the hydrogel precursor solution is added dropwise to a solution of alginate to form an alginate shell on individual droplets of the hydrogel precursor solution. For example, the liquid hydrogel precursor solution is extruded or dispensed from a suitable apparatus for forming droplets. Any suitable apparatus can be used, and will generally comprise a chamber for holding the hydrogel precursor solution, with the chamber being in fluid communication with a fluid passage that terminates in a dispensing outlet or tip. The dispensing tip will have an orifice through which the hydrogel precursor solution is expelled as a droplet. The technique can be executed using a simple apparatus, such as a syringe and needle, as well as machines specifically designed for droplet generation. The desired size of the droplet can be controlled based upon the cross-sectional dimension of the orifice, the viscosity of the hydrogel precursor solution, and relative viscosity of the alginate solution. The invention is particularly suited for droplets having a maximum surface-to-surface dimension (i.e., in the case of a spherical droplet, its diameter) of less than about 5 mm, preferably less than about 2 mm, more preferably less than about 1 mm, and even more preferably ranging from about 50 μm to about 750 μm. In general, the solution of alginate will comprise a sodium alginate bath, and preferably an agitated or stirring bath of sodium alginate dispersed in a solvent system. Other alginate salts (besides calcium alginate) could be used. Various types of gel-forming, but decrosslinkable, alginates can be used in the invention depending on the desired properties. In general, low viscosity/low molecular weight and high-G alginates are preferred, such as those extracted from Laminaria hyperborea. Alginates are commercially available according to their different properties from various sources, including FMC BioPolymer (Philadelphia, Pa.). The amount of alginate in the solution can be varied, but can range from about 0.1% to about 2.0% weight/volume, based upon the total volume of the solution taken as 100%. In general, the viscosity of the alginate solution should be less than the viscosity of the hydrogel precursor solution. The viscosity of an alginate solution depends upon the alginate concentration and average molecular weight of the alginate polymer (i.e., length of alginate molecules or number of monomer units in the chains), with longer chains resulting in higher viscosities at similar concentrations. In one or more embodiments, the viscosity of the alginate solution will range from about 1 to about 20 cP, and preferably from about 1 to about 4 cP at room temperature (˜20 to 25° C.). More specifically, the ratio of viscosity of the hydrogel precursor solution to the viscosity of the alginate solution should be greater than 1 at room temperature. In one more embodiments, the ratio of viscosity of the hydrogel precursor solution to the viscosity of the alginate solution is from about 1:1 to about 1000:1. In one or more embodiments, the ratio of viscosity of the hydrogel precursor is about 20:1. In one or more embodiments, the viscosity of the hydrogel precursor solution is from about 1 up to about 500 cP, with about 40 to about 100 cP at room temperature being particularly preferred. Advantageously, the divalent cation in the hydrogel precursor solution reacts with the alginate to yield a core/shell microparticle for each droplet comprising an alginate shell and a liquid core comprising the hydrogel precursor solution. The core/shell microparticles can be placed in an additional solution containing divalent cation if further hardening of the alginate shell is desired. Regardless it will be appreciated that this approach has a significant advantage in that it creates a temporary (removable), porous, and substantially spherical mold or encapsulant for containing the hydrogel precursor solution. As will be appreciated, “substantially spherical” means that the core/shell microparticle may be spherical with a more “regular” shape, or may have a more irregular shape (ellipsoidal, oblong, etc.). It will be appreciated that for successful encapsulation by the alginate shell, the droplet “cores” must penetrate the surface of the alginate solution after being dispensed. That is, the droplets must have sufficient velocity and/or momentum to break the surface tension of the alginate solution. Those skilled in the art will recognize that several variables can be manipulated to achieve the desired outcome. For example, the alginate solution can be agitated or stirred to reduce the surface tension of the solution. Similarly, it will be appreciated that the relative viscosities of the droplet and the alginate bath can be adjusted to facilitate droplet entry into the alginate bath. Likewise, the requisite droplet velocity will be reduced for larger-sized droplets (i.e., droplets having more mass, thus giving rise to adequate momentum at lower velocity). The height from which the droplets are dispensed can also be varied. In one or more embodiments, the droplets are dispensed from a height (as calculated from the surface of the alginate solution to the dispensing tip) of from about 15 to about 20 cm. In one more embodiments, the target velocity for the droplets is from about 1.5 m/s to about 5 m/s, and preferably from about 1.5 m/s to about 4 m/s. The hydrogel precursor compound in the liquid core is then crosslinked to yield core/shell crosslinked microparticles. Crosslinking can be carried out by various mechanisms depending upon the particular hydrogel precursor compound. In one or more embodiments, the core/shell microparticles are combined with a hydrogel matrix crosslinker, preferably in solution. The crosslinker leaches through the alginate shell into the core/shell microparticles resulting in gelation (crosslinking) of the hydrogel precursor compound to form a 3-dimensional hydrogel matrix. The crosslinker will correspond to the hydrogel precursor compound, but can be varied to control the speed and level of crosslinking achieved within the resulting crosslinked matrix. Suitable crosslinkers include photo- or thermal-initiated crosslinkers, chemical crosslinkers, such as PEG-based crosslinkers (e.g., PEGDA), and the like. Self-crosslinking hydrogel precursors could also be used. It will also be appreciated that a hydrogel crosslinker could be dissolved in the initial alginate bath in order to facilitate somewhat simultaneous hydrogel gelation and formation of the alginate shell in “one-step” without having to transfer the microparticles to a separate container for hydrogel crosslinking. Likewise, the hydrogel crosslinking agent could actually be included in the initial hydrogel precursor solution at a designated pH (e.g., pH<7) in order to effectively pause gelation, whereas the alginate bath could be prepared at pH 8, which significantly reduces gelation time of the hydrogel precursor. Similarly, a photo-crosslinkable hydrogel system can be used, which would involve exposing the core/shell microparticle to activating radiation (e.g., UV light) to initiate hydrogel formation in the liquid cores. It will also be appreciated that different crosslinkers could be used to change the speed of gelation/crosslinking. The data present indicate that even a large (e.g., 3400 Dalton PEG) crosslinkers are capable of diffusing through the alginate shell and reacting with the hydrogel precursor compound to initiate gelation. Regardless of the gelation mechanism used, each core/shell crosslinked microparticle will comprise a distinct alginate shell and now-solidified or “gelled” core comprising a 3-dimensional hydrogel matrix with the biological material suspended, entrapped, or encapsulated in the hydrogel matrix. It will be appreciated that this approach has a significant advantage in that it permits gelation of the hydrogel precursor solution within the porous mold under physiological conditions. The resulting hydrogel matrix is characterized as being a semi-rigid network that is permeable to liquids and gases, but which exhibits no flow and retains its integrity in the steady state. The hydrogel matrix is a 3-dimensional self-sustaining body. The term “self-sustaining body” means that the hydrogel matrix, once formed, retains its shape without an external support structure, and is not susceptible to deformation merely due to its own internal forces or weight. The self-sustaining body is not pliable, permanently deformable, or flowable, like a jelly, putty, or paste, but is resilient, such that the matrix body may temporarily yield or deform under force. In other words, the self-sustaining body will recoil or spring back into shape after minor compression and/or flexing—it being appreciated that the hydrogel matrix will crack, break, or shear under sufficient exertion of external pressure or force. The alginate shell is then removed from the core/shell crosslinked microparticles to yield self-sustaining hydrogel microbeads. In one or more embodiments, the core/shell, crosslinked microparticles are contacted with an appropriate chelating agent, preferably in solution, and for a sufficient period of time to weaken, dissolve, or otherwise disrupt (and thereby remove) the alginate shell. Preferably, the core/shell, crosslinked microparticles are contacted with the appropriate chelating agent under agitation or stirring. Exemplary chelators include citrate, as well as other known chelating agents for the divalent cations (e.g., calcium, barium, or strontium), such as EDTA (ethylene diamine tetraacetic acid), EGTA (ethylene glycol tetraacetic acid), phosphates (e.g., orthophosphate, phosphate salts, etc.), and the like. Additionally, the divalent cation used in formation of the alginate shell could be displaced (albeit very slowly) over time in a saline solution containing, for example, very low calcium concentrations. The alginate shell could also be broken and dislodged using mechanical agitation of the microparticles. The core/shell, crosslinked microparticles can also be collected on a screen or strainer and washed with additional chelator solution if desired, until the alginate shell is removed. Regardless of the embodiment, removal of the alginate shell yields a hydrogel microbead comprising the biological constituent encapsulated, entrapped, or suspended within the 3-dimensional hydrogel matrix. It will be appreciated that this approach has a significant advantage in that it permits dissolution and removal of the porous spherical mold under physiological conditions after successful gelation of hydrogel matrix. The resulting hydrogel microbead is a 3-dimensional (e.g., substantially spherical) matrix-type capsule, meaning that it holds the fill material throughout the bead, rather than having a distinct shell as in a core-shell type capsule. As noted above, the hydrogel microbead is also a self-sustaining body. The resulting hydrogel microbeads can be collected from the solution using a mesh screen or other device, and may be rinsed or suspended in medium or appropriate nutrients, as desired. In one or more embodiments, the resulting microbeads are substantially spherical in shape. Advantageously, the particle size is highly customizable depending upon the capabilities of the selected droplet generator. In one or more embodiments, the resulting hydrogel microbeads or microparticles have an average (mean) maximum surface-to-surface dimension (i.e., in the case of a spherical microbead, its diameter) of less than about 5 mm, preferably less than about 2 mm, more preferably from about 50 μm to about 2 mm, even more preferably ranging from about 50 μm to about 750 μm, and most preferably from about 50 μm to about 500 μm. It will be appreciated that the microbeads or particles formed during the above process may be filtered and/or washed between various steps in the process to isolate the microbeads or particles from the formation solution before proceeding to the next step. In summary, the method described herein allows for the fabrication of non-alginate hydrogel microspheres under biologically relevant conditions. Microencapsulation of living cells or tissue has very broad appeal in tissue engineering and cell-based therapy, yet the materials currently able to be formulated as microcapsules is essentially limited to alginate and in some cases agarose, neither of which possess important biological cues to support or direct cell function. Thus, an important feature of this method is for the microencapsulation of cells or cell clusters in non-alginate hydrogel materials to improve biological activity and biocompatibility. Additionally, alternative material choices (e.g. covalently cross-linked hydrogels, variable cross-linker sizes and reaction chemistry, etc.) could potentially enable a much greater level of control of structural, mechanical, and degradation properties of the resulting microcapsules. Advantageously, because of the temporary alginate shell, microcapsules can be created even when using slow-gelling hydrogel precursors. Another key advantage is that the process is compatible with cells, in contrast to other microsphere fabrication methods that use organic solvents or are otherwise toxic to living tissues. Additional benefits of the described methods include that the hydrogel microbeads improve biocompatibility of the implanted biological material, reducing fibrosis. In addition, the microbeads allow for enhanced control and support of cell and tissue function in the microbeads, improving the bioactivity of any implants created using such microbeads. Furthermore, use of the temporary alginate shell permits a wider variety of hydrogels to be used to create the microbeads, which allows the skilled artisan more control over the desired mechanical properties of the results microbead (e.g., elastic modulus, toughness, etc.). Furthermore, this technique also permits the skilled artisan to selected hydrogels based upon other desired characteristics, such as to control the degradation rate of the hydrogel, once implanted (e.g., by being able to choose between covalent vs. ionic cross-linked gels for long term immunoprotection (cell transplant therapy) or short term cell support (tissue engineering)). Similarly, by being able to select among a range of hydrogels, but still form microbeads, the skilled artisan also has more control of the desired microstructure of the hydrogel matrix, such as the pore size and diffusional properties of the resulting microbeads. The resulting hydrogel microbeads have various uses, including, without limitation reversal of diabetes via encapsulated islets or islet cells, delivery of modified cells or stem cells for bone or cartilage repair, protection of transplanted therapeutic cells or cell clusters from host immune system in general. Additional advantages of the various embodiments of the invention will be apparent to those skilled in the art upon review of the disclosure herein and the working examples below. It will be appreciated that the various embodiments described herein are not necessarily mutually exclusive unless otherwise indicated herein. For example, a feature described or depicted in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present invention encompasses a variety of combinations and/or integrations of the specific embodiments described herein. As used herein, the phrase “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing or excluding components A, B, and/or C, the composition can contain or exclude A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination. The present description also uses numerical ranges to quantify certain parameters relating to various embodiments of the invention. It should be understood that when numerical ranges are provided, such ranges are to be construed as providing literal support for claim limitations that only recite the lower value of the range as well as claim limitations that only recite the upper value of the range. For example, a disclosed numerical range of about 10 to about 100 provides literal support for a claim reciting “greater than about 10” (with no upper bounds) and a claim reciting “less than about 100” (with no lower bounds). The following examples set forth methods in accordance with the invention. It is to be understood, however, that these examples are provided by way of illustration and nothing therein should be taken as a limitation upon the overall scope of the invention. Background work was carried out to evaluate liquid core alginate capsules. Briefly, a solution of 40% glucose and 25 or 50 mM calcium chloride was dropped into low viscosity sodium alginate at various concentrations. Glucose was used to increase the viscosity and density of the liquid core solution, which appears to be necessary in order to form spherical constructs. Calcium was dissolved into the liquid core solution to initiate rapid gelation of the alginate once the drop entered the alginate bath in an inside-out gelation mechanism. Results of the glucose experiments showed clear, smooth interfaces of the liquid core and alginate shell. However, the constructs observed, while generally round, possessed tail like features. This was most likely due to non-optimal rheological properties. The liquid core constructs were then exposed to 50 mM sodium citrate and mildly agitated to assess whether the alginate shells could be dissolved using physiologically relevant concentrations of citrate. Indeed, the alginate shells eventually dissolved, leaving no observable trace of gelled constructs (the liquid glucose core simply dissolved back into the bulk solution). After having identified acceptable reagent concentrations and general technique, a commercially-available HA hydrogel (HyStem) was used as the liquid core solution. Due to its greater viscosity (exact metric unknown), only the hyaluronic acid component, Glycosil, was used to increase the likelihood of success in this experiment. GMP-grade Glycosil was dissolved in HTK solution at 1× (2 mL), and then added to a vial containing calcium chloride so as to achieve a final liquid core solution of 1× Glycosil and 25 mM calcium chloride (HTK solution as solvent). This was added drop-wise through a 27 G blunt tip needle into a stirred 0.25% sodium alginate bath (Protanal LF 10/60, low viscosity, high G content) from a height of 1-2 cm. Generally spherical dual-layered constructs formed in the alginate bath. See FIG. 1. Most appeared to have small tail-like features, similar to those observed in the glucose experiments. However, a small percentage of the constructs appeared significantly spherical without any visible tail, which may indicate the rheological properties of this particular formulation are close to, but not quite appropriately tuned for this technique. The liquid core-shell constructs were left stirring for 5 minutes and then collected with a 250 uM nylon mesh screen, and transferred to PBS containing ˜10 mM calcium chloride to strengthen the alginate shell to protect the integrity of the construct during core hardening. Constructs were later transferred to a vial containing the HyStem crosslinker, PEGDA, at 1× concentration (i.e. 1 vial in 2.5 mL total volume) to recreate normal cross-linker concentration. PEGDA (3400 Dalton) permeates both the alginate shell and the HyStem core, allowing the cross-linker to eventually diffuse throughout the construct. This parameter is clearly subject to variation, as the ratio of total PEGDA to HyStem components was much higher than if the gel were to be made in bulk. In this case, only a couple hundred microliters of Glycosil was in the container, rather than the normal 2.0 mL of Glycosil+Gelin-S. Regardless, this was simply chosen as a starting point, and can be modified. After incubation at room temperature over the weekend, the constructs were observed via light microscopy. The resulting constructs are shown in FIG. 2. Interestingly, it appeared as though either the cores shrunk, or the shells expanded, as there were void spaces between the two layers that were originally in direct contact. The constructs were transferred to a petri dish containing 10 mL of 50 mM sodium citrate in PBS and placed on an orbital shaking at low rpm to create mild agitation and fluid flow. Constructs were observed for 45 minutes, during which time the alginate shells significantly deteriorated and in some cases completely disappeared and the HyStem cores remained unchanged, and in their original spherical geometry. See FIG. 3. After 45 minutes, constructs (see FIG. 4) where collected in a 250 uM nylon mesh screen, returned to the petri dish and rinsed with a fresh 10 mL aliquot of 50 mM sodium citrate. See FIG. 5. Not long after this step (a matter of minutes), virtually all constructs where devoid of any visible shell, with only the hardened HyStem cores remaining. See FIG. 6. The HyStem microspheres were then transferred to clean PBS for further observation. 1. Prepare liquid hydrogel polymer solution at desired final component concentrations, adjusted to contain 25 mM calcium ion. The cation concentration should be adjusted for different droplet sizes. For example, up to 200 mM calcium ion and/or 200 mM barium ion has been used for droplets with an average size range of from about 400-600 microns. 2. Load solution into droplet generator (e.g., syringe/needle, or a microparticle/microencapsulator device) and initiate droplet formation, adjusting instrument settings to achieve desired drop size. 3. Collect droplets in stirring bath containing a solution of 0.25% low viscosity sodium alginate (w/v) in a calcium free buffer or medium (e.g. PBS) at room temperature. The alginate concentration can be adjusted. For example, 0.125% w/v sodium alginate has been used for smaller droplets. The viscosity and density of the hydrogel polymer (core) solution ultimately dictates whether or not truly spherical constructs can be formed upon contact with the alginate bath. Beyond this, the drop height, i.e. the distance from the droplet-generating outlet to the surface of the bath will also play a role in the morphology of the resultant constructs. The specific values of these parameters will also vary depending the size of the droplet, the type of material, concentration and viscosity of the alginate solution, etc. These factors must be considered and will ultimately place constraints on the composition of the target hydrogel polymer solution. 4. Upon collection of all droplets in the bath, continue stirring for ˜5 min to allow alginate shells to fully form and reduce or prevent aggregation of liquid core-shell constructs. Depending on the size (volume) of the original droplet, this step may be optional. For larger spheres (1-3 mm), doing this did appear to limit aggregation of the constructs. The length of stirring is inversely proportional to the divalent cation concentration used in the droplets. In addition, it has been found that droplet deformation can occur with excessive stirring. A stirring time of about 1-2 minutes has been found sufficient for smaller droplets. 5. Separate constructs from alginate solution using an appropriately sized strainer or screen. 6. Cross-link/gel the core hydrogel polymer solution according to the specific requirement of the particular hydrogel solution being used. This step will vary depending upon the type of hydrogel precursor solution used. In some cases it will involve transferring the liquid core-shell constructs to a core gelling bath or vessel containing hydrogel crosslinker. Regardless, it is likely that this step will be very similar to the manufacturer's recommended gelation protocol associated with each material, with only slight modifications to account for the alginate shell and construct geometry. Whatever the case, the medium used herein should contain at least some amount calcium ion to maintain the integrity of the alginate shell during hardening. 