Study on initiating phase of core disruptive accident (Validation study of SAS4A code for the unprotected transient overpower accident)

Abstract

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 SAS4A code was significantly enhanced by applying Phenomena Identification and Ranking Table (PIRT) approach to the Unprotected Loss of Flow (ULOF). SAS4A code has been developed to analyze the early stage of CDA, which is named Initiating Phase (IP). 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. The phenomena were identified by the investigation of UTOP event progression and physical phenomena relating to UTOP were ranked in order of the degree of importance to the figure of merit (FOM), which is the average fuel temperature. 8 key phenomena were identified and the differences in ranking between UTOP and ULOF were clarified. The code validation matrix was completed and an SAS4A model, which was not validated in ULOF, was identified and validated. In this study, SAS4A code became applicable to various scenarios by using PIRT approach to UTOP and the reliability and validity of SAS4A code were significantly enhanced.