MOISTURE OBSERVATIONS IN SEALED TUNNELS AT YUCCA MOUNTAIN, NEVADA

The Topopah Spring Tuff is the host rock for a proposed repository for high-level nuclear waste. Underground tunnels and alcoves in this tuff that have been sealed from ventilation provide potentially useful data on natural moisture conditions and can help address the question of whether significant amounts of percolating groundwater drip into tunnels under present-day conditions. Given the low infiltration rates in the region (Flint et al., 2001; Zhu et al., 2002), natural seepage and dripping in the sealed tunnels would provide evidence of focused flow within fracture networks that could be used to help calibrate seepage models for present-day conditions. These observations can then be used to estimate seepage fluxes during future, wetter climates. In 1999 the Department of Energy (DOE) sealed a nearly 1-km long tunnel bored near the proposed repository area. Four bulkheads isolate four sections of this tunnel, commonly called the Cross Drift, to allow a return to natural, ambient moisture conditions. Alcove 7, which crosses the Ghost Dance Fault, is a niche that has also been sealed with a bulkhead. Observations made in the sealed tunnels during unventilated entries help to ensure that moisture observations will be little affected by the rapid drying effects of ventilation. Extensive evidence of humid conditions has been seen during such unventilated entries, including small puddles apparently produced by condensation dripping. DOE is attempting to systematically collect drips in sample bottles and on plastic sheets so that chemical analyses can be used to identify sources of the water (i.e., natural seepage, condensation, or a mixture). To date two locations of suspected natural seepage have been observed: one in Alcove 7 and the other in a sealed section of the Cross Drift. Both of these drip zones occur outside the proposed repository footprint. DOE is continuing work in the sealed tunnels to address agreements with NRC. Hydrologic data and observations from the sealed tunnels provide a reference point for DOE's performance assessments of deep percolation and seepage. The NRC staff believes that long-term empirical observations in sealed tunnels could be an element of a performance confirmation plan for Yucca Mountain.