Abstract:
An apparatus for generating medical isotopes provides an annular fissile solution vessel surrounding a neutron generator. The annular fissile solution vessel provides for good capture of the emitted neutrons and a geometry that provides enhanced stability in an aqueous reactor. A neutron multiplier and/or a neutron moderator may be used to improve the efficiency and control the criticality of the reaction in the annular fissile solution vessel.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     CROSS REFERENCE TO RELATED APPLICATION 
     BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a system for generating isotopes useful for medical purposes, such as Mo-99, 1-131, Xe-133, Y-90, Cs-137, 1-125, and others and in particular to a system employing an annular aqueous fissile solution vessel. 
         [0002]    Medical isotopes are employed in nuclear medicine where they may be administered to a patient in a form that localizes to specific organs or cellular: receptors where they may be imaged with special equipment. Medical isotopes may also be used in the treatment of disease i exploiting the tissue-destructive power of short-range ionizing radiation after such localization. 
         [0003]    Today, most radioisotopes used in nuclear medicine are produced in nuclear reactors employing highly enriched uranium (HEU). The reactors used for the production of Mo- 99  for the United States are outside of the United States requiring the export of HEU and an attendant risk of nuclear proliferation associated with such out-of-country shipments. 
         [0004]    It has been proposed to generate medical isotopes using low enriched uranium (LEU) which cannot be used directly to manufacture nuclear weapons. Systems for this purpose are described in US patent applications: 2011/0096887 entitled: “Device and Method for Producing Medical Isotopes” and 2010/0284502 entitled: “High Energy Proton or Neutron Source” hereby incorporated by reference. 
         [0005]    In these systems, ions are directed through a target volume holding a gas to generate neutrons. The neutrons may expose a parent material held in solution near the target volume in a fissile solution vessel. In one embodiment the target volume is annular and placed around a cylindrical fissile solution vessel holding the parent material solution. Ions are injected in a spiral through the target volume producing neutrons directed inwardly toward the parent material and outwardly toward a reflector. 
         [0006]    Neutrons received in the neutron rich parent material (such as LEU uranium) experience a multiplication in which neutrons striking the parent material generate additional neutrons which strike additional neutron rich material in a chain reaction. In a nuclear reactor, at steady power, the effective neutron multiplication factor (k eff ) is equal to 1. In a subcritical system, k eff  is less than 1. 
         [0007]    One problem with aqueous reactors is that it can be difficult to maintain a stable power level. This is because there exists strong feedback mechanisms in the neutron multiplication factor as the temperature of the fissile solution rises and as voids are generated (gas bubbles caused by radiolysis breaking water into hydrogen and oxygen). The rapid reduction in the neutron multiplication factor results in a decrease power, which causes the neutron multiplication factor to increase again. In particular, a control system that is trying to maintain constant power in the reactor may not be able to react sufficiently fast to adequately control the system. The result is a system with an unstable power level and potential safety impacts. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides an improved geometry for a fissile solution vessel used to generate medical isotopes. Specifically, the fissile solution vessel is a limited thickness annulus holding an aqueous suspension of a parent material. By controlling the aspect ratio of the annulus, improved reaction stability may be obtained over a conventional cylindrical chamber. In addition, enhanced cooling is possible by employing a cooling jacket on the inner wall of the annular solution vessel. 
         [0009]    Specifically then, one embodiment of the present invention provides a nuclear reaction system having an annular solution vessel for holding an aqueous suspension of nuclear material having an inner wall defining a central opening extending along an axis. A first and second cooling jacket are in thermal communication with the inner wall of the annular solution vessel and an opposed outer wall of the annular solution vessel. 
         [0010]    It is thus an object of at least one embodiment of the invention to provide improved stability to a reaction vessel by increasing the heat transfer area to volume through the use of an annular configuration. 
         [0011]    The annular reaction container may contain low enriched uranium. 
