Hydride reorientation in M5® cladding and its impact on mechanical properties

After the end of their in-PWR life, spent nuclear fuel assemblies (SNF) might be subjected to various stress-temperature-time histories. Ensuring cladding integrity in the SNF is crucial in order to establish regulatory acceptance criteria. Radial hydride formation concomitant to SNA cooling is a key technical issue for zirconium-based fuel rod cladding tubes. Indeed, the precipitation of radial hydrides may degrade the mechanical properties of the cladding and potentially compromise its mechanical integrity. Several studies have been devoted to the investigation of the reorientation of hydrides (HRO) and its impact on cladding mechanical strength for Zircaloy 2 and Zircaloy 4. In this paper, we complement those results with HRO studies conducted on as-received and irradiated M5® cladding. Our results have mainly highlighted : for as-received material, no detrimental effect of radial hydrides ; for irradiated material, a significant impact of irradiation defect annealing which leads to an increase of cladding ductility and a strong decrease of the adverse effect of radial hydrides.