The polished hemispheres of ThO₂ kernels were irradiated by fission fragments, which recoiled from the covered Al-U alloy foils. After irradiation, the lattice parameters of the kernels were measured by micro-beam X-ray diffractometer. It was found that the lattice parameter increased with increasing fission fragment dose and reached a saturation value at doses higher than about 5×10¹⁶ fission fragments-cm^-3. With increasing grain sizes from 4.0 to 19.3 μm, the saturation values of lattice parameter changes increased from approximately 0.07 to 0.18%. The recovery began at about 400°C in all specimens and the temperature of the complete recovery had a tendency to become higher with increasing grain size. These results lead to a conclusion that the grain boundaries act as a sink of the defects created by fission fragments.