REDUCED LIKELIHOOD OF VOLCANIC DISRUPTION OF A GEOLOGIC REPOSITORY AT YUCCA MOUNTAIN

We use past volcanic activity to examine the likelihood of future igneous activity at Yucca Mountain, a potential site for a geologic repository for disposal of high-level radioactive waste. We previously tested claims that basaltic dikes could penetrate the repository with a frequency of 1E-6/yr. Our analysis (Coleman et al., GRL, 2004, doi:10.1029/2004GL021032) raised serious doubts about such claims because they are inconsistent with the small number of volcanoes that formed near Yucca Mountain during the last 1 million yrs and the last 100,000 yrs.

We used new data from the drilling of suspected buried basalts and applied kernel density estimators using NRC's Probabilistic Volcanic Hazard Assessment code (Connor et al., JGR, 2000). Dataset CFB_plio-quat-Mag (Connor et al., 2002) includes 29 Plio-Pleistocene events (14 dated basalts + 15 anomalies formerly assumed to be post-Miocene basalts). The new data eliminate 7 anomalies and reduce the number of basaltic events in this dataset to 22. This lowers the apparent Plio-Pleistocene recurrence rate from 5.5E-6/yr (Coleman et al., GRL, 2004) to 4.2E-6/yr (22 events in 5.3 Myr), consistent with the Pleistocene recurrence rate of 4.4E-6/yr (8 events in 1.8 Myr). The new data yield a repository dike penetration frequency of 4.2E-8/yr. Our previous estimate of dike intersection probability (i.e., 5.4E-8/yr with a 95% upper confidence bound of 9.7E-8/yr) using Pleistocene data is unchanged because none of the drilled anomalies represent Pleistocene basalts. However, factors not included in the model could further reduce the probability of repository intersection. Rising magma tends to follow paths of least resistance and preferentially intrudes pre-existing faults. Current DOE plans call for setback of waste drifts from major faults. Also, local topography can alter the stress field and steer a dike as it nears the surface and, more importantly, significantly affect vent location by influencing the strongest flow to occur at lower (i.e., non-ridge) elevations (Gaffney & Damjanac, GRL, 2006).

[The views expressed are the authors'. They do not reflect an NRC staff position, or any judgment or determination by the Advisory Committee on Nuclear Waste & Materials or the NRC, regarding acceptability of a license application for Yucca Mountain.]