7. Any further desired hardening, curing, or incubation could be done at this point (e.g. if encapsulating living cells that require gentle handling). Otherwise, proceed immediately to step 8. 8. The temporary alginate shell is then removed. For example, the core/shell constructs are then transferred to a bath and/or washed with appropriate chelator to remove the alginate shell. Transfer the now solid core-shell constructs to a solution of 25-50 mM sodium citrate in PBS (or other calcium free buffer), and mildly agitate for 20-40 minutes at 24-37° C., while visually monitoring the progress of shell degradation. The specific volume ratio of microspheres to citrate solution has not been identified. Given the very low cost of citrate, this step should simply be done using a rather large excess of the citrate solution to ensure complete removal of the alginate shell. 9. Rinse constructs using appropriately sized strainer or screen with additional citrate solution, repeat agitation in fresh citrate solution until shells fully dissolved by visual inspection. It is likely that the mechanical shear of the fluid during rinsing is why this step was effective in removing the remaining alginate shell material, rather than the “fresh” citrate solution. Thus, performing this rinsing step earlier could potentially shorten this process. 10. Microspheres (microbeads) are now ready to be used. Transfer to desired medium or buffer for use. 1. GMP grade Glycosil (the thiolated hyaluronic acid polymer component of HyStem-C hydrogels) was reconstituted at 10 mg/mL using a buffered organ preservation solution. The solution is HTK Preservation Solution. 2. Calcium chloride powder was added to the Glycosil solution to achieve ˜25 mM calcium ion concentration. 3. The Ca2+/Glycosil hydrogel polymer solution was then added dropwise to a stirring 0.25% (w/v) alginate bath (Protanal LF 10/60) using a syringe and 27 G blunt tipped needle from a height of 1-2 cm. At times, in order to decrease the droplet size, the droplet was mechanically disturbed be tapping the syringe. 4. Spherical liquid Glycosil core-alginate shell constructs formed instantly upon contact of the droplet with the bath, and were stirred for an additional 5 minutes to allow the shell to fully form and reduce aggregation. While generally spherical, constructs did have tail-like features, some larger than others. These should be easily eliminated by adjusting the viscosity of the Glycosil solution, e.g. concentration, or addition of other viscosity modifying agents. 5. Constructs were collected with a 250 micron nylon mesh screen and stored in 10 mM Ca2+ in PBS for later processing (shell removal). This step may be optional in some cases. 6. Constructs were transferred to a tube containing Extralink (3400 g/mol PEG Diacrylate) at a final concentration of 1× (i.e. 1 vial of GMP grade Extralink in 2.5 mL in said buffered solution used in step 1), and incubated at room temperature for 2 days. Shorter incubation times may be used in certain embodiments. 7. The constructs were then transferred to a solution of 50 mM sodium citrate in PBS and agitated gently on an orbital shaker for ˜45 minutes. Alginate shell degradation was monitored via light microscopy. Not much change was noticed between 20-45 minutes. As alluded to in the general protocol commentary, the rinsing step appeared to facilitate the shell removal process. 8. Constructs were then rinsed in a 250 micron nylon mesh screen with fresh citrate solution, then further agitated in sodium citrate until no visual evidence of the alginate shell remained, leaving only the solid spherical Glycosil core. It is possible that the alginate was completely removed by the rinsing step alone. However, this was not verified via microscopy prior to being returned to the citrate solution and agitated for several minutes. 9. Glycosil spheres were stored in PBS at room temperature. 1. The hydrogel precursor solution was prepared as follows: 1% w/v Glycosil (thiolated HA, MW ˜250 kDa), 200 mM Calcium Chloride, 100 mM Histidine, and 0.25% PEGDA 3400 (pH 6.5, viscosity ˜50 cP). 2. Droplets of hydrogel precursor approximately 600 microns in diameter were dropped into a stirring bath of 0.125% Protanal LF 10/60 Low Viscosity Alginate (viscosity ˜2.5 cP) from a height of 17 cm with an initial velocity of approximately 1.5 m/s at room temperature to form core/shell microparticles. 3. The core shell microparticles were stirred in the alginate bath for 2 minutes, and were then collected using a 250 micron screen. 4. Core/shell microparticles were transferred to PBS (pH 7.4) and incubated overnight to permit crosslinking of the hydrogel (Glycosil and PEGDA). 5. After overnight incubation, the core/shell microparticles were transferred to a stirring bath of 0.85% sodium chloride (or calcium free PBS) containing 50 mM sodium citrate until alginate shells were completely dissolved. 1. The hydrogel precursor solution was prepared with the following composition and physical properties. 1.0% (w/v) Glycosil (hydrogel precursor polymer) 0.5% (w/v) Gelin-S (bioactive agent) 1.0% (w/v) 4-Arm Polyethylene glycol norbornene (hydroxyl radical catalyzed thiol reactive crosslinker, MW: 10 kDa, JenKem USA) 12% (v/v) iodixanol (density modification agent) 200 mM CaCl2 10 mM HEPES pH˜7 at room temperature Density=1.08 g/mL at room temperature Viscosity ˜60 cP at room temperature 2. The alginate bath was prepared with the following composition and physical properties. 0.12% (w/v) Protanal LF 10/60 Low Viscosity Alginate 0.1% Tween 20 (surfactant) 0.1% Irgacure 2959 (photoinitiator, Sigma Aldrich) 10 mM HEPES pH˜7 at room temperature Density ˜1.0 g/mL at room temperature Viscosity ˜2.5 cP at room temperature 3. Droplets of the hydrogel precursor solution were introduced into the stirred alginate bath using droplet generator from a height of approximately 20 cm to generate core/shell microparticles of approximately 1 mm core diameter. 4. Immediately following core/shell particle formation, the stirred bath was irradiated with a longwave ultraviolet lamp (Ultraviolet Products, LLC, Model UVL-56, 6 Watts, 365 nm) through the sidewall of the glass vessel containing the particles and alginate solution with the photoinitiator (Irgacure 2959). Stirring was continued for 5 minutes. 5. After 5 minutes of initial stirring, the alginate bath was diluted 1:1 with saline containing 10 mM HEPES to prevent aggregation and overgrowth of the alginate shells. UV irradiation and stirring was continued for 25 additional minutes. 6. Finally, 1.0 M sodium citrate was added to the bath to a final concentration of 25 mM citrate ion. The alginate shells fully dissolved after about 5 minutes to reveal crosslinked HyStem hydrogel microbeads. HyStem beads were then collected using a 250 micron screen. |
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053533194 | claims | 1. A removable feedwater sparger assembly comprising: an arcuate feedwater sparger having an inlet distal end; a supply pipe having an outlet end for receiving said inlet pipe distal end in flow communication therewith for channeling feedwater from said supply pipe to said inlet pipe to feed said sparger; an annular, radially outwardly extending retention flange disposed on one of said supply pipe outlet end and said inlet pipe distal end; a tubular coupling having an annular band at a proximal end fixedly joined coaxially with the other said supply pipe outlet end and said inlet distal end, and a plurality of circumferentially spaced elongate fingers extending longitudinally from said band, said fingers being flexibly removably joined to said retention flange for maintaining said sparger inlet pipe in flow communication with said supply pipe, wherein said retention flange comprises an annular outer ledge for capturing said coupling fingers thereon; and an annular release collar slidably disposed around said one of said supply pipe outlet end and said inlet pipe distal end adjacent to said fingers, and including a conical cam surface for selectively engaging said fingers upon longitudinal translation of said release collar to displace said fingers radially outwardly from said retention flange for disengagement therefrom, wherein each of said fingers includes an elongate, flexible beam extending integrally from said band and spaced parallel to adjacent ones of said beams, and a hook disposed at a distal end of said beam, said hook being defined by a seat disposed parallel to said outer ledge for retention thereon and a ramp inclined radially outwardly for abutment with said release collar, said ramp being configured to expand said fingers over said retention flange upon coupling of said inlet pipe to said supply pipe until said hooks contract and latch said outer ledge, and said ramp also being effective for engagement with said release collar cam surface upon translation thereof to expand said fingers to disengage said hooks from said outer ledge for removing said inlet pipe from said supply pipe. said inlet pipe includes an intermediate portion spaced from said distal end; said retention flange is disposed on said supply pipe outlet end; and said coupling band is fixed joined to said inlet pipe intermediate portion. said supply pipe comprises a 90.degree. elbow with said supply pipe outlet end facing vertically upwardly, and an integral thermal sleeve extending horizontally from said elbow: and said sparger inlet pipe extends vertically with said distal end thereof facing downwardly toward said supply pipe outlet end. a supply pipe in flow communication with said feedwater nozzle; a header pipe having a plurality of injector nozzles; an inlet pipe in flow communication with said header pipe; and hooking means for coupling said supply pipe to said inlet pipe in a first state and uncoupling said supply and inlet pipes in a second state. a first piping means in flow communication with said feedwater nozzle; a header pipe having a plurality of injector nozzles; a second piping means in flow communication with said header pipe; and first and second latching means for coupling said first piping means to said second piping means in a first state and uncoupling said first and second piping means in a second state, said first latching means being connected to one of said first and second piping means and said second latching means being connected to the other of said first and second piping means, wherein said first latching means comprises a radial retaining surface and said second latching means comprises a plurality of radially outwardly flexible hooking members, said hooking members having a first radial position in said first state whereat said hooking members engage said radial retaining surface and a second radial position in said second state whereat said hooking members are disengaged from said radial retaining surface, said second radial position being located radially outward of said first radial position. 2. An assembly according to claim 1, wherein: 3. An assembly according to claim 2 wherein said retention flange is cylindrical and defines with an outer surface of said supply pipe an annular outer ledge for capturing said coupling fingers thereon, and defines with an inner surface of said supply pipe an annular inner ledge. 4. An assembly according to claim 3 further comprising an annular flexible seal disposed between said inlet pipe distal end and said inner ledge, and sized for being elastically compressed therebetween upon engagement of said fingers and said retention flange. 5. An assembly according to claim 1 wherein: 6. An assembly according to claim 5 wherein said retention flange is sized to fit completely within said coupling axially between said band and said hooks and radially inwardly of said beams. 7. An assembly according to claim 6 wherein said sparger further comprises an arcuate, horizontal header pipe having a plurality of injector nozzles at the top thereof disposed in flow communication therewith, and a coupling tee disposed at an intermediate portion of said header pipe, with the trunk of said tee defining said sparger inlet pipe to channel feedwater through the branches of said tee into said header pipe for discharge from said injector nozzles. 8. An assembly according to claim 6 further comprising an annular support collar fixedly joined to said supply pipe below said release collar for supporting said release collar. 9. An assembly according to claim 6 wherein said coupling has a lower coefficient of thermal expansion than said supply and inlet pipes for further compressing said seal upon heating thereof. 10. An assembly according to claim 6 wherein said seal is a Belleville seal. 11. A removable feedwater sparger assembly for receiving feedwater from a feedwater nozzle of a boiling water reactor, comprising: 12. The removable feedwater sparger assembly as defined in claim 11, further comprising releasing means movable from a first position whereat said releasing means are disengaged from said hooking means to a second portion whereat said releasing means engage said hooking means, said hooking means changing from said first state to said second state in response to movement of said releasing means from said first position to said second position. 13. The removable feedwater sparger assembly as define in claim 12, wherein said releasing means comprise first camming surfaces and said hooking means comprise second camming surfaces, said first camming surfaces bearing against said second camming surfaces during said movement of said releasing means from said first position to said second position. 14. The removable feedwater sparger assembly as defined in claim 13, further comprising a retention flange integrally connected to one of said supply pipe and said inlet pipe, wherein said hooking means comprise a plurality of radially outwardly flexible fingers connected to the other of said supply pipe and said inlet pipe, each of said fingers having a hook for latching onto said retention flange when said hooking means are in said first state. 15. A removable feedwater sparger assembly for receiving feedwater from a feedwater nozzle of a boiling water reactor, comprising: 16. The removable feedwater sparger assembly as defined in claim 15, further comprising releasing means movable from a first position whereat said releasing means are disengaged from said latching means to a second position whereat said releasing means engage said latching means, said latching means changing from said first state to said second state in response to movement of said releasing means from said first position to said second position. 17. The removable feedwater sparger assembly as define in claim 16, wherein said releasing means comprise second camming surfaces and said latching means comprise second camming surfaces, said first camming surfaces bearing against said second camming surface during said movement of said releasing means from said first position to said second position. 18. The removable feedwater sparger assembly as defined in claim 15, further comprising resilient sealing means arranged between said first and second piping means, said resilient sealing means urging said first and second piping means apart in said first state. |
050283790 | claims | 1. A fuel handling system for nuclear reactor plants comprising a reactor vessel having an openable top and removable cover and containing therein, submerged in water substantially filling the reactor vessel, a fuel core including a multiplicity of fuel bundles formed of groups of sealed tube elements enclosing fissionable fuel assembled into units, the fuel handling system comprising the combination of: a fuel bundle handling platform movable over the open top of the reactor vessel; a fuel bundle handling mast extendable downward from the platform with a lower end projecting into the open top reactor vessel to the fuel core submerged in water; a grapple head mounted on the lower end of the mast provided with grappling means for attaching to and transporting fuel bundles; and a camera enclosed within the grapple head which is provided with windows for general distance viewing of the fuel bundles of the fuel core with the camera in several directions, and for close up viewing of fuel bundles, and having a cable connecting the camera with at least one viewing monitor located above the reactor vessel for observing the fuel bundles of the fuel core. a fuel bundle handling platform moveable over the open top of the reactor vessel; a fuel bundle handing mast extendable downward from the platform with a lower end projecting into the open top reactor vessel to the fuel core submerged in water; a grapple head mounted on the lower end of the mast provided with grappling means for attaching to and transporting fuel bundles; and a camera with a prismatic viewing head enclosed within the grapple head which is provided with at least three windows for viewing the fuel bundles of the fuel core from different perspectives, and having a cable connecting the camera with a viewing monitor located above the reactor vessel for observing the fuel bundles of the fuel core and for enabling aiming of the camera prismatic viewing head through each window by an operator. a fuel bundle handing platform moveable over the open top of the reactor vessel; a fuel bundle handing mast extendable downward from the platform with a lower end projecting into the open top reactor vessel to the fuel core submerged in water; a grapple head mounted on the lower end of the mast provided with grappling hook means for attaching to and transporting fuel bundles into and out from the fuel core; and a camera with a prismatic viewing head surrounded by a radioactive resisting quartz cylinder and enclosed within the grapple head which is provided with at least three windows with at least two windows provided with an angled surface for aiming the camera prismatic viewing head in different directions and thereby viewing the fuel bundles of the fuel core from different perspectives, and having a cable connecting the camera with a viewing monitor located above the reactor vessel for observing the fuel bundles of the fuel core and for enabling aiming of the camera prismatic viewing head through the windows by an operator. 2. The fuel handling system for nuclear reactor of claim 1, wherein some windows within the grapple head are provided with an angled surface for aiming the camera view in different directions. 3. The fuel handling system for nuclear reactor plants of claim 1, wherein the camera enclosed within the grapple head is surrounded by a radiation resisting quartz wall. 4. The fuel handling system for nuclear reactor plants of claim 1, wherein the camera containing grapple head is provided with a cooperating pair of hooks for grasping fuel bundles and transporting the bundles within the reactor vessel. 5. A fuel handling system for nuclear reactor plants comprising a reactor vessel having an openable top and removable cover for refueling and containing therein, submerged in water substantially filling the reactor vessel, a fuel core including a multiplicity of fuel bundles formed of groups of sealed tube elements enclosing fissionable fuel assembled into units, the fuel handling system comprising the combination of: 6. The fuel handling system for nuclear reactor plants of claim 5, wherein windows within the grapple head area provided with an angled surface for aiming the camera prismatic viewing head in different directions. 7. The fuel handling system for nuclear reactor plants of claim 5, wherein the camera and prismatic viewing head enclosed within the grapple head are surrounded by a radioactive resisting quartz cylinder. 8. The fuel handling system for nuclear reactor plants of claim 5, wherein the camera containing grapple head is provided with a cooperating pair of grapple hooks for grasping and transporting fuel bundles. 9. The fuel handling system for nuclear reactor plants of claim 5, wherein the fuel bundle handling mast extendable downward from the platform is telescoping whereby it reciprocally moves the grapple head mounted on its lower end down to the fuel core and back up from the fuel core. 10. A fuel handing system for nuclear reactor plants comprising a reactor vessel having an openable top and removable cover for refueling and containing therein, submerged in coolant water substantially filling the reactor vessel, a fuel core including a multiplicity of fuel bundles formed of groups of sealed tube elements enclosing fissionable fuel assembled into units, the fuel handing system comprising the combination of: 11. The fuel handling system for nuclear reactor plants of claim 10, wherein the camera containing grapple head is provided with a cooperating pair of grapple hooks for grasping and transporting fuel bundles. 12. The fuel handling system for nuclear reactor plants of claim 10, wherein the fuel bundle handling mast extendable downward from the platform is mechanically telescoping whereby it reciprocally moves the grapple head mounted on its lower end down to the fuel core and back up from the fuel core to grasp and transport fuel bundles into and out from the fuel core. |
