Report of examination of the sample from core shrouds (K3-H7a) at Kashiwazaki-Kariwa Nuclear Power Station Unit-3. Contract research

The Tokyo Electric Power Company (TEPCO) visually inspected the weld joint of shroud at the Kashiwazaki-Kariwa Nuclear Power Station Unit-3 (BWR, 1100MW) during the 7th periodical inspection of August 2002 by a direction of the Nuclear and Industrial Safety Agency. Cracks were observed at around the inner surface of circumference ring near the weld joint (H7a) of the shroud lower shell and the ring. Core shroud is a cylindrical structure located in reactor pressure vessel and its function are to support in-core components and to form flow route of coolant from lower portion of in-core shroud to in-core by jet pump in normal operation. 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 reviewing the examination with NFD and providing JAERI's own evaluation as a third-party organization for assuring of transparency. The following findings were obtained; (1) The scratches due to grinding and machining were observed on the surface of the boat-shaped sample. (2) Most of the fracture appearance was intergranular except for the region crack initiated at the surface. The fracture appearance near the surface was estimated to be transgranular one from the metallographic crack observation, although the fracture appearance at a distance of about 300 μm from the surface was not obvious by covered oxides. In the transgranular fraction region, the microstructural feature of cold working with increase in hardness was detected. There was no difference of the breadth of the cold working microstructure under the scratches due to grinding and machining, (3) Hardness near the surface within a distance of about 3 mm from the fusion line was decreased due to welding heat. (4) The chemical compositions of the sample satisfied the material specification of Japanese Industrial Standard G 4301SUS 316L. A slight depletion of Cr was observed in the vicinity of the grain boundaries. The observed cracks were concluded to be stress corrosion cracking related to the hardened region at the surface, based on the above results with an expected tensile residual stress near the cracks and dissolved oxygen content in the coolant. (author)