RD&D-Programme 2004 - SKB

17.1.3 Pore geometry
Conclusions in RD&D 2001 and its review
In SR 97, it was assumed that the buffer had a dry density of 1,590 ± 30 kg/m 3 . This gives a
porosity of 41 percent. No questions for further research were identified in SR 97 or its review.
In RD&D-Programme 2001, it is pointed out that other clays than MX-80 could also satisfy the
conductivity requirements that are made on the buffer. A choice of another clay might lead to the
choice of another density and porosity.
Newfound knowledge since RD&D 2001
No new knowledge has been forthcoming.
Programme
The very low hydraulic conductivity of the buffer, despite the fact that its porosity is over
40 percent, suggests that the distribution of the pore volume is of crucial importance for the
properties of the bentonite. Different qualitative and quantitative descriptions of pore structure
– and how it affects properties such as diffusion, conductivity and swelling pressure – are found
in the literature. SKB therefore intends to start a study aimed at quantifying pore structures in
potential bentonite materials under different physical and chemical conditions.
17.1.4 Radiation intensity
The initial dose rate on the canister surface was calculated in SR 97 to be 100–500 mGy/h, and
on the outside of the buffer about 2 mGy/h. The calculations are based on the same assumptions
as the material in section 17.2.2.
17.1.5 Temperature
Buffer and backfill are at ambient temperature at deposition. This varies with repository site
and disposal depth and is approximately 10–15°C. The temperature is dependent to some extent
on the handling sequence, where the buffer blocks have been stored, heat from the deposition
machine, etc. An uncertainty of around 5°C is reasonable.
Determination of the initial buffer temperature is of trivial importance, in contrast to the heat
transport in the buffer after deposition, see section 17.2.12.
17.1.6 Montmorillonite content
Conclusions in RD&D 2001 and its review
The commercial bentonites that are of interest as buffer materials have a given composition.
The montmorillonite content is normally over 80 percent. The delivered product will be qualitytested
before being taken to the repository.
Newfound knowledge since RD&D 2001
New methods for determining the mineral distribution in buffer materials have been used
and developed. Among other things, the important material parameter cation exchange
capacity (CEC) has been determined by means of a simpler and more reliable method /17-2/.
Furthermore, a new analysis method (Rietveld technique) for quantification of minerals from
X-ray diffractograms has been used. Detailed determinations of the mineralogy of MX-80
material have been performed within the Lot project, for reference material and for exposed
material. Other bentonite materials have been studied in the project “Alternative buffer
materials”, see section 17.1.2. New determinations of the montmorillonite content of MX-80
RD&D-Programme 2004 197