Review of Pipe Wall Thinning Mechanism Study and its National Project in Japan

For nuclear power plants, the four major requirements are 1) high safety, 2) high reliability, 3) economical competitiveness, and 4) minimum environmental impact. However, it is still difficult to completely avoid problems concerning structural materials caused by stress corrosion cracking (SCC) and for piping systems caused by flow accelerated corrosion (FAC) and liquid drop impingement (LDI). Since especially FAC and LDI are uncertain phenomena as pipe wall thinning, there are the piping rupture accident risks on all piping systems under the specific conditions. In Japan, after August 2004, the accident of the secondary pipe rupture in Mihama Power Plant Unit 3, The Kansai Electric Power Co., Inc. (KEPCO), R&D projects on pipe wall thinning phenomena and mechanism have been employed by many organizations. On the other hand, evaluation of the safety and reliability of piping systems of long term operating plants and with the special attention to seismic condition have been requested. It was requested to enable evaluation of pipe wall thinning and its reliability with more accuracy. This project was programmed under the government budget from 2006 to 2010 fiscal year according to the Strategy Load-Map for Ageing Management generated by the society of industry, government and academia [1]. As the milestone for the first half decade of the load-map, the project had these achievements: 1) Establish computer program for FAC simulation, 2) Clarify droplet behavior for LDI prediction, 3) Simplified calculation model of pipe wall thinning for seismic evaluation, 4) Evaluate safety margin of thinned piping by FAC or LDI.