A nuclear flask is a shipping container that is used to transport active nuclear materials between many nuclear power stations in the UK. Each flask weighs more than 50 tonnes, and transports usually not more than 2.5 tonnes of spent nuclear fuel.[1]
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Introduced in the early 1960s, Magnox flasks consists of 4 layers; an internal skip containing the waste; guides and protectors surrounding the skip; all contained within the 14-inch-thick (360 mm) steel main body of flask itself, with characteristic cooling fins; and (since the early 1990s) a transport cabin of panels which provide an external housing. Flasks for waste from the later advanced gas cooled reactor power stations are similar, but have less thick steel main walls at 3-inch thickness to allow room for extensive internal lead shielding. The flask is protected by a bolt hasp which prevents the content from being accessed during transit.
All the flasks are owned by the Nuclear Decommissioning Authority, the owners of Direct Rail Services. A train conveying flasks would be hauled by two locomotives, either Class 20 or Class 37, but Class 66 locomotives are increasingly being used. Locomotives are used in pairs as a precaution in case one fails en route. Greenpeace protest that flasks in rail transit pose a hazard to passengers standing on platforms, although many tests performed by the Health and Safety Executive have proved that it is perfectly safe for passengers to stand on the platform while a flask passes by.
The crashworthiness of the flask was demonstrated publicly when a British Rail Class 46 locomotive was forcibly driven into a derailed flask (containing water and steel rods in place of radioactive material) at 100 miles per hour, the flask sustaining minimal superficial damage without compromising its integrity, while both the flatbed wagon carrying it and the locomotive were more-or-less destroyed. Additionally, flasks were heated to temperatures of over 800 °C to prove its safety in a fire. However, critics consider the testing flawed for various reasons. The heat test is claimed to be considerably below that of theoretical worst case fires in a tunnel, and the worst case impact today would have a closing speed of around 170 miles per hour. Nevertheless there have been several accidents involving flasks, including derailments, collisions and even a flask being dropped during transfer from train to road, with no leakage having occurred in any case.
Problems have, however, been found where flasks "sweat", that is, where small amounts of radioactive material absorbed into paint migrate to the surface causing contamination risks. Studies [2][3] identified that 10–15% of flasks in the United Kingdom were suffering from this problem, but none exceeded the international recommended safety limits. Similar flasks in mainland Europe were found to exceed the safety during testing and additional monitoring procedures were put into place. In order to reduce the risk current UK flask wagons are fitted with a lockable cover to ensure any surface contamination remains within the container and all containers are tested before shipment, with those exceeding the safety level being cleaned until they are within the limit.
Probably the most comprehensive safety study was the report Nuclear Waste Trains Investigative Committee: Scrutiny of the transportation of nuclear waste by train through London, October 2001. This report identified risks in the areas of emergency polices and particularly in the area of security of sites such as marshalling yards where there was a risk of terrorist attack.