Microstructural Characterization of RPV Materials Irradiated to High Fluences at High Flux

Abstract

Understanding the embrittlement of reactor pressure vessel (RPV) steels at high fluence region is very important for the long term operation of nuclear power plants. In this study, extensive microstructural analyses were performed on the RPV steels irradiated to very high fluences beyond 1020n/cm2, E>1 MeVat high fluxes under the Pressurized Thermal Shock and Nuclear Power Plant Integrity Management projects in Japan. Three dimensional atom probe analyses were performed to characterize the solute atom cluster formation in these materials. The effects of fluence, flux, and chemical compositions on the characteristics of clusters were analyzed. The formation of dislocation loops was identified in the transmission electron microscopy analyses of high and low Cu steels, and the changes in loop size and number density with fluence were studied. P segregation on grain boundaries was also studied by surface analyses as well as grain boundary chemical analyses. We found that nonhardening embrittlement due to grain boundary fracture is not a major contributor to the embrittlement in these materials and irradiation conditions. The correlation of the microstructural changes and the Charpy transition temperature shifts was studied. The volume fraction of solute atom clusters has an excellent correlation with the transition temperature shifts. The Orowan model calculations of the contributions of dislocation loops to the transition temperature shifts show that in low Cu materials, dislocation loops may be a major contributor, but in Cu containing materials its contribution is weak. Root-sum-square of the contributions of solute atom clusters and dislocation loops seems to be a reasonable model to describe the total ΔRTNDT.

Effects of chemical composition and dose on microstructure evolution and hardening of neutron-irradiated reactor pressure vessel steels

“…Fukuya and coworkers [39] have reported nanostructural evolution of A533B steel with Cu contents of 0.12 and 0.16 wt.% subjected to irradiation doses from 3.1 to 6.8 Â 10 19 n cm À2 under a wide range of flux from 7.8 Â 10 10 to 5 Â 10 12 n cm À2 s À1 , the former of which is the in-service operation condition. A similar study has been made on surveillance specimens weld metals with the Cu content of 0.13 and 0.3 wt.% by Toyama et al [40] and on the various specimens irradiated in testing reactors by Soneda et al [41]. The interesting features are summarized as follows.…”

60th Anniversary of electricity production from light water reactors: Historical review of the contribution of materials science to the safety of the pressure vessel

“…An irradiation-induced grain hardening due to P was also reported for low Cu steels (Cu < 0.1%) (e.g., [53,54,55,56,57]. It has been observed that raising P content increases the number density of irradiation-induced Cu-rich defects (e.g., [55,58]), see Paragraph 8.2.…”

“…Ni alone has a low impact on the formation of Cu-rich clusters [67], while Mn alone strongly increases their number density and volume fraction [133], and ii) for a given Mn content, their number density and volume fraction sharply increase with Ni [67]. The latter result has been confirmed on RPV steels (e.g., [58]). -Long term aging (up to 100,000 hours) at 330°C on high Cu steels (Cu > 0.4 wt%) leads to the formation of Cu-rich precipitates made of a Cu-rich core surrounded by a shell of Mn, Ni and Si atoms, very similar to clusters obtained after neutron irradiation [134,135].…”

“…Cu-rich clusters are only observed when the solid solution Cu is higher than about 0.05-0.06 wt% (e.g., [58,66,125,126,127,119]). Many studies reported that the increase of their average radius and number density is sharp at low fluences and then slows down (e.g., [71,123,127]).…”

“…From thermodynamic considerations, some teams (e.g., [85,137,140] MNPs that do not contain Cu atoms are expected to form from very high fluences (> 10 20 n.cm -2 ) [137] -hence the name Late Blooming Phases -although some authors report their presence from 3 10 19 n.cm -2 in low-Cu steels [58]. This "late blooming effect" might have to be considered for 80-year extension programs of current LWRs.…”