Influence of the Cr2O3 sintering additive on the homogenization of the plutonium distribution inside an heterogeneous MOX pellet

This work has revealed the nature of the Cr2O3 action mechanisms on the development of the microstructure of a MOX pellet and particularly on the improvement of the plutonium distribution. At first, it has been necessary to study thoroughly the description of the interaction phenomena occurring inside the U-Pu-Cr-O system. A model system constituted by the same materials UO2, (U, Pu)O2 and Cr2O3 than those present in a MOX pellet and thermically heated in similar sintering conditions has been carried out. These tests have been completed by studies concerning the reactivity between PuO2 and Cr2O3, the interdiffusion between UO2 and (U, Pu)O2 in presence of chromium and the solubility of chromium in (U, Pu)O2. Then, with all the data acquired, it has been possible to describe the evolution of a MOX pellet in presence of chromium during the sintering of the microstructure. Microstructural characteristics such as the plutonium homogenization degree and the grain size have been studied with temperature and sintering level period. The chromium oxide inside microstructure has been studied too. At last, an interpretation of the influence of the presence of chromium on the development of a MOX pellet microstructure has been given in focusing particularly on the plutonium distribution. This interpretation is based on the formation of the (U, Pu)CrO3 phase and on the plutonium oxidation degree stabilization (+III) by chromium at the grain boundaries level. Advices aiming at optimizing the chromium impact on the development of microstructure are given. In most of the cases, these advices are based on solutions which will contribute, during the sintering thermal treatment, to the presence at lower temperature of the (U, Pu)CrO3 phase and to keep longer a greater quantity of chromium inside the MOX pellet. (O.M.)