RD&D-Programme 2004 - SKB

17.1.13 Impurity levels
Bentonite is the name of a naturally occurring clay material with variations in composition
depending on how it is formed. Bentonite often occurs in different strata with some variation
in composition and with intervening strata that may contain other minerals. In commercial
products, such as MX-80, materials from different strata are often blended to meet specified
quality requirements. Usually, impurities in bentonite consist of minerals that are of little
importance for the performance of the repository (quartz and feldspar). Small amounts of e.g.
amorphous silicon, calcite, pyrite, siderite and gypsum may, however, be of some importance
for the chemical evolution in the repository’s near-field. The majority of the minerals in the
buffer have clear advantages and disadvantages. An example is calcite, which has a favourable
pH-buffering effect, but which may also be enriched at the canister under the influence of a
sharp temperature gradient. At present, however, there is no mineral that is considered to have
a decisive adverse impact on repository performance at normally occurring concentrations. The
composition of the impurities in a buffer material will therefore probably be allowed to vary
within certain limits between different consignments of the material.
Conclusions in RD&D 2001 and its review
SKI considers it important for SKB to formulate acceptance criteria for other minerals and
organic impurities in the clay.
Kasam thinks there is reason not only to determine general limits for impurities, but also to
investigate how combinations of impurities can affect the long-term stability of the buffer
(both positively and negatively).
Newfound knowledge since RD&D 2001
Newly-developed analysis methods (see section 17.1.6) for the mineral distribution in bentonite
permit more efficient material inspection at delivery in terms of costs, time and accuracy. The
accuracy for individual accessory minerals is approximately one percent, which is a considerable
improvement. Different methods have been tried for routine measurement of total carbon
content and organic carbon content.
Programme
It is difficult to set limits on impurity levels in the buffer. Most minerals are of little or no
importance for performance. SKB considers it more important to show that occurring concentrations
of minerals do not seriously diminish repository performance, see section 17.2.18. The
work of improving analysis methods and routine inspections of bentonite continues within the
Lot project and in the investigations of potential buffer materials.
See also section 17.1.2
17.2 Processes in buffer
17.2.1 Overview of processes
On emplacement, the buffer comes into contact with the hot canister surface, the thermal energy
is spread through the buffer by heat transport and the temperature increases. The gamma and
neutron radiation emitted by the canister decreases in intensity due to radiation attenuation in
the buffer.
A negative capillary pressure exists originally in the pores in the buffer, causing water to be
transported in from the surrounding rock. After the buffer has been saturated with water, this
water transport is very slow. Gas transport can occur during the saturation process, when
water vapour can flow from the hotter parts of the buffer to condense in the outer, colder parts.
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