RD&D-Programme 2004 - SKB

18 Backfill
The backfill in the tunnels is not in itself a barrier in the KBS-3 concept. It is, however,
necessary in order for the buffer and the rock to have the desired function. The requirements
made on the backfill are:
• The backfill must have a stiffness that minimizes the upward expansion of the buffer.
The density of the buffer is thereby maintained.
• The backfill must have a hydraulic conductivity that is comparable to that of the surrounding
rock. Otherwise the deposition tunnels may act as conductive pathways that influence the
water flux in the repository.
• The backfill must exert a given swelling pressure against the roof to maintain a swelling
capacity that can seal possible effects of piping and creep movements in the backfill.
The backfill may not have any adverse effect on the barriers in the repository, which imposes
some requirements on its chemical composition.
SKI points out in its review of RD&D 2001 that the backfill is a prerequisite for ensuring that
the buffer performs as intended and that the near-field rock is not short circuited as a barrier
against groundwater flow. SKI therefore urges SKB to rapidly determine the requirements and
criteria that the backfill material must meet so that, prior to licensing, data and methods can
be developed that show how these requirements are met (for example with respect to material
choice, application technology and inspection methods).
18.1 Initial state of the backfill
18.1.1 Variables
More or less the same set of variables is used to describe the backfill as for the buffer, see
Table 17-1.
18.1.2 Geometry
The dimensions of the backfill are given by the dimensions of the tunnels.
18.1.3 Pore geometry
The initial pore geometry (porosity) of the backfill follows trivially from its material
specifications.
18.1.4 Radiation intensity
The initial radiation intensity in the backfill is negligible.
18.1.5 Temperature
A determination of the initial temperature in the backfill is trivial. It will be close to the initial
temperature of the rock.
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