Research program on conditions to failure of high burnup fuel

A fuel failure initiating at the cladding outer surface and penetrating inwards (outside-in cracking) observed in the power ramp test on high burnup BWR fuel has been studied to clarify its onset conditions for the safety review on high burnup fuel integrity under transient conditions. Hydrogen thermal diffusion in the cladding tube radial direction was investigated in terms of radial hydride precipitation which has been considered to be a potential site for the crack initiation. Hydrogen diffusion analyses seemed to provide reasonable prediction of radial hydride initiation conditions in comparison with the hydrogen thermal diffusion test results. Conditions to cause incipient crack were assessed using results of the outside-in cracking simulation test in which irradiated cladding tube specimens were loaded by constant internal pressurization under condition of radial temperature gradient. Results indicated the minimum stress to cause incipient cracks at the cladding outer surface. Crack propagation behaviors were also investigated using estimated incipient crack length and calculated stress intensity factor. (author)