(U, Ce)O2: a relevant simulant material for the study of the oxidative dissolution of MOX MIMAS

In a collaborative study between ICSM (LIME) and DPME (LMPA), the analogy between an unirradiated MOX MIMAS (U, Pu)O₂ and a model mixed oxide based on uranium and cerium has been demonstrated. Model materials with a heterogeneous microstructure similar to that of MIMAS MOX were prepared at ICSM and subjected to alteration experiments in the presence of hydrogen peroxide. Hydrogen peroxide of radiolytic origin is the main species involved in the oxidative dissolution mechanism of MOX. In the case of non-irradiating model materials, hydrogen peroxide was added to the weathering solution. The surface of the weathered pellets was characterized by environmental scanning electron microscopy and Raman spectroscopy. The analysis of the obtained results shows the precipitation of studtite, UO₂O₂·4H₂O, on the surface of the UO₂ grains and its absence on the surface of the mixed oxide grains (U, Ce)O₂. Under comparable conditions, an identical behavior of unirradiated MIMAS MOX (U, Pu)O₂ was previously observed (Figure). Complementary alteration experiments carried out with homogeneous (U,Ce)O₂ materials with variable Ce contents allowed us to understand the role of Ce and to show that the Ce(III)/Ce(IV) redox couple was involved in a catalytic decomposition mechanism of H₂O₂ at the pellet/solution interface All these works are published in njp Materials Degradation and demonstrate the interest of MOX analogs (U, Pu)O₂ as an additional tool in the study of fuel weathering processes in storage conditions.

^{Credit: S. SZENKNECT / ICSM}

Fig.: Microscopic and Raman spectroscopic characterizations of the weathered surface of a non-irradiated MIMAS MOX and a model compound (U, Ce)O2.

For more information, read the article by Théo Montaigne, Stéphanie Szenknect, Véronique Broudic, Paul-Henri Imbert, Florent Tocino, Christelle Martin, Frédéric Miserque, Christophe Jégou, and Nicolas Dacheux. Oxidative Dissolution of (U, Ce)O₂ materials in aqueous solutions containing H₂O₂: a suitable analogue to (U, Pu)O₂ MOX fuel. npj Materials Degradation 7, 34 (2023). DOI: 10.1038/s41529-023-00348-3