Method for thermally treating discharged nuclear fuel

A method for controlling the release of nuclear fission products (including Mo, Tc, Ru, Rh, and Pd) associated with spent nuclear fuel heats the spent nuclear fuel to a predetermined temperature for a predetermined period of time. The predetermined temperature is sufficiently high to diffuse the nuclear fission products within the spent nuclear fuel and sufficiently low to avoid the release of volatile fission products from within the fuel. The predetermined period of time is sufficiently long to cause an optimal degree of phase redistribution of the nuclear fission products within the spent nuclear fuel. The product of the process is spent nuclear fuel characterized by separation of the nuclear fission products into discrete Mo-Tc-Ru-Rh-Pd phases within the spent nuclear fuel. The Mo-Tc-Ru-Rh-Pd phases make the spent nuclear fuel safer for long-term disposal in geologic repositories.