Thermodynamic description of U(IV) solubility and hydrolysis in chloride systems: Pitzer activity model for the system U4+–Na+–Mg2+–Ca2+–H+–Cl––OH––H2O(l)

This study presents updated chemical, thermodynamic, and activity models for the system U⁴⁺–Na⁺–Mg²⁺–Ca²⁺–H⁺–Cl^––OH^––H₂O(l) derived using the Pitzer formalism and a strict ion interaction approach. The models build on comprehensive solubility datasets in dilute to concentrated NaCl, MgCl₂, and CaCl₂ solutions. The Nuclear Energy Agency-Thermochemical Database (NEA-TDB) selection of solubility and hydrolysis constants in the reference state were taken as anchoring point, and were extended further with the solid nanocrystalline phase UO₂∙H₂O(ncr) and the ternary complex Ca₄ [U(OH)₈]⁴⁺. The former was identified in long-term solubility experiments at ambient conditions, whereas the latter has been selected in analogy to Th(IV), Np(IV), and Pu(IV) considering experimental evidences available for these An(IV) in alkaline, concentrated CaCl₂ solutions. These models represent an improved tool for the calculation of U(IV) solubility and aqueous speciation in a variety of geochemical conditions including concentrated brine systems relevant in salt-based repositories for nuclear waste disposal.