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Measurements of water potentials were performed at three niche sites in the ESF. These sites are located on the west side of the ESF main drift at Niche 1 (Niche 3566), Niche 2 (Niche 3650), and Niche 3 (Niche 3107). Niche 1 (Niche 3566) lies between the Sundance fault and a cooling joint branching out from the fault. The primary objective of this effort was to establish the water potential at various points within the three niche sites, to determine whether wet conditions exist at Niche 1 (Niche 3566) near the fault, when other niches are drier. To meet this objective, psychrometers were used as a method to measure water potential in the boreholes. Data from psychrometers in the boreholes were taken over long periods of time after sensor emplacement, so that the sensors were in approximate equilibrium with the moisture in the borehole intervals.
Measurements suggested significant variability in water potentials between the three niches and that wet conditions existed in the vicinity of the Sundance fault. The main observations were:
Additional moisture monitoring activities were conducted within ventilated and non-ventilated areas of the ESF and ECRB. In ventilated sections, no continuous dripping (or seepage) was observed either in the ESF or in the ECRB. This lack of seepage may be explained by the capillary barrier mechanism in which capillary forces hold water within the rock mass. The other explanation pertains to ventilation. Ventilation can remove large amounts of moisture, dry the rock behind the drift walls, and suppress seepage. To determine whether seepage returns when ventilation effects are mitigated, the last one-third of the ECRB Cross-Drift was sealed through use of multiple bulkheads. Although humidity was found to quickly rise upon closing off ventilation, re-equilibration of the rock moisture was found to take much longer. Wet spots were observed and liquid water was collected in sections and, based on chemical analyses of the clean water collected, the presence of water can likely be attributed to condensation. Isotopic signatures indicated that the collected water underwent an evaporation shift. Read more about Micro-climate dynamics in the near-drift environment at Yucca Mountain »
A key issue regarding the performance of the potential high-level radioactive waste repository at Yucca Mountain, Nevada, is the likelihood of precipitation percolating a vertical distance of ~300 m through fractured unsaturated rock into drifts containing waste packages. Water enhances waste package corrosion and is required for transport of released radionuclides. To evaluate the propensity for seepage into tunnels at Yucca Mountain, a 5 m diameter, 2.7 km long tunnel, commonly referred to as the Cross Drift (CD), was excavated in 1998, branching off from the main Exploratory Studies Facility tunnel. Sections of this tunnel have been isolated from ventilation for extended periods over the last four years. We have measured relative humidity and temperature, and made periodic observations of liquid water in the CD. During this observation period, the terminal section of the drift was partitioned into four sections by bulkheads, and ventilation to these sections was minimized to a few days. We compare these observations to results from analytical and numerical models to investigate processes associated with the movement of water vapor between the tunnel bore and the surrounding fractured rock formation. Observations from this effort indicate that fractures can be primary paths for unsaturated zone vapor flow in the immediate vicinity of emplacement drifts. Observations, measurements, and analysis indicate the need for a model that includes fracture-dominated vapor transport as a significant contributor to total water flow into the drifts.