052415732 | description | DESCRIPTION OF THE PREFERRED EMBODIMENT With reference now to the drawings, and in particular to FIGS. 1 to 9 thereof, a new and improved shield apparatus embodying the principles and concepts of the present invention and generally designated by the reference numeral 10 will be described. More specifically, with reference to FIG. 9, the shield apparatus 10 of the instant invention includes a first block 11 formed as a base block, at least one second block formed as an intermediate block 12, and a third block 13 mounted to an upper surface of the second block 12. It should be noted that a plurality of the intermediate blocks may be utilized, as required, depending on a needed height achieved in the assemblage of the block structure together. The block are each defined by an equal predetermined width and an equal predetermined length to permit the wall portions of the blocks to be formed to a modular construction permitting a plurality of such modular constructions to be assembled together to form a wall of any desired length and height. The first block 11 includes the first block cavity 14 coextensively directed within the first block. The first block further includes a first floor 15 spaced from and parallel a first top wall 16. A top wall rib 17 projects medially and orthogonally relative to the top wall and is oriented parallel relative to the first side walls 19 and is coextensively directed along the side walls and extends between the first end walls 20 intersecting the end walls in orthogonal relationship. The first top wall rib 17 includes at least a single first bore 23. The first bore 23 is spaced from one of said end walls a predetermined spacing, and includes a continuous torroidal first "O" ring 24 arranged in surrounding relationship relative to the first bore 23 extending above the first rib top wall 18. One of the first side walls 19 includes a drain conduit 21 directed therefrom to permit drainage of fluid from the first block cavity, as well as the second block cavity 35 (see FIG. 5) and the third block cavity 36 (see FIG. 7). The blocks when in an assembled configuration are also filled with water and the blocks each include intercommunicating passages, to be described in more detail below. The second block 12 includes a second floor 25, with the second floor 25 including a second floor groove 26 extending coextensively between the second end walls 32 and intersecting the second end walls. The groove 26 includes a groove top wall 26a that includes a rigid conduit 31 directed through the groove top wall 26a and spaced from one of the second end walls 32 said predetermined spacing to interfit and be received within the first bore 23 to effect fluid communication between the second block cavity 35 and the first block cavity 14. A second top wall 27 is spaced from and parallel the second floor 25, as well as the groove top wall 26, with the second top wall 27 including a second top wall rib 28. The second top wall rib 28 includes top wall 29, with a second bore 30 directed therethrough. The second bore 30 is spaced from the second wall the predetermined spacing and is arranged for reception of a rigid conduit 41 defined as the third floor groove top wall rigid conduit 41 that is also spaced from the third end wall 47 of the third block said predetermined spacing to effect fluid communication between the third block cavity and the second block cavity. A "O" ring 33 is arranged in surrounding relationship relative to the second bore 30 projecting above the second top wall rib top wall 29 to effect a sealing relationship between the rib 28 and the receiving third floor groove 39. The second side walls 34, as noted above, are spaced apart the predetermined width, as are the first side walls 19 and the third side walls 42 of the respective first and third blocks 11 and 13. The third block 13 is formed with a third top wall 37 spaced from and parallel a third floor 38. The third floor 38 includes the third floor groove 39 directed coextensively between the third block end walls. The third floor groove top wall rigid conduit 41, as noted above, projects through the third floor groove top wall 40 spaced from one of the end walls the predetermined spacing. A third bore 43 is directed through the third top wall 37 and utilizes a filling bore or alternatively as a venting bore. In this vein, a fourth bore 44 is arranged to be formed with a fourth bore conduit 45 that may be utilized as a vent or alternatively, to receive a fluid-fill conduit "F" secured thereto. In this manner, the ease of filling and venting of the structure once assembled is available. It should be noted that the first, second, and third blocks are each formed of a polymeric material of geometric integrity to define a rigid construction to contain the fluid to be directed therewithin and provide for efficient shielding relative to gamma radiation as required. As to the manner of usage and operation of the instant invention, the same should be apparent from the above disclosure, and accordingly no further discussion relative to the manner of usage and operation of the instant invention shall be provided. With respect to the above description then, it is to be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. |
051868903 | abstract | A fuel containment containing nuclear fuel rods of a fuel assembly which will be loaded into a reactor core of a fast breeder reactor is formed narrow in width at middle portion of vertical direction and long in length of vertical direction. And as being used in the reactor core, the middle portion of vertical direction of the fuel containment, wherein both of heat generation and swelling are larger than that of other portion, expands more than other portion, but undesirable contact with adjacent fuel assemblies can be avoided as the middle portion is previously formed narrow. Moreover, the other portions, wherein expansion of the fuel containment and undesirable contact with adjacent fuel assemblies are barely caused, are formed previously wide in width by enlarging the coolant flow path of the fuel containment, whereby pressure drop of coolant of coolant used in the reactor core can be reduced. |
abstract | An X-ray imaging system that generates a large amount of X-rays sufficient for X-ray imaging and collimates X-rays in a direction parallel to each other at high density. The X-ray imaging system includes an X-ray generating apparatus to generate and emit X-rays, a detector to detect the X-rays emitted from the X-ray generating apparatus, and at least one collimator disposed between the X-ray generating apparatus and the detector to prevent dispersion of the X-rays emitted from the X-ray generating apparatus. |
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claims | 1. An automated method for setting parameters of a focused-particle-beam column to acquire a focused image of a sample, comprising: a. Aligning the column, b. Acquiring an image of the sample having a focus-rotation center in the image, c. Setting image contrast, and d. Setting column focus. 2. The method of claim 1 , wherein step b. comprises: claim 1 acquiring an over-focused image of the sample; acquiring an under-focused image of the sample; and calculating a focus-rotation center from an image feature common to the over-focused image and the under-focused image. 3. The method of claim 2 , wherein acquiring the over-focused image and acquiring the under-focused image comprise setting the column off-focus by a predetermined amount. claim 2 4. The method of claim 1 , wherein the focused-particle-beam column comprises a scanning-electron-beam column. claim 1 5. The method of claim 1 , wherein step d. comprises selecting a region of interest of the image around the focus-rotation center and filtering the region of interest with an edge-enhancement filter. claim 1 6. The method of claim 4 , wherein filtering the region of interest with an edge-enhancement filter comprises applying a Sobel filter to pixels of the region of interest. claim 4 7. The method of claim 1 , wherein step d. comprises selecting a region of interest of the image around the focus-rotation center; filtering the region of interest with an edge-enhancement filter; and calculating a value representing image sharpness within the region of interest. claim 1 8. The method of claim 7 , wherein filtering the region of interest with an edge-enhancement filter comprises applying a Sobel filter to pixels of the region of interest. claim 7 9. The method of claim 7 , wherein calculating a value representing image sharpness comprises applying a semi-Kurtosis matrix to pixels within the region of interest. claim 7 10. The method of claim 7 , wherein step d. further comprises performing a Golden Section search over the region of interest to determine an optimum focus setting for the column. claim 7 11. The method of claim 1 , wherein step d. comprises performing a Golden Section search over the region of interest to determine an optimum focus setting for the column. claim 1 12. The method of claim 1 , further comprising: claim 1 e. correcting astigmatism of the column. 13. The method of claim 12 , wherein step e. comprises selecting a region of interest of the image around the focus-rotation center and filtering the region of interest with an edge-enhancement filter. claim 12 14. The method of claim 13 , wherein filtering the region of interest with an edge-enhancement filter comprises applying a Sobel filter to pixels of the region of interest. claim 13 15. The method of claim 12 , wherein step e. comprises selecting a region of interest of the image around the focus-rotation center; filtering the region of interest with an edge-enhancement filter; and calculating a value representing degree of astigmatism within the region of interest. claim 12 16. The method of claim 15 , wherein filtering the region of interest with an edge-enhancement filter comprises applying a Sobel filter to pixels of the region of interest. claim 15 17. The method of claim 15 , wherein calculating a value representing degree of astigmatism comprises applying a semi-Kurtosis matrix to pixels within the region of interest. claim 15 18. The method of claim 15 , wherein step e. further comprises performing a Golden Section search over the region of interest to determine an optimum astigmatism-correction setting for the column. claim 15 19. The method of claim 12 , wherein step e. comprises performing a Golden Section search over the region of interest to determine an optimum astigmatism-correction setting for the column. claim 12 20. The method of claim 1 , wherein step c. comprises: operating the column to acquire a low-contrast image; operating the column to acquire a high-contrast image; and counting saturation levels of pixels in the images. claim 1 21. The method of claim 20 , further comprising the step of averaging the counting of saturation levels of pixels in the images. claim 20 22. The method of claim 20 , wherein step c. further comprises applying a bisection method to set the image contrast at a level meeting pre-set criteria. claim 20 23. An imaging system having a focused-particle-beam column and control apparatus for setting parameters of the column to acquire a focused image of a sample by: a. Aligning the column, b. Acquiring an image of the sample having a focus-rotation center in the image, c. Setting image contrast, and d. Setting column focus. 24. The system of claim 23 , wherein the control apparatus is adapted to acquire an image of the sample having a focus-rotation center in the image by: claim 23 acquiring an over-focused image of the sample; acquiring an under-focused image of the sample; and calculating a focus-rotation center from an image feature common to the over-focused image and the under-focused image. 25. The system of claim 24 , wherein the control apparatus is adapted to acquire the acquire the over-focused image and to acquire the under-focused image comprise setting the column off-focus by a predetermined amount. claim 24 26. The system of claim 1 , wherein the charged-particle-beam column comprises an electron-beam column. claim 1 27. The system of claim 23 , wherein setting column focus comprises selecting a region of interest of the image around the focus-rotation center and filtering the region of interest with an edge-enhancement filter. claim 23 28. The system of claim 27 , wherein filtering the region of interest with an edge-enhancement filter comprises applying a Sobel filter to pixels of the region of interest. claim 27 29. The system of claim 23 , wherein setting image contrast comprises selecting a region of interest of the image around the focus-rotation center; filtering the region of interest with an edge-enhancement filter; and calculating a value representing goodness of focus within the region of interest. claim 23 30. The system of claim 29 , wherein filtering the region of interest with an edge-enhancement filter comprises applying a Sobel filter to pixels of the region of interest. claim 29 31. The system of claim 29 , wherein calculating a value representing goodness of focus comprises applying a semi-Kurtosis matrix to pixels within the region of interest. claim 29 32. The system of claim 29 , wherein setting image contrast further comprises performing a Golden Section search over the region of interest to determine an optimum focus setting for the column. claim 29 33. The system of claim 23 , wherein setting column focus comprises performing a Golden Section search over the region of interest to determine an optimum focus setting for the column. claim 23 34. The system of claim 23 , wherein the control apparatus is further adapted for correcting astigmatism of the column. claim 23 35. The system of claim 34 , wherein correcting astigmatism of the column comprises selecting a region of interest of the image around the focus-rotation center and filtering the region of interest with an edge-enhancement filter. claim 34 36. The system of claim 35 , wherein filtering the region of interest with an edge-enhancement filter comprises applying a Sobel filter to pixels of the region of interest. claim 35 37. The system of claim 34 , wherein correcting for astigmatism of the column comprises selecting a region of interest of the image around the focus-rotation center; filtering the region of interest with an edge-enhancement filter; and calculating a value representing degree of astigmatism within the region of interest. claim 34 38. The system of claim 37 , wherein filtering the region of interest with an edge-enhancement filter comprises applying a Sobel filter to pixels of the region of interest. claim 37 39. The system of claim 37 , wherein calculating a value representing degree of astigmatism comprises applying a semi-Kurtosis matrix to pixels within the region of interest. claim 37 40. The system of claim 37 , wherein correcting for astigmatism of the column further comprises performing a Golden Section search over the region of interest to determine an optimum astigmatism-correction setting for the column. claim 37 41. The system of claim 37 , wherein correcting for astigmatism of the column comprises performing a Golden Section search over the region of interest to determine an optimum astigmatism-correction setting for the column. claim 37 42. The system of claim 23 , wherein setting image contrast comprises: operating the column to acquire a low-contrast image; operating the column to acquire a high-contrast image; counting saturation levels of pixels in the images. claim 23 43. The system of claim 42 , wherein setting image contrast further comprises averaging the counting of saturation levels of pixels in the images. claim 42 44. The system of claim 42 , wherein setting image contrast further comprises applying a bisection method to set the image contrast at a level meeting pre-set criteria. claim 42 |
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abstract | Systems are provided for inspection and tooling submerged in nuclear reactors. Systems mount at the reactor edge, such as on a steam dam, to be independently operable from a refueling bridge or refueling operations. A moveable steam dam clamp may hold a position apparatus at the edge. The positioning apparatus includes a rotatable shoulder and arms move a tool, reactor component, and/or inspection device like a camera or VARD to desired and highly-determinable reactor positions. A float may counter shear and rotation on the shoulder from the arms. Motors at the shoulder with internal transmissions may rotate the shoulder and arms, or manual rotation may be used. The arms may also overlap vertically for installation and removal. Power, controls, and/or data may be provided underwater through an umbilical connection to operators. |
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summary | ||
claims | 1. A device for preparing a captured trace, the captured trace being stored for later use, of I/O workload activity experienced on one or more data storage volumes included with a data storage system for being analyzed by a computerized trace analysis process, the device comprising:a processor in communication with a memory containing software code, which when loaded into said processor enables said processor to execute the steps of:preparing the captured trace for being analyzed by categorizing information from the captured trace into categories related to (i) components in the data storage system experiencing the traced workload activity and (ii) information type including response times and task events;using the categories for access to trace-related information for trace analysis by the computerized trace analysis process; andpresenting the categorized information from the captured trace on a display screen. 2. The device of claim 1, wherein a summary file summarizing the captured trace is created including the categories information and the summary file is used for the access to the trace-related information by the computerized trace analysis process. 3. The device of claim 1, wherein the information type includes I/O activity. 4. The device of claim 1, wherein the components includes logical volume representation of the data storage volumes. 5. The device of claim 1, wherein the computerized trace analysis process includes communication to the computerized trace analysis process for being responsive to the act of a trace being captured. |
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description | The present disclosure relates to a method and a system for delivering radiation to a target tissue, such as a target tissue that comprises and/or is adjacent to a non-target tissue. Radiation therapy and radiosurgery are established methods of treating patients with certain malignant and benign diseases. Radiation therapy, which is also referred to as radiotherapy, radiation oncology, and XRT, involves the administration of ionizing radiation to a patient, such as a patient being treated for cancer. Exposure of malignant cells to radiation inhibits their proliferation and induces cell death. A patient undergoing radiation therapy for cancer treatment is typically treated with ionizing radiation many times, often at defined intervals. A disadvantage of radiation therapy is that normal cells and tissues may be adversely affected by the radiation. Assuming that normal tissue recovers at a faster rate than cancerous tissue, administration of radiation at defined intervals is expected to allow any normal tissue, which has been adversely affected by the radiation, to recover before the next treatment. Radiation therapy also may be combined with surgery, chemotherapy, and/or hormone therapy. Radiation therapy is also used to treat various non-malignant conditions, such as trigeminal neuralgia, severe thyroid eye disease, pterygium, and pigmented villonodular synovitis, and to inhibit keloid scar growth and heterotopic ossification. However, the use of radiation therapy to treat non-malignant conditions is limited by the risk of radiation-induced cancer. Multileaf collimators have been used with linear accelerators (LINAC) to improve the geometric conformation of the radiation treatment to the target tissue as a way of minimizing adverse effects on normal tissue. Multileaf collimators have many rectangular vanes or “leaves” of a material having a high atomic number, such as tungsten, that can be moved independently in and out of the path of a particle beam. When a “leaf” is in the path of a particle beam, the beam is blocked. Therefore, by positioning the “leaves” to form an aperture having a specific geometry, it is possible to shape a beam of radiation to conform to a desired geometric shape. Intensity modulated radiation therapy (IMRT) further improves the conformation by using multiple weighted apertures for each treatment beam to account for geometric variations in the dimensions of the beam projection. Radiosurgery is a medical procedure that allows non-invasive treatment of benign and malignant tumors and other conditions, such as arteriovenous malformations (AVMs) and trigeminal neuralgia. It involves precisely directing highly focused beams of ionizing radiation at a single point (referred to as an “isocenter”). By applying a precise dosage of radiation to the isocenter, tumors and other lesions, which are otherwise inaccessible or inadequate for open surgery, can be ablated. Typically, only a single or a few treatments are necessary. Radiosurgery typically involves the use of multiple small circular collimated radiation beams directed at the isocenter. Consequently, radiation is concentrated at the isocenter through the superposition of multiple small overlapping beams. Adjacent tissues, which are not in the isocenter, are not exposed to the overlapping beams and, therefore, are not subjected to the concentrated dose of radiation. The multiple overlapping beams may be created using many individual sources of radiation with individual collimators, such as in the case of the GammaKnife™ from Elekta (Stockholm, Sweden), or by arcing a single source and collimator about a point. Traditional isocentrically mounted medical LINACs achieve a similar effect of multiple independent beams using a combination of table and gantry angles with circular or multileaf collimators. A complex tissue volume can be treated with radiosurgery using multiple isocenters (or “shots”) at discrete points inside the volume. Radiation is delivered to each isocenter with a specific arrangement of multiple beams and a specific collimator size. The dose of radiation used in radiation therapy and radiosurgery is limited by concerns over toxicity to non-target tissue, i.e., normal tissue, within and/or adjacent to the target tissue, e.g., diseased tissue. The amount of radiation to which non-target tissue is exposed is determined by a number of factors. For example, there is a margin of error associated with the placement of the radiation beam, which is limited by the mechanical accuracy of the treatment device and the knowledge of where the non-target tissue is at the time of treatment. The nature and properties of a single divergent beam of radiation incident on the target tissue also impact the exposure of non-target tissue to radiation. The lateral range of secondary electrons outside the field of radiation, the physical size and shape of the source of the photons, the shape of the collimator edge, photon scatter, and transmission through the collimator affect the fall-off in dosage (i.e., the penumbra of the radiation beam) of high-energy photon radiation away from the edge of the collimator. The interaction of all beams also impacts the exposure of non-target tissue to radiation. Geometrically, all beams overlap in the target tissue, and they also may overlap in the non-target tissue. Such overlap in the target tissue and the non-target tissue makes it very difficult, if not impossible, to deliver a uniformly high dose of radiation to the target tissue while not delivering any radiation to the non-target tissue. Away from the edge of the target tissue, e.g., the tumor, the dose of radiation will also fall off with a varying gradient rather than abruptly. Depending on the techniques used, including the collimator design and the beam arrangement, such dose fall-off can be very steep or very gentle. Such dose fall-off at the target boundary is referred to as the “dose gradient.” The best therapeutic gain can be achieved with a high dose gradient at the edge of the target tissue. For a single divergent high-energy photon beam of radiation covering the target tissue, the “dulling” of dose gradient is caused by scattered photons and electrons along the radial dimension of the beam boundary and the beam's entrance and exit along the beam direction. With multiple beams, the dose gradient is largely influenced by the overlap of beams on the entry and exit sides of the target tissue. The divergence of radiation beams also contributes to making the dose gradient less steep. The use of multiple focal spots (also referred to as “shots”), such as when a volume of target tissue cannot be treated with a single isocenter, results in interaction of beams from adjacent focal spots. Interaction of beams from adjacent focal spots can increase the exposure of non-target tissue within and/or adjacent to the target tissue to radiation. The main reason why the above methods result in exposure of non-target tissue to radiation is that the methods cannot create a sharp dose gradient at the boundary of a target tissue. The collimating