Uranyl nitrate hexahydrate solubility in nitric acid and its crystallization selectivity in the presence of nitrate salts

The solubility of uranyl nitrate hexahydrate was determined as a function of nitric acid concentration and temperature, and the crystallization yield was calculated. Results showed an increase in crystal formation at lower initial nitric acid concentrations upon cooling a saturated solution of U(VI) from 50 °C to 2 °C, with over 70% recovery of U(VI) mass at all nitric acid concentrations and nearly quantitative recovery starting at 4 M HNO₃. A direct correlation between the change in mother liquor volume percent and U mass removal percent was observed. By reducing the cooling rate from roughly 4.0 °C/min to 0.22 °C/min, the separation factor was increased from 3.88 to 15.7 to greater than 81 for the separation of U(VI) from Sr, Cs, and Nd. At the slower cooling rate, the separation factors were measured as a function of acidity for 2.0–4.3 M HNO₃, showing a decrease in selectivity with a decrease in the acidity. There was also no indication that tetravalent metal double-salt precipitation occurred with either Zr⁴⁺ or Ce⁴⁺. These results indicate that a high-yield, high purity hexavalent actinide crystallization scheme may offer attractive benefits for nuclear-fuel recycle in that only a single very simple and well-understood technology is employed, and the use of organic compounds and solvents is avoided.