Weld metal irradiation embrittlement analysis in the range of over-design neutron fluences

The comparison of experimental values of the critical brittle temperature ∆T_F and reference temperature ∆T₀ of VVER-1000 reactor vessel weld metal with an elevated content of manganese and nickel is performed. ∆T_F and ∆T₀ values are defined proceeding from the standard impact bend Charpy and Charpy cracked fracture toughness specimen tests, respectively. Specimens were irradiated in industrial reactors in the frame of surveillance specimen program up to the fast (E ≥ 0.5 MeV) neutron fluences corresponding to the NPP long term operation period. The research results showed the shifts ∆T_F and ∆T₀ to agree with each other. Besides, it was discovered that in the range of over-design fluences the design embrittlement model has a tendency to underestimate the critical brittle temperature shift. (author)