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.
Embrittlement Correlation Method for the Japanese Reactor Pressure Vessel Materials
A new embrittlement correlation method developed for the Japanese reactor pressure vessel (RPV) steels is presented. The Central Research Institute of Electric Power Industry and the Japanese electric utilities conducted a project to develop a new embrittlement correlation method for the Japanese RPV steels based on the understandings on the mechanisms of the RPV embrittlement. In addition to the information from the literatures, we generated new information by characterizing the microstructural changes in the surveillance materials of the Japanese commercial reactors. We found that in low Cu materials, solute atom clusters containing little or no Cu atoms are formed at relatively low fluence of 3×1019 n/cm2, E>1 MeV. The volume fraction of the solute atom clusters has a good correlation with the Charpy transition temperature shift regardless of the Cu content. We also found that the microstructure of the boiling water reactor surveillance material is different from that of the archive material irradiated in material testing reactor. The understandings on the RPV embrittlement mechanisms were formulated using a set of rate equations, and the coefficients of the equations were optimized using the ΔRTNDT values of the Japanese surveillance database. This method considers the effect of neutron flux. Only one set of coefficients was developed, and they are independent of the product form. Predictions of the new embrittlement correlation method were compared with those of the recent U.S. correlation method as well as the U.S. surveillance data. The comparison shows the characteristics of the present method.
A computational microscopy study of nanostructural evolution in irradiated pressure vessel steels
Abstract Nanostructural features that form in reactor pressure vessel steels under neutron irradiation at around 300°C lead to significant hardening and embrittlement. Continuum thermodynamic-kinetic based rate theories have been very successful in modeling the general characteristics of the copper and manganese nickel rich precipitate evolution, often the dominant source of embrittlement. However, a more detailed atomic scale understanding of these features is needed to interpret experimental measurements and better underpin predictive embrittlement models. Further, other embrittling features, believed to be subnanometer defect (vacancy)-solute complexes and small regions of modest enrichment of solutes are not well understood. A general approach to modeling embrittlement nanostructures, based on the concept of a computational microscope, is described. The objective of the computational microscope is to self-consistently integrate atomic scale simulations with other sources of information, including a wide range of experiments. In this work, lattice Monte Carlo (LMC) simulations are used to resolve the chemically and structurally complex nature of CuMnNiSi precipitates. The LMC simulations unify various nanoscale analytical characterization methods and basic thermodynamics. The LMC simulations also reveal that significant coupled vacancy and solute clustering takes place during cascade aging. The cascade clustering produces the metastable vacancy-cluster solute complexes that mediate flux effects. Cascade solute clustering may also play a role in the formation of dilute atmospheres of solute enrichment and enhance the nucleation of manganese-nickel rich precipitates at low Cu levels. Further, the simulations suggest that complex, highly correlated processes (e.g. cluster diffusion, formation of favored vacancy diffusion paths and solute scavenging vacancy cluster complexes) may lead to anomalous fast thermal aging kinetics at temperatures below about 450°C. The potential technical significance of these phenomena is described.