Eddy Current Tests (ECT) were conducted at Mihama Unit 2 during outage in September 2007 and also at Tsuruga Unit 2during outage in October 2007 to verify the integrity of the inner surfaces of the welds (600 Series nickel-based alloy)on the primary coolant inlet nozzle stubs in the steam generators.The tests identified significant signal indications.The subsequent Ultrasonic Tests (UT) confirmed cracks measuring up to 13mm in depth.
　 In response to these two incidents, the Nuclear and Industrial Safety Agency (NISA) instructed operators of Pressurized Water Reactors (PWR) on October 18, 2007 to step up their monitoring of any leakage from the welds on the inlet / outlet nozzle stubs in steam generators that use the same 600 Series nickel-based alloy.It also instructed them, on November 16 of the same year, to inspect the inner surfaces of the welds on the inlet / outlet nozzle stubs in steam generators during the next outage.In view of the facts that all the identified cracks ran in the axial direction, and that 600 Series nickel-based alloy is highly ductile, it has been assessed that, even if the cracks aggravated to cause minor leakage, this would not cause immediate damage to piping*.
　 PWR operators conduct inspections according to the plans.The table below shows the current status (as of June 17, 2008) of inspections on primary inlet nozzle stubs, where 600 Series nickel-based alloy is used for welding.The cracks are so far attributed to primary water stress corrosion cracking (PWSCC) under the primary coolant environment, whereby 600 Series nickel-based alloy, sensitive to stress corrosion cracks, was subject to residual tensile stress generated at the time of welding.No cracks have been found in the welds of outlet nozzle stubs in steam generators.