Like other industrial processes, generating electricity from nuclear power or making nuclear weapons creates waste. These radioactive and chemically toxic wastes result from the mining and processing of uranium as well as from storing or reprocessing spent reactor fuel.
Waste from Uranium Production
The tailings or waste produced by the extraction or concentration of uranium from its ore contain radioactive isotopes of uranium, thorium, and radium as well as significant concentrations of heavy metal including chromium, lead, molybdenum, and vanadium. More than 200 pounds of tailings are produced for each pound of uranium. This sandy waste material must be contained in carefully monitored sites known as tailings piles (Figure 1).
Figure 1 – Containment Site on the Colorado River near Moab, Utah. Courtesy of WISE Uranium Project
Waste from Conversion, Enrichment, and Fuel Fabrication Processes
Uranium production processes do not affect the level of radioactivity and do not produce significant chemical waste. An enrichment process for one ton of uranium hexafluoride produces 130 kg of UF6 (3.5% U-235) and 870 kg of depleted UF6 containing U-238. Depleted uranium has few applications. However, its high density 18.7 g/cm3 makes it useful in armor plating and radiation shielding. It is also a potential energy source for fast breeder reactors.
Waste from Reactors
Spent fuel in the open and closed fuel cycles generates radioactive waste. The components of spent reactor fuel can either be treated as waste (in the open fuel cycle) or reprocessed (in the closed fuel cycle). In either case, spent fuel from the reactor is initially stored in cooling ponds under water at the reactor site to allow a decrease in radioactivity and a corresponding decrease in temperature. The amount of time the spent fuel remains in a pool is determined by whether it is to be kept as waste or reprocessed for either fuel or nuclear weapons. If it is to be treated as waste, it can remain in the pool indefinitely. If it is to be reprocessed to recover plutonium for weapons, it is removed after several months.
Waste from Nuclear Power Generation – The Open Fuel Cycle
In the open fuel cycle, the spent fuel rods remain in the pools under at least 20 feet of water (Figure 2), which protects the surroundings from radiation.
Figure 2 – Storage of Spent Fuel at the Reactor. Courtesy of the Union of Concerned Scientists
After a minimum of one year, the rods may be removed from the pool and placed in a cylinder in a chemically inert atmosphere of helium gas (Figure 3). The cylinder is then sealed and encased in steel and concrete to contain the radiation and enhance security for storage or transportation to a permanent repository. In 2008 there were 160,000 assemblies containing 45,000 tons of spent fuel from nuclear power reactors in the United States. The majority of these are stored at the reactor sites in reactor pools with only about 5 percent in dry casks. Each year about 7,800 additional used fuel assemblies are placed into storage. If all of the current assemblies were collected in a single location they would cover a football field to about a height of five and half yards.
Figure 3 – Dry Cast Storage. Courtesy of the U. S. Nuclear Regulatory Commission
Waste from Reprocessing Spent Fuel – The Closed Fuel Cycle
Spent fuel to be reprocessed for mixed oxide (MOX) fuel remains in the pool for several years before removal. The PUREX (link to unit on plutonium production)LINK process, used both for extracting plutonium and uranium in the closed fuel cycle and plutonium for weapons, generates large volumes of chemical and radioactive waste. In addition, the small amount of highly radioactive material remaining in spent reactor fuel after extraction of the uranium and plutonium poses a significant waste management problem. Lastly, today the mixed uranium and plutonium oxides (MOX) from reprocessing are used only once in thermal reactors due to the buildup of neutron absorbing Pu-240. Thus, this spent MOX fuel becomes waste to be managed.
Waste from Nuclear Weapons Production
The highly radioactive liquid waste from reactors used to produce plutonium for the nuclear weapons of the United States is stored in tanks (Figure 4) at the Hanford, Washington, and Savannah River, South Carolina. The Hanford site manages the largest volume of high-level waste, but the Savannah River site contains more total radioactivity. At Hanford, high-level waste alkaline liquid, salt cake, and sludge are stored in 149 single-shell and 28 double-shell underground tanks, while Savannah River has 51 tanks. These tanks contain approximately 88 million gallons of liquid, which is not only radioactive but also chemically toxic. The composition of the liquid varies from tank to tank. These facilities produced a combined total of 120 tons of plutonium for 20,000 nuclear warheads.
Figure 4 - Waste Storage Tank. (Courtesy Department of Energy)
The U.S. Department of Energy has begun a process of mixing this waste with sand at high temperatures to form a liquid glass mixture, which is poured into stainless steel canisters where it solidifies and is sealed for permanent storage. This method of stabilization, known as vitrification, has also been used to process waste from power reactors.
Types of Nuclear Waste
According to the U.S. Department of Energy (DOE), the four major elements of the environmental legacy of nuclear weapons production are
contaminated environmental media,
surplus facilities, and
materials in inventory.
