Diffusion in the Matrix of Granitic Rock. Field Test in the Stripa Mine
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DIFFUSION IN THE MATRIX OF GRANITIC ROCK. FIELD TEST IN THE STRIPA MINE Lars Birgersson, Ivars Neretnieks Department of Chemical Engineering Royal Institute of Technology S-100 44 STOCKHOLM Sweden SUMMARY A migration experiment with the objective to investigate the existence of a connected pore system in "undisturbed" rock has been performed in the Stripa mine at the 360 m-level. Since all three tracers (Cr-EDTA, I- and Uranine) that were used in the experiment migrated at least 11 cm into the rock under a three month period, this experiment indicates the existence of a connected pore system in "undisturbed" rock. BACKGROUND The radionuclides migrating with flowing water may be considerably retarded if they can diffuse into the rock and sorb on the surfaces of the microfissures in the rock matrix (1). At present there are a series of laboratory experiments being performed with the purpose to determine diffusion coefficients for in
granite,
but
these
various
experiments are not carried out in
tracers
"undisturbed"
rock. It cannot be ruled out that the reduction of the rock stresses which occur when samples are thus necessary to make environment.
It
taken out have induced the microfissures. It is experiments in rock in the natural stress
is also necessary to know if diffusion can occur
through
the fissure filling material, since the radionuclides must pass through it before they can penetrate into the rock matrix. This experiment was performed in the Stripa mine at the 360 m-level. That will give nearly the same conditions as for the planned nuclear waste storage. INFLUENCE OF THE STRESSFIELD Near drillholes and drifts ,the rockstresses will be changed compared to "undisturbed" rock. A general rule in these stresses That is,
are
outside these 2 hole
exists (2).
cases
is
that
the
rock
changed about 2 hole diameters out from and below the hole. diameters
essentially
"undisturbed"
rock
520 Since the diameter of the drift where the experiment has taken
was approximately
m, a 11 m deep 146 mm hole
was
drilled. At this
(11
0146mm,Ll11m 020mm,L=2m
place
5.5
m)
changes in due
the
from
to
the
distance
rock
stresses
drift
the
the
drift
can
be
neglected , i.e. essentially "undisturbed" rock is reached. However, the existence 146
mm
change
further
rockstresses m (2
hole
mm
in
hole
stresses Figure 1. Drilling dimensions and packer positions
outward,
outward
the bottom of the a
20 2
(approximately a
a the
approximately 0.3
in
change
hole
will
the
approximately but
hole
mm
m long) was
drilled. This 20 mm cause
the
cause
diameters)
and below. Thus, in 146
of
will
hole
outside
rock 4
cm this
disturbed zone and 0.3 m below the
larger
hole
essentially
"undisturbed" rock is reached. If
tracers
can
migrate from the little hole past the disturbed zone and
into "undisturbed" rock, this experiment will indicate the existence of a connected pore system in "undisturbed" rock.
521 EXPERIMENTAL ALTERNATIVES : DIFFUSION - FLOW AND DIFFUS
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