Wikipedia
The N-Reactor was a water/graphite- moderated nuclear reactor constructed during the Cold War and operated by the U.S. government at the Hanford Site in Washington; it began production in 1963.
It was a one-of-a-kind design in the U.S., being both a plutonium production reactor for nuclear weapons and, from 1966, producing electricity to feed the civilian power grid via the Washington Public Power Supply System or WPPSS.
In an improvement on the earlier Hanford reactors, N-Reactor was built with a confinement building (although not 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 due to also using moderation from the coolant water, meaning it was thermally stable.
The reactor was shut down in 1987 and placed on cold standby in 1988, with "final disposition" beginning in 1994.