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These tests specifically addressed the conditions under which, and the rate at which, moisture could seep into niches (short drifts) in the repository region. The goals of these tests were to determine the amount of seepage into drifts under a variety of episodic pulse events and to identify potential fast flow paths. Seepage into drifts affects environmental conditions within waste emplacement drifts, the amount of water contacting waste, the rate of waste mobilization, and the transport of radionuclides within the drift and ultimately outside the repository region.
Seepage tests were conducted at three niches in the ESF and one in the ECRB. Air-permeability tests in borehole clusters were performed to characterize the rock and to select borehole intervals for subsequent liquid-release tests. These liquid-release tests involved controlled release of water with dye tracers. Niches were then excavated and examined for the presence of dyed water to determine if and how far the dyed water had traveled.
Tests were repeated at various flow rates and injection volumes to further quantify seepage. Numerical modeling was used to design the tests and predict the outcome, and test results were used to further refine the models. After completion of the active testing phase, long-term monitoring of moisture was conducted, using sensors installed in the niches. Moisture measurements were used to calculate water flow and to quantify the interaction between fast flow paths and rock matrix.
The results or the seepage tests have been used to predict whether water percolating downward above a mined opening (waste emplacement drift or tunnel) will drip into the opening because of gravity, or migrate around the opening due to capillary retention (i.e., a drift acting as a barrier). It is important to determine whether a mined opening acts as a capillary barrier because the barrier may help isolate the waste by preventing water from entering the drift and coming into contact with waste packages.