Study on initiating phase of core disruptive accident. Identification of important phenomena in initiating phase of unprotected transient overpower accident

In the safety study of sodium cooled fast reactor, it is important to appropriately evaluate the event progression of Core Disruptive Accident (CDA). In previous studies, the reliability and validity of the evaluation method of the early stage of CDA, which is named Initiating Phase (IP), was significantly enhanced by applying Phenomena Identification and Ranking Table (PIRT) approach to the Unprotected Loss of Flow (ULOF). In this study, PIRT approach was applied to Unprotected Transient over Power (UTOP), which was one of the most important and typical events in CDA as well as ULOF, and physical phenomena relating to UTOP were ranked in order of the degree of importance. As a result, 8 key phenomena were identified. Compared UTOP with ULOF, both rankings were almost same, however it was clarified in this study that the ranking of the physical phenomena related to the coolant in UTOP was lower than that in ULOF, since in UTOP primary flow remains constant and coolant behavior doesn't cause large reactivity changes. (author)