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Transporting nuclear waste requires a sturdy container that can shield from radioactivity. A specially designed container called a cask is used. There are different cask types for different purposes, but they all have similar overall design to maximize the containment of radioactivity.
A cask is a strong, heavily-shielded, double-walled container. The outer structure is of several inches of high-strength steel. The inner structure is usually made of steel as well. Casks meant to transport used nuclear fuel assemblies have a rack of square openings in this inner structure to provide support for those assemblies. If the cask is being used for transporting used nuclear fuel assemblies, the rack may also contain neutron-absorbing materials to safeguard against the unlikely event of a nuclear chain reaction.
For transporting used nuclear fuel assemblies, the inner canister is dried and filled with an inert gas (usually helium) to prevent long-term corrosion of the fuel assemblies. The casks also usually feature several inches of lead or depleted uranium (which is not radioactive) between the inner and outer structures to provide gamma ray shielding. The inner canister is then sealed, preventing any release of radioactive material. Large honeycomb structures made of wood, foam, or aluminum are placed on the ends of the casks to absorb the force the cask would experience in the event of a drop.
Materials such as used nuclear reactor fuel assemblies, radioactive resins (such as those used to filter radioactive material from water), contaminated clothing or tools, and isotopes used in nuclear medicine are some examples of nuclear waste that may be transported in a cask.
The cask systems are designed to handle a wide range of severe environmental conditions, including earthquakes, flooding, extreme temperatures, and tornadoes. This includes objects that may impact the cask at high speed perhaps during a tornado. The cask systems are able to store waste for several decades, perhaps for more than a century. They are also designed to withstand extreme events, such as a 30 foot drop onto a hard surface, and a 1475° F fire engulfing the cask for 30 minutes. Casks have also been subjected to tests such as being struck broadside by a locomotive travelling at 80 MPH, without failure of containment.
All transportation casks must be very robust, and must be designed to withstand a similar set of extreme events or conditions. However, the requirements are somewhat less stringent for casks that ship less radioactive payloads.
The safety record for the storage and transportation of radioactive materials is exceptional. There have been over 3,000 used nuclear fuel shipments in the US (approximately 24,000 worldwide), over the course of several decades, and there has never been a release of harmful amounts of radiation, or any injuries, deaths or environmental damage.
Center for Nuclear Science and Technology Information of the American Nuclear Society
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