         [0012]    It is thus a feature of at least one embodiment of the invention to provide an apparatus for producing medical isotopes without the risks attendant to handling HEU. 
         [0013]    The annular solution vessel may contain a mixture of water and at least one of uranyl nitrate, uranyl sulfate, uranyl fluoride or uranyl phosphate. 
         [0014]    It is thus a feature of at least one embodiment of the invention to provide a system that may use a variety of different fissile solutions. 
         [0015]    The nuclear reaction system may include a particle source positioned to direct charged particles into a target material proximate to the annular solution vessel for generation of neutrons from the target material to be received in the annular reaction container; and the target material may be contained within a target chamber centered within the central opening and receiving particles along the axis. 
         [0016]    It is thus a feature of at least one embodiment of the invention to provide a target that may be wholly contained within the fissile solution vessel and that will readily produce neutrons passing into the annular fissile solution vessel after excitation by ions generated externally. 
         [0017]    The medical isotope generator may include a neutron multiplier and or moderator material positioned between the target chamber and the inner wall. 
         [0018]    It is thus a feature of at least one embodiment of the invention to convert excess neutron energy obtainable with the ion collision mechanism into additional neutrons. It is another feature of at least one embodiment of the invention to provide for moderation of neutron speeds through a collisional process and thus better control the reaction rate within the annular chamber. 
         [0019]    The medical isotope generator may further include a reflecting material concentrically outside of the annular solution vessel. 
         [0020]    It is thus a feature of at least one embodiment of the invention to permit placement of a reflector outside of the annular fissile solution vessel at a location that allows an arbitrary thickness of material to be employed. 
         [0021]    The aspect ratio defined by a radial thickness of the annular solution vessel perpendicular to the axis to a height of the annular solution vessel along the axis may be substantially greater than 0.1 or between 0.1 and 0.3 or between 0.12 and 0.25. 
         [0022]    It is thus a feature of at least one embodiment of the invention to provide dimensions to the annular chamber realizing an improvement over a cylindrical chamber or other aspect ratio annular chambers. 
         [0023]    The nuclear material may be low enriched uranium having a concentration between 10 and 450 grams of low enriched uranium per liter solution. 
         [0024]    It is thus an object of the invention to provide a reaction system that may work with low concentrations of nuclear materials. 
         [0025]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a perspective view in partial cutaway of one embodiment of the present invention showing concentric chambers including an annular fissile solution vessel assembly around a central cylindrical target chamber, the latter receiving ions along an axis of the cylinder and annulus; 
           [0027]      FIG. 2  is a cross-sectional view of the annular fissile solution vessel assembly including an inner neutron multiplier surrounded by an annular fissile solution vessel, the latter flanked by water cooling jackets; 
           [0028]      FIG. 3  is a simplified elevational cross-section of an alternative embodiment of the invention in which the annular reaction assembly includes sections that cover the top and bottom of the target chamber as well as the sides; 
           [0029]      FIG. 4  is a perspective view in cutaway aligned over a top plan cross-section of yet another alternative embodiment employing a hexagonal annulus and isolated aqueous parent material; and 
           [0030]      FIG. 5  is a top plan cross-section of an alternative embodiment of the invention using a separated neutron multiplier and neutron moderator; 
           [0031]      FIG. 6  is a plot of reactivity change versus aspect ratio of the annular fission solution vessel of  FIG. 2  showing a region of improved stability for the annular fission solution vessel over a cylindrical fission solution vessel; and 
           [0032]      FIG. 7  is a plot of reactivity change versus low enriched uranium concentration for an annular and a cylindrical fission solution vessel. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0033]    Referring now to  FIG. 1 , a medical isotope generator  10  of the present invention may provide a set of nested annular elements including an outer annular reflector chamber  12  surrounding and coaxial with an annular reactor assembly  14 . A cylindrical target chamber  16  fits within the annular reactor assembly  14  so that all three elements of the annular reflector chamber  12 , annular reactor assembly  14 , and target chamber  16  share a common central axis  18 . 
