HTR fuel is based on spherical coated particles inserted in cylindrical graphite blocks, called compact. A fuel simulation code embedded in the PLEIADES platform, ATLAS, describes the behavior of this material under irradiation. This code performs the simulation of mechanics, thermal and fission products diffusion with a finite elements method powered by the CAST3M generic code. This study, which comes within the framework of a PhD thesis, aims at contributing to the development of a multiscale numerical simulation tool. The final objective is to assess the particle failure fraction and the resultant fission product release.