Spray cooling performance during spent fuel pool accident with MAAP code

The effectiveness evaluation of the safety measures under severe accidents is demanded; not only for nuclear reactors but also spent fuel pools (SFPs) after the Fukushima-Daiichi nuclear power plant accident which hit the Tokyo Electric Power Company on March 11, 2011. This paper addresses models of the spent fuel pool with accident analysis code, MAAP and a sensitivity analysis of the spray cooling parameters following a loss of coolant accident (LOCA) to evaluate the fuel coolability. In this study, spray water is injected from the upper space of SFP onto fuel racks using an SFP model attached to a MAAP code. The instantaneous LOCA in SFP was hypothetically assumed, which meant the spent fuels were directly exposed to the atmosphere from the onset of the event. Furthermore, the sensitivity of the spray cooling parameters was analyzed to investigate the influence on how the instantaneous LOCA sequence in the SFP will progress. The spray cooling model of the MAAP code includes a range of input parameters such as the spray-flow rate, spray water temperature, and so on. For example, when the spray-flow rate suffices to cool the fuel, the maximum temperature of the fuel cladding decreases after the onset of spray cooling with a particular time delay. Furthermore, increasing the spray-flow rate helps reduce any time delay and accelerate cooling. Conversely, when the spray-flow rate is insufficient for cooling, the fuel cladding temperature continues to rise and ends up damaging the fuel. The spray-flow rate significantly impacts on the cooling of the fuel accordingly. The MAAP code demonstrates a time trace of the maximum cladding temperature in terms of spray cooling parameters. In addition, certain spray cooling parameters may impact on the behavior of fission products (FP). For example, particles drifting in the atmosphere in aerosol form can be easily adsorbed by spray droplets when the spray-flow rate increases, which further eliminates FP. We confirmed the impact on FP removal of the spray.