Decay heat from spent fuel and HLW is highest at the time the spent fuel is discharged from the reactor and decreases rapidly with storage time. As shown in Fig. 17.8, decay heat is dominated by fission products up to about 60 years after

discharge and by the actinide elements afterwards. Higher decay heat makes storage and disposal of the spent fuel and HLW more difficult, since temperature limits are imposed on these materials to ensure their integrity. In designing a repository for spent fuel, therefore, the distance between adjacent fuel packages is adjusted so that the heat loading does not produce unacceptable temperatures. Typically, these distances are of the order of 10 M. This is a key factor in deciding the volume of rock that must be excavated to create a repository and, therefore, the cost.

By removing the radionuclides that are responsible for the longer-term decay heat, P&T, perhaps combined with long-term interim storage, allows waste packages to be placed closer together so that the volume of rock needing to be excavated is reduced. Compared with direct disposal, we might expect a reduction of gallery length by a factor or 3-6 depending on the length of the (pre-disposal) cooling time.42 Additional gains can be foreseen by separating Cs and Sr and storing them for 100-300 years prior to disposal or, alternatively, through long­term interim storage of vitrified high-level waste without Cs and Sr separation. If longer-term decay heat is a significant issue, after 75-100 years, either from an engineering and operations viewpoint or from a more fundamental perspective in developing a repository concept, actinide P&T and interim storage prior to disposal can be effectively used to lower decay heat in the corresponding waste, allowing for an increased utilization of repository space. This general result of decay heat reduction appears to hold for different types of host rock42 indicating that P&T is an effective means of reducing disposal costs.