THE USE OF QUALITATIVE NATURAL ANALOGUES TO HELP BUILD CONFIDENCE IN PREDICTIVE MODELS FOR SUBSYSTEMS AT YUCCA MOUNTAIN, NEVADA

John S. Stuckless, U.S. Geological Survey, MS 421, Denver, CO 80225 stuckles@usgs.gov

Qualitative natural analogues can contribute to understanding the predicted performance of subsystems of a mined geologic repository for high-level radioactive waste in several ways. Many simple analogues provide a means for the general public to assess the anticipated long-term performance of a repository and other scientific conclusions. Of equal importance, natural analogues provide realistic tests for subsystems at dimensional scales and time spans that cannot be attained by experimental study.

Hydrologic models for ground-water flow at Yucca Mountain predict that most of the flow through the unsaturated zone will stay in the wall rock and move around tunnels. This can be seen qualitatively by the excellent degree of preservation of archaeological artifacts and fragile biological materials recovered from underground openings such as tombs, temples, and caves. Examples include Paleolithic cave paintings in southwestern Europe, murals and artifacts in Egyptian tombs, painted subterranean Buddhist temples in India and China, and painted underground churches in Cappadocia, Turkey.

The probability that a tunnel will remain open over time depends on several variables, but the equations used to calculate that probability are not transparent to most people. However, the same types of analogues that support the predicted water flow in the unsaturated zone also demonstrate that underground openings can remain open for extended periods of time. Caves, such as Lechuguilla in New Mexico, have deposits on their floors that date at more than 11 Ma. Egyptian tombs excavated in limestone have stood open with no evidence of collapse for as long as 3,500 years, and Buddhist temples carved into basalt have been stable for as long as 2,200 years. Even ancient mines excavated in structurally weak rock by the Egyptians, Phoenicians, Greeks, and Romans have remained open with little or no ground support for thousands of years.