Behavior of pre-hydrided Zircaloy-4 cladding under simulated LOCA conditions

To promote a better understanding of high burn-up fuel rod behavior in a loss-of-coolant accident, laboratory-scale experiments were performed varying sample and test conditions with non-irradiated Zircaloy-4 claddings. Short test rods, fabricated with cladding having a wide range of hydrogen concentrations (about 100 to 1,450 ppm), were heated, isothermally oxidized at 1,220 to 1,500 K in steam flow, and quenched in flooding water. Axial shrinkage of the rods during the quench was restrained, controlling the maximum restraint load at four different levels. Test rods ruptured during the heat-up, and slight hydrogen concentration effects were seen on rupture temperature and strain. Depending primarily on the oxidized fraction of the cladding thickness, a part of claddings sustained circumferential cracking and fractured into two pieces during the quench. The fracture/no-fracture threshold of the oxidized fraction decreases as both the initial hydrogen concentration and axial restraint load increase. Consequently, when the restraint load is below 535 N, the fracture threshold is higher than 20% cladding oxidation, irrespective of the hydrogen concentration. This is sufficiently higher than the limit in the Japanese ECCS acceptance criteria. (author)