Zircaloy-4 fuel pin failure under simulated loss-of-coolant-accident conditions: Oxygen embrittlement

Abstract

An extensive experimental investigation was performed to study the oxygen embrittlement of the Indian Pressurized Heavy Water Reactor (PHWR) fuel pin under simulated Loss-of-Coolant Accident (LOCA) conditions. Zircaloy fuel cladding experiences creep and corrosion simultaneously during service and LOCA conditions. Zircaloy-4 fuel pins were pre-oxidized to attain different oxide layer thicknesses, achieving in-service conditions. These pre-oxidized tubes were then subjected to burst tests in the steam environment to mimic the LOCA scenario. The present study aims to improve the understanding of the effect of oxidation on the cladding microstructure and the mechanical response of the fuel pin in a LOCA scenario by accounting for the cross-influence, during transient heating, of oxidation and deformation on the behavior of the clad in the LOCA domain. The oxide layer morphology in pre- and post-burst samples was studied using FESEM, XRD, and Raman spectroscopy. In some cases, the inner oxide layer grew faster than the outer oxide layer when the fuel pin was heated in steam during the burst test. The evolution during transient heating of radial and circumferential crack growth in the oxide layer and the occurrence of delamination facilitated faster oxygen and hydrogen uptake. The hydrogen uptake in pre and post-burst samples was related to the oxygen uptake. The hydrogen concentration increases with the oxygen concentration in the pre-oxidized samples. Small oxygen and hydrogen concentrations were found in the post-burst as-received samples due to the formation of a protective oxide layer. The high-temperature oxide layer was formed at extremely high heating rates.