Safety assessment for a KBS-3H spent nuclear fuel repository at Olkiluoto. Evolution report

The KBS-3 method, based on multiple barriers, is the proposed spent fuel disposal method both in Sweden and Finland. KBS-3H and KBS-3V are the two design alternatives of the KBS-3 method. Posiva and SKB have conducted a joint research, demonstration and development (RDandD) programme in 2002-2007 with the overall aim of establishing whether KBS-3H represents a feasible alternative to the reference alternative KBS-3V. The overall objectives of the present phase covering the period 2004-2007, have been to demonstrate that the horizontal deposition alternative is technically feasible and to demonstrate that it fulfils the same long-term safety requirements as KBS-3V. The safety studies conducted as part of this programme include a safety assessment of a preliminary design of a KBS-3H repository for spent nuclear fuel located about 400 m underground at the Olkiluoto site, which is the proposed site for a spent fuel repository in Finland. In the KBS-3H design alternative, each canister, with a surrounding layer of bentonite clay, is pre-packaged in a perforated steel cylinder prior to emplacement in the deposition drift; the entire assembly is called the supercontainer. Several supercontainers are positioned along parallel, 100-300 m long deposition drifts, which are sealed following waste emplacement using drift end plugs. Bentonite distance blocks separate the supercontainers, one from another, along the drift. Steel compartment plugs can be used to seal off drift sections with higher inflow, thus isolating the different compartments within the drift. The present report describes the repository evolution in successive time frames, including key uncertainties. The description of evolution starts with the initial conditions at the time of emplacement of the first canisters. The repository evolves through an early, transient phase to a state where evolution is far slower. Particular attention is given to describing the transient phase, since this is where most of the important differences between KBS-3H and KBS-3V occur. The description of evolution in this phase addresses in turn the (i), thermal evolution, (ii), groundwater flow and evolution of groundwater composition, (iii), mechanical evolution, (iv), saturation and buffer swelling, (v) evolution of chemical and microbiological conditions and (vi), evolution of the canister surface and interior. A key issue is the local variability of the near-field rock around the KBS-3H repository drifts, which, together with the effects of gas from the corrosion of the supercontainer shells and other steel repository components, results in widely differing saturation times for different drift sections. Nevertheless, even in the tightest drift sections, the buffer is expected to retain its initial water content, and will eventually fully saturate, at which time it is expected to perform its full range of safety functions. In the more distant future, the evolution of conditions at repository depth may be significantly affected by major climate change, and, in particular, by the formation of ice sheets at the ground surface. Key issues, which are common to KBS-3H and KBS.3V, are (i), the possibility that meltwater penetrating to repository depth may lead to some erosion of the buffer, and (ii), the impact of post-glacial earthquakes on canister integrity. The impact of anthropogenic emissions on the magnitude and timing of future major climate changes is a key uncertainty that is also common to the two alternatives. The description of repository evolution provides the basis for the identification of evolution scenarios, an assessment of canister longevity and the analysis of radionuclide release and transport in the event of canister failure. Scenarios involving canister failure and radionuclide release are, however, also identified based on the discussion in this report and in the KBS-3H Process Report. These are initiated, in the first place, by: . the presence of an initial, penetrating defect in one or more of the canisters; . perturbations to the buffer and buffer/rock interface (some of which are specific to KBS-3H), giving rise to an increased rate of transport of sulphide from the geosphere to the canister surface and an increased canister corrosion rate; - penetration of dilute glacial meltwater to repository depth, giving rise to chemical erosion of the buffer, an increased rate of transport of sulphide from the geosphere to the canister surface and an increased canister corrosion rate; and - rock shear movements of sufficient magnitude to give rise to shear failure of the canisters. No new canister failure modes have been identified compared with the KBS-3V design. The consequences of canister failure and radionuclide release are assessed in the KBS.3H Radionuclide Transport Report.