Recovering high-purity uranyl nitrate from simulated used nuclear fuel dissolver solutions by crystallization: rejecting technetium

The separation of U from Tc and other problematic fission product elements like Mo and Ru, along with Sr, Zr, Cs, and Nd, has been achieved via the crystallization of uranyl nitrate hexahydrate (UNH). Rejection of technetium as pertechnetate anion (⁹⁹TcO₄^–) is an especially important feature of this system, as it otherwise tends to follow U(VI) in extractive separations. It also raises the salient question regarding why this oxoanion cannot replace nitrate within the crystalline lattice of UNH. Results showed high-yield (>90 %), high-purity (>99 %) recovery of U as UNH from solutions containing ⁹⁹TcO₄^– by simple reduction of temperature from 60°C to 20°C. There was no observable interaction of ⁹⁹TcO₄^– with UO₂²⁺. The addition of other cations like, Sr²⁺, Zr⁴⁺, Cs⁺, and Nd³⁺, also did not form secondary, contaminant solid phases, leaving the > 99 % of the fission product elements in the mother liquor, while the U was recovered at > 90 %. Similarly, Mo and Ru, when added to the mixture, were shown to behave as the other fission-product elements, remaining in the mother liquor during crystallization. DFT calculations showed that, despite the higher binding strength of TcO₄^–, HMoO₄^–, and BiO₃^– with the UO₂²⁺ cation compared to NO₃^–, the hydrogen-bonding network of the two coordinated ions and four waters of hydration in the UNH crystal structure is the driving force for the high specificity of this separation.