RD&D-Programme 2004 - SKB

10.5.2 Technology for sealing of boreholes
Conclusions in RD&D 2001 and its review
Technology for sealing of boreholes was previously developed and tested within the framework
of the Stripa Project /10-12/ and applied to boreholes in the seabed near SFR. Two different
techniques were used to insert highly compacted bentonite blocks into boreholes up to
200 metres long. Another sealing technique has been tested by Nagra involving blowing in
bentonite pellets through a hose.
SKI points out in its review that the possibility of a failure of a borehole seal or the opening
of flow paths in old borehole locations cannot be completely ruled out. SKI therefore believes
that some kind of respect distance between boreholes and deposition holes is needed. SKB is
developing a concept for sealing of boreholes and dealing with the question of a respect distance
between boreholes and deposition holes in safety assessment and design.
Newfound knowledge since RD&D 2001
A programme for identifying and demonstrating the best available technique for cleaning and
sealing investigation holes is being carried out by SKB and Posiva in cooperation. The first
stage of the programme, which was concluded during 2003, comprises a feasibility study in
which available techniques for sealing of long borehole were identified. In the feasibility study,
clay was identified as a suitable material for sealing most of a borehole. The top part, if the hole
is drilled from the surface, must be sealed so that erosion, glaciation and sabotage will not lead
to a deterioration of the function of the seal in the deeper parts of the hole. Two concepts for
applying clay plugs in boreholes have been identified that should be further investigated: sealing
with pre-compacted blocks and sealing with compacted pellets.
Programme
Additional stages of the ongoing research programme will be carried out during the coming
RD&D period. To start with, stage two will be carried out, including:
• Establishment of performance requirements.
• Formulation of a complete concept for sealing of investigation holes.
• Laboratory studies of potential materials and combinations of different materials.
• Cleaning of the boreholes that will be used in full-scale tests of material and equipment in
the field.
• Planning of and preparations for field tests.
The programme for subsequent stages will be specified when experience is available from the
initial stages.
10.6 Canister retrieval
How retrieval of deposited canisters is accomplished depends on when it takes place. The more
time that has passed after the deposition of a canister, the more work is required. If it is decided
to retrieve the canister immediately after deposition and the deposition tunnel has not been
backfilled, retrieval can be done with the same machine used for deposition. If the deposition
tunnel is backfilled and sealed, however, more work is required. If retrieval takes place several
years after deposition and the bentonite has reached full swelling pressure, even more work is
required due to the fact that deformations can create a risk of falling rock when the backfill is
removed. Furthermore, the canister must be freed from the buffer before it can be lifted up out
of the deposition hole.
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