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Home , Uranium dioxide

Feb. 27, 1962 L. BURRIS, JR., ETA 3,023,097 REPROCESSING 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 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 and 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 have become contaminated by ª fission products in i nu phase consisting of the of metals such as uranium, clear reactors, and more particularly, to a novel method rare earths and 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 , , , naturally occurring condition and when "enriched' by the 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 , 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, , , i praseo solutes the solutions must be kept very dilute in order to dymium, , and may be metal avoid boiling of the , 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 . 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, and , 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 . This can - In the drawings: be done by heat alone, and thereafter a number of other fission products can be removed by introducing 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 , , , , 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 and 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. In this way, the amount of energy' as used herein being understood to be of negative costly uranium required may be held to a minimum, but sign as is the customary thermodynamic practice. The 20 on the other hand, if cost is less of an object, it is more ability of zinc, already referred to, to reduce such fission convenient to introduce the dual purpose metal and the products as antimony is quite in accord with thermo uranium together. dynamic theory as indicated by the graph, and the theoreti The reducing steps of our invention are preferably cal possibility of using calcium to reduce the rare earths carried out at a temperature of 700-800° C., which is also is also evident, although, as explained above, this has dis the preferred temperature for the reductions by means of advantages that make it too impractical to put to use. the dual purpose metal; these temperatures are not critical To be sure, at temperatures of about 500 K, and in a strict sense, but thermodynamic considerations make above, the free energy curve for CeO2 is above that of this the most practical range for achieving the reactions in UO2, which suggests the possibility of using uranium metal question within a reasonable period of time. Vigorous to reduce CeO2; this is quite correct and an initial reduc 30 mechanical agitation should be provided to insure thor tion readily takes place according to the equation: ough contact between the molten metals and the oxide aSS in order to expedite the reducing reactions of both Once this reaction is completed, however, any further the dual purpose metal and the uranium, it is advisable, reduction would not be expected since the sesquioxide 35 as above stated, to reduce the particle size of the oxide product has a free energy curve well below that of mixture before hand. This can best be done by suc uranium dioxide at all temperatures as can be seen by cessive oxidations and reductions of the oxide mixture reference to the graph. Likewise, the curve of plutonium by oxygen gas and gas; the oxygen oxidizes sesquioxide, although it coincides with that of uranium the UO2 to U3O8 quantitatively or substantially so, and dioxide at lower temperatures, diverges away from it at 40 the hydrogen reduces the U3O8 back to UO, and the around 1000 K. so that even if some reducing action at consequent changes in bring about a lower temperatures is theoretically possible, it is too slow crumbling effect; while UO2 has the same crystal struc to be practical. The use, therefore, of uranium to reduce ture in a geometrical sense in all cases, the successive plutonium and the rare earth oxides to the metallic state reductions do not remake of the same size, but would not have seemed practical, but with the molten Smaller ones of the same shape which causes the crum metal solvent system above referred to, our method of bling. The oxidations are carried out at about 300 to using uranium as a reducing metal becomes operative. 500 C. for about 15 minutes or longer for each oxida It has been suggested this is due to the fact that in the tion, except that on the first cycle times of 45 minutes solvent system mentioned the ions of plutonium and of may be required if the initial particle sizes are large. the rare earth elements have very low activity coefficients. 50 The reductions are carried out for about 15 minutes or While we believe that this theoretical explanation is prob longer at about 600 to 800° C.; it is possible that for ably sound, we do not wish to be rigorously bound by it; high burnup and short-cooled material having consid in any event we have found that in a system where a erable radioactive self-heating, it may not be necessary metal of the group above mentioned may be considered as to Supply heat, but rather to cool by suitable means the solvent the oxides of plutonium and the rare earth ele 55 Such as liquid to keep within the ranges men ments may be reduced by uranium when added in stoichi tioned. Considering one gaseous oxidation followed by ometric proportions or in excess thereof, with the result gaseous reduction as a cycle, from two to six cycles, and that a discrete metallic phase forms including plutonium, preferably three, are required to produce oxides of the rare earth metals, and solvent metal. Any excess uranium proper size for the ensuing reductions. Further details that may be present will, of course, enter the metallic 60 of the invention may be gathered from our preferred phase as well, but if proper stoichiometric principles are mode for carrying it out, as set forth in the following observed this difficulty will not be encountered. example: This procedure has the additional advantage of provid Example ing a convenient means for enriching the oxide fuel being reprocessed if the reactor is of an enriched fuel type. In 65 11,350 grams of fresh UO fuel consisting of granules, that case, the uranium used as a reducing agent may be having an average particle size of 3/8 to 4 inch enriched the 