Closure of the Spent Fuel Repository in Forsmark - Studies of alternative concepts for sealing of ramp, shafts and investigation boreholes

In March 2011, SKB submitted applications under the Nuclear Activities Act and the Environmental Code for the construction and operation of a final repository for spent nuclear fuel in Forsmark. An important supporting document for the application under the Nuclear Activities Act was the SR-Site safety assessment. As a part of the work with the application and as background material for SR-Site, SKB prepared production reports, including the closure production report. The closure production report presented definitions, requirements and design premises, a reference design and the initial state for repository closure. SR-Site evaluated the reference design and related design premises that were presented in the closure production report. SR-Site thereby concluded that the design premises on which the reference design were based are adequate. Relaxing the requirements would require additional sensitivity analyses focusing on the hydraulic properties of the access, main and transport tunnels. SR-Site further concluded that the reference design could likely be simplified without violating the current design premises. Furthermore, additional simplifications could probably be made if the design premises could be revised. This has been studied in the project 'Closure - concept studies', whose results are presented in this report. SR-Site also evaluated the reference design for investigation boreholes that is presented in the production report. The evaluation showed that the impact of improper borehole seals is very moderate. Further, SR-Site concludes that the current design premises are appropriate but possibly too strict, since even open boreholes seem to have a limited impact on the groundwater flow in the repository. Since it might be difficult to inspect the outcome of the current design of borehole sealing, it could be of interest to assess whether a solution that may result in higher effective permeability of the borehole seals would provide sufficiently good sealing. Such a solution, which could be based on crushed rock with the right grain size distribution, might also be more robust to control and verify with respect to long-term durability. However, relaxing the design premises in this way would require additional sensitivity analyses. Further, the assessment in SR-Site indicates that the reference design is appropriate for the purpose. However, if the the design premises are relaxed, more robust designs might be worth investigating. This has been studied and the results are presented in this report. Research and development specifically focused on technology for repository closure has not yet been carried out. On the other hand, SKB and other organisations have for many years studied and conducted considerable research on backfilling and sealing of deposition tunnels, including full-scale tests. SKB has also developed and tested technology for sealing of investigation boreholes. Experience and results from these efforts comprise an important basis for this project. The work has focused on sealing of shafts, ramp and investigation boreholes. Alternatives for top sealing and sealing of the central area have not been studied, since their impact on the flow through the repository is judged to be marginal. Nor have alternative concepts for sealing of main and transport tunnels been developed, since it is believed that filling with blocks and pellets of swelling clay is the only alternative that ensures sufficiently low hydraulic conductivity