High level waste is the highly radioactive material resulting from processing of spent nuclear fuel. (The United States no longer reprocesses spent nuclear fuel, but some other countries still do.) Reprocessing spent nuclear fuel at the INL involved dissolving spent fuel in acid and then chemically extracting the uranium. The liquid that remained after this operation is very acidic, highly radioactive and contains chemicals that make it a hazardous waste. (Because it is hazardous and radioactive, it is also called a mixed waste.)

Note: Much of the liquid high-level waste at INL has been treated to make it a solid, using a facility called a calciner. The solid resulting from the treatment is called calcine, which resembles laundry detergent. Making liquid waste into a solid is a significant goal of the environmental management plan at INL, and a requirement of the 1995 Settlement Agreement.