Development of a new method for investigating the mechanisms of solid particle dissolution in nitric acid media: Milli-fluidic study using fluorescent indicators

A key step in existing industrial recycling processes for spent nuclear fuel, after cutting up the spent fuel assemblies, is the continuous dissolution of uranium dioxide in a dissolver containing concentrated nitric acid. The purpose of this study is to enhance our understanding of the phenomena governing the oxide behavior during dissolution, and to acquire new experimental data suitable for modeling and optimizing spent fuel dissolution. The approach adopted involved small scale experiments, which are indispensable for observing, understanding and modeling the phenomena occurring at the solid/liquid interface which control the dissolution reaction. A milli-fluidic reactor of about 1 mL was developed for the continuous dissolution of millimeter or submillimeter solid particles in a nitric acid stream. New experimental and analytical monitoring techniques were used (observation at the interface by confocal microscopy and epi-fluorescence, tracking of the species involved, pH monitoring at the interface by fluorometry). Qualification and experimental implementation of these techniques were first conducted on a nonradioactive surrogate, copper. These observations were used to better understand and quantify continuously and in situ the material dissolution mechanisms and the pH variation at the interface, as well as providing data on the behavior of the off-gases. Intermediate species favorable to the reaction were identified, and made it possible to quantify the coupling between hydrodynamics and dissolution kinetics. We were also able to obtain a pH map of the reaction interface, which is very difficult to implement for this type of reaction in corrosive media. (authors)