RD&D-Programme 2004 - SKB

Furthermore, it must not have an adverse effect on canister and rock, and it must retain its safety
functions for a long time.
In order to retain its safety functions, the buffer must have sufficient density and interact with
the backfill in such a manner that the canister is always surrounded by buffer material. The
buffer must also possess such properties that minor rock movements can be absorbed without
the canister being damaged, and it must be able to dissipate heat from the canister so that the
temperature on the canister surface and in the buffer does not get too high.
The requirements on the buffer for KBS-3H (see section 10.7) are the same as for KBS-3V.
However, the detailed design of the buffer may differ depending on such circumstances as
deposition method and diameter of deposition tunnel.
Furthermore, it must be possible to fabricate, handle and deposite the buffer with acceptable
environmental impact and so that requirements on capacity and quality can be met.
10.2.2 Fabrication technology
Conclusions in RD&D 2001 and its review
In RD&D 2001, SKB concluded that:
• There are primarily two methods available for fabrication of bentonite blocks and rings:
uniaxial pressing and isostatic pressing.
• Blocks have been pressed to both a natural water ratio (10 percent) and an elevated water
ratio (17 percent). Bentonite blocks and rings have been produced for experiments in the
Äspö HRL. They have been fabricated from sodium bentonite (MX-80) by uniaxial pressing.
• Blocks and rings thicker than 0.5–1.0 metre cannot be easily produced by uniaxial pressing.
Thicker buffer units have advantages in connection with handling and emplacement in the
deposition holes.
In its review, SKI finds it laudable that SKB is investigating alternative fabrication methods
for bentonite blocks, since isostatically pressed blocks will probably be more homogeneous in
structure than uniaxially pressed blocks and rings and thereby more likely to meet strict quality
demands.
Newfound knowledge since RD&D 2001
Blocks and rings for nine full-sized deposition holes have been fabricated from sodium
bentonite (MX-80) by uniaxial pressing for use in the Prototype Repository and the Lasgit test
at the Äspö HRL. The blocks and rings pressed for the Lasgit test had a very high initial degree
of water saturation.
There is no isostatic press in Sweden today where full-scale blocks and rings can be fabricated.
No equipment for isostatic pressing of large blocks and rings has been developed during the
period. Twelve blocks on a scale of 1:4 were pressed in a press at Ifö Ceramics in 2000 /10-9/.
The results show that the technique should also be suitable on a larger scale.
In the KBS-3 variant with horizontal deposition (see section 10.7), the bentonite rings and
blocks are the same as in the reference variant, except that they are a few centimetres thicker
and thereby have a lightly larger diameter.
Programme
SKB will study fabrication of the buffer by means of isostatic pressing. The goals of the study
are to specify processes and equipment for material handling, pressing, handling of pressed
blocks, post-press machining and interim storage of blocks and rings prior to deposition.
120 RD&D-Programme 2004