SAS4A analysis on hypothetical total instantaneous flow blockage in SFRs based on in-pile experiments

Fukano, Yoshitaka  

Local fault (LF) has been historically considered as one of the possible causes of severe accidents in sodium-cooled fast reactors (SFRs). Safety assessments on the consequences of three major LF initiators, adventitious-fuel-pin-failures, local over-powers and large flow blockages, have already been performed for the Japanese prototype fast breeder reactor (Monju) and resulted in limited damage of within a fuel subassembly. Although the frequency of hypothetical total instantaneous flow blockage at the coolant inlet of a fuel subassembly (HTIB) is negligible, in-pile experiments on this event have been performed because it gives the most severe consequences among the above-mentioned LF initiators. Therefore a most rapid damage propagation scenario was newly proposed in this study based on past experiments, and a safety evaluation was performed on the consequences of HTIB in Monju using the SAS4A code, additionally validated by an in-pile experiment especially on the HTIB-related phenomena. It was clarified through the evaluation that the consequences of HTIB were much less severe than that of the unprotected loss of flow (ULOF) events which represent the most severe accidents in Monju. Therefore it was concluded that all the consequences of LFs in Monju can be comprised in that of ULOF. This evaluation method would be also applicable to enveloping safety assessments on LFs in arbitrary SFRs.