A fundamental policy for selecting sites for high-level radioactive waste (HLW) geological disposal is to exclude regions of future volcanism to avoid direct damage to the repository due to magmatic intrusion or volcanic eruption. For this purpose, it is important to estimate the potential position, timing, lateral extent and style of future volcanic activity. This estimation is done based on the characteristics of past volcanism and related phenomena that have occurred near the site. Two scales of evaluation should be considered in the context of a deterministic igneous intrusion and volcanic hazard assessment, as will be described in this chapter. These are: (i) evaluation at the scale of an individual volcano, in terms of the potential of migration of magma to the repository from existing volcanoes and their magma plumbing systems; and (ii) evaluation at the scale of a volcano cluster, in terms of the potential for generation of a new volcano in the site area during the performance period of the repository.

In order to make these two evaluations, we should endeavor to understand the geologic processes that give rise to volcanism and to establish plausible geological models for volcanism in specific site regions using available data. These data include spatio-temporal patterns in volcanism, and their relationship to topographical, geological and geophysical data. With regard to geological models, recent research has revealed much useful information about the clustered distribution of volcanism in volcanic arcs, and related geologic processes responsible for the formation of new volcanoes.