Study and modelling of heterogeneous catalytic reactions. application to uranium dioxide dissolution.

P. Marc (1) , F. Charlier (1) , J. Godard (1) , A. Magnaldo (1) , E. Schaer (2)

1 CEA-DES (ex-DEN) - CEA-Direction des Energies (ex-Direction de l'Energie Nucléaire)
2 LRGP - Laboratoire Réactions et Génie des Procédés

Dissolution is a key step in several industrial processes. It is especially a milestone of the head-end of manyhydrometallurgical processes. For example, in recycling of spent nuclear fuel, the solubilization of the chemical elementsis essential before performing the liquid-liquid extraction steps to separate reusable material and final waste. One of themost complex scenarios is that of heterogeneous autocatalytic reactions. Today, there are few satisfying models forthese cases due to a lack of comprehension of their mechanisms.We focus here on the dissolution of uranium dioxide in nitric medium. In order to propose optimized processes fordissolution, this study aims at better understanding the chemical, physico-chemical and hydrodynamic phenomena ofsuch reactions. This study is also part of a modeling approach aiming, on one hand, at expressing the intrinsic reactionrates and describing the physico-chemical phenomena at interfaces and, on the other hand, at developing a generalmodel for dissolution reactors.Results Optical microscopy observation confirmed the highly autocatalytic nature of the reaction and led to measurements, forthe very first time, of true chemical kinetics of the reaction. The acid attack of sintering-manufactured solids occursthrough preferential attack sites. It develops cracks in the solids that can lead to their cleavage. This inhomogeneousattack is made possible by the establishment of bubbling in the cracks which allows periodic renewal of the reagents andthus maintains the reaction within the cracks. This point is a key component of the mechanism a strong link among thedevelopment of cracks, bubbling through the cracks, and overall dissolution kinetics is demonstrated in this work.A model coupling material balance to the structural evolution of the solid and liquid phase compositions, and taking intoaccount the interfacial transport is proposed. The simulations based on this model are close to the experimentalobservations, and allow to replicate the effect of various reaction parameters for the very first time, such as the reductionof overall kinetics when turbulence increases.

Keywords: Chemical kinetics; Hydrodynamics; Mass transfer; Modelling; Multiphase

Domains: Nuclear Experiment [nucl-ex]; Nuclear Theory [nucl-th]

P. Marc, F. Charlier, J. Godard, A. Magnaldo, E. Schaer. Study and modelling of heterogeneous catalytic reactions. application to uranium dioxide dissolution.. ECCE 10 + ECAB 3 + EPIC 5 - 10th European Congress of Chemical Engineering and 3rd European Congress of Applied Biotechnology and 5th European Process Intensification Conferences, Sep 2015, Nice, France. ⟨cea-02489516⟩