Using designs by the Metallurgical Laboratory, DuPont commenced construction of the plutonium semiworks at the Clinton Engineer Works in Oak Ridge on February 2, 1943.

[4] Szilard drafted a confidential letter to the President of the United States, Franklin D. Roosevelt, explaining the possibility of atomic bombs, and warning of the danger of a German nuclear weapon project.

[7] In April 1941, the National Defense Research Committee asked Arthur Compton, a Nobel-Prize-winning physics professor at the University of Chicago, to report on the uranium program.

[10] Emilio Segrè and Glenn Seaborg at the University of California produced 28 μg of plutonium in the 60-inch cyclotron there in May 1941 and found that it had 1.7 times the thermal neutron capture cross section of uranium-235.

[18] For the pilot plutonium separation plant, a site was wanted close to the Metallurgical Laboratory, where the research was being carried out, but for reasons of safety and security, it was not desirable to locate the facilities in a densely populated area like Chicago.

The plutonium pilot facilities needed to be two to four miles (3.2 to 6.4 km) from the site boundary and any other installation, in case radioactive fission products escaped.

While security and safety concerns suggested a remote site, it still needed to be near sources of labor, and accessible by road and rail transportation.

[17][20] Finally, a War Department policy held that, as a rule, munitions facilities should not be located west of the Sierra or Cascade Ranges, east of the Appalachian Mountains, or within 200 miles (320 km) of the Canadian or Mexican borders.

[21] In December, it was decided that the plutonium production facilities would not be built at Oak Ridge after all, but at the even more remote Hanford Site in Washington state.

Compton and the staff at the Metallurgical Laboratory then reopened the question of building the plutonium semiworks at Argonne, but the engineers and management of DuPont, particularly Roger Williams (the head of its TNX Division, which was responsible for the company's role in the Manhattan Project), did not support this proposal.

They felt that there would be insufficient space at Argonne and that there were disadvantages in having a site that was so accessible, as they were afraid that it would permit the research staff from the Metallurgical Laboratory to interfere unduly with the design and construction, which they considered their prerogative.

[23] On January 12, 1943, Compton, Williams, and Brigadier General Leslie R. Groves, Jr., the director of the Manhattan Project, agreed that the semiworks would be built at the Clinton Engineer Works.

In November 1942, the DuPont engineers chose helium gas as the coolant for the production plant, mainly on the basis that it did not absorb neutrons but also because it was inert, which removed the issue of corrosion.

Szilard, in particular, was an early proponent of using liquid bismuth; but the major opponent was Wigner, who argued forcefully in favor of a water-cooled reactor design.

He realized that since water absorbed neutrons, k would be reduced by about 3 percent, but he had sufficient confidence in his calculations that the water-cooled reactor would still be able to achieve criticality.

[18] Although the design of the reactor was not yet complete, DuPont began construction of the plutonium semiworks on February 2, 1943,[33] on an isolated 112-acre (45.3 ha) site in Bethel Valley about 10 miles (16 km) southwest of Oak Ridge, officially known as the X-10 area.

There was an acute shortage of both common and skilled labor; the contractor had only three-quarters of the required workforce, and there was high turnover and absenteeism, mainly the result of poor accommodations and difficulties in commuting.

[39] The fuel slugs were canned to protect the uranium metal from corrosion that would occur if it came into contact with water and to prevent the venting of gaseous radioactive fission products that might be formed when they were irradiated.

General Electric and the Metallurgical Laboratory developed a new welding technique to seal the cans airtight, and the equipment for this was installed in the production line at Alcoa in October 1943.

[47] It consisted of a huge block, 24 feet (7.3 m) long on each side, of nuclear graphite cubes, weighing around 1,500 short tons (1,400 t), that acted as a moderator.

[51] The first permanent operating staff arrived from the Metallurgical Laboratory in Chicago in April 1943, by which time DuPont began transferring its technicians to the site.

A week later the load was increased to 36 short tons (33 t), raising its power generation to 500 kW, and by the end of the month the first 500 mg of plutonium were created.

To load a channel, the radiation-absorbing shield plug was removed, and the slugs inserted manually in the front (east) end with long rods.

[50] The effect of the neutron poison xenon-135, one of many fission products produced from the uranium fuel, was not detected during the early operation of the X-10 Graphite Reactor.

[50] A radioisotope building, a steam plant, and other structures were added in April 1946 to support the laboratory's peacetime educational and research missions.

Studies of these by Emilio G. Segrè and his P-5 Group at Los Alamos revealed that it contained impurities in the form of the isotope plutonium-240, which has a far higher spontaneous fission rate than plutonium-239.

This meant that it would be highly likely that a plutonium gun-type nuclear weapon would predetonate and blow itself apart during the initial formation of a critical mass.

[61] The control room and reactor face are accessible to the public during scheduled tours offered through the American Museum of Science and Energy.

[70] Use of water as a coolant was considered, but there were concerns about the possibility of a catastrophic nuclear meltdown in the densely populated British Isles if the cooling system failed.

[77] Financed through the Belgian uranium export tax, and built with the help of British experts,[78] the 4 MW research reactor went critical on May 11, 1956.