In December 1942, during
World War II, Thomas joined the
National Defense Research Committee (NDRC) as the Deputy Chief of its Division 8, which was responsible for propellants, explosives and the like. Early in 1943, he traveled to the East with
Richard Tolman, a member of the NDRC, and
James B. Conant, the president of
Harvard University and the chairman of the NDRC, to witness a demonstration of a new underwater explosive. Conant and Tolman took the opportunity to quietly investigate Thomas's background. Thomas was then invited to a meeting in Washington DC with
Brigadier General Leslie R. Groves, Jr., the director of the
Manhattan Project, and, as he discovered when he got there, Conant. Groves and Conant were hoping to harness his industrial expertise for the benefit of the project. They offered him a post as a deputy to
Robert Oppenheimer, at the
Los Alamos Laboratory in New Mexico, but he did not wish to move his family or give up his responsibilities at Monsanto. Instead he accepted the role of coordinating the
plutonium purification and production work being carried out at Los Alamos, the
Metallurgical Laboratory in Chicago,
Radiation laboratory in Berkeley, and
Ames Laboratory in Iowa. Monsanto's Central Research Department began to conduct research on behalf of the Manhattan Project as part of the Manhattan Project's
Dayton Project, some of which was conducted on the estate of his wife's family. Initially, there were concerns about the purity of plutonium, an element about which little was known, but Thomas was able to report to Groves and Conant in June 1944 that techniques had been developed that would yield highly pure plutonium, and that the problem was solved. Unfortunately, experiments by
Emilio G. Segrè and his P-5 Group at Los Alamos on reactor-produced plutonium showed that it contained impurities in the form of the
isotope plutonium-240, which has a far higher spontaneous fission rate than
plutonium-239, making it unsuitable for use in the
Thin Man gun-type nuclear weapon design. Thomas attended a series of crisis meetings in Chicago with Connant, Groves,
Arthur Compton,
Kenneth Nichols and
Enrico Fermi. It was agreed that the isotopes could not be separated, so high-purity plutonium would not be required. Thomas therefore decided to disband his plutonium purification team. The Los Alamos laboratory then turned to the technologically much more difficult task of building an
implosion-type nuclear weapon. Monsanto was already working on a key component of the device. In April 1943,
Robert Serber had proposed that instead of relying on spontaneous fission, the chain reaction inside the bomb should be triggered by a
neutron initiator. The best-known neutron sources were
radium-
beryllium and
polonium-beryllium. The later was chosen as it had a 140-day
half life, which made it intense enough to be useful but long-lived enough to be stockpiled. Thomas brought in Monsanto to work on the development of techniques to industrially refine polonium for use with beryllium in the
urchin detonators. He promised the Oakwood City Council that he would return the Runnymede Playhouse building intact after the war, but he was unable to keep this promise because the building became so badly contaminated with radioactivity. Thomas was one of a number of scientists who watched their work come to fruition on July 16, 1945, at the
Trinity nuclear test. For his work on the project, he received the
Medal for Merit from the
president Harry S. Truman in 1946. On May 2, 1945, Groves and Thomas agreed that Monsanto would take over the running of the
Clinton laboratories at Oak Ridge, Tennessee from July 1, 1945. Thomas brought in some 60 new staff from Dayton to help run the Clinton Laboratories, and he persuaded
Eugene Wigner to come from Chicago to work on new reactor designs. Under Wigner, the Laboratories made a pioneering study of
Wigner's disease, the swelling and distortion of the
graphite used as a moderator in reactors due to the neutron bombardment produced in a reactor. Thomas became frustrated with restrictions on spending and the uncertainty about the future of the laboratory. In May 1947, he decided not to renew the contract with the
Atomic Energy Commission to operate the Clinton Laboratories on a month-to-month basis while a new operator was found.
Union Carbide took over the contract in December 1947. Monsanto was, however, given the contract to operate the new Mound Laboratories in early 1948. ==Later life==