Evaluation of post accident molten material relocation time to the core catcher in a fast breeder reactor

• Core-melt relocation time to the core catcher is estimated for severe accidents.
• Numerical heat transfer analysis carried out for pessimistic input conditions.
• Fuel subassembly parts modelled as porous body with effective properties.
• Molten material displacement is accounted region-wise in PLOHS analysis.

Core Disruptive Accident (CDA) is categorized as a Beyond Design Basis Event in a Fast Breeder Reactor (FBR) because of highly reliable, redundant and diverse plant protection systems. Nonetheless, it is analysed with a view to assess its consequences and take remedial measures to mitigate its consequences. In the present paper, core-melt relocation studies have been carried out to assess the time taken for molten core material to get relocated to core catcher, by melting through various structures beneath the active core. Unprotected Loss of Flow Accident (ULOFA) and Protected Loss of Heat Sink (PLOHS) accident are the bounding events arising from different accident initiators. These are considered with the objective of estimating minimum and maximum time for core-melt relocation to the core catcher for the future Indian FBRs. Numerical heat transfer analysis has been carried out for both the postulated accident scenarios with conservative boundary conditions. In the case of ULOFA where the core-melt is envisaged to reach the grid plate within a few seconds, grid plate melt-through occurs in 300 s for constant molten fuel attack. On the other hand, for a PLOHS scenario, molten core relocation from core mid-plane to core catcher takes place in about 5.5 h with very conservative input conditions.