Patent Number: 042017386
Section: summary

This invention relates to a method of preparing U.sub.3 O.sub.8 for use as a nuclear fuel material, in particular, for directly preparing U.sub.3 O.sub.8 having a controlled particle size distribution from an aqueous solution of uranyl nitrate. The invention is particularly useful for the recycle of enriched uranium in the powder metallurgy manufacture of U.sub.3 O.sub.8 -Al nuclear reactor fuel assemblies. In powder metallurgy processes for the preparation of U.sub.3 O.sub.8 nuclear fuel material, control of the particle size is required to provide a compatible blend of the U.sub.3 O.sub.8 fuel material and the aluminum matrix material, and to provide the desired physical and nuclear characteristics of the product U.sub.3 O.sub.8 -Al fuel cores. Careful control of U.sub.3 O.sub.8 particle size distribution is necessary because (1) particles or agglomerates larger than 150 .mu.m tend to form large hard particles that can penetrate nuclear fuel cladding and cause undesirable hot spots and result in melting of the cladding during irradiation, (2) fuel containing 40 wt % or more of particles smaller than 44 .mu.m is susceptible to fission-gas blistering during irradiation, and (3) the particle size distribution of the U.sub.3 O.sub.8 must match the particle size of the aluminum powder matrix material to obtain a sufficiently homogeneous U.sub.3 O.sub.8 -Al blend for isostatic compaction. This last factor has been established empirically as a particle size range of between about 150 and 44 .mu.m, based on the first two factors and on the particle size distribution of commercially available aluminum powder for powder metallurgical processes. One such commercial aluminum powder is Alcoa Atomized Powder No. T-108, available from the Aluminum Company of America, Pittsburgh, Pennsylvania 15219, which has an optimum particle size range for the preparation of the U.sub.3 O.sub.8 -Al fuel cores. Heretofore, U.sub.3 O.sub.8 nuclear fuel material for use in the manufacture of U.sub.3 O.sub.8 -Al powder metallurgy compacts for nuclear fuel has been obtained by calcining UO.sub.3 prepared by the thermal denitration of uranyl nitrate solution. The UO.sub.3 prepared by this conventional process has a particle size distribution of between about 150-600 .mu.m. Calcining the UO.sub.3 to U.sub.3 O.sub.8 does not reduce the particle size to the desired range; thus, the U.sub.3 O.sub.8 prepared by this process must be ground and sized to achieve a particle size distribution compatible with aluminum powder for powder metallurgy processing. See, for example, A Chemical Recovery System for Safeguarding Unirradiated Uranium, USAEC Report Y-MA-3582, July 1, 1970, p.10. However, grinding the U.sub.3 O.sub.8 is undesirable because (1) it is a slow operation, (2) it generates excessive fine particles, (3) it presents a potential for release of radioactive contaminates, and (4) it leads to U.sub.3 O.sub.8 powder buildup in the grinding equipment. In view of the difficulties associated with the conventional process described above, those skilled in the art will recognize that a number of uranium salts, such as uranyl or uranous oxalate and uranyl or uranous formate, prepared from uranyl nitrate may serve as possible intermediate salts for preparing U.sub.3 O.sub.8 having the desired particle size range. However, tests conducted by precipitation and calcination of the uranium oxalates and uranous formate did not produce U.sub.3 O.sub.8 with the appropriate particle size distribution. Further, attempts were made to prepare U.sub.3 O.sub.8 having a uniform particle size range by the evaporation of solvent from an unsaturated uranyl formate solution followed by calcination to U.sub.3 O.sub.8. While this method produced an excellent yield of uranium, the dry product, even when stirred during evaporation, consisted of undesirably large particle agglomerates clearly unsuitable for compatible powder metallurgy use without grinding and particle sizing. One solution that has been suggested for the problem of agglomeration in the precipitation of uranyl formate is disclosed in United Kingdom Patent Specification No. 1,230,937, published May 5, 1971. In this U.K. patent uranyl nitrate is precipitated with formic acid in a vertical column partially filled with glass balls. Stirring the glass balls during the reaction serves to dry and grind the resulting precipitate to yield a dry uranyl formate powder. There is no indication in this patent of particle size, particle size distribution, or whether the product will calcine to a suitable U.sub.3 O.sub.8 powder. SUMMARY OF THE INVENTION Therefore, it is the object of the present invention to provide a method for the direct preparation of U.sub.3 O.sub.8 nuclear fuel material having a controlled particle size distribution from uranyl nitrate solution. It is also an object of this invention to provide a method for the preparation of U.sub.3 O.sub.8 for the powder metallurgical manufacture of nuclear fuel that does not require the grinding of the U.sub.3 O.sub.8. It is a still further object to provide U.sub.3 O.sub.8 in powder form that has a particle size distribution compatible with a matrix material for the manufacture of nuclear fuel material. In accordance with the present invention, U.sub.3 O.sub.8 having a controlled particle size distribution is directly prepared from an aqueous solution of uranyl nitrate by adding formic acid to effect a denitration and form an unsaturated solution of uranyl formate. Additional stoichiometric excess of formic acid is added to the unsaturated uranyl formate to precipitate uranyl formate monohydrate. The resulting crystalline uranyl formate monohydrate is then calcined to produce U.sub.3 O.sub.8 having a controlled particle size distribution. It has been found that U.sub.3 O.sub.8 powder prepared by this technique has the following desirable physical characteristics: (1) no large particles, (2) crystalline particle morphology, and (3) narrow particle size distribution that is compatible with aluminum powder. The foregoing properties are achieved without grinding or sieving operations.