RD&D-Programme 2004 - SKB

SKI observes in its evaluation of RD&D 2001 that SKB should investigate how quantities that
influence matrix diffusion, for example the flow-wetted surface, can be measured in the field,
see section 19.2.12. Further, SKI wants SKB to conduct process-oriented studies of matrix
diffusion in fracture-filling materials and intact rock.
Newfound knowledge since RD&D 2001
By “diffusion” is meant here both diffusion in the flowing water and diffusion from flowing
to stagnant water. The stagnant water tends to be present in the matrix (matrix diffusion) or in
low-conductive parts of the fracture plane.
Diffusion in flowing water is uninteresting in the longitudinal direction, i.e. the flow direction
of the water. In the transverse direction, this diffusion process is of interest for determining
relevant length scales over which mixing takes place on different time scales. For example,
mixing takes place over shorter lengths on experimental time scales than on the time scales of
interest for the safety assessment. These length scales are of interest when radionuclide transport
is to be conceptualized for modelling. The question of whether mixing takes place in fracture
intersections is also dependent on diffusion in the aqueous phase. These issues are being
examined in the ongoing EU project Retrock /19-62/.
When it comes to diffusion into stagnant water, matrix diffusion is the most relevant process.
This also includes diffusion into the fracture-filling material (gouge) and altered rim zones
which are typically present in natural fractures. Field tests carried out at Äspö, True-1 /19-63/
and True Block Scale /19-64 to 19-67/, show that it is presumably mainly diffusion in gouge and
an altered rim zone that are observed on time scales that are typical for field tests. The studies
also confirm that this diffusion is heterogeneous in nature, mainly due to a spatial variation of
the porosity in the materials in which diffusion takes place. A porosity profile perpendicular to
the fracture plane has been observed: The altered rim zone nearest the fracture has a relatively
high porosity, while the intact rock further in has lower porosity /19-68/. Modelling tools have
been developed for describing this type of heterogeneity /19-66, 19-69/.
Since transport experiments in the field with tracers typically measure matrix diffusion into
materials other than the intact matrix, it is also of interest to be able to measure diffusion in the
field in the intact matrix. An experiment called the Long Term Diffusion Experiment (LTDE) is
planned for this purpose /19-70/. Geological characterization of the planned site for the experiment
has been done, and hydrogeological tests will be conducted. The experimental equipment
and the concept for the experiment will be reviewed during 2004.
Electrical methods (electrical conductivity) can be used in situ as an alternative to estimate the
formation factor (which controls the rock’s influence on diffusion). An ongoing doctoral project
has summarized the state of knowledge, identified open questions and presented preliminary
results /19-71, 19-72/. The parameters needed in order to estimate the formation factor are rock
resistivity, pore water resistivity and the estimated contribution made by surface conduction.
The effect of surface conduction decreases as the salinity of the water increases.
Programme
According to current plans, the real measurements in the Long Term Diffusion Experiment will
start during the current period, after characterization has been completed and the experimental
concept has been evaluated.
The doctoral project, which is concerned with electrical methods for estimating the formation
factor in situ, will be finished during the current period. The method is being tested on drill
cores from the site investigation programme. Furthermore, methods are being studied for
estimating the conductivity of the pore water (normally it is assumed that the pore water and
the water in the fractures have the same conductivity). Finally, methods are being investigated
for correcting measurements at low salinity, when the effect of surface conduction can be great.
RD&D-Programme 2004 259