Construction on B Reactor began in August 1943 and was completed on September 13 , 1944 . The reactor went critical in late September and , after overcoming neutron poisoning , produced its first plutonium on November 6 , 1944 . Plutonium was produced in the Hanford reactors when a uranium-238 atom in a fuel slug absorbed a neutron to form uranium-239 . U-239 rapidly undergoes beta decay to form neptunium-239 , which rapidly undergoes a second beta decay to form plutonium-239 . The irradiated fuel slugs were transported by rail to three huge remotely operated chemical separation plants called " canyons " that were about away . A series of chemical processing steps separated the small amount of plutonium that was produced from the remaining uranium and the fission waste products . This first batch of plutonium was refined in the 221-T plant from December 26 , 1944 , to February 2 , 1945 , and delivered to the Los Alamos laboratory in New Mexico on February 5 , 1945 . The material was used in Trinity , the first nuclear explosion , on July 16 , 1945
Trinity
Trinity (nuclear test)
Hanford Site