Lithuanian Energy Institute Experience on Modelling Spent Fuel Pools During Severe Accident Conditions

The tsunami that followed the earthquake at the Fukushima Daiichi nuclear plants in Japan showed that a loss of heat removal in the spent fuel pools (SFPs) may lead to very serious consequences. The consequences of such an accident can be very serious creating a possibility of significant amount of radioactive material release to the environment and can possibly be equivalent to the Chernobyl accident, which has been rated at 7 on the International Nuclear Event Scale. This is because SFPs are in general not housed in a containment with the same integrity as the containment around the reactor core and primary pressure boundary. Thus, the loss of water, which leads to the loss of heat removal in SFP may lead to very serious consequences. In this article the Lithuanian Energy Institute experience and main conclusions on the modelling of SFPs during severe accident conditions are shortly presented. This experience is based on the modelling provided for the Ignalina NPP (RBMK-1500) SFP. In this article 2 cases of the modelling of loss of water in the SFPs are presented related to the Ignalina NPP (RBMK-1500): - The evaluation of the worst possible consequences, assuming the maximal amount of Spent Fuel Assemblies (SFA) in the pools and the maximal possible residual heat of nuclear fuel. This case with the theoretically maximal possible residual heat in SFP is related to the moment before the shutdown of Unit 2 of the Ignalina NPP for the decommissioning (i.e. at the end of 2009). - The second analysis was performed for situation - three years after the permanent shutdown of Unit 2 reactor of Ignalina NPP.