Stress biaxiality in high-burnup PWR fuel cladding under reactivity-initiated accident conditions

Udagawa, Yutaka  ; Sugiyama, Tomoyuki ; Suzuki, Motoe; Nagase, Fumihisa 

In order to promote a better understanding of failure mechanisms of high-burnup PWR fuels under reactivity-initiated accident (RIA) conditions, stress biaxiality in cladding has been estimated for the pellet-cladding mechanical interaction (PCMI) phase. The estimation was based on an analysis of the transient elongations of a pellet stack and a cladding tube measured in RIA-simulating experiments in the nuclear safety research reactor (NSRR) using the RANNS code. Stress biaxiality in the high-burnup PWR fuel cladding during the PCMI phase has been estimated to be 0.7-0.8 on average at the mid wall of the cladding. A comparison to fresh fuel test results and a sensitivity analysis showed that the effects of burnup and pulse width on cladding stress biaxiality are less than 10% for the investigated range. The present analysis also indicated that pellet-cladding friction is strong and that the cladding constraint on pellet stack elongation is significant irrespective of burnup.