A liquid reactant metal alloy includes at least one chemically active metal for reacting with non-radioactive material in a mixed waste stream being treated. The reactant alloy also includes at least one radiation absorbing metal. Radioactive isotopes in the waste stream alloy with, or disperse in, the chemically active and radiation absorbing metals such that the radiation absorbing metals are able to absorb a significant portion of the radioactive emissions associated with the isotopes. Non-radioactive constituents in the waste material are broken down into harmless and useful constituents, leaving the alloyed radioactive isotopes in the liquid reactant alloy. The reactant alloy may then be cooled to form one or more ingots in which the radioactive isotopes are effectively isolated and surrounded by the radiation absorbing metals. These ingots comprise the storage products for the radioactive isotopes. The ingots may be encapsulated in one or more layers of radiation absorbing material and then stored.