Abstract
This study deals with the modelling of uranium dioxide dissolution in nitric medium, which is a key step at the head-end of nuclear fuel reprocessing. This particular dissolution is triphasic (involving solid uranium dioxide, liquid nitric acid and gaseous nitrogen oxide products), and autocatalytic (accelerated by an unidentified reaction product), so that numerous coupled phenomena must be taken into account for a better understanding, modelling and optimization of dissolution reactors. The kinetic parameters were determined for both the catalyzed and non-catalyzed reactions. The kinetic study was realized thanks to a single particle approach and the reaction rates were measured by optical microscopy, thanks to a specific lab-scale device which ensures the absence of any external limitation. Gas–liquid exchanges were shown to have a great impact on the catalyst concentration in the reactor, and evidence of a volume reaction between the dissolution gases (nitrogen oxides) and the catalyst were found. The kinetics of this reaction was estimated from the experimental results. A model was then developed to describe and simulate these phenomena: it takes into account species transport between the particles surface and the bulk media, UO2 dissolution kinetics, the reaction between the dissolution gases and the catalyst, as well as the transport of these gases to the liquid-gas interface. It was shown to be in good agreement with experimental results, at both microscopic and macroscopic scales. Dissolution modelling has now to be upgraded by integrating fragmentation and bubbling models.
