Porosity, Diffusivity and Permeability of EDZ in Crystalline Rock and Effect on the Migration in a KBS-3 Type Repository
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Porosity, Diffusivity and Permeability of EDZ in Crystalline Rock and Effect on
the Migration in a KBS-3 Type Repository
J. Autio and T. Hjerpe
Consulting Engineers Saanio & Riekkola Oy, Laulukuja 4, FIN-00420 Helsinki, Finland
M. Siitari-Kauppi
University of Helsinki, Department of Chemistry, Laboratory of Radiochemistry,
P.O. Box 55, FIN-00014 University of Helsinki, Finland
Summary
Main results of EDZ studies based on using 14C-PMMA method and He-gas diffusion method
carried out using samples from Äspö Hard Rock Laboratory in Sweden, and Research Tunnel
at Olkiluoto in Finland by Posiva and SKB in cooperation are presented together with results
measured using other types of granitic rock samples. The significance of the EDZ around the
deposition hole on migration of radionuclides diffusing out of a waste canister was found
unlikely to be a significant migration route in the absence of large hydraulic gradients when
compared to diffusion through the bentonite barrier. This applies to EDZ adjacent to both
KBS-3V and KBS-3H deposition holes and also to rock surfaces excavated by mechanical
excavation for seals and plugs. The effect of EDZ adjacent to the wall and roof sections of
deposition tunnels on migration is also likely to be negligible if state-of-the-art smooth
blasting techniques are used (as in ZEDEX-tunnel at Äspö) because the fractures are poorly
connected and there is only slight increase in porosity adjacent to the half barrels of blast
holes extending to a depth from 1 to 3 cm into the rock. On basis of experiences the extent and
conductivity of the EDZ adjacent to the floor of deposition tunnels is generally larger than in
roof and wall sections. However, the floor sections can be excavated separately if necessary to
produce similar EDZ as in wall and roof sections.
1. Introduction
Posiva Oy in Finland is planning to construct a KBS-3 type repository for spent nuclear fuel at the
Olkiluoto site at the coast of the Baltic Sea. The abbreviation EDZ (Excavation Damaged Zone) is used
in this document for the zone of irreversible deformation adjacent to the surface of disposal tunnels and
deposition holes. In addition to the damaged zone there is a disturbed zone, which extends from the
damaged zone to the undisturbed “intact” rock and which is characterized as a zone where the changes
of properties are nearly reversible. The properties of excavation damaged and disturbed zone caused by
drill and blast excavation were studied profoundly in the ZEDEX experiment at Äspö HRL [1]. The
results of the study indicated that the properties of the reversible disturbed zone were insignificant for
transport and flow. The effect of excavation disturbed zone on repository performance has also been
assessed negligible in [2] and therefore most of the present interest is focused on the excavation
damaged zone. Results of EDZ studies carried out at Äspö Hard Rock Laboratory in Sweden, and at
Research Tunnel at Olkiluoto in Finland (Fig. 1) by Posiva and SKB in cooperation are presented in
the following.2. EDZ caused by drill and blast excavation
The EDZ caused by drill and blast excavation consists of a zone of intense damage adjacent to the
remaining half barrel of the blast hole, new fracturing induced by blasting and reactivation of pre-
existing fractures. The fracturing is depicted in Fig. 1 where the properties of intense microfracturing
adjacent to the half barrels of remaining blast holes are based on study of rock samples from ZEDEX-
tunnel and its extension at Äspö and mapping of slots cut in the rock in ZEDEX-project [1,3]. The zone
of increased porosity adjacent to half barrels penetrates to a depth of some centimeters, while the
penetration of the radial fractures is an order of magnitude deeper (see Fig. 2). Additional pore
volume in the EDZ adjacent to a half barrel of 4 m long 58 mm diameter blast hole is 11 cm3 and of
distinct fracturing (aperture 10 microns) is 12 cm3 per blasthole. The determination of the void volume
caused by blasting (additional pore volume) adjacent to the remaining half barrel of the blast hole is
presented in [4].
The new distinct fractures formed close to the half barrels could form a flow path if continuous.
However these fractures are not evidently continuous on the basis of the results from the ZEDEX-
tunnel [4]. The only clearly continuous transport path along the tunnel surface is the zone of intense
microfracturing (a few millimeters thick crushed zone). The spatial distribution of microfracturing
and porosity resembles the zone that was found in the samples taken from the full scale
experimental deposition holes in the Research Tunnel at Olkiluoto [5] and at Äspö HRL[6]. The
EDZ in the floor section in both the Research Tunnel at Olkiluoto and ZEDEX-tunnel at Äspö is
clearly larger and the fracturing more connected than in the walls and roof. The floor section can
however be excavated to produce EDZ similar to that in the wall and roof sections if necessary.
Figure 1. EDZ around a KBS-3V
deposition tunnel based on results
from Äspö HRL (left) and the
Research Tunnel in the VLJ
Repository at Olkiluoto (above).Figure 2. Photograph (left) of Sample Z2 from ZEDEX-tunnel and corresponding autoradiograph
(right). Arrows indicate radial microfractures in the rock. The sample width is 102.7 mm. The half
barrel of the blast hole is on top of images and the section is perpendicular to the blast hole. A few
millimeters thick zone around the blast hole is more porous (seen as darker shade of gray).
