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 is analyzed. 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. Radionuclide transport behavior in an islet site is compared to that in an inland site in which a repository would be built in partially saturated fractured media. The total effects of radionuclide transport for the islet site is similar to that for the inland site. Although the lack of information limits more detailed, quantitative predictions, the possibility of islet disposal sites for HLW is worthy of notice, and more research efforts toward investigation of islet sites are warranted.
