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Writer:
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Hartley, L., Hoek, J., Swan, D. & Roberts, D.
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Summary:
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Posiva Oy is responsible for implementing the programme for geological disposal of spent nuclear fuel in Finland. Posiva is currently preparing to submit an application to obtain a construction licence for the Olkiluoto disposal facility by the end of 2012. To prepare for the licensing, the Rock Suitability Criteria (RSC) programme has been set up to develop a classification scheme to define suitable rock volumes for the repository panels, deposition tunnels and deposition holes. The classification scheme considers both long-term safety and engineering aspects. Target properties for the host rock have been defined in terms of the chemical composition and flow of groundwater, as well as transport properties and thermomechanical stability. The work in this report considers the RSC-criteria relating to the fracturing around tunnels and deposition holes and the associated groundwater inflows during open conditions, using the stochastic hydrogeological discrete fracture network (Hydro-DFN) model developed for Olkiluoto Site Descriptive Model 2008.
The modelling work consists of a reference case model combined with a suite of variants to assess specific uncertainties or sensitivities. The variants that considered alternative assignments of fracture transmissivity gave substantially different distributions of the local transmissivity of fractures intersecting the repository, but largely similar distributions of inflows to the repository. The inflows are not generally governed by the local transmissivity of the intersecting fracture, but rather by the flows available in the wider connected open fracture network and the effects of this were well constrained in the calibration of the Hydro-DFN using drillhole PFL measurements.
The model predicts that 87 % of deposition holes do not have any intersecting flowing fractures. Where more than one fracture intersects a deposition hole this may be the result of local clustering of small fractures around a large connected open fracture. Only 18.2 % of 9m tunnel sections (the length of which corresponds to the mean separation of deposition holes along a deposition tunnel) have any inflow, and inflows greater than 1 L/min are found in only 2.4 % of tunnel sections. Only 13.3 % of deposition holes have non-negligible inflows, 3.0 % have an inflow greater than 0.1 L/min and only 1.3 % have inflows greater than 1 L/min. There is a positive correlation between the inflow into 9m tunnel sections and inflow to the adjacent deposition hole, but there are many exceptions where only the 9m tunnel section has an inflow and the deposition hole does not, or vice versa. If a pilot hole is drilled ahead of the deposition holes, then of those pilot holes that have negligible inflow, 29 % of deposition hole at the same location would have an observable inflow. If deposition hole locations are rejected on the basis of an inflow to a pilot hole of above 0.1 L/min, then the percentage of deposition holes with inflows above 0.1 L/min falls from 2.6 % to 0.8 % of the total number of deposition holes that passed the selection criteria. Use of pilot holes to inform selection of deposition hole locations is thus justified. The results also suggest that:
• inflow statistics are not very sensitive to the use of Full Perimeter Intersection (FPI) criteria;
• filtering out deposition holes with inflows under open repository conditions of greater than e.g., 0.1 L/min or 0.01 L/min is an effective screening criterion for avoiding high post-closure flow-rates around the deposition holes;
• a correlation was found between initial Darcy velocities adjacent to deposition holes under closed conditions and inflows under open conditions for the highest values of inflow and Darcy velocity. Therefore, inflows provide a very effective screening criterion for avoiding high post-closure flow-rates around the deposition holes.
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