To identify the microstructural cause of nuclear fuel fragmentation phenomenon in LOCA conditions, CeO2, a surrogate of the UO2, was implanted with xenon and krypton ions. Fine grain-sized CeO2 with a porosity of 10 % and a grain size of about 550 nm, and normal-grain-sized CeO2 with a porosity of 4 % and a grain size of 7 m were used for the irradiation test for the rim part and center part of the nuclear fuel, respectively. The irradiated CeO2 were annealed at temperatures between 300 °C and 1200 °C, and the microstructure of each specimen was analyzed by TEM. Each specimen was then rapidly heated again at a rate of several tens °C / s for the LOCA situation simulation, and then the behavior of the implanted gas bubble was also investigated by TEM. The results of this experiment show how fission gas bubbles behave in the LOCA situation.