RD&D-Programme 2004 - SKB

Oxidizing conditions at repository depth can be broken down into two sub-problems:
• The repository will be oxygenated during construction and operation. Some oxygen will thus
probably remain in and near the repository at closure.
• Fears have been expressed that oxygenated water might penetrate down to repository depth
during periods of greatly changed hydrogeological conditions, for example in conjunction
with a glaciation.
To be able to evaluate the risk that glacial meltwater reaches repository depth, a re-examination
was conducted of existing data from both Äspö/Klipperås (occurrence of redox-sensitive
minerals etc) and other places /19-75/.
Evaluations of both water chemistry /19-76/ and fracture mineralogy /19-77/ show that there are
clear indications that components of a glacial meltwater have reached great depths (deeper than
500 metres). But there is no evidence to indicate that this water might have been oxygenated
below a depth of about 50 metres. There are, on the other hand, strong indications that reducing
conditions have prevailed at depths below 100 metres for a long time /19-77, 19-78/. It is
therefore misleading to associate the presence of glacial meltwater with oxidizing conditions.
In the Rex Project (Redox Experiment in Detailed Scale), studies were made of how oxygen
remaining after closure of a repository can react with minerals and groundwater in the rock
around the tunnel, the deposition holes or along the water-bearing fractures /19-79/. The results
from the in situ experiment were confirmed by a replica experiment in the laboratory. Both
investigations showed that oxygen had been completely consumed after a few days. The agreement
was strikingly good, considering the differences in experimental conditions. The main
conclusions from the project were:
• Oxygen consumption in the geosphere was found to be substantial and speedy.
• Microbial activity contributed substantially to the oxygen consumption.
• Methane and hydrogen gas that diffuse up through the earth’s crust are expected to make a
considerable contribution to reduction capacity.
Besides the oxygen consumption rate, the available buffering capacity is of great importance.
The buffering capacity for inorganic reactions between oxygen dissolved in water and reducing
substances (divalent iron and sulphide) in solution and in fracture-filling and matrix minerals
can be estimated. As far as the capacity of microbes is concerned, quite a bit of fundamental
research remains to be done, see microbial processes, section 19.2.18.
SKI pointed out in its review of RD&D 2001 that a scenario with changed redox conditions
at repository depth can be considered to have a very low probability, but that it cannot be
reasonably ruled out.
Newfound knowledge since RD&D 2001
Experiments have been conducted with weathering of the fracture-filling mineral chlorite
/19-80/. Chlorite is a common mineral in fractures and also occurs in alteration minerals in the
surrounding rock matrix. Chlorite usually contains significant quantities of iron(II) and can
therefore give some of the buffer capacity against intrusion of oxygen-containing groundwaters.
Equip was an EU project conducted during 1997–2000 aimed at using fracture-filling minerals,
mainly calcite, as indicators of current and former groundwater chemistry /19-77/. Generally
speaking, calcite is the most useful mineral, since it can be formed under very different conditions
and furthermore provides information on which type of groundwater the calcite has been
formed from. The composition of isotopes and trace elements is of help in this. Sulphide
minerals and iron oxides or iron oxyhydroxides can also be useful in interpreting redox
conditions. Equip has been succeeded by the EU project Padamot, which will last until 2006.
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