A METHOD FOR IMPROVEMENT OF SAFETY FEATURES OF LARGE FAST BREEDER-REACTORS - NUMERICAL-SIMULATION OF UNPROTECTED LOSS-OF-FLOW ACCIDENT IN AN LMFBR EQUIPPED WITH GAS-EXPANSION MODULES

Numerical simulation of an unprotected loss-of-flow (ULOF) accident has been performed for a large liquid-metal-cooled fast breeder reactor (LMFBR) equipped with gas expansion modules (GEMs) in the radial periphery of the reactor core. The effectiveness of the GEMs in small fast reactors was demonstrated already in the passive safety testing in the Fast Flux Test Facility. According to neutronic calculations based on the transport theory, even in large reactors of electrical power 600 to 1,300 MW, the reactivity worth of GEMs, which replace one layer of radial blanket fuel subassemblies, ranges from -1.9$ to -1.4$, depending on the size of the core. A simulation of ULOF transient was performed with a 5.5s flow-halving time in a 600 MWe LMFBR equipped with GEMs of -1.9$ reactivity worth. The result showed that, if 10% of the rated core coolant flow by pony motors was available following the main pump coastdown, the GEM reactivity alone could bring the reactor subcritical and the predicted maximum coolant temperature was substantially lower than the sodium boiling point. The reactivity worth calculations, a modeling of gas expansion behavior, and ULOF simulation together with needs of further development for the GEM application are described.