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I -F??TI O08'w –Gçanooog –Ggan Feb. 27, 1962 L. BURRIS, JR., ETA 3,023,097 REPROCESSING URANIUM DIOXDE FUES Filed Nov. 23, 1959 2 Sheets-Sheet I-F??TI OOOG–ggan O08’w–gçan no?opa8 (g?20%OG) INVENTOR. Les lie 8urris, Jr. Af r e di S c h n e i der Attorney Feb. 27, 1962 L. BURRIS, JR., ETA 3,023,097 REPROCESSING URANIUM DIOXDE FUELS Filed Nov. 23, 1959 2 Sheets-Sheet 2 - 4O - 5 ? - 6 O - 8O - 9 O - OO - , ? - 2 O - 3 O Nd 2O Gnd Sm2O3 - 4 O - 150 Pr2O3 and Ce2O3 O 3OO 5OO IOOO 5OO 2OOO 25 OO Temperature (K) F I Leslie Burris,INVENTOR. Jr. Alfred Schneid er BY ????? C???-e Attorney - 3,023,097 United States Patent Office Patented Feb. 27, 1962 2 of the dioxide fuel and its commingled substances in prep 3,323,097 aration for the next step about to be described; this size REPROCESSING GUARNUM DIXE FUELS Lesie Burris, Jr., and Alfred Schneider, Naperville, H., reduction is best carried out by successive oxidations and aSsigi nors to the United States of America as rere reductions which cause crumbling by reason of the suc sented by the United States Atomic Energy Cons cessive changes in crystal structure. After the particle ?$$??? size has been reduced sufficiently, a molten reducing i Fied Nov. 23, 1959, Ser. No. 854,989 metal of the group consisting of zinc and cadmium is 4 Claims. (C. 75-84.) introduced to the crumbled mixture, and this causes quite a number of metals to go into the metallic state, where The invention relates to a novel pyrometalurgical O upon they alloy with the reducing metal and the molten method of reprocessing uranium dioxide fuels after they metal phase may then be separated from the solid oxide have become contaminated by ª fission products in i nu phase consisting of the oxides of metals such as uranium, clear reactors, and more particularly, to a novel method rare earths and plutonium which are still not reduced. of removing plutonium and rare earth fission products The molten metal phase may be decanted, filtered or from such fuels. 5 otherwise separated in a manner known to the art. As is well known, uranium dioxide is presently becom Among the metals which are reduced and may be sepa ing widely used as a fuel in nuclear reactors, both in its rated in this manner are antimony, palladium, rhodium, naturally occurring condition and when "enriched' by the molybdenum not removed by the previous steps, and the addition of the dioxide of its 235 isotope. After a a number of others. reactor has been in operation for a period of time, the 20 None of the procedures so far described are effective fuel becomes depleted and, of more practical importance, in separating the rare earth fission products or plutonium contaminated with fission products, some of which have from uranium, both of which separations are necessary comparatively large neutron absorption cross-sections, so for successful fuel reprocessing. None of these will that they interfere with, or "poison,' the nuclear reaction volatilize alone at any practicable temperature, nor is it to such an extent that their removal becomes necessary. 25 possible to volatilize them in the form of any compounds For these reasons reactors are shut down periodically, attainable under practicable pyrometallurgical condi their fuel removed, and fresh fuel substituted; the used tions. So far as reducing them is concerned, their large fuel then must be reprocessed in order to recover its un free energies of formation rule out reduction by either depleted portion as well as the fission products of eco zinc or cadmium, and while theoretically they could be nomic value, and the transmutation products, one of 30 reduced by metallic calcium, this metal is actually too which plutonium-239, is also fissionable by thermal neu effective for the purpose since it reduces all metal oxides, trons and may be used as a fuel. including those of uranium, indiscriminately, thereby de Up to the present, most reactor fuel reprocessing has feating the primary aim of achieving separation. been carried out by “wet methods' in which the fuel and It is the object of our invention to provide a process its commingled fission and transmutation products are 35 whereby uranium dioxide fuels may be metallurgically dissolved in some aqueous solvent such as nitric acid, reprocessed after being depleted in nuclear reactors. and the resulting solution is separated into its components It is a further object to provide a process whereby by various chemical methods such as selective precipita oxides of the rare earth elements or plutonium may be tion, solvent extraction, ion exchange and the like; while metallurgically separated from uranium dioxide and other not altogether unsatisfactory, such methods leave much 40 metallic oxides. to be desired for a number of reasons, both technical and It is a further object to provide a process whereby the economic. Because of the radioactive nature of the oxides of plutonium, lanthanum, samarium, i praseo solutes the solutions must be kept very dilute in order