Fermi's reactor in Tennessee opened the door to the first peacetime use of nuclear technology. When a fissionable material splits, it can produce any of a variety of radioisotopes, unstable isotopes whose decay emits radiation that can be dangerous—as in the fallout of a nuclear bomb. In a reactor, the radiation is contained, and scientists had already discovered that, if properly handled, radioisotopes could have beneficial uses, particularly in medicine. Cancer cells, for example, are especially sensitive to radiation damage because they divide so rapidly, and doctors were learning to use small targeted doses of radiation to destroy tumors. So reaction was swift in the summer of 1946 when Oak Ridge published a list of the radioisotopes its reactor was producing in the June issue of Science. By early August the lab was sending its first radioisotope shipment to Brainard Cancer Hospital in St. Louis, Missouri.

The field of nuclear medicine is now an integral part of health care throughout the world. Doctors use dozens of different radioisotopes in both diagnostic and therapeutic procedures, creating images of blood vessels, the brain, and other internal organs (see Imaging), and helping to destroy harmful growths. Radiation continues to be a mainstay of cancer treatment and has evolved to include not just targeted beams of radiation but also the implantation of small radioactive pellets and the use of so-called radiopharmaceuticals, drugs that deliver appropriate doses of radiation to specific tissues. Because even a small amount of radiation is easily detectable, researchers have also developed techniques using radioisotopes as a kind of label to tag and trace individual molecules. This labeling has proved particularly effective in the study of genetics by making it possible to identify individual DNA "letters" of the genetic code.

From the start, of course, researchers had known that another use for atomic energy was as a power source. After World War II the U.S. government was quick to realize that potential as well. In 1946 President Truman signed a law that created the Atomic Energy Commission, whose mandate included not only the development of atomic weapons but also the exploration of other applications. One of these was to power navy ships, and in 1948 Captain (later Admiral) Hyman Rickover was assigned the task of developing a reactor that could serve as the power plant for a submarine. Rickover, who had been part of the Manhattan Project, would become known as "the father of the nuclear navy." Under his leadership, engineers at the Westinghouse Bettis Atomic Power Laboratory in Pennsylvania designed the first pressurized-water reactor (PWR), which ultimately became the dominant type of power plant reactor in the United States. Rickover's team pioneered new materials and reactor designs, established safety and control standards and operating procedures, and built and tested full-scale propulsion prototypes. The final result was the USS Nautilus, commissioned in 1954 as the world's first nuclear-powered vessel. Six years later the USS Triton became the first submarine to circumnavigate the globe while submerged. Soon a fleet of nuclear submarines was patrolling the world's oceans, able to stay submerged for months at a time and go for years without refueling because of their nuclear power source. Masterpieces of engineering, nuclear submarines and aircraft carriers have operated without accident for nearly 6 decades.