The Hanford Site is a decommissioned nuclear production complex operated by the United States federal government on the Columbia River in Benton County in the U.S. state of Washington. It has also been known as Site W and the Hanford Nuclear Reservation. Established in 1943 as part of the Manhattan Project, the site was home to the Hanford Engineer Works and B Reactor, the first full-scale plutonium production reactor in the world. Plutonium manufactured at the site was used in the first atomic bomb, which was tested in the Trinity nuclear test, and in the Fat Man bomb used in the bombing of Nagasaki.

"Fat Man" was the codename for the type of nuclear weapon the United States detonated over the Japanese city of Nagasaki on 9 August 1945. It was the second of the only two nuclear weapons ever used in warfare, the first being Little Boy, and its detonation marked the third nuclear explosion in history. It was built by scientists and engineers at Los Alamos Laboratory using plutonium from the Hanford Site, and one was dropped from the Boeing B-29 Superfortress Bockscar piloted by Major Charles Sweeney.