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

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 conc...