The K East and K West reactors and associated spent fuel storage basins were built in the mid-1950s in support of Hanford’s plutonium production mission. The fuel storage basins provided temporary storage of irradiated fuel discharged from the reactors prior to shipment to fuel processing facilities at the Hanford Site. In October 2022 workers completed construction of the K East Reactor Interim Safe Storage enclosure, a process known as "cocooning."