Multibarrier systems are commonly proposed for effective isolation of highly radioactive waste (HLW). Presently considered concepts take the host rock as a barrier claiming it to retard migration of possibly released radionuclides from HLW containers to the biosphere. This capacity is small unless water-bearing fracture zones intersecting the blasted waste-containing tunnels and excavation-disturbance zones around them can be sealed by grouting and construction of bulkheads, but this is effective only for a very limited period of time as explained in the paper. The disturbed zones thence make the entire repository serve as a continuous hydraulic conductor causing quick transport of released radionuclides up to the biosphere. The dilemma can be solved by accepting the shortcircuiting function of the disturbed zones along the tunnels on the condition that totally tight waste containers be used. Deep holes bored in the site selection phase through the forthcoming repository can be effective pathways for radionuclides unless they are properly sealed. They are small-scale equivalents of tunnels but do not have any excavation damage and can be effectively sealed by using clay and concrete of new types. Applying this principle to very deep boreholes with a diameter of a few decimeters would make it possible to safely store slim, tight HLW canisters for any period of time.
Radionuclides leaking out from canisters with highly radioactive waste (HLW) must not contaminate the groundwater as required by national and international (IAEA) codes. The commonly applied multibarrier principle implies that the host rock, the waste canisters, and the clay surrounding them shall combine to retard migration of radionuclides escaping from the canisters. Comprehensive research in Sweden and Finland (Swedish Nuclear Fuel and Waste Management Co and POSIVA OY, respectively) has led to the proposal of using thinwalled copper canisters with spent nuclear fuel placed inside an iron core [1,2]. Such canisters, representing a first barrier, will not remain tight if sheared by significant instantaneous or repeated seismic events, and radionuclides being released from the failed canisters would enter the second barrier consisting of very dense smectite-rich clay [
The pathways of contaminated groundwater are interconnected fracture zones intersecting repository rooms, and boreholes made in the site selection and construction phases [
Migration of radionuclides within and from a deep underground repository depends on how the tunnels and shafts are constructed. Common HLW repository concepts imply that tunnels of a few hundred meter length and a cross section area of about 20 m2 are constructed with a spacing of 30 - 40 meters and that holes with a diameter of about 2 m for placing waste canisters are bored from the tunnel floors to about 8 m depth [