3. EDZ caused by mechanical excavation
The EDZ caused by TBM excavation and deposition hole boring is very similar on the basis of the
study of samples taken from the surface of TBM tunnel and experimental deposition holes at Äspö
HRL. The microfracturing and spatial distribution of porosity adjacent to the excavated surface was
studied in laboratory by using 14C-PMMA [5] method and scanning electron microscopy (SEM).
Five rock samples from the TBM-tunnel at Äspö were studied. The porosity in the EDZ with respect to
distance from the excavated surface was similar in all samples, as seen in Fig. 3, in spite of the
variation in rock composition. The average thickness of the EDZ defined as the zone of increased
porosity was about 20 mm. The porphyritic structure of rock samples caused a lot of variation in the
profiles and therefore caused some uncertainty in the depth of the damaged zone.
1.2 3.1
1 4.21
5.21
Porosity (%)
0.8 6.1
7.1
0.6
0.4
0.2
0
0 20 40
Distance (mm)
Figure 3. The porosity in the rock with respect to distance from the surface of TBM-tunnel into the
rock (left). Photograph of the sample (center) and corresponding autoradiograph (right) of a Sample
6.1 taken from the TBM-tunnel at Äspö HRL. Length of the section is 32 mm. The section is
perpendicular to the tunnel surface which is on the left side.
A total of 12 samples were taken from the experimental deposition holes in Prototype Repository
Tunnel in Äspö HRL [6]. The samples representing Äspö diorite were taken from different depths and
orientations with respect to the stress field. According to the results the porosity of EDZ adjacent to the
walls of the experimental deposition holes was clearly higher than the porosity of undamaged rock.
The thickness of the crushed zone was a few millimetres and the depth of the whole damage zone(EDZ) with significant higher porosity than in the undamaged zone extended about 20 mm from the
excavated surface, similar to that in Fig. 3. The extent and structure of the EDZ was very similar to that
found in the experimental deposition holes in the Research Tunnel at Olkiluoto [5].
4. Diffusivity, permeability and porosity in EDZ
Diffusivity and permeability of seven different granitic rock types were measured using He-gas method
[7] and the porosity was determined by using several other techniques including water immersion
techniques and 14C-PMMA method [7]. Results representing these seven different crystalline rock
types (gneissic tonalite, rapakivi granite, muscovite granite, porphyritic granodiorite, tonalite, mica
gneiss and Palmottu granite) are shown in Table I and Figures 4, 5 and 6. The diffusivity and
permeability of samples taken from the experimental deposition holes at Äspö was estimated using this
data and two empirical relationships; one between porosity and permeability and another between
porosity and diffusivity. These were derived from the above mentioned set of results using Darcy’s law
type relationship between porosity and permeability and Archie’s law type relationship between
porosity and diffusivity, shown in Figures 4 and 5.
1E-06
1E-07
De [m2 s-1]
1E-08
1E-09
De = 1.2·10 -8·ε1.61
1E-10
1E-11
0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40
ε [%]
Figure 4. Effective diffusion coefficient (De) versus porosity and curve fit according to Archie’s
law. The dotted curve is the 95% confidence interval for the curve fit.
1E-15
1E-16
1E-17
k [m s-1]
1E-18
1E-19
1E-20
k = 2.53·10 -20 ·e4.62ε
1E-21
1E-22
0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40
ε [%]
Figure 5. Permeability (k) versus porosity (ε) and curve fit according to Darcy’s law. The dotted
curve is the 95% confidence interval for the curve fit.
The average values of the EDZ and Undamaged rock in full-scale experimental deposition holes at
Olkiluoto Research Tunnel [5] are shown as crosses in Fig. 6 (ε = 0.34%, De = 3.97·10-9 m2s-1,
k = 2.96·10-19 m2, and ε = 0.14%, De = 2.63·10-10 m2s-1, k = 5.16·10-21 m2, respectively). The estimatedvalues of undamaged rock, EDZ, and the crushed zone, which is part of the EDZ, in samples taken
from the experimental deposition holes at Äspö (ε = 0.26%, De = 1.4*10-9 m2s-1, k = 8.4·10-20 m2 ;
ε = 0.64%, De = 6.0·10-9 m2s-1, k = 4.9·10-19 m2 and ε = 0.80%, De = 8.5·10-9 m2s-1, k = 1.0·10-18 m2
respectively) are shown as squares with error bars representing the 95% confidence intervals.
Table I. Measured values of porosity, effective diffusion coefficient (De) and permeability (k) for
granitic rock samples. Rock samples from excavation damage zone are classified as ud=undamaged
and d=damaged. Other rock samples are classified as unaltered = u, fairly altered = fa and strongly
altered = sa. Ref. column refers to the reference list.
