SOLUBILITY LIMITS OF DIBUTYL PHOSPHORIC ACID IN URANIUM-NITRIC ACID SOLUTIONS

Abstract

The Savannah River Site has enriched uranium (EU) solution that has been stored since being purified in its solvent extraction processes. The concentrations in solution are approximately 25mM U and 0.1 M nitric acid. Residual tributylphosphate in solution has slowly hydrolyzed to form dibutyl phosphoric acid (HDBP) at concentrations averaging 0.14–0.2450 mM. Dibutyl phosphoric acid, in turn, is in equilibrium with (HDBP)2 and DBP−. Uranium can form compounds with the dibutylphosphate ion (DBP−) which have limited solubility, thereby creating a nuclear criticality safety issue. Literature reports and earlier SRTC tests have shown that it is feasible to precipitate U-DBP solid during the storage and processing of EU solutions. As a result, a series of solubility experiments were run at nitric acid concentrations from 0–4.0 M HNO3, uranium at 0–378 mM, and temperatures from 0–30°C. The data show temperature and HNO3 concentration dependence consistent with what would be expected. With respect to uranium concentration, U-DBP solubility passes through a minimum between 25 and 50 mM U at the HNO3 concentrations and temperatures studied. However, the minimum shows a slight shift toward lower uranium concentrations at lower HNO3 concentrations. The shifts in solubility are strongly dependent upon the overall ionic strength of the solution. These data also reveal a shift to higher DBP solubility above 0.5 M HNO3 for both 25 mM and 50 mM uranium solutions. Analysis of U-DBP solids from the tests identified distinct differences between precipitates from <0.5 M solutions and those from >4 M acid. Analyses identified UO2(DBP)2 as the dominant compound present at low HNO3 concentrations in accordance with literature reports. As the acid concentration increases, the crystalline UO2(DBP)2 shows molecular substitutions and an increase in amorphous content.