Analysis of human intrusion scenarios for the deep disposal of fusion wastes

During the development of the International Thermonuclear Experimental Reactor (ITER) fusion reactor, many efforts are done to minimise the amount of radioactive waste that will arise from the decommissioning of a nuclear fusion power plant. Nevertheless, the most active waste types will have to be disposed in a repository. The impact of possible future human actions on the performance of the repository has to be evaluated. The most active waste types are selected from the inventory of fusion plant model PM-2 with low-activation martensitic steel as main material. After a cooling period of 100 years, the fusion waste is assumed to be disposed off in a repository located in the boom clay layer at the Mol site (Belgium). A systematic approach for the identification of the relevant intrusion scenarios is applied. Three scenarios resulting from borehole drilling are identified as relevant, core inspection, residence and unsealed borehole. This paper will focus on the analysis of the most drastic human intrusion scenario, i.e. the core inspection scenario. The maximum dose is calculated for waste arising from Be-coatings. In the case of a routine inspection, the dose to a geological worker is always under 0.5 Sv. This value, under which serious deterministic health effects are unlikely, can be considered as a reference level for acute exposure. In the case of the very pessimistic close inspection variant a maximum dose of 0.4 Sv is calculated if the intrusion occurs immediately after the disposal. After 60 000 years, the dose becomes lower than 3.5 mSv, which is the average annual background dose. The results of an alternative approach, considering a fusion-specific repository and wells drilling as the main intrusion scenario, are also included in this paper. Finally, a comparison of the consequences due to intrusion scenarios calculated for fusion and fission waste is presented. For the most active fission waste types it lasts 1 million years before the close inspection dose drops under 0.5 Sv.