systems are designed to treat targets of all sizes and shapes. With medical LINACs, the collimators are either fixed or dynamically configurable. Fixed collimators are invariably circular in shape. Dynamically configurable collimators are either composed of four jaws, which can shape any rectangular field, or use multiple leaves of varying thickness, which can shape irregular fields. Because target tissues, such as tumors, are rarely rectangular, the jaws have gradually become obsolete. For multileaf collimators, the beam boundary cannot be very sharp no matter how thin the leaves are. For ablative treatments, circular collimators are typically used. A circular collimator, however, cannot create a sharp dose fall-off between a target tissue and an adjacent non-target tissue when beams are overlapped to form an edge. This is illustrated in FIG. 2a and FIG. 2c. FIG. 2a is a schematic drawing of the movement of a cylindrical radiation beam along a linear interface (see “Brief Description of the Figures” for details). FIG. 2c is a graph of the cumulative radiation intensity (“I”) versus the location along the central axis of overlap between the adjacent cylindrical radiation beams (“y”) of FIG. 2a (see “Brief Description of the Figures” for details). As can be seen in FIG. 2c, the dose fall-off from the central axes and the outer edges of the cylindrical radiation beam along the central axis of overlap between adjacent cylindrical radiation beams is from about 95% to about 5%, which is on the order of the radius of the collimated beam. In other words, a circular collimator makes a “dull” knife for sculpting. External beam radiation therapy typically achieves a dose fall-off of about 90% to about 10% over about 10 mm, whereas radiosurgery with an ablative system typically achieves a dose fall-off of about 90% to about 10% over about 5 mm. Fixed collimators are advantageous in that they have small beam penumbra and precise divergence. Moving a circular beam along a line is a special case. In general, beams are overlapped to shape curved surfaces in two dimensions as illustrated in FIG. 3a and FIG. 3c. FIG. 3a is a schematic drawing of the movement of a cylindrical radiation beam along a convex interface (see “Brief Description of the Figures” for details). FIG. 3c is a graph of the cumulative radiation intensity (“I”) versus the location along the radial dimension (“r”) of FIG. 3a (see “Brief Description of the Figures” for details). As can be seen in FIG. 3c, a circular collimator cannot form a completely “cold” (low dose) hole surrounded by “hot” (high dose) regions. In addition to the above, a patient is held stationary on a patient support system, such as a table, during radiation treatment, and the radiation beam is moved by moving the collimator. The radiation beam can be rotated around the patient with a single isocenter. There is no dynamic coordination of the patient support system with the dynamic delivery of the radiation beam. The only known exception is the management of breathing-induced tumor motion by moving the patient support system exactly opposite that of the tumor in order to keep the tumor location stationary in space (D'Souza et al., “Intra-Fraction Motion Synchronized Adaptive Couch-Based Radiation Delivery: A Feasibility Study,” Phys. Med. Biol. 50: 4021-4033 (2005)). Radiation therapy using external beams as described above typically places the tumor at the isocenter, i.e., the intersection of all rotational axes. This arrangement makes patient set-up much easier. However, in any beam direction, radiation can only be directed at a point in the target tissue through one unique path. This significantly limits how the target tissue is irradiated. Some degree of freedom is possible with CyberKnife (Accuray Inc., Sunnyvale, Calif.). With CyberKnife, an x-band LINAC is mounted on a 6-axes robot, and the patient is kept stationary on a patient support system. Using circular collimators, CyberKnife delivers radiation doses to the target tissue by crossing many (e.g., a hundred or more), variously oriented, cylindrical radiation beams, which do not share a common center of rotation and, therefore, are not isocentric, in the target. While variously angled cylindrical radiation beams can be realized with the CyberKnife, configuring the beams can be complex and, since it involves the use of a robot, expensive. Furthermore, CyberKnife can only use a single source of radiation, which requires more time to treat a patient, and, since CyberKnife uses circular collimators, it cannot create a sharp dose fall-off between a target tissue and an adjacent non-target tissue when beams are overlapped to form an edge. The present disclosure seeks to overcome the disadvantages inherent in currently available methods of radiation therapy. In view of this, it is an object of the present disclosure to provide a method, a collimator, and a system for dynamically sculpting target tissue so that the radiation doses at the boundaries of the target tissue and any non-target tissue(s) have a sharp fall-off, thereby reducing, if not eliminating, “dose spillage” to non-target tissue and delivering a uniform dose to the target tissue. Compared to currently available methods of radiation therapy, the method of the present disclosure is easier to use and more economical. It is another object of the present disclosure to provide a method of planning irradiation of a target tissue in accordance with the present disclosure. These and other objects and advantages, as well as inventive features, will become apparent from the detailed description provided herein. A method of irradiating a target tissue in a patient is provided. The method comprises positioning the patient on a patient support system so that the target tissue in the patient is within irradiating distance of at least one source of a beam of radiation and moving the patient support system relative to the at least one source of a beam of radiation and, coordinately with movement of the patient support system, rotating the at least one source of radiation relative to the target tissue, which comprises and/or is adjacent to a non-target tissue, so that the center of rotation of the beam of radiation is placed at one or more desired locations within the target tissue, while simultaneously and/or sequentially irradiating the target tissue. The beam of radiation can have a D-shaped cross-section, in which case the straight edge of the D-shaped cross-section of the beam of radiation is placed tangentially to the boundary of the target tissue and the non-target tissue as the beam is rotated. A collimator, in particular a collimator with a fixed opening and divergence, is also provided. The collimator (a) shapes a beam of radiation to have a D-shaped cross-section, (b) maintains the central axis of the beam of radiation on or adjacent to the straight edge of the D-shaped cross-section of the beam of radiation, and (c) can fully rotate the beam of radiation in either direction about the beam axis, such that the straight edge of the D-shaped cross-section of the beam of radiation can face any direction. Further provided is a method of making a collimator. The method comprises joining half of a circular (cross-section) collimator with a cone-shaped tunnel with half of a rectangular (cross-section) collimator with a pyramid-shaped tunnel, where both of the circular collimator and the rectangular collimator have the same divergence. Further provided is a system for irradiating a target tissue in a patient. The system comprises (i) a patient support system, which comprises (a) a table or a couch, either of which is optionally padded, (b) one, two or three motors, each of which drives movement of the table or the couch in the direction of a separate axis, (c) optionally, a base, in which case the one or more motors can be housed in the base, and (d) a computerized control system, which can control the movement of the patient support system; (ii) at least one rotatable source of a beam of radiation, wherein each source of a rotatable beam of radiation can be rotated around a target tissue in a patient positioned on the patient support system; (iii) at least one collimator, wherein each collimator is operably aligned with one rotatable source of a beam of radiation; and (iv) a central control unit, which can execute a patient treatment plan including rotation of at least one rotatable source of a beam of radiation relative to a target tissue in a patient positioned on a patient support system, rotation of the at least one rotatable source of a beam of radiation, and movement of a patient support system. Preferably, the at least one collimator (a) shapes the beam of radiation to have a D-shaped cross-section, (b) maintains the central axis of the beam of radiation on or adjacent to the straight edge of the D-shaped cross-section of the beam of radiation, and (c) can fully rotate the D-shaped cross-section of the beam of radiation in either direction, such that the straight edge of the D-shaped cross-section of the beam of radiation can face any direction. Still further provided is a method of planning irradiation of a target tissue in a patient with a system for irradiating a target tissue in a patient. The method comprises: i) determining the volume and the surface contour of the target tissue to be irradiated and, if present, the volume and the surface contour of a non-target tissue located wholly within the target tissue and/or the surface contour and, optionally, the volume of a non-target tissue located partially within the target tissue, (ii) setting the radiation dose to be delivered to the target tissue and limiting the radiation dose to the non-target tissue, (iii) assigning control points to the surface contours identified in (i), (iv) determining the angle of the beam of radiation, the orientation of the collimator, and the position of the patient support system at each control point, (v) assigning “wild card” points within the volume of the target tissue with the proviso that a “wild card” point is not assigned within the volume of any non-target tissue that is located wholly or partially within the target tissue, (vi) determining the path of motion when all control points and, optionally, one or more “wild card” points, are connected and optimizing the weighting of each beam of radiation so as to provide a uniform dose of radiation within the target tissue and a sharp drop-off away from the boundary between the target tissue and any non-target tissue, and (vii) checking the resulting radiation dose distribution against a desired radiation dose distribution and adjusting the path of motion and the weightings of control points accordingly, and, if needed, adding more control points. The present disclosure does not use a single fixed isocenter and a stationary patient support system. Although the collimator and the beams of radiation still rotate around a point, the point is not fixed in relation to the patient. Coordinated motion of a patient support system and rotation of a beam of radiation enables a target tissue in a patient to be traversed by beams of radiation from any orientation, allowing much greater freedom than current radiation treatment equipment that uses a fixed isocenter relative to the patient's anatomy to create overlapping, traversing patterns similar to line art. By coordinating beam rotation and patient movement, such as in all three dimensions, radiation can hit every point in the target from all co-planar directions, if the rotation is coplanar, or from all non-colliding directions, if rotation of the beam is non-coplanar. In view of the foregoing, the present disclosure provides a method of irradiating a target tissue in a patient. The method comprises positioning the patient on a patient support system so that the target tissue in the patient is within irradiating distance of at least one source of a beam of radiation and moving the patient support system, such as in three dimensions, relative to the at least one source of a beam of radiation and, coordinately with movement of the patient support system, rotating the at least one source of radiation relative to the target tissue, which comprises and/or is adjacent to a non-target tissue, so that the center of rotation of the beam of radiation is placed at one or more desired locations within the target tissue, while simultaneously and/or sequentially irradiating the target tissue. “Target tissue” refers to any tissue intended to be irradiated in accordance with the present disclosure. It is anticipated that, at least in some instances, the target tissue will be a malignant tumor. “Non-target tissue” refers to any tissue that is not intended to be irradiated. Non-target tissue can be adjacent to and/or surround a target tissue. Alternatively or additionally, non-target tissue can be located anywhere within the target tissue and can occupy varying volume(s) of the target tissue. For example, the urethra can be surrounded by prostate tissue containing cancer. It is anticipated that the non-target tissue typically will be normal and, in at least some instances, may comprise one or more tissues or structures, such as an organ, that can be critical for the health of the patient and, therefore, should be avoided (an “avoidance structure”). Any suitable patient support system that enables movement of the patient, such as in all three dimensions, can be used in the context of the method. An example is a patient support system, such as a padded table or a couch, the position of which can be driven by one, two or three motors, e.g., analog or digital motors, so that movement in at least one axis, such as the vertical axis, up to three axes of motion can be achieved. Movement in at least the vertical axis (i.e., the axis perpendicular to the top of the patient support system) is preferred. Redundant positional sensors can be used to ensure positional accuracy. The patient support system can comprise a base, in which case the motor(s) can be housed in the base. The movement of the patient support system is preferably controlled by a computerized control system. At least one source of a beam of radiation is used. Preferably, more than one source of a beam of radiation, such as two, three, or even more, is/are used. The number of sources of beams of radiation that can be used is limited by physical space and cost limitations. When more than one source of a beam of radiation is used, patient treatment time can be reduced. The beam of radiation can have, and preferably does have, a D-shaped cross-section. Desirably, the beam of radiation is shaped by a fixed or variable collimator, and has a D-shaped cross-section, such as that which is formed by joining a half circle with a rectangle having a length of the diameter of the half circle and a width not larger than the radius of the half circle. For ease of reference, the term “D-shaped” will be used to refer collectively to D-shaped cross-sections as well as cross-sections having more or less of a “D” shape, unless otherwise indicated or contradicted by context. A “D” shape can be formed by joining a half circle with a rectangle, which has a length equal to the diameter of the half circle and a width that is equal to or less than, even substantially less than, the radius of the half circle. Preferably, and even desirably, the straight edge of the D-shaped cross-section of the beam of radiation is placed tangentially at the boundary of the target tissue and the non-target tissue. The central axis of the D-shaped radiation beam is located on or close to the flattened portion of the “D.” In a preferred embodiment, the central axis of the beam of radiation is kept stationary, i.e., it is focused on a point that is fixed in space. The beam of radiation is rotated around the patient. The collimator is rotated as necessary to alter the orientation of the cross-section, such as a D-shaped cross-section, of a beam. Preferably, and even desirably, the straight edge of a D-shaped cross-section is placed tangentially to the boundary of the target tissue and the non-target tissue, and the tangential placement of the straight edge of the D-shaped cross-section of a beam of radiation is maintained during coordinated rotation of the at least one source of radiation relative to the target tissue and movement of the patient support system. The patient is moved (by means of the patient support system) coordinately with the beam of radiation and the angle of the collimator so that the focal point relative to the volume of the target tissue can be changed. In theory, the beam as well as the patient support system can move independently at the same time, although such an arrangement would be more difficult to implement and unnecessary. Preferably, the movement of the patient support system, the rotation of the beam of radiation around the patient, i.e., relative to the target tissue, and the rotation of the collimator to reorient the cross-section, such as the straight edge of a D-shaped cross-section, of the beam of radiation are coordinated such that the edge of the cross-section, such as the straight edge of the “D,” is kept tangentially to the surface of the target tissue or the surface of a non-target tissue that is within the target tissue and needs to be protected. Because the surface of the target tissue and the surface of the non-target tissue rarely take the shape of a perfect cylinder oriented perpendicularly to the plane of beam rotation, the collimator angle has to be changed at different beam angles in order to keep the straight edge of the “D,” for example, parallel to the interface surface. As the beam is rotated around the patient, such coordination allows the beam to sculpt the volume of the target tissue and to prevent the radiation beam from directly hitting a non-target tissue that is surrounded by the target tissue. Radiation, combined with beam rotation and patient movement, such as in all three dimensions, can traverse every spot in the volume of target tissue from all beam directions, if necessary. By allowing the radiation beam to traverse every point in the volume of target tissue from any direction, the tracks of the beams, which resemble rods with a “D”-shaped cross-section when beams of radiation having D-shaped cross-sections are used, can overlap in the target to form any desirable pattern, as in string arts. The above method is illustrated with reference to the figures. For example, FIGS. 1a-1d compare the cross-section and beam intensity profile associated with a single circular radiation beam and a single D-shaped radiation beam. Specifically, FIG. 1a is a schematic drawing of a single radiation beam with a circular field, the central axis of which is indicated with an “x.” FIG. 1b is a schematic drawing of the cross-section of a single D-shaped radiation beam, the central axis of which is indicated with an “x.” FIG. 1c is a drawing of the beam intensity profile associated with the single circular radiation beam of FIG. 1a, wherein the vertical line indicates the beam intensity at the central axis. FIG. 1d is a drawing of the beam intensity profile associated with the single D-shaped radiation beam of FIG. 1b, wherein the vertical line indicates the beam intensity at the central axis. A typical collimator used in radiosurgery generates a radiation beam having a circular cross-section as shown in FIG. 1a. As shown in FIG. 1c, a radiation beam having a circular cross-section has a beam intensity profile in which the intensity of the beam falls off sharply at the beam boundary. As shown in FIG. 1d, a radiation beam having a D-shaped cross-section also has a beam intensity profile in which the intensity of the beam falls off sharply at the beam boundary, with the fall-off on the straight edge being slightly better due to the fact that the straight boundary is closer to the central axis. Therefore, when considering only single beams, there is little advantage of using a D-shaped beam over a cylindrical beam. When using an external beam of radiation, however, one rarely focuses a single beam of radiation at a target tissue in a single orientation. A single beam, whether one having a circular cross-section or a D-shaped cross-section, generally does not fit the target tissue very well. In radiosurgery the beam size is typically smaller than the target. The use of multiple beams from different orientations, or a single beam from different orientations, allows high doses of radiation to be delivered to the target tissue, while exposing the non-target tissue(s) to as low doses of radiation as possible. In order to deliver a high dose of radiation to a target tissue, the beam would have to be moved over the area of the target tissue and kept within the boundary of the target tissue. Conventionally, this is done by making multiple rotational arcs of a beam to deliver a more or less spherically shaped, high-dose volume before the patient is moved. The task of treatment planning is to pack these “spheres” of high-dose volumes within the boundary of the target. However, in accordance with the present disclosure, movement of the beam of radiation is coordinated with movement of the patient, which abandons the conventional concept of isocenters and enables the use of numerous (e.g., two, three, or more) beams from all directions aimed at different locations in the target. With the method of the present disclosure, treatment planning is given much greater freedom to create the desired dose distribution. When considering the cumulative radiation intensities of superimposing beams of radiation, the use of beams having a D-shaped cross-section is advantageous over beams having a circular cross-section. This advantage is illustrated in FIGS. 2a-2d, which compare the cross-sections and beam intensity profiles associated with the use of multiple circular radiation beams and multiple D-shaped radiation beams. FIG. 2a is a schematic drawing of the movement of a circular radiation beam along a linear interface (the solid horizontal line; e.g., interface between target tissue and non-target tissue), wherein the continuous movement of the beam in the direction of the small horizontal arrows (→) is represented by a series of four overlapping circles, each of which represents the circumference of the cross-section of the circular radiation beam. The horizontal dotted line indicates the central axes of the circular radiation beams. The large vertical arrow (↑) labeled “y” represents the dimension perpendicular to the linear interface between the target tissue and the non-target tissue. “R1” represents the radius of the circular radiation beams, which is also the distance between the central axis and the outer edge of the circular radiation beam along the “y” dimension. FIG. 2b is a schematic drawing of the movement of a D-shaped radiation beam along a linear interface (the solid horizontal line; e.g., interface between target tissue and non-target tissue), wherein the continuous movement in the direction of the small horizontal arrows (→) is represented by a series of three overlapping “D” shapes, each of which represents the circumference of the cross-section of the D-shaped radiation beam. The horizontal dotted line indicates the central axes of the D-shaped radiation beams (i.e., where the half circle joins with the narrow rectangle). The large vertical arrow (↑) labeled “y” represents the dimension perpendicular to the linear interface between the target tissue and the non-target tissue. “R1” represents the distance between