High-level waste contains radioactive components that emit dangerously intense radiation. Radiation is generated through a decay process in which the atoms of a radioactive component (also known as a radionuclide) lose their radioactivity by spontaneously releasing energy in the form of subatomic particles or rays similar to X-rays. Even short but extremely intense exposure to radiation can cause almost immediate health problems such as radiation sickness, burns, and, in severe cases, death. Excessive exposure to these particles or rays damages cells in living tissue and is believed to cause long-term health problems such as genetic mutations and an increased risk of cancer. Because of the intense radiation emitted from high-level waste, the waste must be isolated and handled remotely behind heavy shielding such as a layer of concrete in order to protect humans and the environment. In addition to the intense radioactivity, some of the radioactive components can be very mobile in the environment and may migrate quickly to contaminate the soil and groundwater if not immobilized. Besides radioactive components, DOE high-level waste also generally contains hazardous components added during the process of dissolving used nuclear fuel to remove plutonium and other nuclear materials. These hazardous components include solvents, acids, caustic sodas, and toxic heavy metals such as chromium and lead. Radioactive waste components, when combined with hazardous components, are referred to as “mixed wastes.” DOE has a vast complex of sites across the nation dedicated to the nuclear weapons program, but the high-level waste stemming from reprocessing spent fuel to produce weapons material such as plutonium and uranium has been limited mainly to three sites—Hanford, Washington; the Idaho National Engineering and Environmental Laboratory (“Idaho National Laboratory”) near Idaho Falls, Idaho; and Savannah River, South Carolina. DOE largely ceased production of plutonium and enriched uranium by 1992, but the waste remains. Most of the tanks in which it is stored have already exceeded their design life. For example, many of Hanford’s and Savannah River’s tanks were built in the 1940s to 1960s and were designed to last 10-40 years. (Figure 1 shows waste storage tanks being constructed at the Hanford Site.) These tanks, most of which are underground, are used to store high-level waste. Leaks from some of these tanks were first detected at Hanford in 1956 and at Savannah River in 1959. Given the age and deteriorating condition of some of the tanks, there is concern that some of them will leak additional waste into the soil, where it may migrate to the water table. Treatment and disposal of high-level waste produced at DOE facilities are governed by a number of federal laws, including laws that define the roles of DOE and the Nuclear Regulatory Commission (NRC) in waste management. The Atomic Energy Act of 1954 (AEA) and the Energy Reorganization Act of 1974 established responsibility for the regulatory control of radioactive materials including DOE’s high-level wastes. The Energy Reorganization Act of 1974 assigned the NRC the function of licensing facilities that are expressly authorized for long-term storage of high-level radioactive waste generated by DOE and others. The Nuclear Waste Policy Act of 1982, as amended, defines high-level radioactive waste as “the highly radioactive material resulting from the reprocessing of spent nuclear fuel, including liquid waste produced directly in reprocessing and any solid material derived from such liquid waste that contains fission products in sufficient concentrations, and…other highly radioactive material that the …determines…requires permanent isolation.” The act also established a process for developing and siting a geologic repository (a permanent deep disposal system) for the disposal of high-level waste and spent fuel. Regarding DOE’s high-level waste, the act provided that unless the President determined that a separate repository was required for such waste, DOE should arrange for the use of commercial repositories developed under the act for disposal of its defense waste. In 1985, President Reagan decided that a separate repository for defense waste was not needed. Under amendments the Federal Facility Compliance Act of 1992 made to the Resource Conservation and Recovery Act of 1976 (RCRA), DOE generally must develop waste treatment plans for its sites that contain mixed wastes. These plans are approved by states that the Environmental Protection Agency (EPA) has authorized to administer RCRA or by EPA in states that have not been so authorized. DOE carries out its high-level waste cleanup program under the leadership of the Assistant Secretary for Environmental Management and in consultation with a variety of stakeholders. In addition to the EPA and state environmental agencies that have regulatory authority in states where the sites are located, stakeholders include county and local governmental agencies, citizen groups, advisory groups, and Native American tribes. These stakeholders advocate their views through various public involvement processes including site-specific