High-Burnup BWR Fuel Behavior Under Simulated Reactivity-Initiated Accident Conditions

Abstract

Boiling water reactor (BWR) fuel at 56 to 61 GWd/tonne U was pulse irradiated in the Nuclear Safety Research Reactor (NSRR) to investigate fuel behavior under cold startup reactivity-initiated accident conditions. Current Japanese 8 × 8 type Step II BWR fuel from Fukushima Daini Unit 2 was refabricated to short segments, and thermal energy from 272 to 586 J/g (65 to 140 cal/g) was promptly inserted to the test rods. Cladding deformation of the BWR fuel by the pulse irradiation was smaller than that of pressurized water reactor (PWR) fuels. However, cladding failure occurred in tests with fuel at burnup of 61 GWd/tonne U at fuel enthalpies of 260 to 360 J/g (62 to 86 cal/g) during the early stages of transients, while the cladding remained cool. The failure was comparable to the one observed in high-burnup PWR fuel tests, in which embrittled cladding with dense hydride precipitation near the outer surface was fractured due to pellet cladding mechanical interaction. Transient fission gas release by the pulse irradiation was ~9.6 to 17% depending on the peak fuel enthalpy.