On the effect of dose rate on irradiation hardening of RPV steels

Abstract

The effect of dose rate (DR), or neutron flux (ϕ), on irradiation hardening (Δσy) and embrittlement of reactor pressure vessel (RPV) steels is a key unresolved issue. We report a rigorous evaluation of DR effects based on a very large Δσy database we developed for RPV steels with a wide range of compositions, including a set of split-melt alloys with controlled and systematic variation in Cu, Ni and Mn content. The steels were irradiated at 290°C in three ϕ-regimes to a wide range of overlapping fluences (ϕt). The contribution of copper-rich precipitates (CRPs) to Δσy increases up to a plateau hardening that is a strong function of the alloy Cu, Ni and Mn content, but is relatively independent of DR. However, the pre-plateau region is shifted to higher ϕt with increasing DR. The shift can be approximately accounted for by defining an effective fluence (ϕte) as ϕte ≈ ϕt(ϕr /ϕ)1/2, where ϕr is a reference flux. The ϕ −1/2 scaling is consistent with a vacancy plus self-interstitial-atom (SIA) recombination rate controlling mechanism. The Δσy data are analysed with a combined model describing: (a) the excess vacancy concentration under irradiation as a function of DR, including the effect of solute vacancy traps on recombination; (b) the corresponding radiation enhanced Cu diffusion (RED) coefficient (D*); (c) the resulting accelerated growth of CRPs; and (d) the contribution of CRPs to Δσy. Recombination is shown to increase with higher alloy Ni and Mn content, consistent with a solute–vacancy trapping mechanism. In spite of high recombination rates, however, RED is extremely efficient, with the D* ranging up to a factor of 60 or more times higher than predicted by simple rate theory models. Various explanations of the high diffusion rates are discussed, including large vacancy–solute binding energies that control the vacancy concentrations and jump frequencies near solutes in a way that can enhance both diffusion and recombination.