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. The project was a key part of 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 in the site's T Plant, as an alternative to the Project's uranium enrichment plants. The B reactor was graphite moderated and water-cooled, via a contaminating open cycle with the Columbia River.