The B Reactor at the Hanford Site in the U.S. state of Washington began producing plutonium, with the facility later going on to create more for nearly the entire American nuclear arsenal.

The B Reactor at the Hanford Site, near Richland, Washington, was the first large-scale nuclear reactor ever built, at 250 MW. It achieved criticality on September 26, 1944. This reactor was of vital importance to the Manhattan Project, the United States nuclear weapons development program during World War II. Its purpose was to convert part of its natural uranium fuel into plutonium-239 by neutron activation, for use in nuclear weapons. Pure plutonium was then chemically separated at the site's T Plant, as an alternative to the Project's uranium enrichment plants in Tennessee. The B reactor was graphite moderated and water-cooled, via a contaminating open cycle with the Columbia River.