Annealing for plant life management: Hardness, tensile and Charpy toughness properties of irradiated, annealed and re-irradiated mock-up low alloy nuclear pressure vessel steel

Abstract

Hardness, tensile and Charpy properties of an irradiated (I) and irradiated-annealed-reirradiated (IAR) mock-up pressure vessel steel are presented. Spectrum tailored pressurized light water reactor (PWR) irradiation at 290°C by fast neutrons up to nominal fluences of 5 × 10 19/ cm 2 ( E ≥ 1 MeV ) in a swimming pool type reactor caused the hardness, tensile yield stress and tensile strength to increase. Embrittlement also occurred as indicated by Charpy toughness tests. The optimum annealing heat treatment for the main program was determined using isochronal and isothermal runs on the material and measuring the Vickers microhardness. The response to an intermediate annealing treatment (460°C for 18 h), when 50% of the target fluence had been reached and then irradiating to the required end fluence (IAR condition) was then monitored further by Charpy and tensile mechanical properties. Annealing was beneficial in mitigating overall hardening or embrittlement effects. The rate of re-embrittlement after annealing and re-irradiating was no faster than when no annealing had been perfomed. Annealing temperatures below 440°C were indicated as requiring relatively long times, i.e. ≥168 h to achieve some reduction in radiation induced hardness for example. The overall benefit of an appropriate annealing treatment has been demonstrated on a laboratory scale, and annealing is therefore indicated as being a useful tool in the overall strategy for achieving plant life management goals.