         [0034]    The outer annular reflector chamber  12  may be taller than the annular reactor assembly  14  to provide a substantially equal thickness of reflecting material around the annular reactor assembly  14  in a direction perpendicular to central axis  18 , and above and beneath the annular reactor assembly  14  in directions along central axis  18 . In this embodiment, the annular reactor assembly  14  may be substantially equal in height to the target chamber  16 . 
         [0035]    The target chamber  16  may be a vertically oriented cylindrical shell extending along axis  18  and defining a cylindrical volume that will be charged with a target gas  20 , for example tritium. The cylindrical volume of the target chamber  16  communicates through a vertically extending conduit  22  upward through the outer annular reflector chamber  12  to an ion injector  24  positioned above the target chamber  16  and outside of the outer annular reflector chamber  12 . 
         [0036]    The ion injector  24  is positioned to direct a beam of ions  26 , for example deuterium (D + ), vertically along axis  18  through the conduit  22  into the target chamber  16 . The height of the target chamber  16  along axis  18  and the pressure of the target gas  20  are adjusted to ensure substantially complete collision of the ions with the tritium in the target chamber  16 . In one embodiment the target gas  20  may have a pressure of approximately 10 Torr and having a height within the target chamber  16  along axis  18  of approximately 1 meter. 
         [0037]    The ion injector  24  incorporates an ion source  28  which, in one embodiment, is a cavity receiving deuterium gas through valve  30  to be ionized, for example, by microwave emissions, ion impact ionization, or laser ionization. A generated beam of ions  26  (for example at a rate of approximately 50 milliamperes) passes into an accelerator  32  accelerating the ion beam along the axis  18 . The accelerator, for example, may be an electrostatic accelerator providing 300 kilovolts of acceleration of the ions. 
         [0038]    The beam of ions  26  then passes through a set of baffle chambers  38  bridged by differential pumps  34 . The differential pumps  34  operate to preserve a low-pressure of approximately 50 micro-Torr in the accelerator  32  while permitting the higher 10 Torr pressure in the target chamber  16 . In one embodiment this system employees three pumps  36  each drawing gas from a higher baffle chamber  38  (toward the accelerator  32 ) and pumping it to a lower baffle chamber  38  (toward the target chamber  16 ). The baffle chambers  38  communicate through relatively small openings along axis  18 , for example one centimeter in diameter, to allow passage of the beam of ions  26  while reducing leakage of the tritium in the target chamber  16   
         [0039]    The gas streams through the pumps  36  may be cooled by a moderator fed by chilled water (not shown). The upper pumps may, for example, be turbo pumps operating at less than 5×10 −5  Torr and 5-10 milliTorr respectively, for example, commercially available from Varian, Inc. having offices in Lexington, Mass. The lower pump may be a roots blower, for example, of a type commercially available from Leybold Vacuum Products Inc. having offices in Export, Pennsylvania. Cold traps, getter traps and palladium leaks may be used to remove atmospheric and/or hydrocarbon contaminants from the pump gases. 
         [0040]    The beam of ions  26  strikes target gas  20  in the target chamber  16  to produce neutrons  40  that pass radially and axially outward from the target chamber  16 , for example, with deuterium (D + ) striking the tritium to produce  4 He and a 14.1 MeV neutron. This reaction is predicted to produce approximately 5×10 13  neutrons per second for a 50 milliampere beam of ions  26 . 
         [0041]    Contamination of the target gas  20  by the ions of the beam of ions  26  and helium may be reduced by a purification system  42  such as the Thermal Cycling Absorption Process (TCAP) system developed by the Savanna River National Laboratory (SRNL). 
         [0042]    In an alternate embodiment, the ions may be replaced by electrons and the target chamber may contain a bremsstrahlung converter and photonuclear material such as uranium from which neutrons are produced. 