advisory boards. Over the years, much of the cleanup activity has been implemented under compliance agreements between DOE and the regulatory agencies. These compliance agreements provide for establishing legally enforceable schedule milestones that govern the work to be done. The waste in the tanks at Hanford, Savannah River, and the Idaho National Laboratory is a complex mixture of radioactive and hazardous components, and DOE’s process for preparing it for disposal is designed to separate much of the radioactive material from other waste components. In the tanks, this mixture has transformed into a variety of liquid and semisolid forms. The radioactive components are of many different types; some remain dangerous for millions of years, while others lose much of their radioactivity in relatively short periods of time. Because most of the radioactive components decay relatively rapidly, over 90 percent of the current radioactivity will dissipate within 100 years. DOE plans to isolate the radioactive components and prepare the waste for disposal through the use of an extensive and sequential multi-step treatment process. To fulfill its current commitment to federal and state regulators, DOE expects this process to concentrate at least 90 percent of the radioactivity into a much smaller volume that can be permanently isolated for at least 10,000 years in a geologic repository. DOE plans to dispose of the remaining waste of relatively low radioactivity on-site near the surface of the ground, such as in vaults or canisters, or at other designated disposal facilities. High-level waste generally exists in a variety of physical forms and layers inside the underground tanks, depending on the physical and chemical properties of the waste components. The waste in the tanks takes three main forms: Sludge: The denser, water insoluble components generally settle to the bottom of the tank to form a thick layer known as sludge, which has the consistency of peanut butter. Saltcake: Above the sludge may be water-soluble components such as sodium salts that crystallize or solidify out of the waste solution to form a moist sand-like material called saltcake. Liquid: Above or between the denser layers may be liquids comprised of water and dissolved salts called supernate. As figure 2 shows, 44 percent of the total volume of high-level waste is in saltcake form, followed by liquid and sludges. In addition, a small portion of the waste volume is also in solid form and is stored in facilities other than tanks. At the Idaho National Laboratory, some waste is stored in stainless steel bins, enclosed in concrete vaults, after having undergone a thermal process that converted the liquid into a solid granular substance called calcine. At Hanford, some high-level waste was retrieved from the tanks, dried, and stored as solid material in stainless steel capsules. The various layers of waste in the tanks are not uniformly distributed and often differ from tank to tank and even from place to place within a tank. Depending on how the waste was generated and whether it was mixed or transferred from one tank to another, the layers of waste within any given tank may be unevenly distributed and liquid is interspersed between layers of saltcake. Some tanks contain all three main waste forms—sludge, saltcake, and liquid—while others contain only one or two forms. Tank contents also vary among sites. For example, at the Idaho National Laboratory most tanks contain primarily liquid waste because the waste was kept in an acidic form, while at Hanford and Savannah River, most tanks contain waste in two or three physical forms. The radioactive components of the high-level waste vary greatly in terms of how long they remain radioactive, with the vast majority losing their radioactivity within years or decades. Each radioactive component, or radionuclide, in high-level waste loses its radioactivity at a rate that differs for each component. This rate of decay, which cannot be changed, is measured in “half-lives”—that is, the time required for half of the unstable atoms to decay and release their radiation. The half-lives of major radionuclides in the high-level waste range from 2.6 minutes for barium-137m to 24,131 years for plutonium-239. To illustrate, for any given number of radioactive barium-137m atoms, half will lose their radioactivity within 2.6 minutes. After another 2.6 minutes, half of the remaining unstable atoms will lose their radioactivity, leaving only one- fourth of the original number of unstable atoms still radioactive. The process is the same, but the half-life intervals much longer, for long-lived radionuclides, such as plutonium-239 atoms. For radioactive plutonium- 239 atoms, half will lose their radioactivity within 24,131 years, and half of the remainder will lose their radioactivity after another 24,131 years. Currently, nearly all of the radioactivity in DOE’s high-level waste originates from radionuclides with half-lives of about 30 years or less. As table 1 shows, about 98 percent of the radioactivity of the high-level waste comes from four radionuclides: barium-137m, cesium-137, strontium-90, and yttrium-90.