Impact of metallic Mo nanoparticles on the dissolution kinetics of UO2 in nitric acid under representative PUREX process conditions

This study is dedicated to the dissolution of UO 2 -based model compounds incorporating metallic Mo nanoparticles in 4 mol.L -1 HNO 3 at 353 K (representative for PUREX process conditions) following a dynamic protocol. Compounds of structural and microstructural properties similar to spent nuclear fuel allowed us to evidence the specific impact of Mo in the metallic form on the dissolution kinetics of UO 2 . The dissolution of a pure metallic Mo reference sample under the same dissolution conditions was carried out to gain deeper insights into the oxidation behavior of Mo in nitric acid and to evaluate its influence on the UO 2 dissolution mechanism. The evolution of the U, Mo, and nitrous acid concentrations in solution was measured and the solid residues of dissolution were characterized. The results revealed a mutual inhibitory effect between UO 2 and metallic Mo during dissolution in nitric acid. No precipitates of mixed U and Mo were formed under the studied conditions. The dissolution processes of UO 2 and Mo were found to be interconnected and likely compete for the same auto-catalytic nitrogen species necessary for their oxidation and solubilization in the +VI oxidation state. The results provided crucial insights into how metallic Mo reacts with nitric acid under the representative PUREX process conditions. The findings suggested that metallic Mo(0) is initially completely oxidized to an amorphous brown-red phase with an oxidation state between +4 and +6. Further oxidation, then solubilization of this phase as Mo(VI) species appeared to be facilitated by the auto-catalytic nitrogen species.