Geologic hazards such as volcanism must be assessed when evaluating potential sites for the geologic disposal of high-level radioactive waste. The Japanese islands comprise one of the more volcanically active regions on Earth and the risk of volcanism to a geologic repository is being evaluated as part of the Japanese high-level radioactive waste-disposal program. One hazard that is being evaluated as part of volcanic risk assessment is the possible intersection of a repository by a dike, if a composite volcano were to form near a repository during the long (100 k.y.) performance period of the repository. In this paper we use the characteristics of a well-exposed radial dike system at the Summer Coon volcano in Colorado to define Monte Carlo simulations that estimate the probability of a dike intersection of a repository as a function of volcano distance, dike length and density, and repository area. The models indicate that the probability of intersection declines rapidly as a function of distance from a volcano, as dike density decreases due to the radial dike geometry. The probability of intersection for a mafic dike set with shorter average length declines more rapidly than that of a silicic dike set with longer average length. However, mafic dikes have a higher probability of intersecting a repository close to a volcano (<5–6 km) because they are more numerous than longer silicic dikes. The probability of a silicic dike intersecting a repository is ≤10−2 at distances greater than 15 km from the volcano, decreasing to ≤10−4 at distances greater than 30 km.

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