An Approach to Controlling Inaccessible Regions for Inspection Defined by Rules on Fitness-For-Service in Japan

On September 2007, Primary Water Stress Corrosion Cracking (PWSCC) flaws were found on the dissimilar metal weld of the steam generator (SG) inlet pipe nozzles of Tsuruga-2. Following the Tsuruga-2, similar cases were found in some other plants. These cracks were located in inaccessible regions by Ultrasonic Testing (UT) examination applied from the outer surface. Triggered by these cases, the Nuclear and Industrial Safety Agency (NISA), Japanese regulator of nuclear industries, directed nuclear power plant owners to investigate current status of these inaccessible regions for inspection defined by rules on fitness-for-service in Japan, and required to show developing plan on alternative measures of inspection for UT-exempted welds. On the other hand, the study to manage inaccessible regions in the rules on fitness-for-service has been started. The authors consider and propose a possible approach for modified rules of inspection in order to make control of these inaccessible regions.