Comments regarding the bentonite barrier - SR 97 Post-closure safety

This review only deals with the role of the bentonite for ensuring the integrity of the canister from the start and throughout the lifetime of the repository. It seems as the technical reports in question deliver convincing data and answers to most of the questions that can be raised concerning the bentonite and the interplay between the different processes at hand. However, there is one area where further analysis and discussions seem inevitable. This area is related to the thermo-hydrological behavior of the bentonite, especially during the first phase of the repository. This is discussed in some detail below. The bentonite is compacted to a high density before installation and has a degree of saturation of about 80 % when it is placed in the bedrock and around the canister. The bentonite is then expected to gradually increase its water content by uptake of water from the surrounding bedrock. Thereby, the bentonite will swell and completely fill the gap between the canister and the bentonite and between the bentonite and the bedrock as well as exert radial pressure on the canister itself. It is well known that the hydraulic conductivity and the heat conductivity of the bentonite to a large extent depend on the degree of saturation. In the reports it is obvious that the pore pressure in the surrounding bedrock is expected to be large, close to 500 kPa. This high pressure, together with the suction in the bentonite, is expected to result in a rather quick saturation of the bentonite around the canister. The same processes are expected to rather quickly saturate the backfill, consisting of a mixture of crushed bedrock and bentonite in the tunnels above the deposition holes. These processes have been analyzed by means of finite element analysis. The time required is comparatively short and the resulting temperatures in the bentonite close to the canister will be acceptable. Questions can, however, be raised regarding the boundary conditions assumed for the pressures and the flow capacity of the bedrock. This is important as, if the degree of saturation of the bentonite becomes too low, the temperature of the canister might force a movement of water away from the canister, thus lowering the degree of saturation of the bentonite. Then the thermal conductivity of the bentonite will decrease, and the temperature of the bentonite could start increasing, whereby irreversible processes could occur in the bentonite. This could, in turn, result in a much higher hydraulic conductivity for the bentonite, making the barrier less effective than anticipated. During the tests in the Stripa project, it was found that there was a possibility of holes being fairly 'dry', as very little water came from the bedrock and hardly any cracks crossed the hole. This has been accounted for in the analysis by studying the case where the bentonite is saturated by flow of water from the backfill in the tunnel above. This analysis also assumes 500 kPa pore water pressure at the boundary of the tunnel. If this is the case, then the analysis seems reasonable and the wetting procedure is comparatively rapid. However, this begs the question of how long it will take to restore the pore pressures that existed around the tunnel in the bedrock before the tunnel was excavated? After the tunnel is excavated, it will take some 40 years before the whole site is filled with canisters and the tunnel is closed off. The problem that needs to be given some more attention is how the sequence of using the holes should be planned and how the pore pressures can be expected to be restored to its original values or at least parts of it. If the time delay is too large, there might be a risk of obtaining too high temperatures in the bentonite, which might cause unexpected and unwanted changes in the properties of the bentonite