Report of examination of the sample from core shroud (K1-H4) at the Kashiwazaki-Kariwa Nuclear Power Station Unit-1 (Contract research)

At the Kashiwazaki-Kariwa Nuclear Power Station Unit-1 of the Tokyo Electric Power Company (TEPCO), cracks were confirmed at the weld joint (H4) in the middle of core shroud, by the visual inspection test for the weld joint of core shroud during the 13th periodic examination of September, 2002 by a direction of the Nuclear and Industrial Agency. TEPCO has conducted a material examination with Nippon Nuclear Fuel Development Co., Ltd. (NFD) on the specimen including cracks sampled from the core shroud. The present research has been performed with the objective to independently investigate and evaluate the materials by jointly attending the examination with NFD from the planning stage, receiving the final data given by the examination and providing JAERI's own evaluation report as a third-party organization for assuring of transparency. This report summarizes the examination results and following results were obtained; (1) Cracks initiated at inner surface branched three-dimensionally as they propagated. The crack was over through the extracted sample. (2) Almost all cracks were intergranular cracking. (3) Some cracks reached the weld metal. (4) Corrosion products clung to crack surface, and some of them were propagated into grains. (5) The thickness of the surface hardening layer was about 50-70 micro-meters. (6) Distinct chromium depletion such as degradation of thermal sensitized materials was not detected at grain boundaries. (7) The values of chemical composition for examined material were near the minimum of type 304L stainless steel in Japan Industrial Standard (JIS G4304). From the above results, the considerations of residual stress induced with welding process and dissolved oxygen concentration in core cooling water, it was concluded that the cracks were initiated by stress corrosion cracking and propagated three-dimensionally through grains, and some cracks reached weld metal. (author)