Analysing the impact of autocatalysis on the dissolution kinetics of uranium and plutonium mixed oxide powders by optical microscopy

Abstract

Spent fuel dissolution is a key step in the recycling of nuclear fuel as it must ensure the production of a liquid solution consisted of concentrated nitric acid and the reusable fissile materials (uranium and plutonium), and decrease the fissile material hold-up in the system. This approach needs the understanding of chemical kinetic phenomena involved in the solid-liquid reaction. Optical microscopy was implemented to determine the dissolution rates of U-Pu-mixed oxides in nitric acid at different concentrations and temperatures for the first time. Mass transfer resistance was found negligible during dissolution indicating a process limited by the chemical reaction. Results showed a decrease in dissolution rate of the different oxides studied mainly due to the increment of plutonium content in the solids. Moreover, dissolution kinetics of solid U0.70Pu0.3O2 seems to improve when a catalyst is present in solution. Thus, the autocatalytic behaviour for the dissolution of spent fuel might occur for solids with 0% and up to 30%PuU+Pu of content.