Dissolution of uranium dioxide in nitric medium, towards a macroscopic model of reactors
Résumé
Dissolution is a key step in several industrial processes. It is especially a milestone of the head-end of many
hydrometallurgical processes. For example, in recycling of spent nuclear fuel, the solubilization of the chemical elements
is essential before performing the liquid-liquid extraction steps to separate reusable material and final waste. One of the
most complex scenarios is that of heterogeneous autocatalytic reactions. Today, there are few satisfying models for
these 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 for
dissolution, this study aims at better understanding the chemical, physico-chemical and hydrodynamic phenomena of
such reactions. This study is also part of a modeling approach aiming, on one hand, at expressing the intrinsic reaction
rates and describing the physico-chemical phenomena at interfaces and, on the other hand, at developing a general
model for dissolution reactors.
Optical microscopy observation confirmed the highly autocatalytic nature of the reaction and led to measurements, for
the very first time, of "true" chemical kinetics of the reaction. The acid attack of sintering-manufactured solids occurs
through preferential attack sites. It develops cracks in the solids that can lead to their cleavage. This inhomogeneous
attack is made possible by the establishment of bubbling in the cracks which allows periodic renewal of the reagents and
thus maintains the reaction within the cracks. This point is a key component of the mechanism: a strong link among the
development 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 into
account the interfacial transport is proposed. The simulations based on this model are close to the experimental
observations, and allow to replicate the effect of various reaction parameters for the very first time, such as the reduction
of overall kinetics when turbulence increases.
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