FEATURE ARTICLE
Managing the Environmental Legacy of U.S. Nuclear-Weapons Production
Although the waste from America's arms buildup will never be "cleaned up," human and environmental risks can be reduced and managed
Kevin Crowley, John F. Ahearne
Nuclear-Weapons Production
The U.S. nuclear weapons complex is massive in scale and highly dispersed: some 5,000 facilities located at 16 major sites and more than 100 smaller sites, ranging from mills to recover uranium from mined ore to facilities for weapons assembly, maintenance and testing. The largest facilities in the complex are those built for materials production and processing—Hanford, the Idaho National Engineering and Environmental Laboratory (INEEL), Oak Ridge and Savannah River in South Carolina. At these locations, nuclear materials (enriched uranium, plutonium and tritium) were produced for use in weapons, naval fuel and civilian nuclear applications. Their massive scale was dictated by the need for secrecy and safety as well as the physics of nuclear-materials production.
Manhattan Project scientists believed that an atomic weapon could be constructed using either of two radioactive isotopes, uranium-235 or plutonium-239. But obtaining sufficient quantities of these materials (tens of kilograms per bomb) proved a technically difficult and expensive challenge. Uranium-235, the principal isotope of a uranium weapon such as that used at Hiroshima, makes up about 0.7 percent by mass of natural uranium, most of the remainder being uranium-238. To be usable in a fission weapon, uranium-235 must be concentrated (enriched) to greater than 20 percent and preferably, for greatest efficiency, to over 90 percent. Uranium enriched to 20 percent or more uranium-235 is "highly enriched uranium" (HEU).
Uranium enrichment during the Manhattan Project exploited the small mass differences between uranium-235 and uranium-238 using electromagnetic separation and gaseous diffusion—the latter a process still used in the United States to enrich uranium for civilian power plants. Thousands of separation stages were required to obtain sufficient uranium-235 enrichment, and the facilities housing these processes were scaled to yield the required quantities.
Two plants were built during the Manhattan Project to obtain the roughly 50 kilograms of enriched uranium for the "Little Boy" weapon that was dropped on Hiroshima: the K-25 building at the Oak Ridge Gaseous Diffusion Plant and the Y-12 electromagnetic-separation plant at Oak Ridge. After the war, the Atomic Energy Commission (AEC) expanded the gaseous diffusion plant and built two additional enrichment facilities, the Paducah plant in Kentucky and the Portsmouth plant in Ohio, to meet growing civilian demands.
Plutonium-239, the principal isotope in "weapons-grade" plutonium, was produced by irradiating uranium-238 fuel rods with neutrons in large production reactors built along the Columbia River at the Hanford site. The capture of a neutron by uranium-238 produces uranium-239, which subsequently decays to neptunium-239 and then plutonium-239.
Once a plutonium-239 atom is produced, additional neutron captures can cause it to fission or produce heavier "reactor grade" plutonium (primarily plutonium-240 and -241), which is not as suitable for weapons. To minimize additional captures, the Hanford operators removed the fuel rods from the reactor after a short irradiation time, but this required large throughputs of uranium to obtain the needed quantities of plutonium. Three reactors and two chemical processing plants were built at Hanford to produce plutonium for the war effort. Following the war, the AEC built six additional reactors and three chemical processing plants at Hanford and five reactors and two chemical processing plants at Savannah River to meet the growing plutonium demands.
Tritium, an isotope of hydrogen and a key component of fusion weapons ("hydrogen bombs"), which were first developed by the United States in the early 1950s, was produced by the irradiation of lithium-6 targets in the production reactors at Savannah River. An isotope-separation facility was constructed at the Y-12 site at Oak Ridge to produce lithium-6 for this purpose.
During the roughly 45 years of nuclear-materials production in this country, about 103 metric tons of weapons-grade plutonium were obtained from the production reactors at Hanford and Savannah River; 994 metric tons of highly enriched uranium were obtained from the enrichment plants at Oak Ridge, Portsmouth and Paducah. Some of these materials have been declared to be surplus to U.S. defense needs (see table). Plans are now in place to turn most of this excess into fuel for use in nuclear power plants. The spent fuel will be disposed of in a geologic repository. Dealing with the by-products of nuclear-materials production is another matter.
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