Medium-deep or very deep disposal of highly radioactive waste?

• Several of the commonly proposed concepts for disposal of highly radioactive waste (HLW) imply construction at medium depth (400-600 m) in granitic rock, which is excellent for constructing a stable repository since it provides effective mechanical protection of the waste. A drawback is that major water-bearing fracture zones are frequent and must be avoided in the site selection process since they can undergo large deformations caused by seismic and tectonic events and cause failure of waste containers located in or near them. The effect of such events can be minimized by surrounding them with ductile “buffer” clay that retards groundwater-driven adflow. An alternative concept is placement of HLW in very deep boreholes (VDH) where the rock is much less permeable and where the very salt, heavy groundwater is stagnant. The boreholes are proposed to be 4 km deep and grouped in a small number of sites. The upper 2 km parts, with temperatures lower than about 100oC, are sealed by being filled with perforated supercontainers with dense clay blocks, while the lower part contains supercontainers with waste canisters and dense clay blocks, raising the temperature between 2 and 4 km to 100-150oC. The holes are kept filled with clay mud into which the supercontainers are inserted where the rock contains few fractures, while concrete is cast where the rock is fracture-rich. In the upper part clay migrates through the perforated supercontainers and consolidates the mud. In the lower part clay the same process takes place where the clay block in each supercontainer is located, while the rest of the mud retains its original low density but undergoes stiffening. In the upper, sealed part of the hole, the consolidated clay will be much tighter than the surrounding rock, while in the lower part the mud will be more permeable but still capable of limiting water circulation within the hole. The paper compares the two repository principles and recommends closer examination of the very deep hole concept, which has obvious advantages respecting both performance and cost.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Samhällsbyggnadsteknik -- Infrastrukturteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Civil Engineering -- Infrastructure Engineering (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Samhällsbyggnadsteknik -- Geoteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Civil Engineering -- Geotechnical Engineering (hsv//eng)

Nyckelord

Structural Engineering

Konstruktionsteknik

Geoteknik

Soil Mechanics

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