Validation study of SAS4A code for the unprotected loss-of-flow accident in an SFR

Abstract

Core Disruptive Accident (CDA) has been considered as one of the important safety issues in the severe accident evaluation of Sodium-cooled Fast Reactor (SFR), since SFR core is not in the most reactive configuration. Initiating Phase (IP) is the earliest stage of CDA, and SAS4A code was designed to precisely model the IP event progressions. Phenomena Identification and Ranking Table (PIRT) approach was applied to the SAS4A code in order to enhance its reliability in this study. This paper describes the study on the validation method and results of the validation of SAS4A using PIRT approach in a typical SFR. In this study, SAS4A validation was based on the generic PIRT process: (1) definition of issue of SAS4A code, (2) definition of PIRT objectives of using the PIRT approach, (3) selection of potential plant designs, (4) selection of potential scenarios, (5) selection of the figure of merit (FOM), (6) partition of scenario into convenient time phases, (7) definition of considerable physical regions, (8) identification of the phenomena, (9) ranking the important phenomena, (10) development of the code validation test matrix, and (11) test analyses for validation corresponding to the test matrix. Unprotected Loss Of Flow (ULOF) is selected as the specific scenario, since it is one of the most important and typical events in CDA. The average fuel temperature which corresponds to the energy release is selected as the FOM, since the energy release due to power excursion is important to evaluate whether the influence of CDA can be confined in the vessel. The phenomena are identified by the investigation of ULOF event progression and by making the system decomposition and hierarchy. These phenomena are ranked according to importance to the FOM. In the test matrix, the key phenomena are associated with SAS4A models, and the test cases used for validation of each model are shown. The results of the test analysis corresponding to this matrix show that the SAS4A models required for the IP evaluation were sufficiently validated. Furthermore, the validation with this matrix is highly reliable, since this matrix represents the comprehensive validation that also considers the relation among physical phenomena. In this study, the reliability and validity of SAS4A code were significantly enhanced by using PIRT approach to the sufficient level for CDA analyses in SFR.