Evaluation of ECCS actuation events

Since commercial nuclear power plants were installed, a number of incidents have occurred. Actuation of the emergency core cooling system (ECCS), which maintains core safety in the case of coolant accidents, occur very few times in Japan, but occur rather often abroad. Learning from these experiences is one effective means for improving the reliability of nuclear power generation. In this report, information on ECCS actuation events is retrieved from data bases on accidents experienced at operating nuclear power stations in Japan and abroad up to the present. Through analysis of the causes of these events, various means of preventing the recurrence of similar events are discussed. First, the Secondary Data Base for ECCS Events (SDBEE) was established from the data retrieved, to facilitate cause analysis. Next, the causes were classified into the following two categories and analyzed by adapting SDBEE. (1) Actual Signal: ECCS actuation events due to an actual signal triggered by changes in such actual plant parameters as pressure, temperature etc. (2) Spurious Actuation: ECCS actuation events due to an improper safeguard signal without an accompanying change in such actual plant parameters as pressure, temperature etc. It has been reported that the dominant causes of actual signal events are improper plant operation due to malfunction of such equipment as valves, and one kind of spurious actuation event is incorrect blocking of the ECCS actuation signal during maintenance work. Some means for preventing the recurrence of major events are presented. (author)