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 (Δσ_y) 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 Δσ_y 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 Δσ_y 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 (ϕt_e) as ϕt_e ≈ ϕt(ϕ_r /ϕ)^1/2, where ϕ_r is a reference flux. The ϕ ^−1/2 scaling is consistent with a vacancy plus self-interstitial-atom (SIA) recombination rate controlling mechanism. The Δσ_y 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 Δσ_y. 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.

Acknowledgements

The authors gratefully acknowledge the many people who have contributed to the UCSB IVAR program. Phil Simpson (University of Michigan) and Dennis Heatherly (ORNL) have been instrumental in making IVAR facility a great success. We thank David Gragg (UCSB) for his tireless contributions to the preparation of the specimens and irradiation capsules. We also appreciate many helpful discussions with our colleagues Brian Wirth (UC Berkeley), Gene Lucas (UCSB), Tim Williams (RRA), Randy Nanstad (ORNL) and Colin English (AEA Harwell), and also thank them for other contributions too numerous to list. The sets of alloys supplied by Wayne Pavanich (consultant), Randy Nanstad, Tim Williams and P. Tipping (Paul Scherrer Institute) were also a key element of this work. This research was funded under NRC Contracts #04-94-049 and 04-01-064. Thus we also express thanks to our NRC program monitors, Mike Vassilaros, Carolyn Fairbanks and Tanny Santos, for providing continuing encouragement and support. Finally, although not the focus of the present paper, we also acknowledge the outstanding facilities and staff at the NIST Center for Neutron Research, where we carried out our SANS measurements.