Preliminary Assessments for Disposal of High-Level Waste Within Small Islets

This paper analyzes, for the final disposal of high-level waste (HLW), the possibility of a repository sited below the fresh/saline groundwater interface within islet rock formations. Because of their relatively stable tectonics, the offshore islets of some countries (such as those of Taiwan) are worthy of being considered as potential repository sites. Before the emplacement of radwastes in such a repository, however, the mass exchange across the fresh/saline groundwater interface must be limited and the horizontal movement of advective saline ground-water must be extremely low. Theoretical equations for the location and shape of the interface are derived. When radwastes are buried in rock formations, the temperature effect of the decay heat could cause buoyant convection flow of saline groundwater upward across the groundwater interface. This could carry released radionuclides across the groundwater interface to upper formation layers where fresh groundwater flows. The radionuclides could then be carried by the fresh groundwater to the sea. Although basic HLW repository designs should eliminate the significance of this temperature effect, it is incorporated into this preliminary analysis for the purpose of conservative estimations.