A method for sealing cored boreholes using dense montmorillonite clay in perforated copper tubes where the rock has only very fine fractures, and concrete cast where the holes intersect water-bearing fracture zones, has been used for closing 100 to 500 m deep boreholes made for site investigation in Sweden and Finland. Application of this technique to even deeper boreholes requires performance analysis respecting placeability and long-term function of the clay and concrete, individually and in combination, as discussed in the paper. Placeability requires that maturation of the initially unsaturated clay is neither too fast nor too slow and means of controlling the rate are described. The proposed concrete material has a very low cement content and talc as fluidizer, which gives slow but ultimately very significant strengthening and low solubility of the cement reaction products. At a few kilometers depth the temperature can be more than 60oC, which has to be sustained by the sealing materials, and this makes saponite and mixed-layer smectite/illite possible alternatives to montmorillonite clay. The concept is judged to be applicable also to backfilling of wider holes for disposal of highly radioactive waste.