Speciation-Improved Modeling of U(VI) Extraction by the Monoamide MOEHA Using the Mikulin–Sergievskii–Dannus Approach

The development of sustainable alternatives to conventional PUREX reprocessing is a major challenge in the nuclear fuel cycle, and monoamide-based solvent extraction has emerged as one of the promising approaches for next-generation spent nuclear fuel reprocessing. The process study requires a reliable thermodynamic model to simulate and optimize solvent extraction operations. In this study, batch extractions of nitric acid and uranium(VI) were performed using 1.2 mol·L–1 MOEHA in Isane IP185, with various nitric acid and uranium concentrations. UV–Vis spectroscopic spectra decompositions were carried out to determine uranium speciation in the organic phase. The spectra were decomposed assuming the formation of two major complexes during the extraction of uranium: the predominant dinitrate species (MOEHA)2(UO2)(NO3)2― and the minor trinitrate species (MOEHA)x(HNO3)(UO2)(NO3)2―whose concentration increases with nitric acid content. The speciation data were incorporated into the thermodynamic model implemented in the PAREX+ simulation tool. The model was able to efficiently reproduce uranium, nitric acid, and water extraction data. Including the concentration gap of the dinitrate complex in the parameter optimization improved the prediction of uranium speciation while maintaining good agreement with experimental distributions. Validation using independent data sets confirmed the robustness of the model across varying extractant concentrations. This work improves the physicochemical description of uranium extraction by MOEHA and strengthens the reliability of the thermodynamic model for process design and optimization within the spent nuclear fuel reprocessing framework.