Confirmatory investigations on the flux effect and associated unstable matrix damage in RPV materials exposed to high neutron fluence

This paper provides additional experimental data on the neutron flux effect on RPV hardening and embrittlement and on the so-called unstable matrix damage that was suggested to occur at high flux. Six materials taken from the first irradiation surveillance capsules of Belgian PWRs with a fluence not exceeding about 1.5 E19 n/cm² were further irradiated in the BR2 high flux reactor to additional fluences ranging between about 1 and 1.5 E20 n/cm² at 290 °C. Eight additional RPV materials were selected to investigate the flux effect on irradiation hardening. No statistically-significant difference in irradiation hardening for low and high flux could be evidenced from the null hypothesis test applied with the general linear model. This is confirmed by additional experiments where fourteen irradiated specimens of various RPV materials consisting of low to high Cu and Ni contents were annealed at 350 °C for 5 h to eventually reveal some recovery of the unstable matrix damage. The results did not show any recovery upon heat treatment, which indicates that unstable matrix defects did not appear in these materials during irradiation at high flux.