Emergency Preparedness for Spent Fuel Shipments during Loss of Cooling Function in Ships

Rokueimaru is a ship, which is owned and managed by Nuclear Fuel Transport Co., for spent fuel shipments from nuclear power plants to the reprocessing facility in Japan. She satisfies with the requirements of Class INF 3 ship of IMO. She has cooling devices in her holds, because of spent fuel heat. We have evaluated hazards of natural events related to nuclear material transport, one of the hazards is “loss of cooling function” in case of ships rides on land due to tsunami. This hazard is to be occurred as a result of the cooling device stopping because of inability to take seawater by riding on land, however probability of this hazard occurring is very low. We evaluated the hazard as the following process. First, we evaluated the influence by analysis for this hazard assessment. In the assessment, we analysed under conditions maximum number (four) of the packages loaded in a hold and cooling devices stopped. As a result, it was shown that the seal of the packages could be broken after a certain period of time because of high design heat generation rate of the packages and conservative analysis model (not considering heat radiation from the side of the hold). Next, we studied a mitigation measure to open all apertures from inside of the hold to outside directly. We changed the analysis model to more realistic, and analysed by the model in consideration of the mitigation measure. As the result, we confirmed that the seal integrity could be maintained in case of up to two packages in a hold if the mitigation measure was carried out. Furthermore, we conducted parameter survey on the heat generation rate for maintaining the seal integrity in case of loading 3 or 4 packages in a hold, and we obtained the acceptable heat generation rates. Based on these results, we confirmed the seal integrity can be maintained at the “loss of cooling function” event by restricting the number of loaded packages in a hold or heat generation rate of the packages, and to take mitigation measure as emergency preparedness.