Today ultracentrifuges are used for uranium enrichment. A centrifuge is routinely used in medical labs to separate materials of different density. The test tubes are spun rapidly and the denser materials move away from the center of rotation because it requires more centripetal force to pull them toward the center. A gas ultracentrifuge uses a rapidly rotating drum to separate the UF₆ with the two isotopes. Gas centrifuges supply about 54% of the enriched uranium today. Each centrifuge is a more effective separator than a stage in a gaseous diffusion plant and requires only 6% of the electrical energy of gaseous diffusion.

Because of the relatively small size and lower energy needs of a centrifuge-based separation plant, there are serious concerns about the use of these plants to produce weapons-grade uranium. Pakistan used such a plant for its bomb, and there are indications that North Korea and Iran have similar plants.

In September 1942, General Groves and Robert Oppenheimer chose Los Alamos, New Mexico, as the site for the top-secret laboratory at which weapons would be developed. Thirty-five miles northwest of Santa Fe, it was almost totally isolated and the site was occupied only by a school. During World War II hastily erected housing held Nobel Prize winning scientists, younger scientists and engineers recruited into the project, wives and children, and soldiers.

After determining the “critical mass,” the minimal amount of enriched uranium needed to create a bomb, they designed and built the uranium-based weapon called “Little Boy.” The uranium was divided into two halves and placed in a cannon-like container. An explosive charge drove the two masses together, forming a large enough mass of uranium to sustain a rapid chain reaction and explode. This weapon was never tested. It contained 64 kilograms (141 pounds) of uranium, about 2.5 times the critical mass. Less than 1 kilograms (2 pounds) of the uranium fissioned. Only 0.6 grams (0.001 pounds) was converted into energy, but the result was the equivalent of 15,000 tons of TNT. Little Boy was dropped on Hiroshima, Japan, on August 6, 1945. Over 100,000 people were killed in the blast, resulting fires, and effects of radiation.

The second task of Los Alamos was to design and build a weapon using plutonium. Originally they had expected to use the cannon-type method used with the uranium bomb, but when the plutonium was produced by the Hanford reactors it was found to contain ²⁴⁰Pu, and another design had to be developed. They arranged a sub-critical plutonium mass in the shape of a sphere and used specially designed explosive charges to simultaneously compress the plutonium, increasing its density above the critical point. Scientists were uncertain that the design would work, so they decided to test the device first.