Thermal shock behavior of PHWR fuel cladding subjected to LOCA integral test

During a hypothetical loss of coolant accident (LOCA) in a pressurised heavy water reactor (PHWR), cladding temperature may rise up to 1500 °C due to coolant positive void coefficient of reactivity. At such a high temperature, β-Zr phase absorbs significant amount of oxygen and hydrogen. This paper provides novel data on the synergistic effects of oxygen, hydrogen and prior β-Zr layer thickness on the crack initiation and propagation during thermal shock in a LOCA scenario. Integral LOCA experiments were performed on the PHWR cladding. The results showed the presence of non-uniform distribution of oxygen, hydrogen and prior β-Zr layer thickness along the circumference and axial direction of cladding. Significant amount of localised deformation and oxidation in the burst region lead to a high oxygen pick up and thinning of prior β-Zr layer, making it a highly brittle region. Substantial amount of hydrogen (up to 5000 wppm) was picked up by the cladding away from the burst opening due to inner surface oxidation by steam. Two types of hydride morphologies were noticed, hydride blisters near the burst tip and fine hydride platelets away from it. Based on the integral tests, threshold concentration of oxygen in the prior β-Zr for crack initiation ahead of the burst tip was observed to be 1.2 wt%. Hydrogen played an indirect role of increasing the embrittlement of cladding by enhancing the oxygen solubility in the β-Zr. Hydrogen has minimal role on the thermal shock failure below 900 wppm. Circumferentially averaged more than cladding half thickness of prior β-Zr layer prevents the crack propagation.