Thorium Research in the Manhattan Project Era

Research on thorium as an energy source began in 1940 under the direction of Glenn Seaborg at the University of California, Berkeley. Following the discovery of plutonium-239 and its fissile qualities, similar experiments demonstrated that uranium-233 bred from thorium was also fissile. Seaborg viewed uranium-233 as a potential backup to plutonium-239, whose production was one of the Manhattan Project's primary efforts. The central appeal of U-233 was that the chemistry of uranium was well understood, unlike plutonium, but plutonium-239 had the potential to be produced from natural uranium in a critical nuclear reactor. Natural thorium lacked fissile isotopes and so a critical nuclear reactor (to produce U-233) from thorium alone was not possible. Not until the X-10 graphite reactor was constructed at Oak Ridge in 1943 was sufficient U-233 created to conclusively assess its nuclear properties, which were found to be superior to Pu-239 in a thermal-spectrum reactor. Early production of plutonium at X-10 showed significant contamination by Pu-240, which made plutonium unsuitable for simple "gun-type" nuclear weapons. Researchers in the "Metallurgical Laboratory" at the University of Chicago, which included Seaborg's chemistry group, suggested that the plutonium produced be used as a fuel in a special reactor to convert thorium to uranium-233 for weapons. This effort encountered many severe difficulties in fuel fabrication and dissolution. Seaborg also recognized the severe issue that uranium-232 contamination would play in any effort to use uranium-233 for weapons. Through tremendous effort, weapons designers at Los Alamos were able to design workable weapons using the implosion principle, which accommodated for the impure plutonium produced. Interest in U-233 for weapons effectively disappeared by 1945, but the Metallurgical Laboratory continued to investigate the potential of a thorium-U-233 "breeder" reactor, based on a homogeneous mixture of uranium salts in heavy water. This effort also came to an end in early 1945. With the end of World War II, the United States was fully focused on growing its nuclear weapons stockpile, and thorium/uranium-233 lacked relevance to that mission as the Manhattan Project concluded at the end of calendar year 1946.