The Plutonium Finishing Plant, also known as the Z Plant, was part of the Hanford Site plutonium production complex in Washington state. During World War II, Hanford produced plutonium nitrate (Pu(NO3)2), which was shipped to the Manhattan Project's Los Alamos Laboratory, where it was turned into metallic plutonium and made into pits for nuclear weapons. After the war ended, the Los Alamos Laboratory moved to divest itself of production activities in order to devote itself to research. The Plutonium Finishing Plant was built at Hanford to carry out the purification and reduction of the plutonium to metallic form, known as "buttons", and then perform the casting, grinding and lathing of the plutonium to turn it into pits.
The plant operated from 1949 to 1989. In 1953, it began shipping plutonium buttons to the new Rocky Flats Plant in Colorado, which fabricated pits. Plutonium production peaked in 1965, when 4,500 kilograms was produced. Between 1957 and 1961, nine different types of pits were produced at Hanford. Pit production ended in 1965, when the Atomic Energy Commission announced that henceforth this work would be undertaken at the Rocky Flats Site. As demand for weapons-grade plutonium declined after 1964, the Plutonium Finishing Plant began producing mixed plutonium-oxide uranium-oxide (MOX) fuel for Hanford's Fast Flux Test Facility and reactor-grade plutonium.
Plutonium was valuable, and reducing waste saved landfill dispatches and preserved the long-term radiological safety of the area by not burying the highly contaminated waste. The Plutonium Finishing Plant reclaimed solid waste in its RECUPLEX facility, combustible waste in the 232-Z Incinerator, and liquid waste in the 242-Z Waste Treatment Facility. A multi-purpose Plutonium Reclamation Facility opened in 1964. A serious accident at the 242-Z Waste Treatment Facility occurred in 1976, when the contents of a glove box containing americium and plutonium exploded, seriously injuring an operator, Harold McCluskey. This accident prompted a series of reviews and evaluations that led to the 1978 Department of Energy decision to close most of the Plutonium Finishing Plant's facilities. Before the plant could be demolished, approximately 18 metric tons of plutonium-bearing material was stabilized between 1996 and 2004. Legacy plutonium from plant systems was removed by 2005, and all weapons-grade plutonium was shipped to the Savannah River Site by 2009. Demolition work on the plant began in 2017 and was completed in 2021.