SAFETY MARGINS AFTER FAILURE OF FUEL CLADDING DURING PROTECTED LOSS-OF-HEAT-SINK ACCIDENTS IN A SODIUM-COOLED FAST REACTOR

摘要

Protected loss-of-heat-sink (PLOHS) events are identified as one of the most dominant sequences in a sodium-cooled fast reactor. Safety margins for significant core damage in PLOHS events were therefore studied in this paper assuming fuel cladding failure. The following three possible mechanisms leading to degradation of the core were then identified to be scrutinized by a thorough and state-of-the-art review of open papers on the phenomena anticipated to occur under cladding failure conditions: (1) Fuel melting due to fuel-sodium reaction product (FSRP) formation, (2) Thermal transient due to FP gas impingement from adjacent failed fuel pins, and (3) Mechanical load due to the same FP gas impingement. Following results were thus obtained through simulation analyses on each phenomenon mentioned above using the FUCALF code: (1) No fuel melting occurs due to FSRP formation, (2) No fuel melting occurs due to FP gas impingement from adjacent failed fuel pins, and (3) No mechanical pin failure occurs due to the same FP gas impingement. Moreover, these are valid at the coolant temperatures of up to 950 degree C. It was therefore concluded that large safety margins are provided during PLOHS events even in failure of fuel cladding. These results will be effectively used in formulating the safety criteria for SAs or BDB As as one of the supporting evidences to be seriously considered.