Loss-of-cooling accident calculations for the wet storage pool of NPP Gösgen/Däniken

•Severe accident calculation for first wet storage pool in Switzerland.•Definition of event table for accident management measures.•Influence of storage policy on accident progression.•Possible molten core concrete interaction in late phase of a severe accident in a wet storage pool. The wet storage building of the nuclear power plant Gösgen/Däniken, a 3002 MWth pressurized water reactor, is the first building for the long term wet storage of spent nuclear fuel in Switzerland. Due to the Swiss moratorium against the export of spent nuclear fuel in 2006 and the non-availability of a nuclear repository it was necessary to store the spent fuel locally at the nuclear power plant site. The prolongation of this moratorium in 2016 does not allow reprocessing of spent nuclear fuel in the next years. As a direct conclusion from this moratorium it was decided to build a wet storage pool at Gösgen/Däniken which entered operation in 2008 with a capacity of 504 fuel assemblies. In a second stage of completion another 504 fuel assemblies can be stored in the pool. Two cooling towers will be available for the passive cooling of the wet storage pool with a maximum cooling capacity of 1.5 MW after the second stage of completion. In this wet storage pool only spent nuclear fuel elements will be stored, which have been stored before more than four years in an at reactor spent fuel pool. This means that the nuclear heat of a single 15x15 rod nuclear fuel assembly (FA) used in the NPP Gösgen is at 1.6 kW maximum, when it is stored in the wet storage pool. The present work describes postulated severe accidents with total loss of cooling in the wet storage pool (WSP) of the Gösgen/Däniken nuclear power plant. The accident progression in a spent fuel pool and even more in a wet storage pool is very slow due to the low nuclear decay heat. Therefore the investigation of such kind of accident was not assumed to be of high importance for a long time. The overall accepted assumption of an energy recovery after at least 24 h would not lead to boiling of a spent fuel pool due to the low decay heat. The severe accident in Fukushima Daiichi (2011.03.11) following the great Eastern earthquake and the related tsunami has shown that the power recovery can be delayed much longer, e.g. for more than a week. This long time of loss of cooling could endanger a spent fuel pool especially if the whole core is unloaded into the pool like in Fukushima Daiichi unit 4. Main goal of the work is to achie