Résumé
$\beta$-Thorium phosphate diphosphate polycrystalline ceramic is considered as a promising candidate for the immobilization of actinides in the field of a long-term storage. In order to study the behavior of the ceramic in relation with the evacuation of helium produced by actinides and daughters disintegrations, the release of radiogenic helium from a sintered pellet aged over an about six year period was studied by thermal desorption and high resolution mass spectrometry. The apparent diffusion coefficients are determined for different annealing sequences and are approximately the same for zircon and britholite at low temperature and uranium dioxide ceramic at high temperature. The apparent diffusion coefficients are found to be (7 ± 3) x 10^-22 m^2 s^-1 and (1.5 ± 0.5) · 10^-20 m^2 s^-1 at 20 °C and 120°C, respectively. The activation energy for the apparent diffusion process encompassing both volume and inter-granular diffusion is estimated at 45 ± 15 kJ mol^-1 in the studied temperature range (20–1020° C).