Spent fuel transportation risk assessment: routine transportation†

Abstract The RADTRAN model for calculating radiation doses is based on the well understood behaviour of ionising radiation. Absorption of ionising radiation depends on the energy and type of radiation and on the absorbing material. The casks that are used to transport spent nuclear fuel have walls that absorb most of the emitted ionising radiation and thereby shield the public and the workers. For routine transportation, RADTRAN models the cask as a sphere and assumes that the longest dimension of the trailer or railcar carrying the cask is the same as that of the cask. The dose rate in Sv/h at one metre from the cask is modelled as a virtual source at the centre of a sphere whose diameter is the longest dimension of the actual spent fuel cask. People who live along the cask’s route and the people in vehicles that share the route are exposed to external radiation from the cask. The dose to workers and the public from a cask during routine transportation depends on the time that the workers or public are exposed to the cask, the distance from the cask, and the cask’s external radiation. When the vehicle carrying the cask is travelling along the route, the faster the vehicle goes, the less exposure to anyone along the vehicle’s route. Therefore, an individual member of the public receives the largest dose from a moving vehicle when he or she is as close as possible to the vehicle, and the vehicle is travelling as slowly as possible. In the present analysis, these doses are in the range of four to seven nanosieverts. Collective doses along the route depend on the size of the exposed population. In this study, such doses were of the order of 0·1 person-millisieverts. The appropriate comparison between the collective dose from a shipment of spent fuel is not a comparison between the radiation dose from the shipment and zero dose, but between the background radiation dose in the presence and absence of a shipment, e.g. 8·810096 person-Sv if there is a shipment and 8·81000 person-Sv if there is no shipment.