Failure of high burnup fuels under reactivity-initiated accident conditions

Sugiyama, Tomoyuki ; Umeda, Miki ; Fuketa, Toyoshi; Sasajima, Hideo ; Udagawa, Yutaka  ; Nagase, Fumihisa 

Pulse irradiation tests of high burnup fuels were performed to assess the fuel failure limit in a postulated reactivity-initiated accident (RIA). A BWR-UO rod at a burnup of 69 GWd/t failed due to pellet-cladding mechanical interaction (PCMI). The fuel enthalpy at failure was close to those for PWR-UO rods of 71 to 77 GWd/t with more corroded cladding. Comparison of cladding metallographs between the BWR and PWR fuels showed the morphology of hydride precipitation, which depends on the cladding texture, affects the failure limit. Two tests with PWR-MOX rods of 48 and 59 GWd/t also resulted in PCMI failure. The fuel enthalpies at failure were consistent with results obtained in the previous tests with UO fuel rods, if the failure enthalpy is plotted as a function of the cladding oxide thickness. Therefore, the PCMI failure limit under RIA conditions depends on the cladding corrosion states, and the same limit is applicable to UO and MOX fuels below 59 GWd/t.