THE ROLE OF UNDERGROUND OPENINGS IN THE UNSATURATED ZONE AS A BARRIER IN ISOLATING HIGH-LEVEL RADIOACTIVE WASTE

Understanding the interaction of groundwater with buried high-level radioactive waste is a critical issue in characterizing the proposed repository at Yucca Mountain, Nev. Water moving through the unsaturated zone (UZ) is preferentially diverted around underground openings by capillary action, as can be demonstrated both mathematically and experimentally. In addition, there are abundant examples from natural analogues that lend strong support to the theoretical predictions and short-term experiments. Examples include: 1) multi-year infiltration and seepage studies in caves, 2) preservation in caves of delicate archeological materials for thousands of years, and of biological remains for more than 1,000,000 years, and 3) preservation of paintings and other artifacts in subterranean tombs and temples. Also, visual observations in underground openings show that much of the water that does seep in tends to flow down walls. Thus, tunnels in the UZ are a form of barrier because they inhibit water contact with the radioactive waste.

Effectiveness of tunnels as a barrier can be enhanced by using them for ventilation. The forced ventilation planned for Yucca Mountain to control heat during the active operation phase of the proposed repository would have the potential of removing several orders of magnitude more water each year than would be supplied to the entire proposed repository block by infiltration. Passive ventilation after closure of the proposed repository could continue to remove large volumes of water. Analogue studies in caves show that passive ventilation without leaving access for human intrusion is possible. Most caves lose moist air through openings large enough to permit human entry either by barometric pumping or by flow between openings at different elevations. However, at Kartchner Cavern, Ariz., moist air is exhausted from a network of fractures. Raises driven into, but not through, the highly fractured Tiva Canyon Tuff that caps Yucca Mountain could provide similar ventilation to remove moisture from the proposed repository without creating an access route for human intrusion.