Embrittlement Issues in RPV Materials; Might They Be Used Beyond Their Current Fluence Range?

Abstract

Reactor Pressure Vessel (RPV) materials are well proven suitable materials for nuclear applications. Surveillance data from both pressurized Water Reactors (PWR) and Water-cooled Water-moderated Energy Reactor (WWER) up to 0.08–0.45 dpa and research data up to approx 1–2 dpa are available for irradiation temperature of 300°C. The response to radiation is very well understood as well as the effect of deleterious impurities. The data have been analysed in details, showing the strong role of impurities like copper and phosphorus besides basic matrix damage (MD). Reliable embrittlement models have been developed for the range of pressure vessel application. In fact, nowadays, very ‘clean’ materials with minimum radiation sensitivity are utilized. For ‘clean’ RPV steels, basic matrix damage is the main damage mechanism, described as a power function of the dose with exponents normally from 1/3 to 1/2. New advanced ferritic-martensitic (FM) materials have been designed for fusion and GEN IV applications, as for example Eurofer steel. Irradiation embrittlement data at high doses for this material are already available, also at 300°C. Both RPV and FM materials may be suitable also for GEN IV applications; in particular for the pressure vessel. In this study, the embrittlement curves of RPV and Eurofer are compared showing continuity and a common embrittlement trend till intermediate doses. For very high doses, over 1–2 dpa where only Eurofer data are available, damage rate saturation tendency is observed. The observed continuity and partial overlapping can be explained by a common damage underlying mechanism of the different steels, e.g. ferrite damage, in spite of different steel structures and content (e.g. from 1% Cr for PWR, 2.25% Cr for WWER and 9% Cr for Eurofer steel. Form the modeling point of view the matrix term need to be adjusted for doses far higher than those experienced for RPV applications; in fact the predictions are far too conservative and a new descriptive term need to be developed. In conclusion, clean RPV materials together with new developed steels can be conveniently utilised at 300°C also outside their envisaged dose range.