Preliminary risk assessment for the transport of reactor decommissioning waste in the United Kingdom

It has been decided to decommission the UKAEA Windscale Advance Gas-cooled Reactor (WAGR) to a 'green field' site to provide information on procedures and costs. This paper describes a risk assessment for the transport to a suitable repository of 200 large concrete packages containing about 700 t of resulting waste material. The packages are cubic with external dimensions a little over 2 m. Prototypes have been drop tested from heights of 0.65 m and 5 m and they sustained insufficient damage to impair the containment and shielding significantly. An impact equivalent to a 15 m drop was calculated to be necessary to expose the package contents. An event tree approach was adopted to determine the frequency of severe mechanical and thermal loading during road and rail transport. Collisions with a second vehicle or fixed object, falls from bridges, serious fires and crane failure during unloading were considered. The frequency of rail accidents sufficiently severe to expose the package contents was calculated to be about 5x10-7 per year. The frequency of these severe impacts in conjunction with a serious fire was found to be two orders of magnitude smaller. The package weight would limit the vehicle speed on UK roads and no accident which could expose the package contents was considered plausible. Preliminary calculations indicated that the consequences of a transport accident (increased gamma radiation and dispersion of flammable waste by fire) would be relatively minor. It was concluded that transport risks should not present obstacles to the decommissioning of the WAGR. (author)