Influence of High Fluence Irradiation on Reference Temperature of VVER-440 RPV Surveillance Specimens

Master curve (MC) testing of VVER-440 RPV surveillance specimens treated for 27 years (∼200,000 h) in a surveillance channel of Metsamor-NPP was performed to investigate the influence of long term high fluence irradiation on RPV embrittlement. The surveillance chain consisted of both thermal aged specimens (above the core level) and irradiated specimens (inside the core). The reference temperature (To) values obtained from irradiated specimens are compared with the results from thermal aged specimens to characterize irradiation induced shifts in To values for both base and weld metal specimens. It was found that the high fluence irradiation up to a nominal fluence of 3.2 × 1025 n.m-2 at E > 0.5 MeV resulted large embrittlement with To shift values greater than 300 °C for both base and weld metal specimens. The obtained shifts in To values at these high fluence values were used to compare with the predictions from PNAE procedure in the Russian regulatory guide outside its validity range. It was found that the measured shift in reference temperature for the weld metal was well below the predicted value while, the shift in reference temperature for base metal was largely under-predicted at these high fluence values.