Development and Validation of Evaluation Method on Hypothetical Total Instantaneous Flow Blockage in Sodium-Cooled Fast Reactors and its Application to a Middle Size SFR

Local faults (LFs) have been considered to be of greater importance in safety evaluation in sodium-cooled fast reactors (SFRs) because fuel elements were generally densely arranged in the subassemblies (SAs) in this type of reactors, and because power densities were higher compared with those in light water reactors. A hypothetical total instantaneous flow blockage at the coolant inlet of an SA (HTIB) gives most severe consequences among a variety of flow blockages. Although an evaluation on the consequences of HTIB using SAS4A code was also performed in the past study, SAS4A code was further developed by implementing analytical model of power control system in this study. An evaluation on the consequences of HTIB in Monju by this developed SAS4A code was performed in this study. Furthermore SAS4A code was newly validated using an in-pile experiment which simulated HTIB events. The validity of SAS4A application to safety evaluation on the consequence of HTIB was further enhanced in this study. It was clarified by the analyses considering power control system that the reactor would be safely shut down by the PPS triggered by either of 116% over power or DND trip signals. Therefore the conclusion in the past study that the consequences of HTIB would be much less severe than that of ULOF was strongly supported by this study.