Mathematics – Logic
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.v14a..06c&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #V14A-06
Mathematics
Logic
6309 Decision Making Under Uncertainty, 8400 Volcanology, 8499 General Or Miscellaneous, 9350 North America
Scientific paper
Yucca Mt. (YM) is a potential repository site for high-level radioactive waste and spent fuel. One issue is the potential for future igneous activity to intersect the repository. If the event probability is <1E-8/yr, it need not be considered in licensing. Plio-Quaternary volcanos and older basalts occur near YM. Connor et al (JGR, 2000) estimate a probability of 1E-8/yr to 1E-7/yr for a basaltic dike to intersect the potential repository. Based on aeromagnetic data, Hill and Stamatakos (CNWRA, 2002) propose that additional volcanos may lie buried in nearby basins. They suggest if these volcanos are part of temporal-clustered volcanic activity, the probability of an intrusion may be as high as 1E-6/yr. We examine whether recurrence probabilities >2E-7/yr are realistic given that no dikes have been found in or above the 1.3E7 yr-old potential repository block. For 2E-7/yr (or 1E-6/yr), the expected number of penetrating dikes is 2.6 (respectively, 13), and the probability of at least one penetration is 0.93 (0.999). These results are not consistent with the exploration evidence. YM is one of the most intensively studied places on Earth. Over 20 yrs of studies have included surface and subsurface mapping, geophysical surveys, construction of 10+ km of tunnels in the mountain, drilling of many boreholes, and construction of many pits (DOE, Site Recommendation, 2002). It seems unlikely that multiple dikes could exist within the proposed repository footprint and escape detection. A dike complex dated 11.7 Ma (Smith et al, UNLV, 1997) or 10 Ma (Carr and Parrish, 1985) does exist NW of YM and west of the main Solitario Canyon Fault. These basalts intruded the Tiva Canyon Tuff (12.7 Ma) in an epoch of caldera-forming pyroclastic eruptions that ended millions of yrs ago. We would conclude that basaltic volcanism related to Miocene silicic volcanism may also have ended. Given the nondetection of dikes in the potential repository, we can use a Poisson model to estimate an upper-bound probability of 2E-7/yr (95% conf. level) for an igneous intrusion over the next 1E4 yrs. If we assume one undiscovered dike exists, the upper-bound probability would rise to 4E-7/yr. Higher probabilities may be possible if conditions that fostered Plio-Quaternary volcanism became enhanced over time. To the contrary, basalts of the past 11 Ma in Crater Flat have erupted in four episodes that together show a declining trend in erupted magma volume (DOE, TBD13, 2003). Smith et al (GSA Today, 2002) suggest there may be a common magma source for volcanism in Crater Flat and the Lunar Crater volcanic field, and that recurrence rates for YM could be underestimated. Their interpretation is highly speculative given the 130-km (80-mi) distance between these zones. A claim that crustal extension at YM is anomalously large, possibly favoring renewed volcanism (Wernicke et al, Science, 1999), was contradicted by later work (Savage et al, JGR, 2000). Spatial-temporal models that predict future intrusion probabilities of >2E-7/yr may be overly conservative and unrealistic. Along with currently planned site characterization activities, realistic models could be developed by considering the non-detection of basaltic dikes in the potential repository footprint. (The views expressed are the authors' and do not reflect any final judgment or determination by the Advisory Committee on Nuclear Waste or the Nuclear Regulatory Commission regarding the matters addressed or the acceptability of a license application for a geologic repository at Yucca Mt.)
Abramson Louis
Coleman N.
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