Use of Natural Analog and Modeling Studies to Constrain the Effects of Magmatic Activity on Long-Term Geologic Repositor
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USE OF NATURAL ANALOG AND MODELING STUDIES TO CONSTRAIN THE EFFECTS OF MAGMATIC ACTIVITY ON LONG-TERM GEOLOGIC REPOSITORIES GREG A. VALENTINE, N.D. ROSENBERG, B.M. CROWE, AND F.V. PERRY MS F665, Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545 USA. ABSTRACT Examples of the application of natural-analog studies to the estimation of the consequences of a volcanic eruption penetrating a radioactive waste repository are given, including the criteria for analog selection and new data from ongoing studies. Examples of early modeling results focusing on the spatial and temporal scale of subsurface processes are also provided. All of these examples are taken from studies of the potential Yucca Mountain repository, Nevada, but similar approaches could be applied in other areas. In addition, studies of subsurface processes initiated by magmatic events serve as useful analogs for repository thermal loading studies. INTRODUCTION Many countries pursuing long-term geologic disposal of radioactive waste have experienced little or no magmatism for very long time intervals and are thus able to practically eliminate any risk associated with these processes. In some countries, such as the United States and Japan, the risk from magmatic events (including intrusive, volcanic, and hydrothermal processes) is significant enough to require detailed studies. In this paper we describe some of the approaches that are being used to determine the consequences of magmatic processes to the potential Yucca Mountain repository in Nevada. The approaches are general and are applicable beyond the specific volcanic region surrounding Yucca Mountain. The Yucca Mountain region has experienced varying degrees of magmatic activity since the late Miocene, when large calderas were formed and numerous large-volume silicic ignimbrites were erupted 1 . Since about eight million years ago volcanic activity has consisted entirely of the formation of relatively small (< 3 km 3 ) basaltic centers and associated shallow intrusive complexes. Characterization of this activity has been reported in numerous papers over the past 15 years, but is described most comprehensively by Crowe et al. 2 . The following probabilistic framework has been used to estimate the risk of magmatic activity to the potential Yucca Mountain repository: Prdr = Pr(E3given E2,E1)Pr(E2given E1)Pr(E1)
(1)
where Prdr is the probability of magmatic activity causing releases that exceed the regulatory requirements for licensing a repository. El is the recurrence rate of magmatic events in the region surrounding Yucca Mountain (based on the estimated recurrence rate during the Quaternary and Pliocene), E2 is the probability that a future magmatic event intersects the potential repository or is close enough to it to have a significant effect on its performance, and E3 is the probability that a given magmatic event will cause release of waste in quantities that exceed regulatory limits (i.e., the consequences of the event). Calculations of El and E2 have
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