We will focus on the first two components. As we have seen in previous modules, nuclear weapons production in the United States was a complex series of manufacturing operations that generated large quantities of nuclear and chemical wastes. The term “waste” is defined as solids or liquids that are radioactive, chemically hazardous, or both. This waste consists of materials that have been disposed of previously, await disposal, or have been retrieved in site cleanups and are currently in storage. Waste is measured in terms of its volume (cubic meters) and its radioactivity (curies). Waste from nuclear weapons production managed by DOE includes 24 million cubic meters containing 900 million curies.
The major categories of waste are
mixed low-level waste,
11e(2) byproduct material,
hazardous waste, and
High-level waste is the highly radioactive waste resulting from spent nuclear fuel from production or power reactors, as well as from the chemical processing of spent nuclear fuel and irradiated target assemblies. The radioactivity comes from fission fragments and their daughter products resulting from the fission of U235 in production reactors. Although radiation from short-lived fission products (fragments and their daughters) will decrease dramatically in the next hundred years, radiation risks associated with the long-lived products will remain high for thousands of years. In the initial decay period, most of the radioactivity is due to Cs-137, Sr-90, and their short-lived daughter products. Plutonium, americium, uranium, and their daughter products are the major contributors to long-term radioactivity (Figure 5).
Figure 5 – Radioactive Decay of High-level Waste from Reprocessing One Tonne of Spent Reactor Fuel. (Courtesy OECD NEA)
Transuranic (TRU) waste contains alpha-emitting transuranic elements or actinides with half-lives of greater than 20 years and a combined activity of greater than 100 nanocuries per gram of waste. Because of the long half-lives of many TRU isotopes, TRU waste can remain radioactive for hundreds of thousands of years. Some common isotopes found in TRU are Pu-238, Pu-239, Pu-240, Pu-241, Pu-242, Am-241, and Cu-244. TRU waste results from the fabrication of plutonium components, recycling of plutonium from scrap, retired weapons, and chemical separation of plutonium. Unlike high-level waste, which results from a few specific processes with a narrow range of physical matrices and chemical characteristics, TRU waste exists in many forms with a spectrum of chemical properties.
A small percentage of TRU waste exhibits high direct exposure hazards and is referred to as "remote-handled" TRU waste. The majority of TRU waste emits low levels of direct radiation and is called "contact-handled" waste. The chief hazard of "contact-handled" waste is due to alpha radiation. Alpha particles cannot penetrate the skin but cause serious localized tissue damage when inhaled or ingested. When inhaled, TRU elements tend to accumulate in the lungs; soluble TRU compounds migrate through the body, accumulating in the bone marrow and liver.
Mixed low-level waste contains both chemically hazardous waste subject to the Resource Conservation and Recovery Act (RCRA) and radioactive materials. The radioactive component of mixed low-level waste is similar to low-level waste and thus less radioactive than high-level or TRU waste. Hazardous chemical components present in mixed waste include toxic heavy metals, explosives, halogenated organic compounds, and acids.
By-product materials include waste from uranium production described above. The other category is defined by government regulations. A variety of materials not covered previously fall into these categories. These materials include polychlorinated biphenyls, asbestos, and byproduct materials that have been mixed with chemically hazardous substances.
Waste Repositories in the United States
Two locations in the United States have been identified as repositories for nuclear waste. The operational Waste Isolation Pilot Plant (WIPP) (Figure 6) located in southeastern New Mexico is a geologic repository for the disposal of waste such as clothing, equipment, rags, and other items contaminated with transuranic (TRU) elements resulting from nuclear weapons production. This TRU waste is defined as having activity greater than 100 nanocuries per gram due to transuranic isotopes. These isotopes have long half-lives, extending from 20 to thousands of years but much lower levels of radioactivity than the high level waste. The waste is packaged in containers and placed in salt beds approximately 2,000 feet below ground. The salt will slowly close around the waste, permanently isolating it from the accessible environment.
Figure 6: WIPP Courtesy of the Uranium Information Center
Yucca Mountain, located about 100 miles northwest of Las Vegas, Nevada, has been selected as the site of a national geological repository for high-level spent nuclear fuel from civilian power plants and defense-related activities (Figure 7). This site is being studied carefully by the Department of Energy (DOE) to ensure public health and safety. If DOE determines that the site is suitable, it will submit a construction application to the Nuclear Regulatory Commission (NRC).
Figure 7 – Yucca Mountain Site in Nevada. Courtesy of the U.S. Department of Defense
As the licensing agency, the NRC will use standards currently being developed by the U.S. Environmental Protection Agency. However, conflicting scientific and technical information as well as strong political opposition from Nevada cloud the future of the site. As of 2009, no nation has opened a permanent repository for the storage of high-level nuclear waste. Most nuclear waste remains stored on the site at which it was produced.
Complete Bibliographies on Nuclear Waste from the Alsos Digital Library for Nuclear Issues
Frank A. Settle (Washington and Lee University)