Smectite clay, especially montmorillonite, is proposed for isolating canisters containing highly radioactive waste (HLW) like spent reactor fuel placed in deep boreholes. It is used for minimizing groundwater flow around and along waste packages (“Buffer Clay”) and for providing them with ductile embedment for eliminating risk of canister damage caused by displacements in the host rock. The clay has the form of heavily compacted blocks of granules that swell in conjunction with water uptake until their full hydration potential has been utilized. The dense clay blocks are fitted in perforated supercontainers that are submerged in smectite mud. The long-term chemical stability of the clay is sufficient for providing the required waste-isolating capacity, which is primarily supplied by the heavyness of stagnant, very salt groundwater at depth.