SAS4A analyses of SCARABEE in-pile experiments simulating hypothetical total instantaneous flow blockages in SFRs

Fukano, Yoshitaka  

A hypothetical total instantaneous flow blockage at the coolant inlet of an SA (HTIB) gives most severe consequences among a variety of initiating event for local faults (LFs) such as adventitious fuel pin failure, local overpower and flow blockages. An evaluation on the consequences of HTIB was performed in the past study as an enveloping event analysis among a wide spectrum of initiating events for LFs. Although the SAS4A code has been validated by a number of in-pile experiments in the French CABRI and U.S. TREAT reactors, most of them were performed under loss-of-flow (LOF) combined with transient-overpower (TOP) conditions. The changing rate of flow in HTIB is much more rapid than that in these LOF test. Therefore additional and more expedient validation was undergone in this study using the TIB experiments which were performed in the French SCARABEE reactor especially for its modeling of coolant boiling, cladding melting, molten cladding motion, fuel melting and wrapper tube failure anticipated to occur during an HTIB condition. Four TIB experiments were performed with 19 or 37 pin bundles in the SCARABEE reactor. SAS4A analyses on these experiments showed good agreement with those experimental results in the following phenomena which were anticipated to occur during an HTIB condition: (1) Timing and progress of coolant boiling and cladding dryout; (2) Timing of cladding melting and behavior of molten cladding relocation; (3) Timing and progress of fuel melting, disruption and relocation; (4) Timing of wrapper tube melt-through. Therefore it can be concluded that the validity of SAS4A application to safety evaluation on the consequence of HTIB in the past study is enhanced in this study.