The N-Reactor was a graphite-moderated nuclear reactor constructed during the Cold War and operated by the U.S. government at the Hanford Site in Washington.
It was a one-of-a-kind design in the U.S., being both a power reactor used to feed the civilian electrical power grid via the Washington Public Power Supply System (WPPSS), and a plutonium production reactor for nuclear weapons.
N-Reactor was built with a confinement building instead of a containment building. In the event of an accidental release of steam, air and steam would vent through filters that confined any radioactive particles present. It was partially moderated with graphite, but had a negative void coefficient, meaning it was thermally stable.
The reactor was placed on cold standby in 1988, with "final disposition" beginning in 1994.