         [0043]    Referring now to  FIG. 2 , the beam of neutrons  40  from the target chamber  16  may pass into the annular reactor assembly  14 . The annular reactor assembly  14  includes an initial one-centimeter thick (in a radial direction) annular water jacket  44  receiving circulated chilled light water provided through one or more conduits  51  from an external water chiller, recycler. The annular water jacket  44  is followed by a coaxial annular neutron multiplier/moderator  46 , the latter being in one embodiment an aluminum-clad beryllium metal that multiplies fast neutrons passing outward from the target chamber  16  and moderates fast neutrons traveling inward from the annular fissile solution vessel  50  (to be described) by “cooling” those neutrons, a process that reduces their speed in exchange for an increasing of the temperature of the neutron multiplier/moderator  46 . The excess heat of the neutron multiplier/moderator  46  is removed by water jackets  48  and  44  which allow control the temperature of the neutron multiplier/moderator  46  to ensure the escape of sufficient neutrons from the target chamber  16  while moderating neutrons received from the annular fissile solution vessel  50 . Alternatively, the neutron multiplier/moderator  46  may be constructed of depleted uranium or other similar material. The neutron multiplier/moderator  46  may provide 1.5-3.0 multiplication factor such as may be adjusted by adjusting its thickness. 
         [0044]    Neutrons emerging from the neutron multiplier/moderator  46  pass through a second annular chilled water jacket  48  similar to water jacket  44  and then into annular fissile solution vessel  50 , the latter having walls comprised, in one embodiment, of zircaloy-4. The annular fissile solution vessel  50  includes a solution  52  of a parent material such as Uranyl Nitrate or Uranyl Sulfate in a light water solution. The solution  52  contains nominally 19.75 percent  235 U and thus is low enriched uranium (LEU). 
         [0045]    Production of the desired  99 Mo isotope occurs by fission of  235 U in the solution  52  which also produces additional neutrons. 
         [0046]    Solution  52  may be extracted from the annular fissile solution vessel  50  via one or more conduits  54  where the desired isotopes may be chemically extracted from the fissile solution. These isotopes may be purified via the LEU-modified Cintichem process to provide a source of the desired medical isotopes, particularly  99 Mo. The fissile solution may be cleaned using the UREX process to extend the useable lifetime of the solution. Access conduits  54  also allow control of the height of the solution  52  for control of the reaction as well as initial filling, subsequent drainage, and flushing of the annular fissile solution vessel  50 . The access conduits  54  also allow introduction and removal of nitrogen for space filling and for feed makeup for water, fissile solution, and pH control (when using uranyl nitrate). 
         [0047]    Concentrically surrounding the annular fissile solution vessel  50  is another water jacket  56  similar to water jackets  48  and  44  having chilled light water circulating therein. 
         [0048]    Outside of the annular reactor assembly  14  is the annular reflector chamber  12 , for example being an aluminum walled chamber filled with a reflector material  60  which in one embodiment may be heavy water having a volume, for example, of 1000 liters. The reflector material  60  increases the generation efficiency by reflecting neutrons back into the annular fissile solution vessel  50  and therefore may also permit reaction control by draining the annular reflector chamber  12  and thus reducing the neutron reflection into the annular fissile solution vessel  50 . Control of the reaction rate may also be had by changing the height of solution  52  in the annular fissile solution vessel  50 . 
         [0049]    Referring now to  FIGS. 2 and 6 , during operation, thermal energy generated by the fission reactions causes solution  52  to rise in temperature, for example, from 20 degree Celsius to 60 degrees Celsius and can promote the generation of voids formed by radiolysis of hydrogen or oxygen or from other gases such as ammonia and NO x  (in the case of use of uranyl nitrate) as well as krypton and xenon produced by fission. Generally, these gases are diluted by nitrogen fill and drawn off for processing. 
         [0050]    The increase in temperature and the formation of voids can significantly reduce the neutron multiplication factor k eff  in the chamber  50 . This effect, however, is reduced by the annular form of the annular fissile solution vessel  50  as compared to cylindrical chamber of similar volume. 
