Euradwaste '08 - EU Bookshop - Europa
Euradwaste '08 - EU Bookshop - Europa Euradwaste '08 - EU Bookshop - Europa
1.650 1.600 1.550 1.500 1.450 1.400 1.350 1.300 1.250 bulk wet density bulk dry density A B C Cw D Dw E Ew A 100 % coarse rounded granular material, embedded in two layers B 92 % coarse, 8 % fine, two layers C 85 % coarse, 15 % fine, two layers Cw 85 % coarse, 15 % fine, two layers D 70 % coarse, 30 % fine, two layers Dw 70 % coarse, 30 % fine, repeat run, two layers E 64 % coarse, 28 % fine, 8 % briquettes, two layers Ew 64 % coarse, 28 % fine, 8 % briquettes, repeat run, only one layer Figure 3.3.4: Results of the emplacement tests, reached total bulk (wet) density and dry density with different granulate fractions 3.4 Seal saturation rate and gas permeability testing (GRS) Seal concept Currently, highly compacted bentonite buffers are studied in the frame of several concepts for the final disposal of high-level waste (HLW). In 2000, GRS started investigations on the suitability of moderately compacted clay/sand mixtures as a sealing material in clay repositories [6]. Such mixtures may represent a reasonable alternative to highly compacted bentonite buffers, especially for the safe closing of repository rooms containing gas generating waste, since they will act as a gas vent thereby avoiding the development of undesired high gas pressures in the disposal cells. In a clay repository, this granular sealing material may be used as buffer and/or as sealing backfill in disposal boreholes or disposal drifts containing either Nuclear Spent Fuel (NSF) or vitrified highlevel waste (HLW). The buffer/backfill material will be installed in drifts or boreholes as a slightly compacted embankment. Functional characteristics of the seal material and test programme In contrast to highly compacted buffers, moderately compacted clay/sand mixtures exhibit: (1) a high permeability to gas in the unsaturated state, and (2) an adequate low permeability to water or self-sealing potential against water, respectively due to swelling of the clay minerals after water uptake from the rock. Because of their low cohesion they also show a low gas entry/break- 250
through pressure in the saturated state enabling gases to migrate out of the disposal cell at reasonably low pressure without any damage of the host rock. The objective of the test programme is to test and demonstrate that the sealing properties of clay/sand mixtures determined preliminarily in the laboratory can be technically realized and maintained under repository relevant in-situ conditions. The test programme consists of three steps: (1) laboratory investigations/mock-up tests for selection of suitable seal material mixtures and for testing material installation techniques, (2) scoping calculations for in-situ test preparation with an assessment of seal saturation under in-situ conditions, and (3) in-situ tests at the Mont Terri Rock Laboratory (MTRL). Laboratory Investigations for the Selection of Suitable Material Mixtures In the GRS-laboratory in Braunschweig, different clay/sand mixtures with mixing ratios between 35clay/65sand and 70clay/30sand have been investigated with regard to their sealing performance [7]. These investigations have shown that the functional requirements given above are best met by 35clay/65sand and/or a 50clay / 50sand mixtures (see Table 3.4-1) which therefore have been selected for further testing under in-situ conditions at the MTRL. Large-Scale Mock-up at GRS’s Laboratory in Braunschweig/Germany Before going in situ, both the installation techniques and the required saturation time for the material mixtures being considered were to be investigated and optimized in large-scale mock-up tests in GRS’s laboratory in Braunschweig. Table 3.4-1. Measured material parameters at installation conditions (averages in parentheses) Sample Gas permeability under dry conditions [m 2 ] 35/65 1.2E-13 50/50 7.5E-14 Initial water permeability at full saturation [m 2 ] 3.3E-17 - 9E-18 (5.2E-18) 1.1E-18 - 4.3E-18 (2.2E-18) Gas breakthrough pressure [MPa] 0.4 - 1.1 (0.75) 0.4 - 2.8 (1.83) 251 Gas permeability after gas break-through [m 2 ] 1.1E-17 - 1.6E-17 (1.4E-17) 5.5E-18 - 6.2E-18 (5,9E-18) Swelling pressure [MPa] 0.2 - 0.4 (0.28) 0.3 - 0.5 (0.35) The mock-up tests (Figure 3.4.1) are performed in vertically arranged steel tubes and they are designed as a full-scale replica of the envisaged in the in-situ experiments (Figure 3.4.2). The lower fluid injection volume is filled with a porous material (stone chips or sand). At top of the porous medium a filter frit is placed for ensuring a homogeneous distribution of the injected water over the entire borehole cross section. Above the filter frit, the clay/sand-seal is installed in several layers to a height of 1 m. The predetermined installation density of about 1.9 g/cm 3 for the 35/65 clay/sand mixture has been realized by using an electric vibrator for material compaction. A further filter frit is installed above the seal for water and gas collection. The whole test set-up is sealed against the ambient atmosphere by a gastight packer on top of the upper filter frit. In contrast to the in-situ experiments where instruments are not installed in the seal itself to avoid any negative impact on the sealing properties, the mock-up is equipped with sensors in the test tube wall to monitor the pore and total pressure evolution at three levels along the seal. Additionally, two total pressure sensors are installed at the bottom of the upper frit. Furthermore, the mock-up is equipped at the inlet and
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1.650<br />
1.600<br />
1.550<br />
1.500<br />
1.450<br />
1.400<br />
1.350<br />
1.300<br />
1.250<br />
bulk wet density bulk dry density<br />
A B C Cw D Dw E Ew<br />
A 100 % coarse rounded granular material, embedded in two layers<br />
B 92 % coarse, 8 % fine, two layers<br />
C 85 % coarse, 15 % fine, two layers<br />
Cw 85 % coarse, 15 % fine, two layers<br />
D 70 % coarse, 30 % fine, two layers<br />
Dw 70 % coarse, 30 % fine, repeat run, two layers<br />
E 64 % coarse, 28 % fine, 8 % briquettes, two layers<br />
Ew 64 % coarse, 28 % fine, 8 % briquettes, repeat run, only one layer<br />
Figure 3.3.4: Results of the emplacement tests, reached total bulk (wet) density and dry density with<br />
different granulate fractions<br />
3.4 Seal saturation rate and gas permeability testing (GRS)<br />
Seal concept<br />
Currently, highly compacted bentonite buffers are studied in the frame of several concepts for the<br />
final disposal of high-level waste (HLW). In 2000, GRS started investigations on the suitability of<br />
moderately compacted clay/sand mixtures as a sealing material in clay repositories [6]. Such mixtures<br />
may represent a reasonable alternative to highly compacted bentonite buffers, especially for<br />
the safe closing of repository rooms containing gas generating waste, since they will act as a gas<br />
vent thereby avoiding the development of undesired high gas pressures in the disposal cells. In a<br />
clay repository, this granular sealing material may be used as buffer and/or as sealing backfill in<br />
disposal boreholes or disposal drifts containing either Nuclear Spent Fuel (NSF) or vitrified highlevel<br />
waste (HLW). The buffer/backfill material will be installed in drifts or boreholes as a slightly<br />
compacted embankment.<br />
Functional characteristics of the seal material and test programme<br />
In contrast to highly compacted buffers, moderately compacted clay/sand mixtures exhibit: (1) a<br />
high permeability to gas in the unsaturated state, and (2) an adequate low permeability to water<br />
or self-sealing potential against water, respectively due to swelling of the clay minerals after water<br />
uptake from the rock. Because of their low cohesion they also show a low gas entry/break-<br />
250
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