to dymium, yttrium, neodymium and cerium may be metal avoid boiling of the water, decomposition of the organic lurgically separated from uranium dioxide. solvents and ion exchange resins, emulsification between 45 It is a further object of the invention to provide a aqueous and organic liquids, and other effects due to heat method of reprocessing uranium dioxide fuels without and radiation. This dilution requirement results in ex the use of aqueous solvents. tremely large quantities of solutions to be handled, with All the foregoing objects are attained by our discovery consequent high costs for equipment and shielding, since that, contrary to what might have been expected from all operations with the radioactive materials involved 50 available thermodynamic data, in a system where the must be carried out by remote control or in "glove boxes.” solvent is a molten metal of one or more of the group If a method could be worked out for the reprocessing consisting of zinc and cadmium, the oxides of plutonium, of reactor fuel without the need for dealing with such the rare earth fission products, barium and strontium, large volumes of fluid, a very great saving, could be made are reducible to the metallic state by uranium metal it by the reduction of the cost of shielding as well as in 55 self, with the result that an alloy of these elements and other respects. 3. the solvent metal separates into a discrete phase from Most attempts, however, to reprocess uranium dioxide the other oxides. The molten metal phase may then be fuels by other than wet methods have not been satisfac removed, leaving the uranium dioxide relatively free of tory. As might be expected, pyrometallurgical methods those elements which have thermal neutron cross-sections are quite successful in remoiving the more volatile fission 60 of sufficient size to make them objectionable, so that it products such as the gases krypton and xenon, the non can be returned to the reactor for an indefinite number metals such as iodine and bromine, and metals with low of re-runs. boiling points such as cesium and rubidium. This can - In the drawings: be done by heat alone, and thereafter a number of other fission products can be removed by introducing oxygen 65 FIG. 1 is a flow sheet drawing showing the process of so as to form volatile oxides and other volatile com our invention; and pounds which are then removed by heat. In this second FIG. 2 is a graph showing the free energies of forma category are ruthenium, selenium, technetium, tellurium, tion in kilocalories per gram-atom of combined oxygen of and molybdenum; the removal of molybdenum is not a number of oxides plotted against temperatures in de always complete. After the removal of all the fission 70 grees Kelvin. products which will volatilize, either alone or with the In order to illustrate the thermodynamic aspects of the help of oxygen, it is necessary to reduce the particle size invention, attention is directed to FIG. 2 which is a Super 3,028,097 3 4 imposed graph of certain curves taken from the first three group consisting of zinc and cadmium have been reduced. graphs appearing in the United States Atomic Energy The last step may be followed optionally by separating Commission publication, ANL-5750, "The Thermochemi the metal phase from the unreduced oxide phase and then cal Properties of the Oxides, Fluorides, and Chiorides to taking a fresh amount of molten solvent metal to carry 2500 K.” The free energies of formation of a number 5 out the reduction of plutonium and rare earth oxides of oxides in kilocalories per gram-atom of combined by means of uranium as a reducing agent. However, it is oxygen are plotted as ordinates against abscissae of tem more economical to let the same metal from the group peratures in degrees Kelvin; as can be seen, the most easily mentioned serve the dual purpose of a reducing metal for reducible oxides such as AgaO and Hg2O, which go to the those oxides reducible by it, and as a solvent metal for metallic form merely on mild heating, have the smallest O the reduction by means of uranium of the invention. The negative free energies near the top of the graph, whereas two steps may be carried out in the same vessel simultane at the opposite extreme, the curve for CaO is nearest to ously and the order in which the oxide mixture, the dual the bottom. By thermodynamic principles only a metal purpose metal and the uranium is introduced is not impor of an oxide lower in the scale is capable of effectively tant from a theoretical point of view, although from an reducing an oxide above it; thus calcium metal could be 5 economic point of view it is sometimes advisable to intro expected to be a powerful reducing agent, as it is, whereas duce the dual purpose metal first and allow a sufficient silver and mercury are weak reducing agents because of time to elapse to permit it fuily to reduce all oxides which the small free energies of their oxides, the term “free it is capable of reducing.
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