Analysis of the Processes in Spent Fuel Pools in Case of Loss of Heat Removal due to Water Leakage

The safe storage of spent fuel assemblies in the spent fuel pools is very important. These facilities are not covered by leaktight containment; thus, the consequences of overheating and melting of fuel in the spent fuel pools can be very severe. On the other hand, due to low decay heat of fuel assemblies, the processes in pools are slow in comparison with processes in reactor core during LOCA accident. Thus, the accident management measures play a very important role in case of some accidents in spent fuel pools. This paper presents the analysis of possible consequences of fuel overheating due to leakage of water from spent fuel pool. Also, the accident mitigation measure, the late injection of water was evaluated. The analysis was performed for the Ignalina NPP Unit 2 spent fuel pool, using system thermal hydraulic code for severe accident analysis ATHLET-CD. The phenomena, taking place during such accident, are discussed. Also, benchmarking of results of the same accident calculation using ASTEC and RELAP/SCDAPSIM codes is presented here. 1. Introduction According to the NRC data [1], the probability of a loss of coolant from spent fuel storage pool is about 10？6 per pool per year. The consequences can be very severe and a huge amount of radioactive materials can be released to the environment because the spent fuel pools are not covered by containment as the reactor core. As it is noted in the Operating Experience Feedback Report “Assessment of Spent Fuel Cooling” [2], during operation time, two losses of spent fuel pool coolant inventory events occurred, and decrease of water level by 1.5？m was registered. These real events were terminated by operator action, when approximately 6？m of water remained above the stored fuel. In the case without operator actions, the water loss could have continued, that could lead to the severe accident in spent fuel pool. During station blackout accident at Fukushima NPP after tsunami disaster in March 11, 2011, the water level in SFP of Unit 4 decreased more than 6.5？m, down to the 1.5？m above the top of SFA. The mitigation measures, water injection from nondesign sources, allow to prevent a severe accident (damage of fuel rods) in SFP of Fukushima NPP Unit 4. The possible consequences of water loss due to leakage and late operator actions (water injection to the spent fuel pool after fuel heat up) are evaluated in this paper. The evaluation of this accident was performed for Ignalina NPP Unit 2 spent fuel pool, assuming the possible highest decay heat generation in SFP (such situation was at the moment of final