Sample type Porosity
2 2
Rock type (designation) Ref. D e [m /s] k [m ] [%]
Gneissic Tonalite ud (B3) 9,2 3.50E-10 5.57E-21 0.20
from Olkiluoto Research ud(B4) 9,2 1.10E-10 3.60E-21 0.20
Tunnel d(B4) 9,2 1.45E-09 7.40E-20 0.70
" ud(D2) 9,2 2.30E-10 4.65E-21 0.20
" d(D2) 9,2 5.20E-09 4.00E-19 0.90
" ud(D4) 9,2 4.00E-10 8.46E-21 0.20
" d(D4) 9,2 7.20E-09 7.59E-19 1.00
" ud(D12) 10 6.12E-09 1.04E-18 0.20
" ud(D13) 10 5.55E-09 5.77E-19 0.20
Rapakivi Granite, Pegmatitic u 11 1.01E-09 1.15E-19 0.20
" u 11 1.49E-08 2.04E-18 0.30
" sa 11 1.26E-08 1.19E-18 1.30
Muscovite Granite u 11 - 2.51E-17 1.00
" fa 11 9.70E-09 1.33E-18 0.60
Porphyritic Granodiorite u 11 7.80E-09 9.03E-19 0.40
Tonalite u (Olkiluoto) 11 3.60E-10 2.96E-20 0.20
" u (Sy1) 13 1.16E-09 9.78E-20 0.20
" fa (sy7) 13 3.10E-09 3.57E-19 0.50
Mica Gneiss u (Sy1) 13 3.40E-09 6.69E-18 0.20
Palmottu Granite u (Wgranite) 12 1.50E-09 1.50E-19 0.20
" u (Wgranite) 12 1.00E-09 6.80E-20 0.10
" fa (Wgranite) 12 8.90E-11 1.60E-21 0.10
" fa (Egranite) 12 2.70E-08 2.30E-17 0.70
" u (Gneiss) 12 8.90E-11 1.60E-21 0.10
10-16
10-17
e
ag
m
10-18
f da
k [m2]
Crushed zone at Äspö
e eo
e gr EDZ at Äspö
10-19 gd EDZ at Olkiluoto
Porosity, ε
sin
r ea Undamaged rock at Äspö 1.3%
I nc
10-20
Undamaged rock at Olkiluoto
10-21
0%
10-10 10-9 10-8 10-7
De [m2s -1]
Figure 6. Results of effective diffusion coefficient (De), permeability (k) and porosity (ε, in gray
scale) measurements in Table I representing eight different crystalline rock types (marked as dots).5. The effect of EDZ in deposition holes on the migration radionuclides Analyses of EDZ caused by the boring of the experimental full-scale deposition holes in the Research Tunnel at Olkiluoto included determinations of the porosity, diffusivity and permeability of the undamaged rock and EDZ using two novel methods: the 14C-polymethylmethacrylate (14C-PMMA) and the He-gas method. The results have been presented by [5]. Since the hydraulic conductivity of compacted bentonite is low, the predominant mode of migration of nuclides through such material will be diffusion. The effect of the excavation-damaged rock zone on the transport of radionuclides was analysed based on this assumption in [8] and the measured permeability of EDZ. The result of the analysis showed that flow of 0.85 l/a through the excavation-damaged zone surrounding a deposition hole is required to obtain a significant transfer of radionuclides and this would require a hydraulic gradient of 19, however the actual hydraulic gradient is more likely to be of the order of 0.019 (1.9 %) resulting on a flow of 8.5·10-4 l/a. More detailed description of the evaluation can be found in [8]. According to this evaluation, the excavation-damaged zone is not likely to be of importance as a migration route for radionuclides. The results apply also to KBS-3H concept and plug structures. The question could be reversed. What should be the hydraulic conductivity (critical conductivity) of EDZ for it to become a significant transport route based on the above mentioned approach. The calculated critical conductivity was 7·10-9 m/s, over two orders of magnitude higher than the realistic value of 10- 11 m/s, for both deposition hole and a plug structure with similar type of high density sealing. 6. Discussion and conclusions The effect of EDZ caused by mechanical excavation on migration of radionuclides seems to be insignificant in the above described conditions on the basis of the evaluation. This applies to EDZ adjacent KBS-3V and KBS-3H deposition holes and rock surfaces excavated by mechanical excavation for seals and plugs. The effect of EDZ adjacent to the wall and roof sections of deposition tunnels on migration is also likely to be negligible if state-of-the-art smooth blasting techniques are used because the fractures are poorly connected. The extent and conductivity of the EDZ adjacent to the floor of deposition tunnels is evidently larger than in roof and wall sections and could in principle cause significant transport as shown by [14]. However, in theory it should be technically feasible to excavate the floor sections to produce similar EDZ as in wall and roof sections. Backfilling and sealing are important components in all geological disposal concepts and have effect on the EDZ. Their functions are to prevent the excavations and EDZ from becoming major transport routes for radionuclides released from the repository. Therefore an evaluation of the effects of EDZ on migration of nuclides should consider the backfilling and sealing structures, too. 7. Acknowledgements This work was commissioned and supported by Posiva Oy (Finland) and the Swedish Nuclear Fuel and Waste Management Co. (Sweden).
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