Compounds of Hexavalent Uranium and Dibutylphosphate in Nitric Acid Systems

Storage tanks at the Savannah River Site (SRS) contain highly enriched uranium (U) and dibutyl phosphoric acid (HDBP), formed from the hydrolysis and radiolysis of tributylphosphate (TBP) in the PUREX process. Highly enriched dibutyl phosphoric acid, a relatively strong acid with a pKa (acid dissociation constant) of about one, forms insoluble compounds with U and other actinides in acid solutions. Accumulation of solids in these storage tanks presents a criticality hazard and must be avoided. In this work, U and HDBP compounds were formed from batch reactions in 0.2–6.0 M nitric acid (HNO3) and two HDBP concentrations, forming either a homogenous or heterogeneous solution based on solubility. A 4:1 HDBP:U ratio was used in all experiments. The chemical composition of the compounds was characterized using liquid scintillation, ion chromatography, and stannous chloride reduction. The physical characteristics of the U and HDBP compounds were characterized using a combination of infrared spectrometry (IR), electronic spectrophotometry (UV/Vis), and Phosphorous‐31 Nuclear Magnetic Resonance (31P NMR). The physical appearance of the solids ranged from a pale yellow powder at 0.2 M HNO3 loading to a bright yellow gel at 6.0 M HNO3, with constant modifications as the acid concentration increased. The U and HDBP precipitate from 0.2 M HNO3 was characterized as UO2(DBP)2 (a polymer with ten units) in homogenous U and HDBP solutions and UO2(DBP)2(HDBP) x (where x = 1 or 2) in heterogeneous U and HDBP solutions. Indications of nitrate complexation appeared in precipitates from 0.8 M HNO3 for both HDBP loadings, however a 1:1 uranium: nitrate ratio was not reached at that acid concentration. UO2(NO3)(H(DBP)2)(HDBP)2 precipitated from ≥ 3.0 M HNO3 with high HDBP concentrations. This compound was not formed until 6.0 M HNO3 was used in lower HDBP concentrations. It is assumed that the change in physical characteristics and addition of nitrate coincides with the breakdown of the UO2(DBP)2 polymer. Supporting data for these findings are presented and discussed.