The formation mechanisms of high burnup structure in UO₂ fuel

The formation process of high burnup structure (HBS) in UO₂ fuel is accompanied by the subdivision of original grains and the formation of fission gas bubbles, which can accelerate the fission gas swelling and release, and correspondingly raises some safety concerns over the extension of operating life of the nuclear fuel in the reactor. The UO₂ fuel specimens were irradiated up to a maximum burnup of 120 GW·d/tU in High Flux Engineering Testing Reactor (HFETR) of Nuclear Power Institute of China (NPIC). High resolution scanning electron microscope (SEM) observations were carried out in the fuel specimens with different local burnups of 48–120 GW·d/tU. Our results show that there exist two restructuring mechanisms during the formation of the HBS in UO₂ fuel matrix: polygonization and recrystallization. Furthermore, a comparison with earlier experiments published at the CEA of France suggests that the deposition of fission fragments and recoil particles is the main mechanism responsible for the formation of small grains on the inner wall of large bubbles.