the central axis and the curved edge of the D-shaped radiation beam, which is also the radius of the half-circle. “a” represents the distance between the central axis and the straight edge of the D-shaped radiation beam along the “y” dimension, which is also the width of the rectangle. FIG. 2c is a graph of the cumulative radiation intensity (“I”) vs. the distance from the linear interface between the target tissue and the non-target tissue along the “y” dimension of FIG. 2a, wherein the central axes of the circular radiation beams are represented by a vertical dotted line, and the maximum cumulative radiation intensity is indicated by I0. FIG. 2d is a graph of the cumulative radiation intensity (“I”) vs. the distance from the linear interface between the target tissue and the non-target tissue along the “y” dimension of FIG. 2b, wherein the central axes of the D-shaped radiation beams are represented by a vertical dotted line, and the maximum cumulative radiation intensity is indicated by I0. If a line were drawn parallel to the target boundary at a distance of y from the boundary, the line would be intersected by the cross-section of the beam (i.e., the circle or the D-shaped circumference). The length between the two points of intersection is defined as the cord length (“c”). The cumulative intensity can be expressed as I=I0*c/(2R1), where I0 is the maximum intensity at the central axis of the beam and “R1” is the radius of the circle or the height of the “D”. By comparing FIG. 2c with FIG. 1c and FIG. 2d with FIG. 1d, respectively, it can be seen that the dose fall-off is more gentle (i.e., less sharp) when a single beam is dynamically moved or multiple such beams are horizontally combined. By comparing FIG. 2d with FIG. 2c, it can be seen that the dose fall-off is sharp at the boundary between the target tissue and the non-target tissue when a D-shaped radiation beam is moved, whereas the dose fall-off is gentle at the boundary between the target tissue and the non-target tissue when a circular radiation beam is moved. This is why the use of circular collimators in conventional ablative radiation treatments cannot create a sharp edge along a straight line. In contrast, the use of a D-shaped collimator, in accordance with the present disclosure, can create a sharp edge along a straight line. On the other hand, the dose fall-off on the opposite side of the straight edge is just as gentle as the combination of the circular beams. This is advantageous inasmuch as it allows easier abutment of the beam with other beams and easier shaping of corners simply by rotating the “D” to face in any direction in space. Rotation of the “D” to face in any direction in space allows a target tissue with a complex volume to be sculpted and irradiated with a uniform dose. Thus, while a circular beam provides a “dull knife” in radiosurgery, a D-shaped beam provides a “sharp knife.” The advantage of superimposing beams having a D-shaped cross-section is further illustrated in FIGS. 3a-3d. FIG. 3a is a schematic drawing of the movement of a circular radiation beam along a convex interface between a target tissue and a non-target tissue. The large circle represents such an interface and the surface of a non-target tissue. The clockwise movement of the beam in the direction of the upper arrows is represented by a series of three circles, each of which represents the circumference of the cross-section of the circular radiation beam. The convex dotted line comprising the arrows indicates the path of the central axes of the circular radiation beams. The vertical arrow (↑) labeled “r” represents the distance from the center of the non-target tissue in the radial direction, whereas the arrow labeled “R1” represents the radius of the non-target tissue and the arrow labeled “R2” represents the radius of the cross-section of the circular radiation beam. FIG. 3b is a schematic drawing of the movement of a D-shaped radiation beam along a convex interface between a target tissue and a non-target tissue. The large circle represents such an interface and the surface of a non-target tissue. The clockwise movement in the direction of the lower arrows is represented by a series of three “D” shapes, each of which represents the circumference of the cross-section of the D-shaped radiation beam. The convex dotted line comprising the arrows indicates the central axes of the D-shaped radiation beams, and “a” represents the distance between the central axis and the straight edge of the D-shaped radiation beam, which is also the width of the rectangle. The vertical arrow (↑) labeled “r” represents the distance from the center of the non-target tissue in the radial direction, whereas the arrow labeled “R1” represents the radius of the non-target tissue and the arrow labeled “R2” represents the distance between the central axis and the outer curved edge of the D-shaped radiation beam, which is also the radius of the half circle. FIG. 3c is a graph of the cumulative radiation intensity (I) along the radial direction “r” of FIG. 3a, wherein the central axes of the circular radiation beams are represented by a vertical dotted line and the maximum cumulative radiation intensity is indicated by I0. FIG. 3d is a graph of the cumulative radiation intensity (I) along the radial direction “r” of FIG. 3b, wherein the central axes of the D-shaped radiation beams are represented by a vertical dotted line and the maximum cumulative radiation intensity is indicated by I0. If concentric curves to the surface of the non-target tissue (i.e., the circle with radius R1) were drawn across the cross-section of the circular radiation beam in FIG. 3a, the curves would be intersected by the circumference of the beam (the circle with radius R2). The length between the two intersection points is defined as the cord length (“c”). The intensity can be expressed as I≈Io*c/(2R2){(R1+R2)/r}, wherein (R1+R2/r) is a geometric skewing factor (due to movement of the circular radiation beam along a convex interface as opposed to a flat interface). The geometric skewing factor causes the intensity profile to be asymmetric. However, by comparing FIG. 3d with FIG. 3c, it can be seen that the dose fall-off is sharp at the boundary between the target tissue and the non-target tissue when a D-shaped radiation beam is moved, whereas the dose fall-off is gentle at the boundary between the target tissue and the non-target tissue when a circular radiation beam is moved. This is why the use of circular collimators in conventional ablative radiation treatments cannot create a sharp edge along a straight line. In contrast, the use of a D-shaped collimator, in accordance with the present disclosure, can create a sharp edge along a straight line. The advantage of superimposing beams having a D-shaped cross-section is still further illustrated in FIGS. 4a-4d. FIG. 4a is a schematic drawing of the movement of a circular radiation beam along a concave interface between a target tissue and a non-target tissue, wherein the movement is represented by a series of three overlapping circles, each of which represents the circumference of the cross-section of the circular radiation beam. The arc with radius R1 represents part of the surface of the target tissue. The dashed concave line indicates the moving path of the central axes of the circular radiation beams. The arrow (↓) labeled “r” represents the distance from the center of the non-target tissue in the radial direction, whereas the arrow labeled “R2” represents the radius of the cross-section of the circular radiation beam. FIG. 4b is a schematic drawing of the movement of a D-shaped radiation beam along a concave interface between a target tissue and a non-target tissue, wherein the clockwise movement is represented by a series of three “D” shapes, each of which represents the circumference of the cross-section of the D-shaped radiation beam. The arc with radius R1 represents part of the surface of the target tissue. The dashed concave line comprising the arrows indicates the moving path of the central axes of the D-shaped radiation beams, and “a” is the width of the rectangular portion of the cross-section of the D-shaped radiation beam. The arrow (↓) labeled “r” represents the distance from the center of the target tissue in the radial direction, whereas the arrow labeled “R2” represents the distance between the central axis and the center of the outer curved edge of the D-shaped radiation beam, which is also the radius of the half circle. FIG. 4c is a graph of the cumulative radiation intensity (I) along the radial direction “r” of FIG. 4a, wherein the central axes of the cylindrical radiation beams are represented by a vertical dotted line, and the maximum cumulative radiation intensity is indicated by I0. FIG. 4d is a graph of the cumulative radiation intensity (I) along the radial direction “r” of FIG. 4b, wherein the central axes of the D-shaped radiation beams are represented by a vertical dotted line and the maximum cumulative radiation intensity is indicated by I0. If concentric curves to the surface of the target (i.e., the large arc with radius R1) were drawn across the cross-section of the circular radiation beam, the curves would be intersected by the circumference of the beam (the circle with radius R2). The length between the two intersection points is defined as the cord length (“c”). The intensity can be expressed as I=Io*c/(2R2){r/(R1−R2)}, wherein (r/R1−R2) is a geometric skewing factor (due to movement of the circular beam along a concave interface as opposed to a flat interface). The geometric skewing factor causes the intensity profile to be asymmetric. Here, again, however, by comparing FIG. 4d with FIG. 4c, it can be seen that the dose fall-off is sharp at the target boundary when a D-shaped radiation beam is moved, whereas the dose fall-off is gentle at the target boundary when a circular radiation beam is moved. Arcs or many beams are often used to create a high dose volume. For a complete 360-degree arc, the high-dose volume created with circular beams is a sphere. Moving such a sphere to form a flat surface (i.e., interface) separating a target tissue from a non-target tissue is even harder. The radiation intensity along the line perpendicular to the interface is proportional to the area of the intersection of a plane parallel to the interface and the sphere, i.e., I=Io*c2/(2R)2, where R is the radius of the beams, causing an even gentler fall-off than moving a circular beam along a line. With D-shaped beams, a complete arc produces a dome-shaped, high-dose volume with a flat bottom and a hemispherical top. The flat bottom can shape a flat surface (i.e., interface) separating a target tissue from a non-target tissue, whereas the hemispherical top can shape a round surface. More generally, multiple coplanar beams of different orientations are used to create a high-dose gradient at the boundary of the target tissue and the non-target tissue. This is illustrated in FIG. 5a, which is a schematic drawing illustrating the projection outline of multiple intersecting D-shaped beams at different beam angles (α) and a fixed collimator angle (β=0), wherein the vertical arrow (↑) represents the central axis of a 0° beam and the other arrow represents the axis of rotation, and wherein the beams can be created with multiple sources of radiation or a single arcing beam. The flat edges of all of the D-shaped beams are coplanar, such that the axes of all beams intersect at the same point. The flat edges of the D-shaped beams form a plane, one side of which is always shielded and the other side of which has a high radiation intensity due to the overlapping of multiple beams. The curved side of the D forms a rounded edge of the high dose volume. Depending on the interface surface the beams try to shape, D-shaped beams have the flexibility of using different shapes of edges to match that of the interface. In contrast, FIG. 5b is a schematic drawing illustrating the projection outline of multiple intersecting cylindrical beams at different beam angles (α) and a fixed collimator angle (β=0), wherein the vertical arrow (↑) represents the central axis of a 0° beam and the other arrow represents the axis of rotation, and wherein the beams can be created with multiple sources of radiation or a single arcing beam. There is no flat edge as with the D-shaped beams. Conventional radiation treatments have been implemented by only rotating the radiation beam around a fixed point in the patient. This is illustrated in FIG. 6a, which is a schematic drawing illustrating the conventional rotation of a beam (Beam Positions 1, 2, and 3) around a fixed point or isocenter (“x”), which is adjacent to a non-target tissue (delineated by the circle). As shown in FIG. 6a, optimal sparing of non-target tissue is not possible with this arrangement when fixed collimators are used (as often are) in radiosurgery. In accordance with the method of the present disclosure, the motion of the patient positioning system is coordinated with the rotation of a radiation beam. When the radiation beam is D-shaped, the straight edge of the D-shaped beam is kept tangential to the boundary of the high-dose volume, thereby maintaining a sharp dose fall-off. This is illustrated in FIG. 6b, which is a schematic drawing illustrating that coordination of rotation of a beam with a D-shaped cross-section (Beam Positions 1, 2, and 3) around a fixed point or isocenter (“x”), which is adjacent to a non-target tissue (delineated by the circle), with movement of a patient (represented by the Non-Target Tissue Positions 1, 2 and 3). A sharp dose drop-off results at the interface between the target tissue and the non-target tissue, thereby protecting the non-target tissue without underdosing the target tissue. As shown in FIG. 6b, when the beam is in position 1, the non-target tissue is in position 1. When the beam is in position 2, the patient is moved such that the non-target tissue is moved from position 1 to position 2. Likewise, when irradiating the target tissue with the beam in position 3, the non-target tissue is moved to position 3. This arrangement allows the sparing of non-target tissue within and/or adjacent to (e.g., surrounding) the target tissue. As shown in FIG. 7, which is a schematic drawing illustrating that coordination of rotation of a beam (Beam Positions 1, 2, and 3) with movement of a patient, such that each beam position is associated with a different patient position, a high dose surface covering the target tissue (represented by the large oval) can be shaped. In view of the above method, also provided is a collimator, in particular a collimator with a fixed opening and divergence, which (a) shapes a beam of radiation to have a D-shaped cross-section, (b) maintains the central axis of the beam of radiation on or adjacent to the straight edge of the D-shaped cross-section of the beam of radiation, and (c) can fully rotate the beam of radiation in either direction about the beam axis, such that the straight edge of the D-shaped cross-section of the beam of radiation can face any direction. Such a collimator can be made by joining half of a circular collimator with a conical-shaped tunnel with half of a rectangular collimator with a pyramid-shaped tunnel. The two halves should have the same divergence. FIG. 12 shows half of a circular (cross-section) collimator with a cone-shaped tunnel and half of a rectangular (cross-section) collimator with a pyramid-shaped tunnel having the same divergence as the half of a circular (cross-section) collimator with a cone-shaped tunnel. The two halves can be joined to create a D-shaped collimator. Such collimators should be made from a high density material, preferably tungsten alloy. Different methods can be used for joining the two halves, such as, for example, welding, screw-tightening, clamping, and the like. Once they are joined together, the cross-section of such a collimator at every height level would show a D-shaped opening, formed by a half-circle and a narrow rectangle. The movement of the collimator can be coordinated with movement of the patient support system and rotation of the beam of radiation. As illustrated in FIGS. 3a-3d and 4a-4d, such a collimator can create a sharp dose gradient along a line as well as a “cold” hole surrounded by “hot” regions. This ability is analogous to a sharp knife, which can sculpt an intricate high-dose volume of complex shape. The collimator can rotate 360°, such that the flattened edge of the “D” can face all directions. By tangentially moving the straight edge along the direction of a linear interface, for example, such as an interface between a target tissue and a non-target tissue, the maximum dose is not moved away from the edge, thereby maintaining a sharp dose drop-off when multiple beams are overlapped or abutted, as is often the case in modern radiation treatments. The reason for this is that the straight edge is kept as close as possible to the axis of the radiation beam, which is where the beam of radiation is straight, thereby minimizing beam divergence. In contrast, movement of a circle along a linear interface results in the maximum dose being delivered at the center of the circle, which is at a distance away from the edge equal in length to the radius of the circle. When trying to preserve a non-target tissue within a target tissue, such as a urethra within a cancerous prostate, the straight edge of the “D” is kept tangential to the outer surface of the non-target tissue, i.e., the urethra in this example, such that the non-target tissue is always shielded and the beam is kept inside the target tissue, i.e., in the cancerous prostate in this example. Accordingly, a system for irradiating a target tissue in a patient is also provided. The system comprises (i) a patient support system, which comprises (a) a table or a couch, either of which is optionally padded, (b) one, two or three motors, each of which drives movement of the table or the couch in the direction of a separate axis, (c) optionally, a base, in which case the one or more motors can be housed in the base, and (d) a computerized control system, which can control the movement of the patient support system; (ii) at least one rotatable source of a beam of radiation, wherein each source of a rotatable beam of radiation can be rotated around a target tissue in a patient positioned on the patient support system; (iii) at least one collimator, wherein each collimator is operably aligned with one rotatable source of a beam of radiation; and (iv) a central control unit, which can execute a patient treatment plan including rotation of at least one rotatable source of a beam of radiation relative to a target tissue in a patient positioned on a patient support system, rotation of the at least one rotatable source of a beam of radiation, and movement of a patient support system. Preferably, the at least one collimator (a) shapes the beam of radiation to have a D-shaped cross-section, (b) maintains the central axis of the beam of radiation on or adjacent to the straight edge of the D-shaped cross-section of the beam of radiation, and (c) can fully rotate the D-shaped cross-section of the beam of radiation in either direction, such that the straight edge of the D-shaped cross-section of the beam of radiation can face any direction Also provided is a computer optimization method to plan dynamic radiation sculpting in accordance with the present disclosure. As with line arts, where the tangent lines to a two-dimensional or a three-dimensional surface can be determined analytically or numerically, the beam lines can be determined once the volume (and its surface) of the target tissue and the adjacent or internal non-target tissue(s) are known. When a radiation beam can traverse every point in the target from every direction, the number of parameters to optimize is enormous. Thus, points on the surface of the target tissue and, if present, its enclosed non-target tissue and other points inside the target tissue are treated separately. The surface of the target tissue and any non-target tissue in and/or adjacent to the target tissue are modeled as a connection of grid points in the computer. For each point on the surface of the target tissue, there are two unique beams from two opposing directions that can be tangential to the surface and with the straight edge of the “D” facing the surface. When the central axis of the beam is kept stationary such that the beam is tangential to the surface of the target tissue, it is easy to compute the unique corresponding coordinates of the patient support system for each of the beams. Because the beam parameters, including orientation, collimator angle, and the coordinates of the patient support system are relatively fixed for the surface points, the planning system can determine the control parameters for these points in a deterministic fashion. Inside the target tissue and in regions away from the surfaces, there is no restriction as to the beam direction, collimator angle, or where the straight edge of the “D” should intersect. The planning system has more freedom to determine these parameters as needed for creating the desired dose distribution. Because the radiation beam is straight at the beam axis and diverges away from the beam axis, in order to keep the beam edge sharp, the straight edge of the “D”-shaped collimator should stay on or adjacent to the central axis of the beam. One method of describing such a set of lines that define a curve or curved surface is using Bezier curves that are defined by a set of parametric equations. The basic Bezier curve is defined by four control points. However, this method can easily be extended to curves of higher degree. The parametric equation describing a Bezier curve is: b n ( t ) = ∑ i = 0 n b i ( n i ) t i ( 1 - t ) n - i where b0 is the first point and n is the number of points. A second degree Bezier function describes a curve, as illustrated in FIG. 8. FIG. 8 is a schematic drawing illustrating the parametric points bn(t) defining a hull at a convex surface, wherein “b” represents the point, “n” represents the index number of points, and “t” represents time. The time interval between two successive points is the irradiation time, which is optimized to create the desired dose distribution. In each case the parametric lines define a convex hull surrounding the curve but by definition not crossing into the function. A third degree function describing a surface and a fourth degree function can be used to model smooth motion. Bezier functions are often used to describe computer-generated surfaces. These functions can be used to define equidistant points on the surface. A simple strategy to define a sculpted path is to determine the tangent to these points with a margin normal to the tangent. FIG. 9 is a flowchart setting forth key steps of a treatment plan for dynamic sculpting in accordance with the present disclosure. The first task of planning the dynamic radiation sculpting is to find all such beams. The corresponding Bezier surface description is computed for the surface of the target tissue and the non-target tissue in and/or adjacent to the target tissue. A set of equidistant points induced by these Bezier surfaces are then used to determine the “D”-shaped beams, which are tangential to the surface. Each of these beams forms a control point, which defines the radiation beam angle, the orientation of the “D” collimator, and the patient support system coordinates. The points inside the target are “wild cards.” A beam needs to pass these points, but it can come from any direction. For a given beam direction, the coordinates of the patient support system also have some freedom. FIG. 10 is a schematic drawing illustrating the concept of using fixed (•) and “wild card” (×) points to determine treatment delivery. In this example, fixed points are placed at the surface of the target tissue (represented by the large oval) and at the surface of a non-target tissue (represented by the small circle) within the target tissue. The second task of planning the dynamic radiation sculpting is to find the motion path by connecting the control points. Motion constraints for the beam angle and movement of the patient support system are applied to connect all of the control points. Optimization algorithms, such as the algorithm for solving the “traveling salesman problem” in three-dimensional Euclidean space, is used to minimize the motion and to ensure that all control points are visited with minimal repeating of a traveled path. The “wild card” points inside the target can be used to connect the surface points when necessary. The weightings of each beam are optimized so that the dose of radiation is uniform within the surface boundaries. This optimization problem can be modeled as a constrained nonlinear programming and can be solved by using techniques such as simulated annealing. After sculpting the surfaces of the target tissue and non-target tissue(s), the most difficult task is completed. The optimization system then checks the resulting radiation dose distribution against the desired dose distribution. More control points can be added to make up the difference between the desired dose distribution and the delivered dose distribution. The motion path computed in the previous step is then adjusted accordingly. So far, the beam with a “D”-shaped cross section is regarded as emitted from one radiation source. It also can be from two or more sources. The use of two or more sources allows the dose rate to be shared by all of the sources. When using accelerated electrons to generate the radiation beams, this allows the tube current of each source to be smaller. When such sources are radioactive materials, the use of two or more sources also allows each of the sources to be smaller, resulting in a narrower penumbra and a sharper dose fall off at the edge of the beam. In view of the above, also provided is a method of planning irradiation of a target tissue in a patient with a system for irradiating a target tissue in a patient. The method comprises: (i) determining the volume and the surface contour of the target tissue to be irradiated and, if present, the volume and the surface contour of a non-target tissue located wholly within the target tissue and/or the surface contour and, optionally, the volume of a non-target tissue located partially within the target tissue, (ii) setting the radiation dose to be delivered to the target tissue and limiting the radiation dose to the non-target tissue, (iii) assigning control points to the surface contours identified in (i), (iv) determining the angle of the beam of radiation, the orientation of the collimator, and the position of the patient support system at each control point, (v) assigning “wild card” points within the volume of the target tissue with the proviso that a “wild card” point is not assigned within the volume of any non-target tissue that is located wholly or partially within the target tissue, (vi) determining the path of motion when all control points and, optionally, one or more “wild card” points, are connected and optimizing the weighting of each beam of radiation so as to provide a uniform dose of radiation within the target tissue and a sharp drop-off away from the boundary between the target tissue and any non-target tissue, and (vii) checking the resulting radiation dose distribution against a desired radiation dose distribution and adjusting the path of motion and the weightings of control points accordingly and, if needed, adding more control points. By “weighting” is meant the amount of time spent at a control point. When more time is to be spent at a control point, the control point has more weighting. When less time is to be spent at a control point, the control point has less weighting. By “sharp drop-off” is meant to have a high dose gradient where a large change in the radiation dose occurs over a short distance. For example, a change in the radiation dose from 90%, for example, to 50%, for example, over a distance of a few millimeters, such as 5 mm or less, is considered to be a sharp drop-off. The radiation dose to the non-target tissue can be limited by establishing maximum dose, mean dose, and doses allowed to certain percentages of the volume of the non-target tissue, for example. The following example serves to illustrate the present disclosure. The example is not intended to limit the scope of the claimed invention in any way. This example describes a simulation of the method in accordance with the present disclosure. For the simulation, a target shaped as half of a doughnut with a cylindrical “critical structure” located at the center hole of the doughnut is placed inside a larger cylinder of uniform density resembling the torso of the human body. The goal is to deliver as high a dose as possible to the entire target, while minimizing the dose to the critical structure and other normal tissues. FIGS. 11a and 11b show the isodose distributions resulting from computer-optimized treatment plans using beams collimated with a D-shaped collimator (FIG. 11a) and beams collimated with a traditional circular collimator. When the D-shaped collimators are used (FIG. 11a), a higher dose gradient is created at the boundary of the target tissue, as indicated by the closely spaced isodose lines around the target boundary. More than 95% of the target is covered by the 70% isodose line. The “hot spot” within the normal tissue anterior to the target and the critical structure receive 50% of the maximum dose. The critical structure receives the highest dose of 40%. When the traditional circular beams are used (FIG. 11b), 95% of the target is covered by the 55% isodose line. The “hot spot” within the normal tissue anterior to the target and the critical structure receive 90% of the maximum dose. The critical structure receives the highest dose of 45% of the maximum dose. In radiation treatments, the target must receive the prescribed dose. If both plans are to deliver the same prescription dose, the critical structure would receive 57% and 82% of the prescription dose resulting from the use of D-shaped beams and circular beams, respectively. The hot spot at the anterior region inside the torso would be 71% and 164% of the prescription dose resulting from the use of D-shaped beams and circular beams, respectively. This example illustrates the significant advantage of using D-shaped collimators in creating sharp dose gradients. All patents, patent application publications, journal articles, textbooks, and other publications mentioned in the specification are indicative of the level of skill of those in the art to which the invention pertains. All such publications are incorporated herein by reference to the same extent as if each individual publication were specifically and individually indicated to be incorporated by reference. The invention illustratively described herein may be suitably practiced in the absence of any element(s) or limitation(s), which is/are not specifically disclosed herein. Thus, for example, each instance herein of any of the terms “comprising,” “consisting essentially of,” and “consisting of” may be replaced with either of the other two terms. Likewise, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Thus, for example, references to “the method” include one or more methods and/or steps of the type described herein and/or apparent to those ordinarily skilled in the art upon reading the disclosure. The terms and expressions, which have been employed, are used as terms of description and not of limitation. In this regard, where certain terms are defined and otherwise described or discussed elsewhere herein, all such definitions, descriptions, and discussions are intended to be attributed to such terms. There also is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized that various modifications are possible within the scope of the claimed invention. Thus, it should be understood that, although the present invention has been specifically disclosed in the context of preferred embodiments and optional features, those skilled in the art may resort to modifications and variations of the concepts disclosed herein. Such modifications and variations are considered to be within the scope of the invention as defined by the appended claims. |
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abstract | The transmissivity of an fθ lens which is used as a means for converging laser light differs in the center and in the edge thereof. As a result, when the fθ lens is used as it is with the purpose of crystallizing by laser irradiation, energy distribution of the laser light which is irradiated on the semiconductor film is not uniform so that the whole surface of the semiconductor film could not be irradiated uniformly. Therefore, the present invention provides a laser irradiation apparatus including a galvanometer mirror and an fθ lens that can offset the change of the energy due to the change of transmissivity of the fθ lens and can scan the laser light while controlling the change of the energy on the object to be irradiated. Moreover, the invention provides a manufacturing method of a semiconductor device including the laser irradiation apparatus described above. |
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051805278 | description | DESCRIPTION OF THE PREFERRED EMBODIMENTS In the nuclear fuel pellets according to the present invention, a deposition phase of substances having high thermal conductivity is continuously present in the grain boundaries in the pellets. Thus, the thermal conduction in the pellets is efficiently performed through the continuous deposition phase. As a result, the average thermal conductivity of the pellets is increased, and then the distribution of temperature in the pellets becomes more uniform than that of the conventional nuclear fuel pellets. Further, the nuclear fuel pellets according to the present invention are manufactured in the following manner. Specifically, high-thermal conductivity substances, at least a part of which liquefies at a temperature near or below the sintering temperature thereof, are added to nuclear fission substances, and sintered. Thus, the high-thermal conductivity substances are melted into liquid when they are sintered. As a result, the thus liquefied substances are deposited in the grain boundaries of uranium oxide or mixed oxides, and become continuous grain layers after cooling. Moreover, the following substances are preferable as the above-described high-thermal conductivity substances. Specifically, they include beryllium oxide alone, or a mixture of beryllium oxide and at least one of or one oxide of titanium, gadolinium, calcium, barium, magnesium, strontium, lanthanum, yttrium, ytterium, silicon, aluminum, samarium, tungsten, zirconium, lithium, molybdenum, uranium, and thorium or an eutectic matter being obtained by heating the above mixture, whereby the melting point thereof being decreased. Hereinafter, nuclear fuel pellets of embodiments according to the present invention will be described with reference to the accompanying drawings. FIG. 1 is an enlarged schematic diagram illustrating one of nuclear fuel pellets, which is observed commonly in respective embodiments according to the present invention. In FIG. 1, a deposition phase 8 of substances having high thermal conductivity is continuously deposited in the grain boundaries of nuclear fission substances 7. First Embodiment Beryllium oxide (BeO) powder was added to uranium oxide (UO.sub.2) powder, and mixed therewith. The amount of beryllium oxide was 1.5 wt. % at a maximum (5.0 vol. % at a maximum) with respect to the total amount of the uranium oxide powder and the beryllium oxide powder. The thus mixed powder was molded by pressing with a pressure of about 2.5 through about 3.0 t/cm.sup.2, and a mold of about 50 through about 55% TD was obtained. The mold was sintered at about 2100.degree. C., the temperature being higher than the eutectic point thereof. As a result, pellets having an average grain diameter of about 110 through 160 .mu.m were obtained. In the process of sintering, at least a part of the pellets liquefied and covered at least half the grain boundaries. As the grain boundary-covering factor of the thus liquefied pellets increases, the thermal conductivity of the pellets increases monotonously. Next, the relative thermal conductivities of such pellets were measured varying the amount of beryllium oxide to be added. The measurements thereof are shown in FIG. 2. Second Embodiment Beryllium oxide powder and titanium oxide powder were mixed, and the mixture thereof was melted at a temperature higher than the eutectic point thereof, and then ground. The thus ground powder was mixed with uranium oxide powder, and molded by pressing with a pressure of about 2.5 through 3.0 t/cm.sup.2. Thereafter, the thus obtained mold was sintered in a reduction atmosphere at about 1700.degree. C., the temperature being higher than the eutectic point (about 1670.degree. C.). As a result, pellets having an average grain diameter of about 50 through about 110 .mu.m were obtained. In this embodiments, the beryllium oxide powder and the titanium oxide powder were added to the uranium oxide powder in the following proportions expressed by wt. % with respect to the total amount of the pellets: ______________________________________ Beryllium oxide powder Titanium oxide powder ______________________________________ 1.5 wt % 1.0 wt % 3.0 wt % 2.0 wt % 1.0 wt % 1.0 wt % 1.0 wt % 4.0 wt % 1.0 wt % 8.0 wt % ______________________________________ The thermal conductivities (K) of the pellets obtained by use of the above-described proportions were compared with the thermal conductivity (Ko) of conventional pellets. The comparison results are shown in FIG. 3. Third Embodiment Beryllium oxide powder and gadolinium oxide powder were mixed, and the mixture thereof was melted at a temperature high than the eutectic point thereof, and then ground. The thus ground powder was mixed with uranium oxide powder, and molded by pressing with a pressure of about 2.5 through about 3.0 t/cm.sup.2. Thereafter, the thus obtained mold was sintered in a weak oxidation atmosphere (moist hydrogen of an oxidation potential of about -300 kJ/mol) at about 1700.degree. C., the temperature being higher than the eutectic point (about 1500.degree. C.). As a result, pellets having an average grain diameter of about 15 through about 20 .mu.m were obtained. The thermal conductivity of the thus obtained pellets was about 1.11 through about 1.13 times that of the conventional pellets such that gadolinium oxide powder of about 10 wt. % was added to the uranium oxide powder (the comparison was made at a temperature of about 1000K). In this embodiment, the adding proportions of the beryllium oxide powder and the gadolinium oxide powder to the uranium oxide powder were about 1.5 wt. % and about 1.0 wt. %, respectively to the total amount of the pellets. Fourth Embodiment Beryllium oxide powder and silicon oxide powder were mixed, and the mixture thereof was mixed with uranium oxide powder. The thus mixed powder was molded by pressing with a pressure of about 2.5 through about 3.0 t/cm.sup.2. Thereafter, the thus obtained mold was sintered in a reduction atmosphere at about 1700.degree. C., the temperature being higher than the eutectic point (about 1670.degree. C.). Further, besides the above, beryllium oxide powder and silicon oxide powder were mixed, and the mixture thereof was melted at a temperature higher than the eutectic pint, and then ground. The thus ground powder was mixed with uranium oxide powder, and the mixture thereof was sintered in the same manner as above. As a result, pellets having an average grain diameter of about 40 through 50 .mu.m were obtained. The thermal conductivity of the thus obtained pellets was about 1.08 times that of the conventional pellets of uranium oxide (the comparison as made at a temperature of 1000K). In this embodiment, the beryllium oxide powder and the silicon oxide powder were added to the uranium oxide powder in the following proportions expressed by wt. % with respect to the total amount of the pellets: ______________________________________ Beryllium oxide powder Silicon oxide powder ______________________________________ 0.9 wt % 0.1 wt % 0.9 wt % 0.3 wt % ______________________________________ Fifth Embodiment Beryllium oxide powder and aluminum oxide powder were mixed, and the mixture thereof was mixed with uranium oxide powder. The thus mixed powder was molded by pressing with a pressure of about 2.5 through 3.0 t/cm.sup.2. Thereafter, the thus obtained mold was sintered in a reduction atmosphere at about 1900.degree. C. or about 2000.degree. C., the temperatures being higher than the eutectic point (about 1840.degree. C.). Further, besides the above, beryllium oxide powder and aluminum oxide powder was mixed, and the mixture thereof was melted at a temperature higher than the eutectic point, and then ground. The thus ground powder was mixed with uranium oxide powder, and the mixture thereof was sintered in the same manner as above. As a result, pellets of two different kinds were obtained. Specifically, the thermal conductivity of the pellets obtained by sintering at about 1900.degree. C. was about 1.08 times that of uranium oxide. Further, the thermal conductivity of the pellets obtained by sintering at about 2000.degree. C. was about 1.12 times that of uranium oxide (the comparison was made at a temperature of 1000K). In this embodiment, the beryllium oxide powder and aluminum oxide powder were added to the uranium oxide in the following proportions expressed by wt. % with respect to the total amount of the pellets: ______________________________________ Beryllium oxide powder Aluminum oxide powder ______________________________________ 0.9 wt % 0.1 wt % 0.9 wt % 0.3 wt % ______________________________________ Further, the average grain diameters in the case of 0.9 wt. % - beryllium oxide powder and 0.1 wt. % - aluminum oxide powder were as follows depending on the sintering temperatures; about 60 .mu.m when sintered at about 1900.degree. C., and PA0 about 110 .mu.m when sintered at about 2000.degree. C. PA0 about 90 .mu.m when sintered at about 1900.degree. C., and PA0 about 140 .mu.m when sintered at about 2000.degree. C. Moreover, the average grain diameters in the case of 0.9 wt. % - beryllium oxide powder and 0.3 wt. % - aluminum oxide powder were as follows depending on the sintering temperatures; Sixth Embodiment Beryllium oxide powder, titanium oxide powder and gadolinium oxide powder were mixed, and the mixture thereof was melted at a temperature high than the eutectic point, and then ground. (Besides this, the mixture thereof was not melted depending on conditions). The thus obtained powder was mixed with uranium oxide powder, and this mixed powder was molded by pressing. Thereafter, the thus obtained mold was sintered in a weak oxidation atmosphere. The average grain diameter was about 30 .mu.m, and the thermal conductivity of the thus obtained pellets was about 1.11 through 1.13 times that of the conventional pellets consisting of uranium oxide and gadolinium oxide. In this embodiment, the beryllium oxide powder, titanium oxide powder and gadolinium oxide powder were added to the uranium oxide powder in the following proportions expressed by wt. % with respect to the total amount of the pellets: ______________________________________ Beryllium oxide Titanium oxide Sadolinium oxide powder powder powder ______________________________________ 1.5 wt % 0.5 wt % 10 wt % 1.5 wt % 1.0 wt % 10 wt % ______________________________________ In all of the above-described embodiments, a part of the additive substance having high thermal conductivity is liquefied during the sintering. Further, at least a half of the grain boundaries is continuously covered with the thus liquefied high-thermal conductivity substance. As the grain boundary-covering factor of the liquefied substance increases, the thermal conductivity of the pellets increases monotonously. In all cases, when the pellets have the same density, the thermal conductivity thereof is increased in proportion to the increase of the amount of the additives. Further, even when a very small amount of additive (e.g., beryllium oxide of 0.3 wt. %) is added, high-density pellets can be obtained, and the thermal conductivity thereof can also be increased. Moreover, the relative densities of the molds with respect to the theoretical densities were about 50% TD. The relative densities of the thus obtained sintered pellets were about 95 through 99.7%. Furthermore, nuclear fuel pellets having the same advantages as above can also be obtained by use of the following high-thermal conductivity substances, a part of which or the entire of which is melted at a temperature near or below their sintering temperatures. Specifically, such substances include beryllium oxide alone, or a mixture of beryllium oxide and at least one of or one oxide of barium, calcium, magnesium, strontium, aluminum, lanthanum, yttrium, ytterbium, silicon, titanium, uranium, zirconium, tungsten, lithium, molybdenum, samarium, thorium, and gadolinium. As described above, according to the present invention, the thermal conductivity of nuclear fuel pellets can be significantly increased. Thus, the temperature in the center of the nuclear fuel rod can be reduced, whereby the discharge amount of gases generated on the nuclear fission can be efficiently reduced. Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. |
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047643390 | abstract | A high flux reactor is comprised of a core which is divided into two symetric segments housed in a pressure vessel. The core segments include at least one radial fuel plate. The spacing between the plates functions as a coolant flow channel. The core segments are spaced axially apart such that a coolant mixing plenum is formed between them. A channel is provided such that a portion of the coolant bypasses the first core section and goes directly into the mixing plenum. The outlet coolant from the first core segment is mixed with the bypass coolant resulting in a lower inlet temperature to the lower core segment. |
