On flux effects in a low alloy steel from a Swedish reactor pressure vessel

This study aims to investigate the presence of Unstable Matrix Defects in irradiated pressure vessel steel from weldments of the Swedish PWR Ringhals 4 (R4). Hardness tests have been performed on low flux (surveillance material) and high flux (Halden reactor) irradiated material samples in combination with heat treatments at temperatures of 330, 360 and 390 °C in order to reveal eventual recovery of any hardening features induced by irradiation. The experiments carried out in this study could not reveal any hardness recovery related to Unstable Matrix Defects at relevant temperatures. However, a difference in hardness recovery was found between the low and the high flux samples at heat treatments at higher temperatures than expected for the annihilation of Unstable Matrix Defects–the observed recovery is here attributed to differences of the solute clusters formed by the high and low flux irradiations. •Hardness testing is combined with post irradiation annealing at 330, 360 and 390 °C.•Unstable matrix defects is studied in a reactor pressure vessel steel.•Comparison between surveillance material and accelerated irradiation.•No evidence of unstable matrix defects, i.e. not present in studied material.•Difference in hardness recovery between irradiation conditions found at 390 °C.