Contribution to the understanding of brittle fracture conditions of zirconium alloy fuel cladding tubes during LOCA transient

Highlights: • Cladding fracture under hypothetical LOCA investigated using semi-integral testing. • Tight competition between burst and out-of-burst fracture. • Competition strongly related to microstructure-related fracture mechanisms. • Fracture thresholds as a function of applied loading and layered microstructure. - Abstract: This work addressed fracture mechanisms of nuclear fuel claddings in hypothetical Loss-Of-Coolant-Accident (LOCA) conditions. To this aim, laboratory-scale "semi-integral" thermal-mechanical tests were carried out, using an oxidation temperature of 1200 °C, slow cooling down to 700 °C and a final quench by reflooding under an axial load. Two fracture mechanisms were observed. Fracture in the burst zone was due to oxide formation and subsequent transformation of ductile β into brittle oxygen-enriched α(O). The second fracture mechanism affected a region both enriched in oxygen and in hydrogen due to secondary hydriding. In the investigated conditions, fracture location was independent of burst morphology, oxidation time and axial loading. A fracture scenario as a function of the reflooding transient was proposed. Physical fracture mechanisms were elucidated with respect to microstructural evolution in these two regions. Based on the local distribution of chemical elements and phases, a fracture threshold function of the local, layered microstructure was suggested for each fracture location.