The U.S. Environmental Protection Agency (EPA) has determined that deep geologic disposal is appropriate for three types of radioactive waste generated in the United States: spent fuel, high-level waste, and transuranic waste. Spent fuel is nuclear fuel that has been discharged from a reactor after irradiation. High-level waste (HLW) is the highly radioactive material that remains after the reprocessing of spent fuel to recover uranium or plutonium. Transuranic (TRU) waste is any waste material contaminated with more than 100 nCi/g of elements having atomic numbers greater than 92 and half-lives longer than 20 years. Spent fuel and HLW can result from either commercial or governmental activities, although no commercially generated spent fuel has been reprocessed since 1972. TRU waste results primarily from the design and manufacture of nuclear weapons, not from nuclear power plants.
The physical characteristics of TRU waste differ substantially from those of spent fuel and HLW. This imposes different requirements on materials associated with containment and isolation, so TRU waste will be discussed separately from spent fuel and HLW Because all three are judged to be particularly dangerous to human beings and the environment, the EPA standard requires a demonstration of adequate 10,000-year performance of geologic repositories for these radioactive wastes. The U.S. Department of Energy (DOE) is responsible for implementing the standard by designing, siting, and building the repositories.
This article briefly describes TRU waste, HLW, and spent fuel and the two repositories currently planned by DOE. It con cludes by offering some observations on materials compatibility among waste, container materials, and host rock.