Some recent research results and their implications for RPV irradiation surveillance under long term operation
During the last decade the irradiation behaviour of characteristic RPV steels used in Pressurized Water Reactors has been extensively investigated in the international research programs CARISMA, CARINA and LONGLIFE. Whereas the CARISMA and CARINA programs were mainly focused on material testing of German RPV steels and data base applications by the RTNDT and Master Curve concepts, the main emphasis of the LONGLIFE project was placed on microstructural examinations of a wide range of RPV steels used in European NPP. Atom Probe Tomography, Small Angle Neutron Scattering, Transmission Electron Microscopy and others belonged to the techniques applied. Four of those RPV steels which are particularly interesting under the point of view of long term irradiation behavior have been studied at different irradiation levels up to 8×10 n/cm (E > 1 MeV) in all three research programs. In this way it was possible to get a comprehensive understanding of the material behavior induced by neutron irradiation and the responsible mechanisms behind. By means of the material properties and microstructural data obtained, in particular the impact of chemical composition (copper and nickel), and neutron flux on the material’s fracture toughness was quantified. Moreover, the most important mechanisms such as formation of element specific precipitations, segregations, and matrix defects could be demonstrated in relation to the change of material properties. For one material the occurrence of late irradiation effects has been observed in both material properties and microstructural appearance. All the available data are discussed thoroughly by consulting the relevant literature in the light of the practical implications they have for RPV irradiation surveillance under long term operation of 60 years and beyond. Specific issues like material specific application of reference temperature concepts, data scattering, prediction of high fluence behavior and use of material test reactor data are addressed. Finally, based on the results obtained by the research programs concerned, best practices for RPV irradiation surveillance programs are suggested and exemplified by industrial experience and adopted measures already in use.
A Physically Based Correlation of Irradiation-Induced Transition Temperature Shifts for RPV Steels
This paper presents a physically-based, empirically calibrated model for estimating irradiation-induced transition temperature shifts in reactor pressure vessel steels, based on a broader database and more complete understanding of embrittlement mechanisms than was available for earlier models. Brief descriptions of the underlying radiation damage mechanisms and the database are included, but the emphasis is on the model and the quality of its fit to U.S. power reactor surveillance data. The model is compared to a random sample of surveillance data that were set aside and not used in fitting and to selected independent data from test reactor irradiations, in both cases showing good ability to predict data that were not used for calibration. The model is a good fit to the surveillance data, with no significant residual error trends for variables included in the model or additional variables that could be included.
On the effect of dose rate on irradiation hardening of RPV steels
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.