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
Environmental Consequences
Although the production of nuclear materials generated huge quantities of waste, good records of radioactive and chemical waste production and environmental discharges generally were not kept until the 1970s. What is known of that early history today is based on reviews of written records supplemented by process knowledge and mass-balance calculations. We have been selective in our use of data in the following discussion, preferring more recent sources that contain documentation for the estimates. We also have rounded the data except where there is demonstrated support for greater precision.
The uranium-enrichment process produced low-level solid and liquid wastes and other process liquids, and up to 200 kilograms of depleted uranium (enriched in uranium-238) for every kilogram of HEU. There are on the order of half a million metric tons (metal equivalent) of depleted uranium in storage at several sites, and although this material is not classified as waste, most of it has no agreed-upon disposition pathway.
The separation of lithium-6 for tritium production used on the order of 10,000 metric tons of mercury, of which about 900 metric tons is unaccounted for. The DOE estimates that about 110 metric tons was discharged into East Fork Poplar Creek at Oak Ridge, and some of this contamination has migrated offsite and into the Clinch River–Watts Bar Reservoir system that is used for recreation and municipal water supply. The inorganic mercury compounds in this waste are not thought to be toxic, but they can pose a hazard to human beings if transformed to methylmercury by soil and water microorganisms.
Plutonium production also produced large quantities of waste: During the 50 years of operation of the Hanford site, for example, about 67 metric tons of plutonium were produced from almost 97,000 metric tons of irradiated uranium fuel. Chemical processing of that uranium to recover plutonium produced some 2 million cubic meters (500 million gallons) of highly radioactive, chemically toxic waste, and another 1.7 billion cubic meters (450 billion gallons) of process liquids. The DOE reports that about 76,000 cubic meters of solid waste contaminated with actinides (principally plutonium) and an additional 1.2 million cubic meters of other solid low-level wastes have been buried at the site.
Some of the waste generated by nuclear materials production was released directly to the environment. Volatile gases from chemical processing were vented directly into the atmosphere, sometimes without filtering, especially during the early years of production. Reactor cooling water contaminated with conditioning chemicals such as chromium and with radioactive isotopes produced in the reactor (neutron-activation products) were also discharged. Waste liquids were discharged into large surface ponds or into subsurface soil and groundwater through injection wells and other drainage structures. Radioactive and chemically contaminated solid waste was burned or dumped into shallow pits and trenches.
Indeed, there are thousands of "release sites" that are current or potential future sources of contaminant releases to the environment. As a result of such releases, soil and groundwater at many sites are extensively contaminated with industrial solvents, toxic chemicals, metals and radionuclides.
Large volumes of waste remain in storage at several sites and could become significant sources of future environmental contamination if not managed properly. At Hanford, for example, there are about 200,000 cubic meters of high-level waste in storage in 177 large underground tanks. These tanks have been in service between 16 and 58 years; under current plans, the last tank will not be closed until about 2046. The older, single-containment tanks were designed with service lives of 10 to 20 years, although no one really knew how long they would last. One tank began leaking just six years after it was put into service, and to date 67 of these tanks are suspected to have leaked up to 5,700 cubic meters, or 1.5 million gallons, and possibly more than a million curies of high-level waste into the subsurface. (Curies are a measure of radioactivity in a material; for comparison, a ton of uranium-238 has 0.3 curies.) Some of this contamination has reached groundwater.
At Savannah River, there exist some 130,000 cubic meters (34 million gallons) of high-level waste stored in 48 underground tanks. Nine of the tanks have leaked waste into their secondary containments, and a few tens of liters of waste leaked into the environment from one tank when the secondary containment overflowed. Efforts are now under way to immobilize the sludge fraction of this waste in a borosilicate glass matrix.
The Idaho site processed naval spent fuel and some research reactor fuel to recover enriched uranium, but here, unlike at Hanford and Savannah River, the high-level waste was immobilized as a powdered ceramic (calcine), about 4,000 cubic meters of which are being stored in stainless steel bin sets inside steel-reinforced concrete silos. These structures were designed to contain the waste for up to 500 years. Additionally, another 4,000 cubic meters of so-called "sodium-bearing waste" liquids await disposition in some of the site's 11 underground storage tanks.
Past practices for managing the large volumes of waste generated by nuclear materials production, when judged by today's standards, appear ill-informed at best, bordering on reckless at worst. It is important, however, to judge these practices against the prevailing environmental attitudes and practices during the Second World War and Cold War. The Manhattan Project was created during a national emergency at a time when the future of Europe and Asia hung in the military balance. National priority was given to weapons production at the expense of waste management. This sense of urgency, and a shroud of secrecy that hid production activities from public view, carried over into the Cold War, although increasing effort was given to minimizing environmental releases as time went on.
Wartime shortages of materials such as stainless steel created further difficulties. Carbon steel was employed to construct the waste tanks at Hanford and Savannah River, with the result that the high-level waste, which was highly acidic, had to be neutralized with alkaline chemicals such as sodium hydroxide to reduce tank corrosion. The addition of these chemicals to the waste increased volumes and produced solid precipitates. Later chemical processing and volume-reduction operations to reduce radioactive heat generation and conserve tank space further increased physical and chemical heterogeneity. Characterization of this waste and removal of the precipitates from the tanks will be difficult and expensive, especially at Hanford.
The waste-management decisions made during the Manhattan Project and ensuing Cold War created the environmental problems that the nation now confronts. These decisions continue to exact a steep price, both in the high annual costs of managing the stored waste and environmental contamination, and also in the loss of trust by citizens in their government as the consequences of waste-management practices carried out in secrecy for almost five decades have become public knowledge.
» Post Comment