RD&D-Programme 2004 - SKB

The wetting of the backfill from the rock is measured by emplaced psychrometers and provides
valuable information on the hydraulic interaction between the rock and the backfill.
The goal of the project “Backfill and Closure of Tunnels and Rock Caverns” is to develop
materials and technique for backfilling and closure of a deep repository for spent nuclear fuel
of the KBS-3 type. The goal of phase 1, which has consisted solely of technical studies and was
concluded in early 2004, was to describe the potential of the proposed backfilling concepts for
meeting the requirements formulated by SKB and Posiva, select the most promising concepts
for further studies and describe methods for verifying the performance of the backfilling concepts.
The proposed backfilling concepts differ with regard to backfill materials and installation
method (compaction of the material in situ in the tunnel, emplacement of blocks, or a combination
of the two methods) and are described in brief in section 18.1.6.
The assessment of the concepts has been based on performance requirements formulated
to ensure a safe environment for canister and buffer during the deep repository’s operating
period. These requirements, along with assumed conditions in the deep repository (salinity of
groundwater etc), comprise the basis for the design premises used to compare and discriminate
the concepts. The design premises pertain to requirements on the backfill’s compressibility,
hydraulic conductivity, swelling pressure, long-term stability, negative effects on the barriers in
the final repository, and technical feasibility. The risks involved and the need for further studies
for the individual concepts have also been dealt with in phase one.
The principal recommendations can be summarized as follows: Concepts A and B (see
Figure 18-1), which are based on compaction in situ in the tunnel, will be further studied in the
next phase of the project to determine if high enough densities can be achieved for the materials
in question. Concept D has good potential for meeting the requirements, but it is a new backfilling
concept and some uncertainties regarding material choice, manufacture and emplacement of
blocks therefore need to be investigated in the next phase. The heterogeneous concept E differs
in terms of design and performance from the other concepts. This concept will not be further
investigated for the time being, but some of the work that will be done for the other concepts
can be applied to assess concept E as well, if it should prove to be of interest. Concept C, where
most of the backfill material consists of non-swelling clay, will not be further studied since it
has been judged unlikely that it will be able to meet the stipulated requirements.
An alternative backfill material that has been thoroughly studied is the natural montmorillonite
clay Friedton. Studies have been conducted in the form of laboratory tests /18-5/ and compaction
tests in the field /18-6/.
The results show that the material’s hydraulic conductivity is sufficiently low at the density
measured in the field tests and for the salinity foreseen in the groundwater. However, the
achieved field density gives a compressibility that is too high to prevent excessive upswelling
of the buffer in the deposition holes. The field tests also showed that it is difficult to apply the
material so that a gap at the roof is avoided. However, the preparation of the material and the
compaction technique should be able to be improved so that adequate density can be achieved
in the field.
Programme
Studies of backfilling concepts will continue within the three described projects.
Now that the backfill is water-saturated, the Backfill and Plug Test has entered a new phase
with flow and compression tests. The water pressure in the backfill, which is 500 kPa today as a
result of pressurizing in the filter mats, will gradually be reduced and the flow between the mats
measured. The mats near the roof and near the floor have been separated from the mat in the
central part of each section, enabling the hydraulic conductivity of the backfill to be measured
in these different parts. After concluded flow tests, compressibility will be measured by means
of the four pressure cylinders attached to the floor and the ceiling.
232 RD&D-Programme 2004