Report of examination of the samples from primary loop recirculation piping (K1-PLR) at Kashiwazaki-Kariwa Nuclear Power Station Unit-1 (Contract research)

At the Kashiwazaki-Kariwa Nuclear Power Station Unit-1 (BWR, 1100 MW) of the Tokyo Electric Co., Inc. (TEPCO), indications of cracks were identified in a weld joint portion of the primary loop recirculation piping by the ultrasonic inspection test during periodic inspections. The portion where cracks were found is in important piping consisting pressure boundary of nuclear reactor involving high pressure water. To investigate the cause of cracks, samples were cut from the representative portion judged by TEPCO. TEPCO has conducted a material examination with Nippon Nuclear Fuel Development Co. Ltd. (NFD) on the specimen including cracks. The present investigation was carried out with the objective to ensure transparency of the examination by attending the examination with NFD from the planning stage and providing JAERI's own evaluation report as a third party organization. As the results of the present examination, the following findings were made; (1) A crack was observed near the weld region of the inner surface of piping and its depth was about 7 mm. (2) Intergranular cracking was observed at almost whole fracture surface. (3) At the crack opening region of the inner surface of piping, transgranular cracking was observed in the depth of about 100 μm. Microstructure formed by cold work and increases of hardness were also observed in this region and the crack was initiated near the region where hardness value showed the highest. (4) Chemical compositions of the material satisfied the material specification of Japanese Industrial Standard G 3214, SUS F316. However, Content of Cr was very slightly depleted in the vicinity of grain boundary. The present observed cracks were concluded to be stress corrosion cracking related to hardened region of the sample surface, based on the above examination results with the expected tensile residual stress near the cracks and dissolved oxygen content in cooling water. (author)