MOX (mixed uranium and plutonium oxide) is a civil nuclear fuel. It comes in the form of cylindrical pellets with a diameter and height of approximately 10 mm. It is produced using a process that includes a high-temperature sintering step in a controlled atmosphere. During the sintering of MOX fuel pellets—formed via uniaxial pressing from a previously prepared granular feedstock—a phenomenon of dedensification resulting in swelling may occur under certain conditions. This swelling can persist during thermal stability tests intended to simulate specific in-reactor irradiation conditions.

The aim of this thesis is to identify the physicochemical mechanisms underlying this dedensification phenomenon and to formulate them mathematically in order to develop a model.

The thesis therefore includes an experimental component—to be conducted at Cadarache (using UO2) and Marcoule (using MOX)—to provide data for the model, which will be mathematically formulated during a theoretical phase carried out in collaboration with modelers at Cadarache.