Study of Scenarios Related to Design Extension Conditions of PHWR Using PSA Methodology

Studies to establish the event classification framework and acceptance criteria for an event that exceeds the existing DBAs in order to enhance the safety operation of nuclear power plants in accordance with the domestic nuclear safety laws amended recently have been doing briskly in the country. In this perspective, a study of DECs for domestic PWRs has already begun, and accordingly, studies to cope with BDBAs including DECs for domestic CANDU plants also are required. In this paper, a postulated event which might be included into DECs for domestic CANDU is analyzed by CATHENA code, and Probabilistic Safety Assessment(PSA) data needed for selecting events for DECs are drawn. In this paper, based on the assumption that all the heat sinks and power are not available, countermeasure capability against loss of safety functions such as power supply system in domestic CANDU are assessed. The results and available power and heat sink source which are the required coping facilities for mitigating the accident are presented and the available limited time also are assessed. Provided that ways of long-term core cooling are secured later, it is expected that the safety functions of a plant can be secured even in the extremely severe natural disasters