Research to understand the embrittlement behavior of Yankee/BR3 surveillance plate and other outlier RPV steels

The reactor pressure vessels at the Yankee Rowe and Belgian BR3 nuclear plants were constructed by Babcock and Wilcox in 1958. The plates of an open-hearth fabrication were welded using a submerged-arc process with Linde 80 flux as the filler. The original surveillance programs at the two plants were limited to representative A302B specimens; they feature similar chemistries as the ASTM reference plate, but coarser microstructure. The present testing program includes sixteen Charpy-V and four tensile specimens of the surveillance plate, irradiated at BR3 at a dose rate of ∼ 7 E10 cm-2.s-1 (> 1 MeV) over a period of 25 years; the investigation also addresses annealing and notch orientation effects. The new experimental results are compared to previously published data for the same and/or related melts. The Yankee/BR3 surveillance plate displays an anomalously large 41J Cv-shift as compared to the ASTM reference plate and to Regulatory predictions. Some of the Linde 80 welds investigated by the BR3 program are also found to behave as outliers. The data are evaluated in the light of state-of-the-art damage modeling and fracture micromechanics concepts, which are currently being incorporated into a new, consolidated strategy for improved RPV surveillance. The approach makes extensive use of the information contained in the load-deflection response of the instrumented Cv test. The implications of such analysis in terms of RPV steel embrittlement trend curve development are discussed. 90 refs