the b reactor at the hanford site, near richland, washington, was the first large-scale nuclear reactor ever built. the project was commissioned to produce plutonium-239 by neutron activation as part of the manhattan project, the united states nuclear weapons development program during world war ii. the b reactor was fueled with metallic natural uranium, graphite moderated, and water-cooled. it has been designated a u.s. national historic landmark since august 19, 2008 and in july 2011 the national park service recommended that the b reactor be included in the manhattan project national historical park commemorating the manhattan project. visitors can take a tour of the reactor by advance reservation.