Effect of debris material composition on post accidental heat removal in a sodium cooled fast reactor

Following a core meltdown accident in a pool type sodium cooled fast breeder reactor (SFR), a debris bed is formed over the core catcher. The influence of initial material composition of the debris bed (consisting of fuel and steel) on the post accidental heat removal processes has been investigated in view of reactor safety. A one dimensional transient multiphase thermal model has been developed and employed for this purpose. The model includes sodium boiling, steel melting, fuel melting, slumping of molten particles and formation of molten pool. The overall steel fraction in the debris bed, the extent of steel stratification in the bed due to density difference between the debris particles and the overall porosity of the debris bed are considered as main parameters. It is seen that the assumption of a homogeneous initial mixture of fuel and steel in the debris bed yields a low core catcher plate (CCP) temperature compared to stratified debris beds where steel fraction increases with debris height. In a debris mixture with a low steel content (20% steel and 40% fuel), the peak temperature experienced by the CCP increases only marginally, from 872 K for homogeneous mixture to 900 K for a highly stratified mixture. But, in a debris mixture rich in steel content (30% steel and 30% fuel) the corresponding peak temperatures experienced by CCP are 886 K and 932 K respectively, suggesting that material stratification has significant effect in steel-rich debris mixture. For identical steel stratification profiles, the peak CCP temperature in a steel lean debris mixture is lower than that in a steel rich debris mixture. For 20% overall steel fraction, the peak temperature experienced by CCP is less than the creep limit of 923 K even for the worst stratification. But for 30% overall steel fraction, increasing the extent of steel stratification above a specific value results in CCP peak temperature exceeding the safe limit based on creep failure. Thus, the thermal load on the CCP increases with steel content though the thermal conductivity of steel is much higher than that of the fuel.