The radioactive residues remaining after nuclear reprocessing of spent nuclear fuels and which contain a high level of fission products. These are not able to be further processed and are therefore stored in managed environments for very long periods of time. This includes the process of vitrification in which the waste is converted to a glassy material and stored in containers in geologically stable shafts deep below the surface of the Earth. Over the period of storage, the fission products decay. The rate of decay depends on the half-life of the fission products.