RD&D-Programme 2004 - SKB

Kasam says that disposal in Very Deep Holes is not a realistic method such as is required in the
environmental impact assessment according to the Environmental Code. The alternatives to the
KBS-3 method which are supposed to be described according to the Environmental Code should
be sought in the category “built repositories within the uppermost kilometre”.
Newfound knowledge since RD&D 2001
A literature review was recently conducted to supplement previously gathered geoscientific
information about conditions at depth in the earth’s crust /23-22, 23-23/. The focus was the same
as before, i.e. to provide information as a basis for judging the possibility of depositing radioactive
waste in four-kilometre deep vertical boreholes. The update consisted of geoscientific
information published in the open literature since 1997, and the emphasis was on crystalline
rock of relevance to SKB’s disposal concept.
The literature shows that scientific investigations in existing deep boreholes in the world have
continued. Examples are the KTB Field Laboratory in Germany and the Kola SG-3 Deep
Geolaboratory in Russia. New deep boreholes are also being drilled, for example the geothermal
borehole in Skåne down to a depth of about four kilometres, of which about two kilometres are
being drilled in underlying crystalline bedrock.
It is generally expected that fracture frequency, hydraulic conductivity and porosity will
decrease with depth in crystalline rock due to increasing rock stresses. At a depth of four
kilometres it can be expected that groundwater circulation will virtually cease, that hydraulic
conditions can be regarded as static, and that diffusion will be the dominant mechanism for
mass transport. Another contributory reason for this would be the increasing salinity gradients
that are normally observed with increasing depth. Solutions of such high salinity that they can
be designated as brines are often encountered. The higher density of this saline water should
inhibit vertical circulation for purely physical reasons. Petrologists have maintained that free
fluids are probably absent at even greater depths. Long-term geochemical reactions can be
expected to have consumed the free fluids. It is important to note – after the completed literature
review – that this simplistic view of how conditions ought to be at great depth has not been
borne out by the studies that have been carried out on the Kola Peninsula in Russia and in
Germany /23-23/.
The literature review summarizes the most recent results and draws a series of general
conclusions regarding what this means for disposal in Very Deep Holes. As far as thermal
properties are concerned, it is clearly difficult to estimate temperature and thermal conductivity
at great depths, particularly if the bedrock is of heterogeneous composition. Hydraulically
speaking, much of the information suggests virtually stagnant conditions in the groundwater at
great depths, and the high salinities would contribute to this. At the same time, however, there
are observations that indicate that relatively rapid transport of solutions may be possible even in
environments with brine concentration /23-24/. Claims that flow and transport occur over great
distances down at a depth of several kilometres may therefore be difficult to prove wrong. This
is of special interest for SKB, since the rock would be an important barrier in a concept with
disposal in Very Deep Holes.
The presence of bacteria at great depths is also discussed in the report /23-23/. It is assumed that
the temperature at a depth of four kilometres will not exceed 115°C and that bacterial life will
therefore be fully possible. In other words, a sterile environment cannot be guaranteed.
Programme
SKB will continue to follow development work in the field of VDH. Four years ago the German
company Deutag was commissioned by SKB to conduct a review of the technical prospects for
VDH /23-25/. This review may possibly be redone during the coming three-year period, but this
needs to be discussed, since the value of additional studies is limited.
320 RD&D-Programme 2004