Micro-XANES and X-ray microtomography study of oxidation State, morphology, and chemistry evolution during nuclear fuel sintering

Nuclear waste management has led to recycling PuO₂ into mixed oxide fuel nuclear fuel (MOX). Its manufacturing route involves a sintering process. A multiscale model is under construction in order to predict the evolution of the sample characteristics during sintering and to improve sintering conditions. The purpose of the small scale model is to simulate grain size and chemistry evolution of micrometer large samples. In the green state, crystallites are 0.3 μm in size and are made either of UO_2+x or of PuO_2-y. After sintering (U,Pu)O_2-z solid solutions are formed with crystalline grains close to 5 μm in size. At high temperature, sintering is governed by diffusion phenomena. Diffusion coefficients vary with point defect concentration. These defects are related to the oxidation state of the cations, the spatial distribution of which can be observed by means of micro X-ray absorption spectroscopy (XAS). Micro X-ray absorption near edge structure spectroscopy (micro-XANES) and high resolution X-ray microtomography study experiments have been performed at the European Synchrotron Radiation Facility beamline ID22, Grenoble. The applicability of these techniques has been demonstrated for testing simulation validity of (U, Pu)O_2-z samples along the MOX sintering route.