When nuclear fuel has been burned in a reactor and there is no permanent disposal site, there is an alternative to storing the spent fuel rods after they have cooled off in a pool of water for at least a year. Spent fuel rods can be stored in steel cylinders in what is called ‘dry cask’ storage.
Nuclear fuel rods are tubes of uranium oxide pellets. About two hundred of the tubes are packed together in an ‘assembly.” These assemblies are then inserted into the core of a reactor.
When they are spent, they are removed to a spent fuel pool. After a year, the assemblies can be removed from the pools and sealed into steel cylinders called canisters that are around 16 feet long and 6 feet in diameter. The canisters are either welded or bolted shut. Then the atmosphere in the canister is exhausted and replaced with an inert gas which is typically helium. Finally the steel canister is placed inside a ‘cask.’
The cask containing a welded canister consists of a concrete cylinder with walls over two feet thick. The cask containing the bolted canister is a six inch shell of boron doped metal that acts as a neutron shield .
The casks are stored in what are referred to as ‘Independent Spent Fuel Storage Installation’ which are licensed by the United States Nuclear Regulatory Commission. The first storage facility licensed by the NRC was the Surry Nuclear Power Plant in Virginia.
The casks may be stood upright in concrete or steel buildings. Alternatively, they may be placed horizontally in chambers about the size of a one-car garage. As of the end of 2010, there were forty eight dry cask storage facilities licensed at reactor sites and fifteen dry cask storage facilities not located at reactor sites. About sixty two metric tons of spent nuclear fuel currently exist in the United States. Of that, about twenty two percent, or fourteen tons, is stored in dry casks.
Dry casks can also be used to transport spent nuclear fuel to sites for permanent disposal when such a site is available. A typical design for truck transport is similar to the dry casks intended for storage but there are some differences. The diameter of the cask diameter is four feet which is expanded to six feet on the ends by disk shaped ‘impact limiters.” The overall length including the impact limiter disks is twenty feet. There is an inner steel canister surrounded by a lead gamma shield which in turn is enclosed in a second steel shell. The second steel shell is encased in a neutron shield and a final shell. Average weight is twenty five tons.
Rail transport casks very similar in terms of shells and shields but are much bigger. Cask diameter is eight feet with overall diameter at eleven feet. The overall length is twenty five feet. The average weight of such an cask is about one hundred and twenty five tons so it can transport five times as much as a truck cask.
These casks should last at least a hundred years but they were only intended as temporary storage. The existing dry cask storage facilities may be targets for terrorists or vulnerable to natural disasters. Unfortunately with the cancellation of the Yucca Mountain Repository, the United States currently has no permanent spent fuel storage site.
