Although the Nuclear Regulatory Commission (NRC) would allow used fuel with as little as one year of cooling to be moved from the used fuel storage pool into dry storage, there currently are no NRC-licensed dry storage casks that could handle used fuel cooled for only one year. Since the heat generating rate of used fuel decreases very rapidly (exponentially) as the cooling time of the used fuel assemblies increases, designing and using dry cask storage systems is far easier and safer for used fuel that has been cooled longer in the used fuel storage pool. The shortest cooling time for used fuel assemblies in dry storage systems licensed by the NRC is three years and is only approved for a few cask designs; for the majority of NRC-licensed systems, the minimum cooling time is five years. In practice, most used fuel loaded into dry storage casks is cooled for seven or more years.
Learn more about nuclear energy and related topics in NEI’s “Ask an Expert” section.