Effectiveness of Thermal Annealing in Recovery of Tensile Properties of Compositionally Tailored PWR Model Steels Irradiated in LYRA-10

Understanding the mechanical behaviour of reactor pressure vessel (RPV) steels at high fluences has become an important topic in regard to Long-Term Operations (LTO) of existing nuclear power plants (NPP). The effectiveness of thermal annealing treatments to recover the mechanical properties of compositionally tailored pressurised water reactor (PWR) model steels irradiated to high neutron fluences, up to 1.22 × 1020 n·cm−2, is analysed in this study. Tensile testing of four different PWR RPV steels was performed after irradiation and subsequent recovery annealing treatment at 450 °C for 40 h. Irradiation-induced hardening and the effectiveness of recovery thermal annealing have been assessed by comparing the strength and ductility properties of irradiated and irradiated and subsequently annealed samples with unirradiated reference samples for all four model steel. The annealing treatment resulted in a significant recovery of the yield strength (~75–89%) and the ultimate tensile strength (~78–96%) of all four PWR model steels. This study proves that substantial irradiation-induced hardening (up to ~389 MPa) observed in steels containing high Ni and Mn contents can still be recovered using the thermal annealing treatment. No influence of annealing on ductility properties has been observed for all four model steels. Microscopy analyses of these steels to understand the underlying irradiation damage and recovery mechanisms are planned for the near future