Patent Number: 042343836
Section: description

Referring specifically to FIG. 2, a nuclear reactor is indicated generally at 10, said reactor having a graphite core 11 and a surrounding heat exchange unit 12 provided with inlet and outlet means 13 and 14, respectively, for the heat exchange fluid. A body 15 of molten tin containing dissolved uranium and fission product metals is contained in the core, while a critical uranium nitride mass 16 rests on the core bottom. The space 17 above the molten tin solution contains a nitrogen or nitrogen-argon atmosphere, and slidably mounted control rods 18 project into the reactor core. A portion of the molten tin solution 15 is continuously or intermittently withdrawn through line 20 for passage through heat exchanger 21 and then through line 22 to a cleanup zone 23 wherein fission product impurities are removed, as shown in FIG. 3, before the solution is returned to the reactor through line 24. If desired, the cleanup zone 23 can be bypassed by returning the recycle stream through lines 25 and 24 directly to the reactor. A line 26 leading from line 24 into the bottom portion of the core permits part or all of the recycle stream to be directed into the lower portion of the molten tin solution to provide any degree of agitation between the uranium nitride particles and the surrounding molten tin solution, if desired. New uranium fuel or fertile fuel material can be added to the system via line 27 which discharges into the recycle stream in line 24. The nitrogen atmosphere above the molten tin, supplied through line 32, can be continuously or intermittently withdrawn via line 30 for passage through a gaseous fission product removal zone 31 before being returned to the reactor through said line 32 along with any required makeup nitrogen supplied through line 33 and any argon or other inert gas employed through line 34. A vent gas line 35 leads from the zone 32 through an absorber 36 and into the atmosphere. If desired, the gas leaving the reactor through line 30 need not be passed through zone 31 but can be directed through lines 30' and 32 back to the reactor, reinforced with additional nitrogen and/or argon, as required. Referring now more particularly to FIG. 3, there is shown a unitary system which is adapted to remove metal fission product impurities from a plurality of spent uranium fuels including uranium and uranium oxide, as well as from the uranium-tin recycle stream removed from reactor 10 through lines 20 and 23. Spent uranium oxide fuel enters the system through line 38 for discharge into a graphite vessel 40 containing a body 41 of molten tin as supplied through lines 42 and/or 69', said body being heated by a reboiler 43 and agitated by a stirring device 44. Nitrogen entering unit 40 through line 45 reacts with uranium oxide and a portion of the fission products present to convert the uranium and said impurities to solid nitride particles which fall to the bottom of the unit as indicated at 46. The nitrogen gas which passes unreacted through the molten tin 41 sweeps with it the carbon monoxide formed during the reaction and passes from vessel 40 through line 47 and absorber 48 to the atmosphere. The solid nitrides 46, along with molten tin containing dissolved fission product metals, pass through line 49 to a refractory, inert vessel 50. Also directed into the vessel 50, through line 23 as noted above, is the molten tin recycle stream containing dissolved uranium and a variety of fission product impurities, and perhaps some solid UN particles. (If no such solid particles are present, the recycle stream in line 23 can be introduced directly into vessel 60.) Vessel 50, heated by a reboiler 51 and provided with a stirrer 52, operates under a relatively high vacuum and at temperatures above 1485.degree. C. The nitrogen evolved from the metal pool 53 (as nitrides present therein are decomposed to uranium and fission product metals dissolved in the tin) is exhausted through line 54. The molten metal 53 is withdrawn through line 55 for discharge into a graphite vessel 60. At this point it is appropriate to note that spent uranium fuel may be introduced into the system through line 56 which also discharges into vessel 60. Nitrogen is supplied to said vessel through line 57, and the temperature and nitrogen pressure maintained in the vessel are such as to induce precipitation of UN, as indicated at 61. The unused nitrogen gas introduced into vessel 60 is discharged to the atmosphere through line 62. The solution 63 of molten tin in vessel 60, now free of dissolved uranium but containing dissolved fission product metals, is removed from the vessel through line 64 along with any solid zirconium nitride shown at 65 as floating on the solution 63. The solid UN particles, now free of fission product impurities, are removed from vessel 60 through line 66 by appropriate mechanical means (not shown), and said UN product may either be returned to the reactor through lines 67 and 24, along with molten tin supplied through line 69, or it may be passed to succeeding portions of the unit, i.e., to vessel 80 for conversion to a uranium-in-tin solution or to vessel 90 for conversion to pure uranium, as discussed below. The fission product-containing molten tin solution discharged into vessel 70 is given a vigorous, relatively high pressure nitriding treatment therein by means of nitrogen supplied through line 71, thereby converting most of the fission product metals present to insoluble nitrides. Some of the latter float on the molten tin body 75, as indicated at 72, while others drop to the bottom of said body, as indicated at 73. All solid nitrides present are then withdrawn from the system through line 74 for ultimate disposal. The unreacted nitrogen supplied to vessel 70 is withdrawn through line 77 and, after passing through absorber 78, is discharged to the atmosphere. The remaining molten metal 75, which contains trace amounts of fission product impurities, is then passed via line 76 to vessel 100 where, by means of reboiler 101, the solution is heated sufficiently to distill the tin overhead through line 69, while the fission products remaining are withdrawn as bottoms through line 103 for disposal. The tin vapors in line 69 are condensed by passage through a condenser 104, and the resultant molten tin is passed through line 69 back to the reactor 10 via line 24, though if desired it can be directed in whole or in part through line 69' to vessel 40, thereby supplementing or replacing the tin supplied to this portion of the assembly through line 42. The UN particles, as passed through line 66 into an inert refractory vessel 80, encounter therein a mass of molten tin. This portion of the apparatus is kept under vacuum, with the nitrogen evolved from the tin solution being withdrawn through line 82. The ensuing solution 81 of uranium in molten tin can be passed via line 83 to another inert, refractory vessel 90 where the mass 91 of molten metal contained therein is heated by reboiler 92 to distill the tin vapors overhead through line 93. Said vapors are cooled to the molten state by passage through a heat exchanger 94 and the molten tin is then returned to vessel 80. The pool 95 of pure uranium which collects at the bottom of vessel 90 is withdrawn through line 95 for future use either as such or in the form of its oxide. Alternatively, the uranium-in-tin solution 81 in vessel 80 can be recycled back to the reactor via lines 84 and 24, this being the preferred method for returning the uranium removed from the reactor via lines 20 and 23 back to the reactor 10. In presenting FIGS. 2 and 3 of the drawing, conventional items of equipment such as instrumentation, pumps and certain valves, as well as liquid handling and liquid-solid separating means, have been omitted in the interests of simplicity and of a more ready understanding of the invention. Further, while the present invention has been described with respect to particular embodiments thereof, it is not intended to limit the invention to details of description and illustration. Reference is made to the appended claims for a precise definition of the invention.