Understanding post-irradiation creep behavior of M5Framatome zirconium alloy

Abstract

After in reactor use, the nuclear fuel assembly of pressurized water reactors is stored in water and is eventually transported, in a dry cask, to a long-term storage facility or to a reprocessing plant. During transportation in dry environment the fuel rod temperature and internal pressure both increase. This leads to simultaneous creep deformation and annealing of the cladding tubes made of M5Framatome11M5 and M5Framatome are trademarks or registered trademarks of Framatome or its affiliates, in the USA or other countries. zirconium alloy. This study shows that post-irradiation creep, or heat treatment, leads to the recovery of the radiation induced hardening and also to the recovery of the uniform elongation because of <a>-loop annealing. On the other hand, it is shown that cladding tubes made of M5Framatome alloy exhibit low post-irradiation creep rates, especially at low applied stress. This strain rate is significantly lower than for the unirradiated material and similar to the as-irradiated material, despite the significant <a> loop annealing. The low creep rates are mainly attributed to the Nb-rich nano-precipitates that act as dispersed strengthening particles. These nano-precipitates appear under irradiation and are stable at temperatures up to 450 °C explaining their strengthening effect even after significant annealing.