CHARACTERIZATION OF FRACTURES AT THE NOPAL I SITE AND COMPARISON TO FRACTURE CHARACTERISTICS OF YUCCA MOUNTAIN, NEVADA

The Nopal I uranium (U) deposit in the Peña Blanca district, Mexico, is a natural analog of some aspects of the proposed repository at Yucca Mountain, Nevada. The U.S. Nuclear Regulatory Commission (NRC) has independently conducted activities at Nopal I to better understand the analogous nature of source term and contaminant transport aspects of the two sites. The U deposit consists of a series of mined horizontal benches, some of which have been cleared to allow detailed mapping. The benches are vertically separated by 10 m (32.8 ft). The vertical walls between benches, and cleared benches, were mapped at a scale of 1:25. Uranium transport through the fractured silicic tuff at the site has been described (Pearcy et al., 1995), however that study focused only on the cleared benches. In general, fracture density on the vertical walls appears to be higher than on the cleared benches. Fracture density (total trace length per unit area) determined in ArcGIS indicates density at Nopal I can be greater than 6 m/m² within the ore deposit, but is not significantly different than outside the ore deposit. At Yucca Mountain the fracture intensity is much lower at 0.29 m/m² for the lower lithophysal unit of the Topopah Spring Member. This difference could be due to a longer cutoff length for mapped fractures at Yucca Mountain. At Nopal I fracture density was also calculated by frequency (i.e., number of fractures/m²) and it shows a general increase close to the deposit and somewhat higher values within the deposit. The fractal dimension of the fracture pattern on cleared benches, determined using a box-counting method, was 1.8, while at Yucca Mountain the fractal dimension of the Topopah Spring Member is 1.7 (Barton et al., 1993). Fractal dimension of rock blocks at Yucca Mountain range for 2.37-2.69 (La Pointe, 1988), while at Nopal fracture frequency density has a fractal dimension of 2.4 and a fracture length density of 2.6. The frequency of fractures having a mineral coating at Yucca Mountain has been used by the U.S. Department of Energy to support the active fracture model and this approach could be tested using the Nopal data.

The NRC staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed or of the acceptability of a license application for a geologic repository at Yucca Mountain.