235 isotope or an enriched isotope mixture containing to 50% U385, are placed in a . Before it. In the case of reactors a slightly being placed in the reactor this fuel has a total uranium may advisably be used in order to bring content of 10,000 grams, the balance of 1,350 grams be up the proportion of uranium-235 to the level found in 70 ing oxygen, and after withdrawal from the reactor, the lature. U288 content of the spent fuel is 5,000 grams, the U235 Our discovery, it will be understood, is to be carried out content 4,800 grams, the fission products content 200 after the volatile fission products have been removed by grams, the oxygen content remaining about unchanged, volatilization, alone or with the help of oxygen as above as indicated in the process flow sheet of FIG. 1. It will explained, and the oxides reducible by a number of the 75 be understood that in the flow sheet the quantities not 8,028,097 E. 6 otherwise designated are to be considered to represent is partially reduced to UO, the said oxidations and re grams; F.P. means fission products, Group A means ductions being repeated for a sufficient number of cycles the volatile elements or oxides such as Kr, Rb, Tc, Ru, to reduce the particle size, then immersing it in a dual Te, I, Xe, and Cs, Group B means Mo, and Group C purpose liquid selected from the group consisting of means elements soluble in U-Zn such as Y, Zr, Nb, Mo, molten zinc and molten cadmium under an inert at Rh, Pd, Ag, Cd, In, Sn, Sb, Ba, Sr, and the rare earths. mosphere to bring about the reduction to the metallic The spent fuel is then placed in a stainless vessel state of the oxides reducible by the liquid, adding to with an oxygen atmosphere and electrically heated to the liquid uranium in the stoichiometric amount to bring about 500 C. for 45 minutes. This drives off the vola about the reduction of the oxides of the class consisting tille elements and volatile compounds of Kr, Rb, Tc, Ru, of the oxides of the rare earth elements and plutonium Te, I, Xe, Cs, and Se, and also removes about 50% of O to the metallic state, then separating the resulting liquid the molybdenum content. The vessel is evacuated and metal phase from the solid oxide phase. then the temperature of the vessel is raised to 700 C. 2. The method of claim 1 where the heating to drive and the vessel is filled with hydrogen for 15 minutes, after . off the volatile fission product compounds is carried out which the vessel is again evacuated and cooled by cooling at about 300 to 500 C., where the cyclic oxidation of coils to 500° C. and the oxidation repeated for 15 min 15 said product comprising UO is carried out at about 500 utes. This oxidation-reduction cycle by means of oxygen C. and the cyclic partial reduction of the product com and hydrogen gases is carried out three times. The re prising UO is carried out at about 600 to 800 C., sulting mixture of oxides is then transferred to a where reduction by the dual purpose liquid and the re vessel and 40,000 grams of liquid zinc containing in solu duction by the uranium within the solvent liquid and the tion 236 g. of fully enriched uranium are introduced into 20 decantation are carried out at about 700 to 800 C., the vessel at a temperature of about 750 C. Vigorous where the distillation is carried out at about 600 to 740 agitation by means of a tantalum stirrer is provided in C. at a pressure of about 10 to 100 mm. Hg. order to intermix the molten zinc and the oxide mass 3. A method of reducing the oxides of the class con for a period of about 8 hours at 750 C. in an argon sisting of the oxides of the rare earth elements and plu atmosphere. At the end of this period the agitation is 25 tonium to the metallic state, comprising immersing them. discontinued but the temperature of the vessel still main in a solvent selected from the class consisting of molten tained at about 750° C. until the contents separate into zinc and molten cadmium and adding thereto metallic a lighter liquid metal phase and a heavier solid oxide uranium in the stoichiometric amount needed to reduce phase which settles to the bottom; the liquid phase is the oxides to the metallic state to bring about a reducing then removed by pressure siphoning it out of the tan reaction which is carried out at 700 to 800 C. and ac talum vessel and the tantalum vessel containing the oxide companied by agitation. phase is transferred under an inert atmosphere to a still 4. A method of separating the oxides of the class in which at a pressure of 10-100 mm. Hg and a tempera consisting of the oxides of the rare earth elements and ture of 600 to 740° C. the residual zinc, about 400 plutonium from a mixture comprising oxides of uranium, grams, is distilled out of the oxide mass. The composi 35 comprising immersing the mixture in a solvent selected tion of the latter is 5010 grams of U8 and 5006 grams from the group consisting of molten zinc and molten of U235. Material of this analysis is entirely suitable for cadmium, adding thereto metallic uranium in the stoichio reuse in the enriched type of nuclear reactor from which metric amount needed to reduce the oxides to the me the spent fuel was originally taken. 40 tallic state to bring about a reducing reaction, main What is claimed is: taining the temperature of the reducing reaction at 700 1. A method of reprocessing spent oxide fuel con to 800° C. accompanied by agitation, and after comple sisting of uranium dioxide, and the oxides of the fission tion of the reaction separating the resulting liquid metal products and transmutation products from nuclear re phase from the solid oxide phase by decantation of the actors, comprising heating said fuel in an oxygen at 45 liquid metal phase followed by distillation of the resid mosphere to drive off the volatile fission product elle ual liquid metal from the oxide phase. ments and compounds, then cyclically oxidizing the re sulting product at 300-500 C, with oxygen gas where References Cited in the file of this patent by substantially all the UO of the product becomes oxi A.E.C. Research and Development Report HW 51748, dized to UO, and partially reducing it with hydrogen 50 "Fabrication Behavior of Some Uranium Dioxide Pow gas at 700-800° C. whereby substantially all the UOs derS.” Dec. 1, 1957,