The ceramics fuel of UO2, irradiated in power reactors, has restructuring at high burnup, around 70MWd/kgU, which is near in the scope of future fuel utilization with improving economy. The original grains are divided into sub-grains of 100 nm scale and segregation of fission gas (Xe, Kr) grows bubbles resulting in volumetric swelling of the fuel. This restructuring was investigated in the field of nuclear engineering of power reactor fuel industry. This presentation briefly provides observations of this restructuring and some of engineering consequences. In a mechanism study a new approach, combing accelerator irradiation and computational science has been started and some of initial results are presented.