VOLCANIC HAZARD ASSESSMENT AT THE PROPOSED YUCCA MOUNTAIN HIGH-LEVEL RADIOACTIVE WASTE REPOSITORY, SOUTHERN NEVADA, USA: BASALTIC VOLCANISM IN THE YUCCA MOUNTAIN REGION

Yucca Mountain lies within the central Basin and Range Province, a region characterized by predominately basaltic volcanism during the past five million years. Three episodes of small-volume, alkali-basaltic volcanism have occurred within 50 km of Yucca Mountain during the Quaternary. The presence of basaltic volcanism near Yucca Mountain has led to more than 20 years of volcanic hazard studies sponsored by the Department of Energy (DOE), the Nuclear Regulatory Commission, and the State of Nevada. These studies continue today, with emphasis on reducing the uncertainty in assessments of the probability of volcanic disruption, and a better understanding of the consequences of a volcanic eruption at the repository site. Estimates of the probability of volcanic disruption of the repository depend on the recurrence rate and spatial distribution of past episodes of volcanism in the region. Estimates of the probability of disruption generally fall within the range of 10^-7 to 10^-9 disruptions per year, including the results of an expert elicitation sponsored by DOE in 1995-1996. Values for the disruption probability are dominated by a volcanic recurrence rate ranging from 10^-5 to 10^-6 volcanic events per year (equating to recurrence intervals of several hundred thousand years), a rate that is lower than most other basaltic volcanic fields of the Basin and Range interior. Uncertainties in the recurrence rate are due to uncertainties in estimates of the total number of volcanic events in the past 5 m.y. Recently published aeromagnetic data indicate up to twelve previously undetected volcanoes may be buried in alluvial-filled basins south and west of Yucca Mountain. Sensitivity studies indicate that the potential presence of these volcanoes could increase the hazard by 40%, an insignificant change compared to the range of previous hazard estimates. A key remaining question is whether undetected buried volcanoes exist to the east of Yucca Mountain, an area where no Plio-Pleistocene volcanoes are known to exist. A planned improved-resolution aeromagnetic survey followed by drilling and sampling of selected aeromagnetic anomalies will constrain the number and age of buried volcanoes within 10-15 km of the repository site.  Based on these results, DOE plans to update the 1996 expert elicitation following collection of new data.