Patent Number: 045029873
Section: summary

A commonly assigned and copending U.S. patent application Ser. No. 223, 547(79), now U.S. Pat. No. 4,397,778, filed Jan. 9, 1981, by Milton H. Lloyd, and entitled "Co-Processed Nuclear Fuels Containing (U,Pu) Values as Oxides, Carbides, or Carbonitrides", describes a method for coprocessing nuclear fuels in a secure and diversion resistant manner. According to that method, uranium, selected fission products, and plutonium constituents of a product stream from a nuclear reprocessing facility are first stabilized by maintenance of the stream in excess of 0.5 M HNO.sub.3 concentration. Then, the stabilized stream is concentrated by vacuum distillation while being maintained at about 40.degree. C. and a pressure of about 10 to 40 Torr. Thereafter, the stream is made acid deficient by neutralization with a predetermined amount of ammonium hydroxide. Finally, the acid-deficient stream is fed to a spherule-forming operation employing an internal gelation process to produce sol-gel spherules. Inasmuch as many of the operational details and conditions of the method described in assignee's copending patent application may be utilized herein, it is incorporated herein by reference. In the practice of the invention of the aforementioned patent application, it was discovered that the crystal morphology of the precipitated phase, or phases, during spherule-forming operations could provide a crucial process parameter for control of numerous product qualities, such as density and the like. While this discovery was first made in pure urania systems of a single phase, the problem was found to be greatly exacerbated in mixed-fuel systems of uranium and plutonium involving complex phases. Because mixed-fuel systems have more utility in Liquid Molten Fuel Breeder Reactor (LMFBR) and like reactors, there is a need for a method of controlling the phenomena of crystallite size, and hence crystal morphology, in nuclear-reactor fuel spherules. To date, however, the art has found that product qualities, and particularly density, have varied unexpectedly for reasons that cannot be readily explained or fully understood. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an efficient method for optimizing crystallite growth during spherule-forming operations by an internal gelation process to effect air-dried spherules of a tap density of about 1.0 g/cc which can effectively be pelletized into good quality ceramic pellets of 93 to 95% theoretical density. It is another object of this invention to provide spherules of urania-plutonia mixtures whereby crystallite size is effectively controlled within a range suitable for preparation of the gel-derived-pellet form of nuclear reactor fuels. These objects and other advantages of the invention may be ascertained by reference to the following disclosure and are provided in an improved method for preparing internally gelated spherules via a solgel process wherein a concentrated HMTA-urea solution is employed in spherule-forming operations. The improvement in the method comprises the step of heat-treating the hexanethylene-tetramine(HMTA)-urea solution for a sufficient duration without causing excessive urea decomposition prior to conducting spherule formation. Excessive urea decomposition occurs if the HMTA-urea feed is boiled for too long a duration which adversely affects the feed broth stability at 0.degree. C. Uranium will precipitate when an HMTA-urea solution is mixed with the metal feed even at 0.degree. C. The teachings in the aforementioned copending patent application disclose that concentrated feed solutions of HMTA and urea employed in a spherule-forming operation of an internal gelation process should be prepared at about ambient temperature, or even be chilled down to about 0.degree. C., because of the inverse temperature-solubility relationship of HMTA. However, it has been found that this previously disclosed procedure is less desirable because it restricts the formation of large urania crystallites and thereby prevents the formation of low density microspheres. It has been found more advantageous, according to the present invention, to heat-treat the HMTA-urea solutions by boiling for a sufficient duration and then cool the resultant solution to about ambient temperature before admixture with a solution of metals selected from the group consisting of uranium, plutonium, thorium, and mixtures thereof for subsequent spherule formation by passage through a formation nozzle. Heat-treated solutions of the present invention are preferable over those of the aforementioned patent application because they allow for the preparation of microspheres over a wide range of sphere densities in a controlled manner.