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Under unsaturated conditions, the rate of water dripping into a waste emplacement drift is expected to be less than the downward percolation rate because the drift cavity acts as a capillary barrier. If percolating water encounters the cavity, capillary forces in the formation tend to retain the water, preventing or limiting the water from seeping into the drift. Water encountering the drift will tend to divert laterally as a result of the capillary barrier. If the lateral hydraulic conductivity is insufficient to divert the water, gravitational forces can exceed capillary forces, leading to seepage into the drift.
The permeability of the rock matrix in the repository host horizon is quite small relative to the fracture network, leading to fracture-dominated flow under all but the lowest percolation flux conditions. Because of the strong capillary barrier of the rock matrix in the repository horizon, virtually no seepage into drifts can occur through the rock matrix. Only flow through the fractures can lead to significant seepage. On this basis, the ambient drift seepage model was constructed as single-continuum, unsaturated flow model for fractures in the repository host rock.
Numerous drift seepage tests conducted in the ESF and ECRB provided data used for calibration of the ambient seepage model. Model validation was conducted using independent sets of test data, as well as data from a natural seepage event in the ESF South Ramp that occurred following heavy rainfall during the winter of 2004-2005. The sensitivity of drift seepage to the permeability and capillary strength of the fracture network and the local percolation flux, led to detailed quantification of the effects of spatial variability and uncertainty of these factors on drift seepage predictions.
Seepage of water into waste emplacement drifts affects the release and migration of radionuclides towards the accessible environment. As such, the results of the drift seepage model were an essential input to the total system performance assessment.