Analyses of Severe Accident Sequences in the Spent Fuel Pool of the VVER-1000 Type of Reactor With MELCOR 1.8.6

Since the Fukushima nuclear disaster in 2011, much attention has been paid to investigation of severe accidents (SA) progression in spent fuel pools (SFP) of various types of nuclear power plants (NPP). In Czech Republic, 4 VVER-440 and 2 VVER-1000 types of reactors (at the Dukovany and Temelin NPPs, respectively) are currently under operation. In order to enhance their safety, especially with respect to station black-out (SBO) events, numerical analyses have been carried out evaluating the risks associated with accidents occurring also in the SFP. The present paper deals with analyses of 2 postulated scenarios (loss of cooling and loss of coolant) and is mainly focused on the input deck preparation for the integral, lumped parameter (LP) code for SA analyses — MELCOR 1.8.6. Emphasis is put on description of correct implementation of the complex geometry of the SFP, consisting of 3 distinct pools separated by concrete walls (lined with steel plates) in which fuel assemblies (FA) are stored in an absorber grid (rack). In the description of the prepared numerical model, light is shed on the encountered modeling issues, and corresponding hints and solutions are proposed in order to provide guidance for preparing adequate models for various types of SFP designs. Finally, some of the most characteristic results are presented for both postulated scenarios.