ULOF transient behaviour of metal-fuelled fast breeder reactor cores as a function of core size and perturbation methods

•Metal fuel FBR safety can be assessed by its response to unprotected transients.•Safety during unprotected loss of flow accident (ULOF) is important for FBR cores.•ULOF analyses are carried out as a function of core size and perturbation method.•Smaller metal cores are found to be safer with respect to the ULOF accidents.•1st order perturbation method gives conservative results in an ULOF accident. The safety behaviour of metal-fuelled fast breeder reactor cores may be assessed by their transient behaviour during anticipated unprotected transients. Out of such transients, unprotected loss of flow accident (ULOFA) has been recognized as an event important for determining reactor safety due to the positive sodium void coefficient of reactivity and the remote possibility of complete power failure as initiator. Reactor safety under ULOFA condition is particularly based on the inherent feedbacks, which is calculated using the removal worths and Doppler constants. As the removal worth is a strong function of reactor size, ULOF analyses are carried out in three different reactor size viz. 120MWe, 500MWe and 1000MWe. The study reveals that smaller metal cores are safer than larger cores with respect to the ULOF accidents in the pre-disassembly phase. The present study also shows that the use of exact perturbation based reactivity worths introduce no significant changes in the safety behaviour of metal fuel reactor compared to that with the use of first order perturbation worths in pre-disassembly phase. The first order approximation is found to be valid as the expansion of materials in the core during ULOFA is small before the core enters the disassembly phase.