Application of probabilistic safety assessment to the pipe rupture incident at Hamaoka Unit-1

On November 7, 2001, a pipe rupture occurred in the steam condensation line of the residual heat removal system at the Hamaoka Unit-1 while operating at rated power, resulting in steam release with radioactivity into the reactor building and the high pressure coolant injection being unavailable. The reactor was manually shut down immediately after the pipe rupture and there was no radioactive release into the environment. Although this incident was not safety significant, the type of rupture was unprecedented. Thus, the regulatory body, the Nuclear and Industrial Safety Agency (NISA), investigated event causes and examined corrective actions for preventing recurrence, and issued the report describing the investigation results, on May 13, 2002. At the NISA's request, the Nuclear Power Engineering Corporation (NUPEC) evaluated the safety significance of the incident and the effectiveness of corrective actions from the risk point of view by carrying out the Probabilistic Safety Assessment (PSA), that is, the estimation of the core damage frequencies (CDFs). The results showed that the incident has no risk significance and that the individual corrective actions would not lead to any significant increase in CDFs. The present study is the first PSA application to the regulatory decision-making process in Japan. (author)