Research on high burnup fuel behavior under accident conditions

Sugiyama, Tomoyuki ; Fukuda, Takuji; Nagase, Fumihisa 

To confirm the reactor safety design, safety reviews are performed under accident conditions as well as under normal operation conditions. One of such design basis accidents is the reactivity-initiated accident (RIA); a reactor excursion typically caused by rapid ejection of the control rod. In case of high burnup fuel, mechanical failure of corroded cladding can occur due to the rapid thermal expansion of fuel pellet. Cladding mechanical test is effective to evaluate the failure limit, but the conventional tests using axial tension or hydraulic pressure cannot represent the biaxial stress state during RIA, which is generated in the cladding chemically bonded with the fuel pellet. Therefore, the biaxial stress testing apparatus was developed, which simulates the stress condition in RIA by the simultaneous axial load and hydraulic pressurization. Mechanical properties of zircaloy-4 cladding are being acquired with the apparatus to improve the accuracy of fuel failure limit evaluation.