In-pile Xe diffusion coefficient in UO2 determined from the modeling of intragranular bubble growth and destruction under irradiation

Intragranular bubbles grow in the nuclear fuel by diffusion and precipitation of fission gases, mainly xenon; and are ultimately destroyed, under irradiation, by fission fragments. This article will attempt to determine the in-pile bubble distributions taking into account the evolution of the concentration profile around a bubble during its growth and the destruction process by fission fragments. From these distributions a relation between the bubble mean radius and the diffusion coefficient of xenon can be established, allowing the determination, from experimental measurements of intragranular bubble sizes, of the in-pile Xe diffusion coefficient in UO₂. The estimated activation energy (0.9 eV) is about one order of magnitude lower than the widely used value of 3.9 eV determined from out-of-pile experiments. This effect can be attributed to the presence of point defects created by the irradiation.