TESTING CLAIMS ABOUT VOLCANIC DISRUPTION OF A POTENTIAL GEOLOGIC REPOSITORY AT YUCCA MOUNTAIN, NEVADA

A special use of geologic data is to evaluate low probability/high consequence events. We examine volcanism near Yucca Mountain (YM), a potential repository for disposal of high-level radioactive waste. Recent studies suggest that basaltic dikes could penetrate the repository with a frequency as high as 1E-6/yr. Our analysis raises doubts about such claims. More realistic models are developed based on non-detection of dikes in the potential repository footprint and analyses of known Pleistocene volcanism. First, using a statistical analysis, penetration frequencies > 2E-7/yr are not consistent with the non-detection of dikes in the footprint. Second, using NRC's Probabilistic Volcanic Hazard Assessment (PVHA) code [Connor et al., JGR, 2000], we analyzed 10 datasets, 8 of which include 5-15 magnetic anomalies that are assumed to be buried basalts. Claims of high penetration frequency fail recurrence tests at time scales of 1 Myr and 100 kyr. For a repository intrusion frequency of 1E-6/yr in the last 1 Myr, an expected 40-96 volcanos would have erupted in the region (80-192 without gravity weighting); only 8 Pleistocene events are known (recurrence rate 4.4/Myr). We also test whether the 80 kyr Lathrop Wells volcano began a new volcanism pulse. For a penetration frequency of 1E-6/yr, the PVHA code indicates 4-10 (8-19 without gravity weighting) volcanic events would be expected in the last 100 kyr; only 1 is known. There is uncertainty about the numbers of Pliocene and Miocene events near YM because of the longer time available for physical erosion and burial by alluvium and younger volcanics. Using the more reliable Pleistocene data (8 events in 1.8 Myr) and the PVHA code with zero gravity weighting, the frequency of dike intersection is 5.4E-8/yr with a 95% upper confidence bound of 9.7E-8/yr. If additional Pliocene basalts exist undetected in the alluvial basins, that would be further evidence of the decline in post-Pliocene activity. [The views expressed herein are the authors’. They do not reflect an NRC staff position, or any judgment or determination by the Advisory Committee on Nuclear Waste or the NRC, regarding the matters addressed or the acceptability of a license application for a geologic repository at Yucca Mountain.]