Research program on conditions to failure of high burnup fuel

Hydrogen-induced cladding failure has been studied in terms of outside-in cracking, which initiates at outer surface and propagates inwards, and hydrogen-induced cladding embrittlement for the assessment of fuel integrity under transient conditions in the safety review on high burnup fuel. Various tests and analyses have been carried out to understand the outside-in cracking mechanism, such as hydrogen thermal diffusion, crack initiation and its propagation. Its onset conditions were investigated by dividing the phenomenon into processes of; (1) hydride precipitation at the cladding outer rim, (2) incipient crack formation, and (3) crack propagation. Dominating factors were clarified and quantified for each of the processes utilizing test and analysis results accumulated so far. Effects of significantly increased cladding hydrogen content on the structural integrity of high burnup fuel were examined by internal pressurization burst tests on PWR and BWR cladding tubes. Hoop strength and circumferential elongation were measured on irradiated and unirradiated cladding tubes having various hydrogen contents at elevated temperatures. (author)