Helium thermal diffusion in a uranium dioxide matrix

The behavior of helium and its diffusion mechanisms in uranium dioxide significantly impact the possible evolution of the spent fuel matrix in interim storage or in a disposal repository, and are therefore an essential aspect of the R&D, regarding fuel matrix behavior in a closed system. A specific experimental study has been conducted, the first part being devoted to thermal diffusion and applied on implanted samples. The analysis procedure used is based on the spreading (under thermal annealing conditions) of a helium-3 profile initially created in the material. The profile is investigated by the resonant nuclear reaction 3He(d,p)4He. The measured helium-3 diffusion coefficients obtained indicate two types of behavior between 1123 and 1273 K, depending on the fluence, with a common activation energy of 2 eV. The values determined experimentally demonstrate that the thermal component of helium diffusion in spent fuel will not be significant over the time/temperature range of interim storage and disposal. Vacancy defects can lead to changes in the behavior and in the diffusion of helium. They will be quantified in the second part of this study.