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.