Abstract
Deep investigation boreholes in crystalline rock for site selection of repositories for high-level radioactive waste are proposed to be sealed by installing a series of dense concrete and clay plugs. These should prevent radionuclides from leaking canisters at depth to migrate to the biosphere through the holes. The concrete seals will be installed where the holes intersect water-bearing fracture zones to serve as stable and low-permeable supports for adjacent clay plugs. Low porosity and microstructural stability must be guaranteed for many thousands of years and ordinary Portland cement with organic superplastizer will not fulfill the requirements since the high pH will cause degradation of contacting clay and the organic additive can produce colloids with a capacity to carry radionuclides up to the biosphere. Very cement-poor concrete (
Highlights
Deep boreholes made in conjunction with site investigations for locating repositories for high-level radioactive waste (HLW) should be effectively sealed for up to a hundred thousand years [1]
Deep investigation boreholes in crystalline rock for site selection of repositories for high-level radioactive waste are proposed to be sealed by installing a series of dense concrete and clay plugs
For holes passing through low-permeable rock with fine fractures only, sealing with clay is suitable, whereas concrete seals are proposed for those parts of the holes that intersect permeable fracture zones [2], cf
Summary
Deep boreholes made in conjunction with site investigations for locating repositories for high-level radioactive waste (HLW) should be effectively sealed for up to a hundred thousand years [1]. For holes passing through low-permeable rock with fine fractures only, sealing with clay is suitable, whereas concrete seals are proposed for those parts of the holes that intersect permeable fracture zones [2], cf Figures 1 and 2. The average number of clay and concrete seals in 500 to 1000 m long holes is on the order of 10 - 20. The concrete, which is cast down in the holes, does not have to be water-tight but serve as a filter for preventing particles from adjacent clay seals to migrate through it and further into the fracture zones where they can be dispersed and lost. Since the concrete seals require a certain time to mature and obtain sufficient bearing capacity for carrying the clay segments without yielding, the whole sealing campaign must be planned with respect both to the time-dependent increase in strength of the concrete and of the clay seals, which establish bonds to the rock that makes them carry their own weight
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