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claims | 1. A loading and unloading elevator for an autoclave sterilizer in a radioactive environment, the loading and unloading elevator comprising:a table top separating a processing space from a maintenance space;an elevation system positioned within the maintenance space;at least two cart rails configured to support a cart, the cart rails positioned within the processing space; anda plurality of loading elevator rails coupled to the cart rails, the loading elevator rails extend from the elevation system within the maintenance space through the table top to the processing space, wherein the loading elevator rails are configured to adjust the height of the cart rails. 2. The loading and unloading elevator of claim 1, wherein the elevation system is configured to adjust the height of the loading elevator rails and the cart rails. 3. The loading and unloading elevator of claim 2, wherein the elevation system comprises a platform coupled to the loading elevator rails, wherein the platform is configured to adjust the height of the loading elevator rails and the cart rails. 4. The loading and unloading elevator of claim 3, wherein the elevation system further comprises a motor coupled to the platform, wherein the motor is configured to adjust the height of the platform, the loading elevator rails, and the cart rails. 5. The loading and unloading elevator of claim 4, further comprising a plurality of bearings extending through the table top, wherein the bearings guide the loading elevator rails through the table top. 6. The loading and unloading elevator of claim 5, further comprising a plurality of bellow sleeves, each bellow sleeve surrounds one loading elevator rail and extends from the bearings to the cart rails. 7. The loading and unloading elevator of claim 6, wherein the elevation system is configured to adjust a height of the cart to a plurality of predetermined heights. 8. The loading and unloading elevator of claim 7, wherein the elevation system further comprises a programmable logic controller configured to control the motor, wherein the programmable logic controller is configured to adjust the height of the cart to each of the predetermined heights. 9. The loading and unloading elevator of claim 8, further comprising a worm drive, two right-angle gearboxes, and two screws, the worm drive being coupled to the motor and to the two right-angle gearboxes, each right-angle gearbox being coupled to one of the screws, the screws being coupled to the platform, wherein a motor turns the worm drive, the worm drive turns the two right-angle gearboxes, each right-angle gearbox turns one of the screws, and the screws adjust a height of the platform. |
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claims | 1. A nuclear reactor, comprising:a vessel that houses a core immersed in a primary fluid in the vessel for cooling the core, the core including a bundle of fuel elements that extend along respective parallel longitudinal axes and that are each provided with an active part, a head, and, between the active part and the head, expanders arranged radially in a direction perpendicular to the respective parallel longitudinal axes;wherein the expanders each include low thermal expansion elements, high thermal expansion elements alternating vertically with the low thermal expansion elements such that each high thermal expansion element is vertically interposed between two low thermal expansion elements, a terminal closing element covering the low thermal expansion elements and the high thermal expansion elements, and a fastening member axially securing the low thermal expansion elements and the high thermal expansion elements to the terminal closing element, the low thermal expansion elements being made of a first material having a first thermal expansion coefficient and the high thermal expansion elements being made of a second material having a second thermal expansion coefficient greater than the first thermal expansion coefficient;wherein the low thermal expansion elements alternately engage with the high thermal expansion elements to amplify radial expansion of terminal parts of the expanders which, when a predetermined temperature is exceeded, shift laterally relative to each other to space the fuel elements from one another and radially expand the core;wherein each fuel element of the bundle of fuel elements is provided with a plurality of expanders that project radially from a shaft of the fuel element and are angularly spaced around the respective parallel longitudinal axis of the fuel element; andwherein the low thermal expansion elements are Z-shaped and the high thermal expansion elements are in the shape of a parallelepiped. 2. The nuclear reactor of claim 1, wherein each head of the bundle of fuel elements includes multiple peripheral faces and each fuel element of the bundle of fuel elements is provided with a number of expanders equal to a number of the multiple peripheral faces of the head of the fuel element. 3. The nuclear reactor of claim 1, wherein for each fuel element of the bundle of fuel elements, radial expansion of the expanders flexes the shaft of the fuel element and spaces the active parts of the fuel elements, thereby expanding the core by radially moving respective feet of the fuel elements, positioned at respective lower axial ends of the fuel elements, while respective heads of the fuel elements, positioned at respective upper axial ends of the fuel elements, remain substantially stationary. 4. The nuclear reactor of claim 1, wherein expansion of the expanders spaces the fuel elements, thereby expanding the core by rotation of the fuel elements around respective feet of the fuel elements effective to radially move the respective feet, positioned at respective lower axial ends of the fuel elements, with respective heads of the fuel elements positioned at respective upper axial ends of the fuel elements and spaced from one another; said head of each one of the fuel elements being radially constrained by flexible containment elements. 5. The nuclear reactor of claim 1, wherein each one of the expanders extends perpendicular to the respective parallel longitudinal axis of the respective fuel element. 6. The nuclear reactor of claim 1, wherein each one of the expanders includes a terminal closing element that covers the low thermal expansion elements and the high thermal expansion elements and projects radially to an outside of the low thermal expansion elements and the high thermal expansion elements. 7. The nuclear reactor of claim 1, wherein:for each expander of the plurality of expanders, the shaft of each of the fuel elements includes a radial extension that projects radially from the shaft and has a radially external end and a radially internal end joined to the shaft opposite to the radially external end;the low thermal expansion elements include a first low thermal expansion element having an axially bent radially external terminal part engaged with the radial extension and a radially internal end;the high thermal expansion elements include a first high thermal expansion element having a radially internal end engaged with the radially internal end of the first low thermal expansion element and a radially external end engaged with a radially external terminal part of a second low thermal expansion element. 8. A nuclear reactor, comprising:a vessel that houses a core immersed in a primary fluid in the vessel for cooling the core, the core including a bundle of fuel elements that extend along respective parallel longitudinal axes and that are each provided with an active part, a head, and, between the active part and the head, expanders arranged radially in a direction perpendicular to the respective parallel longitudinal axes;wherein the expanders each include low thermal expansion elements, high thermal expansion elements alternating vertically with the low thermal expansion elements such that each high thermal expansion element is vertically interposed between two low thermal expansion elements, a terminal closing element covering the low thermal expansion elements and the high thermal expansion elements, and a fastening member axially securing the low thermal expansion elements and the high thermal expansion elements to the terminal closing element, the low thermal expansion elements being made of a first material having a first thermal expansion coefficient and the high thermal expansion elements being made of a second material having a second thermal expansion coefficient greater than the first thermal expansion coefficient;wherein the low thermal expansion elements alternately engage with the high thermal expansion elements to amplify radial expansion of terminal parts of the expanders which, when a predetermined temperature is exceeded, shift laterally relative to each other to space the fuel elements from one another and radially expand the core;wherein each fuel element of the bundle of fuel elements is provided with a plurality of expanders that project radially from a shaft of the fuel element and are angularly spaced around the respective parallel longitudinal axis of the fuel element; andwherein each one of the expanders includes a terminal closing element that covers the low thermal expansion elements and the high thermal expansion elements and projects radially to an outside of the low thermal expansion elements and the high thermal expansion elements. 9. The nuclear reactor of claim 8, wherein the low thermal expansion elements are Z-shaped and the high thermal expansion elements are in the shape of a parallelepiped. 10. The nuclear reactor of claim 8, wherein:for each expander of the plurality of expanders, the shaft of each of the fuel elements includes a radial extension that projects radially from the shaft and has a radially external end and a radially internal end joined to the shaft opposite to the radially external end;the low thermal expansion elements include a first low thermal expansion element having an axially bent radially external terminal part engaged with the radial extension and a radially internal end;the high thermal expansion elements include a first high thermal expansion element having a radially internal end engaged with the radially internal end of the first low thermal expansion element and a radially external end engaged with a radially external terminal part of a second low thermal expansion element. 11. A nuclear reactor, comprising:a vessel that houses a core immersed in a primary fluid in the vessel for cooling the core, the core including a bundle of fuel elements that extend along respective parallel longitudinal axes and that are each provided with an active part, a head, and, between the active part and the head, expanders arranged radially in a direction perpendicular to the respective parallel longitudinal axes;wherein the expanders each include low thermal expansion elements, high thermal expansion elements alternating vertically with the low thermal expansion elements such that each high thermal expansion element is vertically interposed between two low thermal expansion elements, a terminal closing element covering the low thermal expansion elements and the high thermal expansion elements, and a fastening member axially securing the low thermal expansion elements and the high thermal expansion elements to the terminal closing element, the low thermal expansion elements being made of a first material having a first thermal expansion coefficient and the high thermal expansion elements being made of a second material having a second thermal expansion coefficient greater than the first thermal expansion coefficient;wherein the low thermal expansion elements alternately engage with the high thermal expansion elements to amplify radial expansion of terminal parts of the expanders which, when a predetermined temperature is exceeded, shift laterally relative to each other to space the fuel elements from one another and radially expand the core;wherein each fuel element of the bundle of fuel elements is provided with a plurality of expanders that project radially from a shaft of the fuel element and are angularly spaced around the respective parallel longitudinal axis of the fuel element;wherein for each expander of the plurality of expanders, the shaft of each of the fuel elements includes a radial extension that projects radially from the shaft and has a radially external end and a radially internal end joined to the shaft opposite to the radially external end;wherein the low thermal expansion elements include a first low thermal expansion element having an axially bent radially external terminal part engaged with the radial extension and a radially internal end;wherein the high thermal expansion elements include a first high thermal expansion element having a radially internal end engaged with the radially internal end of the first low thermal expansion element and a radially external end engaged with a radially external terminal part of a second low thermal expansion element. 12. The nuclear reactor of claim 11, wherein the low thermal expansion elements are Z-shaped and the high thermal expansion elements are in the shape of a parallelepiped. 13. The nuclear reactor of claim 11, wherein each one of the expanders includes a terminal closing element that covers the low thermal expansion elements and the high thermal expansion elements and projects radially to an outside of the low thermal expansion elements and the high thermal expansion elements. |
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description | 1. Field of the Invention The present invention relates to a radiation source, lithographic apparatus, and a device manufacturing method. 2. Related Art A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern on an impinging light beam. In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one pass, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”—direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. The size of features that can be imaged by a lithographic projection apparatus is limited by the wavelength of the radiation used. To image smaller features requires a shorter wavelength and so UV, deep UV (DUV) or extreme UV (EUV) radiation is used. The wavelengths that can be used are limited by the sources available, for example, HG vapor lamps for UV, excimer lasers for DUV, and plasma discharge sources for EUV. Semiconductor light sources, such as LEDs and laser diodes, are extremely efficient light sources that are in widespread use in many fields, but as yet there is no such device useful to provide exposure radiation for lithography. The wavelength of the light output by most current LEDs is determined by the specific semiconductor band structure in the pn-junction. The application of an electrical supply to the junction causes electrons to be injected into the n-type region where they occupy the conduction band of this region (e.g., higher energy bands), as the valence band is full. As the occupancy of this band increases electrons will also be pushed into the conduction band of the p-type region. However, the valence band of the p-type region has some vacancies, so electrons will fall into these lower energy states, emitting light of frequency characteristic of the band-gap in the p-type region in order to conserve energy. Another type of device produces radiation from a pn-junction using the avalanche effect. The avalanche effect is a process whereby a reverse bias is applied to a suitably doped pn-junction diode. Electrons tunnel across the forbidden depletion region and multiply rapidly, unless strictly controlled. The usual result is thermal emission in the infra-red region. Further information regarding such physical processes can be obtained from: www.tpub.com/neets/book7/26.htm, which is incorporated by reference herein in its entirety. Relevant information can also be found in “Light Emission In Silicon in the Visible Range From Nanoscale Diode Anti-fuses” by V E Houtsma et al., Proceedings from the 3rd International Workshop on Materials Science, which is incorporated by reference herein in its entirety. This discusses the use of reverse biasing to create avalanche breakdown and subsequent emission of visible light in pn-junctions. Therefore, what is needed is a new radiation source that is useful in lithography, as well as, lithography apparatus and device manufacturing methods using the source. According to an embodiment of the invention, there is provided a radiation source for use in lithography. The radiation source comprises a pn-junction disposed on a substrate that can be reverse-biased to cause avalanche breakdown and emission of UV or DUV radiation by deceleration of electrons accelerated into the n-type region of the pn-junction. This embodiment of the present invention also provides a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, the lithographic apparatus comprising a radiation source. The radiation source comprising a pn-junction disposed on a substrate that can be reverse-biased to cause avalanche breakdown and emission of UV or DUV radiation by deceleration of electrons accelerated into the n-type region of the pn-junction. A further embodiment of the present invention provides a lithographic apparatus comprising an illumination system, a support structure, a substrate table, and a projection system. The illumination system provides a beam of radiation. The support structure supports a patterning device. The patterning device imparts the beam with a pattern in its cross-section. The substrate table holds a substrate. The projection system projects the patterned beam onto a target portion of the substrate. The illumination system comprises a radiation source and an illumination optical system arranged to project an image of the radiation source onto a pupil plane of the projection system. The support structure is arranged to support the patterning device at a plane that is a Fourier transform of the plane of the radiation source. A still further embodiment of the present invention provides a lithographic apparatus comprising a radiation source, a substrate table, and a projection system. The radiation source serves as patterning means and comprises a plurality of selectively addressable elements, each element comprising a pn-junction disposed on a substrate that can be reverse-biased to cause avalanche breakdown and emission of UV or DUV radiation by deceleration of electrons accelerated into the n-type region of the pn-junction. The substrate table holds a substrate. The projection system projects an image of the radiation source onto a target portion of the substrate. According to a still further embodiment of the invention, there is provided a device manufacturing method comprising projecting a patterned beam of radiation onto a substrate. The beam of radiation is generated by reverse-biasing a pn-junction disposed on a substrate to cause avalanche breakdown and emission of UV or DUV radiation by deceleration of electrons accelerated into the n-type region of the pn-junction to generate the beam. FIG. 1 schematically depicts a lithographic apparatus, according to one embodiment of the invention. The apparatus comprises an illumination system IL, a support structure MT, a substrate table WT, and a projection system PL. The illumination system (illuminator) IL is configured to condition a radiation beam B (e.g., UV radiation or DUV radiation). The support structure (e.g., a mask table) MT is constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters. The substrate table (e.g., a wafer table) WT is constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W in accordance with certain parameters. The projection system (e.g., a refractive projection lens system) PL configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. The support structure MT supports, i.e., bears the weight of, the patterning device MA. It holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in a target portion C of the substrate W. It should be noted that the pattern imparted to the radiation beam B may not exactly correspond to the desired pattern in the target portion C of the substrate W, for example, if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam B will correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit. The patterning device MA may be transmissive or reflective. Examples of patterning devices MA include, but are not limited to, masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types, such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix. The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including, but not limited to, refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system.” As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask). The lithographic apparatus may be of a type having two (e.g., dual stage) or more substrate tables WT (e.g., two or more mask tables). In such “multiple stage” machines the additional tables WT may be used in parallel, or preparatory steps may be carried out on one or more tables WT, while one or more other tables WT are being used for exposure. The lithographic apparatus may also be of a type wherein at least a portion of the substrate W may be covered by a liquid having a relatively high refractive index (e.g. water) so as to fill a space between the projection system PL and the substrate W. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask MA and the projection system PL. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems PL. The term “immersion” as used herein does not mean that a structure, such as a substrate W, must be submerged in liquid, but rather only means that liquid is located between the projection system PL and the substrate W during exposure. With reference again to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus maybe separate entities, for example when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source SO may be an integral part of the lithographic apparatus, for example when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system. The illuminator IL may comprise an adjuster AD for adjusting an angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (e.g., σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator IL can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator IL may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section. After passing through the illumination IL, the radiation beam B is incident on the patterning device MA, which is held on the support structure MT. The radiation beam B is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PL, which focuses the beam onto the target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, capacitive sensor, etc.), the substrate table WT can be moved accurately, so as to position different target portions C in the path of the radiation beam B, for example. Similarly, the first positioner PM and another position sensor (not shown) can be used to accurately position the mask MA with respect to the path of the radiation beam B, after mechanical retrieval from a mask library, or during a scan, for example. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (e.g., coarse positioning) and a short-stroke module (e.g., fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper, as opposed to a scanner, the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (e.g., scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies. The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure. 2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously, while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion C in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion C. 3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device MA, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. In this mode, generally a pulsed radiation source SO is employed and the programmable patterning device MA is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device MA, such as a programmable mirror array of a type as referred to above. Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. FIG. 2 is an energy level diagram of a reverse-biased pn-junction that is used as the radiation source SO, in one embodiment of the present invention. Such a radiation source SO has a low operating voltage, a high switching speed, and provides great design freedom. High intensity can be provided by the use of large or multiple sources, for example. The pn-junction can be doped with impurities to increase emission of radiation at a desired frequency, and hence increase the efficiency of the device. For protection, the pn-junction may covered by a layer of transparent oxide. By reverse biasing the pn-junction with a potential difference of at least 4V, or in one example 5V, radiation of wavelength 300 nm or less can be obtained. In one example, the potential difference should be as high as can be tolerated by the device so as to produce radiation of as short a wavelength as possible. A filter for selecting a desired range of wavelengths from the radiation emitted by said pn-junction may be used to create a (quasi-)monochromatic source. In one example, the radiation source SO comprises a plurality of selectively addressable elements, each element comprising a pn-junction as hereinbefore defined. This provides a spatially addressable light source SO, which may be used for control of the illumination of a lithographic projection apparatus or as a patterning means MA in a lithographic apparatus. In this embodiment, the pn-junction is shallow and covered by a thin layer of transparent oxide ox, e.g., with a thickness Tox of about 20 nm. The n-type region n is nearer the surface. When the junction is reverse-biased by a voltage source (not shown) with a potential difference of about 5V, broadband radiation is emitted. This occurs because there is an avalanche breakdown leading to electrons being accelerated into the n-type region where they decelerate and emit “bremstrahlung” (braking radiation). The emitted radiation is a continuous spectrum with a lower cut-off wavelength determined by the energy of the electrons, as follows: λ min = hc U = 1.24 * 10 - 6 U ( 1 ) where λmin is the lower cut-off wavelength, c is the speed of light, h is Planck's constant and U is the potential difference across the junction. Thus, in one example a potential difference of 5V can generate radiation at 248 nm. In one example, by including appropriate impurities in the junction, peaks in the emission spectrum can be created, so that the efficiency of emission at a desired wavelength can be increased. In one example, filters can be used to select a desired wavelength band for use to form the beam. In various examples, the power of the device can be increased by increasing its area and/or by the use of an array of sources SO. The latter arrangement also enables power control by selective application of some or all sources SO in the array. In one example, electrode and addressing schemes used in displays can be used to selectively activate individual sources in the array. FIG. 3 shows an array of sources SO used to allow for control of an angle of illumination impinging on a patterning means MA, according to one embodiment of the present invention. In the Kohler illumination arrangement, the illumination system IL is arranged to image a secondary light source onto a pupil plane of the projection system PL. The secondary light source is located in a plane of the illumination system IL, which is a Fourier transform of the mask plane, which in turn is the object plane of the projection system PL. With such an arrangement, the angular distribution of the radiation illuminating the mask MA is determined by the spatial distribution of the light in the secondary light source. Various means, such as zoom-axicons, masks, diffractive optical elements, and optical fiber guides, have been used to define the spatial distribution of light in the secondary light source. According to this embodiment of the invention, a light source SO formed by an array of individually switchable reverse-biased pn-junctions as described above, each pn-junction forming in effect a pixel, is placed at the object plane of the illumination system IL and the mask (or other patterning means) MA is placed at a pupil plane of the illumination system IL. The image plane of the illumination system IL is arranged to be coincident with a pupil plane of the projection system PL. In effect, Kohler illumination is achieved with the primary light source, formed of an array of reverse-biased pn-junctions, at a plane that is a Fourier transform of the mask plane. Any desired angular illumination distribution a at the mask MA can therefore be obtained by selective energizing of pixels of the light source SO having a spatial distribution p. The angular distribution a can therefore be set to any desired amount, allowing optimization of illumination for every exposure with no throughput loss, nor the delay and expense required for the manufacture of custom diffractive optical elements. FIG. 4 shows an array of reverse-biased pn-junctions used as a patterning device MA, according to one embodiment of the present invention. The patterning device MA consists of an array of individually addressable pixels, each formed by a reverse-biased pn-junction arranged to emit UV or DUV radiation, as described above. Each junction can be regarded as a pixel. By selective activation of the pixels of patterning device MA, a mask pattern can be displayed on the patterning device MA through a contrast between pixels that are ON, i.e., emitting radiation, and those pixels that are OFF, not emitting radiation. This image can then be projected onto the substrate W to expose a radiation-sensitive layer thereon with the mask pattern. The projection system PL may have a magnification of ¼ or ⅕ or less so that a pattern having a minimum feature size of less than about 250 nm can be projected from a patterning means MA with a pixel size of about 1 μm2 or less. If reduction of the mask pattern is not necessary, the projection system PL may be omitted and the lithography apparatus configured as a simple contact, or near contact printer. Although specific reference maybe made above to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. For example, other applications can be, but are not limited to, the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein may be processed, before or after exposure in, for example, a track (e.g., a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool, and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers. Although specific reference may be been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications. Another application can be, but is not limited to, imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured. The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5–20 nm), as well as particle beams, such as ion beams or electron beams. The term “lens,” where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g., semiconductor memory, magnetic, or optical disk) having such a computer program stored therein. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. |
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claims | 1. A method of constructing a 4D tilt series of images of a moving object, the method comprising:(a) irradiating the moving object with an optical pulse;(b) directing an electron pulse to impinge on the moving object;(c) detecting the electron pulse;(d) processing the detected electron pulse to form an image of the moving object;(e) capturing the image of the moving object associated with a time step; and(f) repeating (a)-(e) to construct the 4D tilt series of images of the moving object. 2. The method of claim 1 wherein the time step is on a scale of nanoseconds. 3. The method of claim 1 wherein the electron pulse consists of a single electron. 4. The method of claim 1 wherein the electron pulse comprises a plurality of electrons. 5. The method of claim 1 wherein the moving object comprises a nanomechanical cantilever. 6. The method of claim 1 wherein the moving object comprises a biological entity selected from at least one of a cell, a protein, a nucleic acid, a virus, or a complex thereof. 7. The method of claim 1 wherein the 4D tilt series of images is a tomographic tilt series of images capturing a portion of the moving object. 8. The method of claim 1 further comprising measuring a difference between each image in the 4D tilt series of images at a microscale level. 9. The method of claim 1, further comprising measuring a difference between each image in the 4D tilt series of images at a nanoscale level. |
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abstract | A method is provided for operating a nuclear reactor. The method includes operating the nuclear reactor for at least one plutonium equilibrium cycle during which the core contains plutonium-equilibrium nuclear fuel assemblies; subsequently, operating the reactor for transition cycles, at least some of the plutonium-equilibrium nuclear fuel assemblies being progressively replaced with transition nuclear fuel assemblies and then with uranium-equilibrium nuclear fuel assemblies; and then operating the nuclear reactor for at least one uranium equilibrium cycle. |
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description | This is a continuation of application Ser. No. 14/524,660, filed Oct. 27, 2014, and claims the benefit of GB 1319001.2, filed on Oct. 28, 2013, both of which are incorporated herein by reference. The present invention relates to image guided radiation therapy (IGRT) apparatus. More particularly the invention provides an IGRT apparatus in which components can be conveniently and efficiently maintained and repaired. Radiation therapy is a localised treatment designed to treat an identified tissue target (such as a cancerous tumour) and spare the surrounding normal tissue from receiving doses above specified tolerances thereby minimising risk of damage to healthy tissue. Prior to delivery of radiation therapy, an imaging system can be used to provide a three dimensional image of the target from which the target's size and mass can be estimated and an appropriate treatment plan determined. Many factors may contribute to differences between the dose distribution determined in the treatment plan and the delivered dose distribution. One such factor is an inconsistency between the patient position at the imaging stage and the patient position in the radiation treatment unit. Image guided radiation therapy (IGRT) is known. The method involves the use of an imaging system to view target tissues whilst radiation treatment is being delivered to the target tissue. IGRT incorporates imaging coordinates from the treatment plan to ensure the patient is properly aligned for treatment in the radiation therapy device. Various medical imaging technologies are used to identify target tissues in radiation therapy planning and IGRT. These include (without limitation); Computed Tomography (CT), Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI). MRI is ideal for on-line position verification during radiotherapy, it is able to make fast 2D images of soft tissues with orientation along and perpendicular to the field axis, allowing imaging at critical locations which are defined during the treatment planning procedure. MRI also provides excellent contrast between tissue types giving a sharp image of the target. The Applicant's prior published international patent application no. WO03/008986 describes a device for use in IGRT which includes the functions of an MRI device in a radiation therapy treatment apparatus and proposes technology for overcoming the problems in doing so. The large scale of these combined devices will be appreciated. Such devices are typically of the order of 2-3 meters in diameter and they weigh several tons. It will be appreciated they cannot be easily transported or manoeuvred for maintenance and repair. In accordance with the present invention there is provided an image guided radiation therapy apparatus comprising a medical imaging device integrated with a linear accelerator, the linear accelerator configured for emitting a radiation beam which is shaped by a beam shaper, wherein the position of the beam shaper is adjustable between a first position and a second position wherein the first position is a treatment position and the second position is a non-treatment position. The inventors have recognised that the cumbersome proportions of a combined medical imaging and radiation therapy treatment device present maintenance and repair engineers with a challenge in accessing component parts of the device for service. Preferably, the second position is a service position. Preferably, the IGRT apparatus comprises a gantry and the first position is within the gantry and the second position is removed from the gantry. Whilst the beam shaper component must be carefully aligned for treatment, its position within the gantry is inconvenient for the purposes of servicing. As one of the more complex components of the apparatus, it is important the beam shaper can be readily accessible for maintenance purposes. The provision of adjustment means for adjusting the position of the beam shaper enables the beam shaper to be moved between an operational position and a service position, the service position being much more easily accessible for the service engineer. Optionally, the device provides for a range of servicing positions. Preferably, the position of the linear accelerator is moveable with the beam shaper between the first and second positions. The radiation beam emitter of the linear accelerator is transported around the target tissue by means of a gantry. When in operation, the beam shaper must be positioned adjacent the radiation beam emitter within the gantry. The beam shaper is typically a multi-leaf collimator (MLC). MLC comprises multiple inter-engaging metal leaves which can each be moved independently by means of multiple electro-mechanical positioning mechanisms. The medical imaging device is desirably an MRI device. Since such devices generate a very strong magnetic field it is advantageous to distance a predominantly metal component such as an MLC from the magnetic field for servicing since any ferromagnetic material in the MLC will be drawn to the magnetic field potentially resulting in damage or disassembly of sub-components. Adjustment of the beam shaper position can be achieved by means of an adjustment arm to which the beam shaper is mounted and a linkage connecting the adjustment arm to a fixed body and operable to move the beam shaper between the first and second position. For example, the fixed body might be the wall, floor or ceiling of a room in which the IGRT apparatus is installed. Alternatively the fixed body might comprise a support beam fixed to any of the walls, floor or ceiling of a room in which the IGRT apparatus is installed. In another alternative, the fixed body is a gantry or framework of the IGRT apparatus itself. The adjustment arm may be adjusted manually or by a mechanical or electro-mechanical actuation means. In some embodiments the linkage comprises a pivot operable to pivot the beam shaper from a position within the gantry to a position removed from the gantry. In such embodiments, the beam shaper can be caused to travel through a simple arc from the first to the second position and desirably also in reverse from the second back to the first position. The pivot can comprise a simple hinge located at or adjacent a first end of the adjustment arm distal from a second end to which the beam shaper is mounted. Actuation means may comprise any conventional mechanical or electro-mechanical means; for example (but without limitation) a hydraulic or pneumatic system may be used. In another alternative, one or more electrically operated actuators may be used. The actuators may comprise rotary actuators, linear actuators or a combination thereof. The actuation means may further comprise a gearing system to facilitate leverage of the heavy components. Actuation means may be removably attachable to the adjustment arm or may form an integral part of the arm. In more complex embodiments, the linkage may comprise a multi-axis joint. A multi-axis joint allows the arm to be pivoted as described above but also permits rotation of the adjustment arm about its own axis allowing greater manoeuvrability in positioning the beam shaper for servicing. The adjustment arm may be provided with one or more joints operable to present the beam shaper in an increased number of positions and orientations. Joints in the adjustment arm may comprise simple hinge joints, multi-axis joints or any combination thereof. The adjustment arm may incorporate a linear actuator allowing the length of the arm to be adjusted thereby providing further flexibility in the positioning of the beam shaper when removed from the gantry. A variety of potential configurations for IGRT apparatus in accordance with the invention are possible. For example, the imaging device could be an MRI device of an open ring configuration or a drum configuration. An open system may require more sophisticated engineering but may provide benefits to the subject in providing for less intimidating, more comfortable treatment. In such a design, an open ring MRI system is integrated with a rotating linear accelerator mounted on an additional ring. The additional ring may also support a beam stopper and a megavoltage imaging system. Preferably the apparatus is configured such that magnetic imaging device and the linear accelerator may be operated both independently and simultaneously. Desirably the magnetic imaging device and linear accelerator are arranged to share an isocentre. A preferred and probably more economical design solution may use a dosed drum design based on the conventional drum MRI design. Active or passive magnetic shielding in the integrated system may provide a minimal field strength at the mid plane around the MRI magnet. This shielding can prevent magnetic distortion of the accelerator tube and will also assist in minimising disturbance of the other accelerator systems in the dose proximity of the MRI system. Inclusion of the shielding results in a system necessarily of wider diameter than a conventional system and thus in a larger distance between isocentre and focus. Alternatively the magnets can be designed in order to minimise the field strength at the point(s) in space where the accelerator will operate. Preferably, the IGRT apparatus comprises a light source for projecting a light beam through the beam shaper when the beam shaper is in the second position. This can be used to confirm configuration and correct working of the beam shaper during servicing. For example, the light beam can be projected onto a ceiling or other surface. Preferably, the linear accelerator is positioned such that the path of radiation from the linear accelerator to a patient is linear. Put another way, it is preferred that the path of radiation is not bent; rather the radiation path passes directly from the linear accelerator, through the beam shaper and to the patient. An embodiment of the invention will now be described with reference to the accompanying figures. As can be seen from FIG. 1, an integrated device is provided in a dosed drum arrangement which comprises an inner MRI portion 1 and an outer gantry portion 2 which incorporates the linear accelerator having a head including a radiation gun 3, acceleration tube 4 and an X-ray emission target (not shown). The resultant radiation is used to bombard a target 6 in a body 7 contained in the bore of the gantry portion 2. A beam shaper 8 uses data from the MRI to focus the radiation beam emitted by the linear accelerator onto the target 6. The body 7 is introduced to and guided through the isocentre on a sliding table 9. In use, the gantry is rotated about the isocentre to enable bombardment of the target 6 from multiple directions. The table 9 may also be tiltable to expose the target 6 to the direct line of the emitted beam in another plane. FIG. 2 shows two orthogonal views of an adjustment arm 18 which comprises a mechanical arm 10 having a first end 11 which can be connected to a framework in the gantry of a suitably arranged IGRT apparatus by means of a pivotal linkage passing through a bore 16 passing through the end 11. A second end of the arm 10 embodies a housing 12 which houses an MLC having a leaf driving section 13 and multiple leaf section 17. An actuator 14 is operable to cause the mechanical arm 10 to rotate about a pivot point at the centre of bore 16. The assembly is powered by cabling 15. FIG. 3 shows schematically an adjustment arm 18 pivotally mounted in the gantry portion of an IGRT apparatus of substantially similar design to the apparatus shown in FIG. 1. When it is desired to access the MLC 17 for maintenance or repair, the gantry is rotated to position the MLC at an appropriate height for the service engineer. One suitably positioned, the actuator 14 is operated to rotate the mechanical arm about the pivotal linkage through bore 16 allowing the assembly to be tilted outside of the outer circumference of the gantry 2 and the MLC revealed to the engineer in a safe and convenient position a good radial distance from the magnetic field present in the MRI portion 1. Desirably the assembly is lockable in position in the gantry when radiation treatment is being delivered. This may be achieved by incorporating a locking mechanism into the actuator or linkage. In an alternative, a lockable panel is provided on an outer circumference of the gantry 2 for containing the assembly 18 during delivery of radiation treatment. With reference to FIGS. 4a and 4b, the IGRT apparatus may include a light source 19 for projecting a light beam 20 through the beam shaper when the beam shaper is in the second position. This can be used to confirm configuration and correct working of the beam shaper during servicing. For example, the light beam 20 can be projected onto a detector 21 which is connected to calibration circuitry 22. With particular reference to FIG. 4b, the light source 19 is bounced off a mirror 23 so that it follows the same path as radiation from the radiation source 24 through the leaves 25 of the MLC. As will be evident from FIG. 4b, the radiation source is provided in line with the subject to be treated such that there is no need for a bending magnet. Other embodiments and simple design variations of the embodiments disclosed herein will no doubt occur to the skilled addressee without departing from the true scope of the invention as defined in the appended claims. |
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047145846 | abstract | A device for use in a nuclear reactor which includes a pressure vessel having a vessel head, a core in the vessel, elements for controlling the reactivity of the core, drive rods which pass through the vessel head for displacing the elements, and a plurality of head adapters which pass through the vessel head, each head adapter forming part of a drive rod housing enclosing a respective drive rod, each housing enclosing a region which communicates with the interior of the vessel and which is closed at the top. The drive is associated with a respective housing and constitutes a housing extension including a component for connecting the device to the head adapter forming part of the respective housing, and a component for forming, around the associated drive rod and within the associated housing, a fluid passage having a cross-sectional area not exceeding a selected value at least upon the occurrence of a leak in the respective housing. |
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