A comparative study of unprotected loss of flow accidents in 500 MWe FBR metal cores with PFBR oxide core

Analyses of unprotected loss of flow accidents for 500 MWe U-Pu-6%Zr and U-Pu-10%Zr metal fueled sodium cooled reactors are presented and compared with that of the 500 MWe (U-Pu) MOX Prototype Fast Breeder Reactor (PFBR - under construction in Kalpakkam, India). A flow halving time of 8 s is considered for all the cases. It is found that the results of the metal fuel cases are close to each other. The loss of flow accident is benign for the metal fueled reactors where it is found that sodium coolant boiling is delayed up to 900 s, without credit for safety grade decay heat removal systems. In contrast, the oxide fueled reactor shows much earlier onset of sodium boiling and fuel slumping, leading to near prompt criticality and entry into the disassembly phase. Thus it is concluded that unprotected loss of flow accidents in metal fueled reactors are benign and allow sufficient time for operator action, if safety grade decay heat removal systems are able to remove the decay heat.