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.

Keywords:

• Radioactive materials transport, Risk assessment, Cask design, Radiation absorption

Notes

† Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

‡ The term ‘routine transportation’ is used throughout this document to mean incident- or accident-free transportation.

§ The currently registered RADTRAN users are listed on a restricted-access Web site at Sandia National Laboratories.

** Neuhauser et al.² is the technical manual for RADTRAN 5 and is cited because the basic equations for the incident-free analyses in RADTRAN 6 are the same as those in RADTRAN 5. The technical manual for RADTRAN 6 is not yet available.

†† The wording of the regulation is actually ‘two meters from a plane surface perpendicular to the outside edge of the vehicle carrying the cask’.¹⁹ The vehicle, in the case of SNF transportation, is the trailer on which the cask is mounted. The RADTRAN calculation takes no credit for any offset of the cask from the trailer edge.

‡‡ Thirty metres is typically as close as a person on the side of the road can get to a vehicle travelling on an interstate highway.

§§ Appendix II of Ref. 15 contains a detailed discussion on the collective dose.

*** The five persons per railcar in urban areas are assumed to include occupants of passenger trains. Passenger trains carry more than five per car, but the majority of railcars even in urban areas carry freight only. This estimate is consistent with estimates made in past studies.

††† 2004 is the most recent year for which data have been validated.

‡‡‡ The States of Oregon and New Jersey still require gas station attendants to refuel cars and light duty vehicles, but heavy truck crews do their own refueling.