[en] An improved nuclear fuel composition characterized as a mixture of the dioxide of uranium and plutonium with pores of specified sizes and volumes of each size is described. The volume of pores of each size are adjusted so that as each group of pores of each size is removed by the nuclear fission induced process of densification, the volume removed is balanced by the volume added by the nuclear fission induced process of solid state swelling. This fuel composition is dimensionally stable in-pile to high burnups because the rate of pore removal is matched to the rate of swelling