Effect of pre-hydriding on thermal shock resistance of Zircaloy-4 cladding under simulated loss-of-coolant accident conditions

摘要

Experiments simulating loss-of-coolant accident (LOCA) conditions were performed to evaluate the effect of prehydriding on the thermal-shock resistance of oxidized Zircaloy-4 cladding. Test rods fabricated with 580-mm long claddings were isothermally oxidized at temperatures ranging from 1,220 to 1,530 Kin steam and then were quenched with flooding water. Both artificially hydrided (400 to 600 ppm) and non-hydrided claddings were subjected to these tests. Since cladding fracture on quenching primarily depends on the amount of oxidation, the fracture threshold was evaluated in terms of "equivalent cladding reacted (ECR)." Under an axially unrestrained condition, the fracture threshold is about 56% ECR, and the influence of pre-hydriding is not observed. The fracture threshold is decreased by restraining the test rods on quenching, and it is more remarkable in pre-hydrided claddings than in non-hydrided claddings. Consequently, the fracture threshold was about 20% ECR and 10% ECR for non-hydrided and pre-hydrided claddings, respectively, under the fully restrained condition. These results indicate a possible decrease of fracture threshold of high burn-up fuel claddings under LOCA conditions.