This study is dedicated to the dissolution of UO2-based model compounds incorporating metallic Mo nanoparticles
in 4 mol.L-1 HNO3 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 UO2. 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 UO2 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 UO2 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 UO2 and Mo were found to be interconnected and likely compete for the same autocatalytic
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 brownred
phase with an oxidation state between +IV and +VI. Further oxidation, then solubilization of this phase as
Mo(VI) species appeared to be facilitated by the auto-catalytic nitrogen species.