         [0051]    As shown generally in  FIG. 6 , a calculated reactivity curve  70  as a function of aspect ratio for the annular chamber  50  shows a lower magnitude reactivity change (values closer to zero in the chart) for the annular volume  50  then a comparable reactivity curve  72  for a cylindrical volume at aspect ratios above approximately 0.11. Lower magnitude of reactivity change equates to a desirable improved stability of the reaction system. 
         [0052]    The aspect ratio is the radial thickness of the volume  50  divided by the height of the volume  50 . Reactivity change is change in neutron multiplication factor k (i.e., Δk)divided by k. Generally it will be therefore desirable that the volume  50  have an aspect ratio of between 0.1 and 0.3 and alternatively between 0.12 and 0.25 or substantially greater than 0.15. 
         [0053]    Referring now to  FIG. 7 , a calculated reactivity curve as a function of concentration of low enriched uranium shows an improved stability within the range of 102-450 grams of low enriched uranium per liter of solution when compared to a cylindrical chamber, finding acceptable operating concentration within this range. It is believed that this data can be extrapolated to indicate an acceptable operating range from 10-450 grams of low enriched uranium per liter of solution. 
         [0054]    Referring now to  FIG. 3 , the top and bottom of the target chamber  16  also may be surrounded by the neutron multiplier/moderator  46  and portions of the annular chamber  50  for improved efficiency in capturing neutrons  40 . It will thereby be understood that the term annular should be understood to include an annulus having an upper and lower solid base. 
         [0055]    Referring now to  FIG. 4 , it will further be appreciated that the annular chamber  50  need not be a cylindrical annulus but may take on other annular shapes such as a polygonal annulus  80  having an inner and outer periphery providing a polygonal cross-sectional such as a hexagon. Further, the solution  52  within the annular fissile solution vessel  50  need not be homogenously distributed, but may be, for example, contained within separate reactant columns  84 , for example, passing in a serpentine path through the water bath of the annular fissile solution vessel  50 . Such reactants columns can further provide reduced thermal resistance and moderate the effect of voids. 
         [0056]    Referring now to  FIG. 5 , the neutron multiplier/moderator  46  of  FIG. 2  may desirably be split into two components, the first being primarily a neutron moderator  92 , for example, constructed of beryllium or the like as described above, and positioned coaxially inside the water jacket  48  and coaxially outside the water jacket  44  both previously described. In this embodiment, a separate neutron multiplier  90  may be positioned coaxially within the water jacket  44 , constructed, for example, of and cooled both by its contact with water jacket  44  coaxially surrounding the neutron multiplier  90  and a water jacket  94  coaxially within the neutron multiplier  90  and surrounding the target chamber  16 . The separation of functions allows independent temperature control of the neutron moderator  92  and the neutron multiplier  90  as well as constructing these components of different materials (if desired) and tailoring their thicknesses to the particular roles they play. 
         [0057]    The temperature of the water jacket  44  and  94  may be monitored by temperature probes  96  and  98  and provided to a feedback control system  100  controlling intake valves  102  and  104  for the water jackets  44  and  94  respectively (outlet valves not shown). The valves  102  and  104  may control the circulation of chilled water within the water jackets  44  and  94  thereby controlling the temperature of the neutron moderator  92  and its effect in moderating the nuclear reaction. The feedback controller  100  may control the temperature of the water jackets  44  and  94  to a predetermined value or to a dynamic value based on a monitoring of the general reaction rate by other means. In addition the feedback controller  100  may manage other control variables such as control of height of the solution  52  to moderate the reaction rate. 
         [0058]    Generally, the medical isotope generator  10  will be further shielded with concrete and water according to standard practices. Other isotopes such as  131 I,  133 Xe, and  111 In may also be produced by a similar structure. 
         [0059]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0060]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0061]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.