A Radio Toxicity Index for Fusion Waste

With current designs of future fusion power plants, it is evident that a significant quantity of operational radioactive waste will be produced over the lifetime of the plant. This waste will be mostly due to the replacement of in-vessel components (IVCs) on a regular basis, currently assumed to be every five years. This potentially large quantity of waste raises issues about its ultimate disposal, particularly the nature of the disposal facility required to accommodate it. The term invessel component includes the divertor and the breeding blanket, the “fuel” in a fusion reactor. In this perspective only, the waste resulting from IVC replacement is analogous to the fuel waste arising from fission power plants, and this comparison, whether justified, or not, could prejudice the fusion waste disposal solution. As fusion in-vessel component waste is significantly and fundamentally different from fission reactor fuel waste, it is essential that the fusion disposal solution be based solely on its needs. To highlight this fundamental difference between the fusion and fission operational waste, a radio toxicity index has been defined, which may prove to be of value in defining appropriate requirements for the disposal of fusion operational waste. Uranium has been the basis of the fission power industry and it is found in nature in concentrations varying, typically, from 0.1 to 1%, and in some cases ore bodies with concentrations up to 25% have been found. Because uranium is a radioactive element, and is quite common in the earth’s crust, it offers an opportunity to be used as a benchmark for comparing potential fusion and fission power reactor radioactive waste. As U-238 is the most abundant isotope of uranium found in nature (>99%), it is proposed that the radio toxicity of U-238 be used to assess the relative radio toxicity of relevant fusion and fission waste. The ratio of the radio toxicity of a given material to that of U-238 is referred to as the radio toxicity index. Therefore, a waste material with a radio toxicity index equal to one would be considered acceptable for disposal in the earth’s crust, in the same manner that uranium tailings are disposed of in the mining industry. The results of studies performed for typical fusion breeder material indicate that there is no compelling economic reason for reprocessing. Furthermore, the radio toxicity index for such materials indicates that there are no technical reasons — i.e., there does not appear to be a need for deep, geological disposal of spent fusion breeder material. On the contrary, the application of the radio toxicity index to spent fission fuel has demonstrated, from a waste disposal perspective, that there are compelling reasons for reprocessing to separate low radio toxicity fission products from the high radio toxicity actinides, which can be reused. This conclusion augurs well for a future fusion power industry and goes a significant distance in demonstrating the potential environmental advantages of fusion power.