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Uranium dioxide dissolution in nitric acid is a complex reaction. On the one hand, the dissolution produces nitrous oxides (NOX), which makes it a triphasic reaction. On the other hand, one of the products accelerates the kinetic rate; the reaction is hence called autocatalytic. The kinetics for these kinds of reactions need to be formalized in order to optimize and design innovative dissolution reactors. In this work, the kinetics rates have been measured by optical microscopy using a single particle approach. The advantages of this analytical technique are an easier management of species transport in solution and a precise following of the dissolution rate. The global rate is well described by a mechanism considering two steps: a non-catalyzed reaction, where the catalyst concentration has no influence on the dissolution rate, and a catalyzed reaction. The mass transfer rate of the catalyst was quantified in order to discriminate when the reaction was influenced by catalyst accumulated in the boundary layer or uncatalyzed. This first approximation described well the sigmoid dissolution curve profile. Moreover, experiments showed that solutions filled with catalyst proved to lose reactivity over time. Results pointed out that the higher the liquid-gas exchanges, the faster the kinetic rate decreases with time. Thus, it was demonstrated, for the first time, that there is a link between catalyst and nitrous oxides. The outcome of this study leads to new ways for improving the design of dissolvers. Gas-liquid exchanges are indeed a lever to impact dissolution rates. Temperature and catalyst concentration can be optimized to reduce residence times in dissolvers.

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 liquidgas 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. [ABSTRACT FROM AUTHOR]