AMMONIUM POLYURANATE PRECIPITATION FROM FLUORIDE SOLUTIONS: AN EXPERIMENTAL STUDY OF THE UO_3-HF-NH_3-H_2O SYSTEM AS APPLIED IN UF_6 PROCESSING

摘要

In the ammonium diuranate (ADU) process, UF_6 is reacted with water, and the acidic solution of uranyl fluoride is treated with aqueous ammonia to precipitate ammonium polyuranate for subsequent reduction to UO_2 and production of fuel pellets for commercial nuclear reactors. Our experiments simulated adding aqueous ammonia to the reaction products of UF_6 and water in typical ADU processes. Chemical and X-ray diffraction analysis of products from the experiments are consistent with postulated chemical equilibria in which solids with structures close to that of ammonium polyuranate are formed from co-precipitation of the NH_4~+(aq) cation with (previously unreported) anions of the form UO_2F_(3-x)(OH)_x~-(aq). More efficient separations of solidrnproducts were obtained at NH_4H:UF_6 ratios of 19 or greater, with x closer to the value of 3 for the hypothetical formation of pure ammonium polyuranate. Supplementary experiments in the current study and a previous study in our laboratory indicated that nominal uranium concentrations of 90 mg/e in the filtrate resulting from such separations could be reduced to microgram per liter levels by batch mixing a l-to-2.5 aqueous diluate of the filtrate with the Diphonix~® ion exchange resin. Our study further demonstrated that reaction of the purified NH_4OH-NH_4Fdiluate with aqueous Ca(OH)_2 at 80 to 90℃ could produce essentially uranium-free CaF2 and an ammonia distillate, as useful waste-conversion end products from a modified ADU process.