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Gas threshold pressure and gas permeability of silo concrete specimens for a low- and intermediate-level waste disposal facility in Korea
Annals of Nuclear Energy 55 (2013) 1–8
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Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Gas threshold pressure and gas permeability of silo concrete specimens for a low- and intermediate-level waste disposal facility in Korea Juyub Kim a,⇑, Juyoul Kim a, Haeryong Jung b, Jae-Chul Ha b, Eun-Hee Kim c a
FNC Technology, 46 Tabsil-ro, Giheung-gu, Yongin 446-902, Republic of Korea Korea Radioactive Waste Management Corporation, 1045 Daedeok-Daero, Yuseong-gu, Daejeon 305-353, Republic of Korea c Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 28 September 2012 Received in revised form 3 December 2012 Accepted 5 December 2012 Available online 11 January 2013 Keywords: Gas threshold pressure Gas permeability Gas migration Concrete silo LILW disposal
a b s t r a c t In order to simulate gas migration at an underground disposal facility, the characteristics of a medium such as the gas threshold pressure and gas permeability need to be measured in advance. In this study, the gas threshold pressure and gas permeability of silo concrete specimens for a Korean LILW (Low- and Intermediate-Level Waste) disposal facility were measured. The concrete specimens had the same composition as the concrete used in the construction of the silo. The gas threshold pressure was measured by injecting a constant gas flow into cross sections of the specimens. To measure the gas permeability, selected pressures were applied to the specimens and the apparent permeability was calculated using the Hagen-Poiseuille equation. The intrinsic permeability was calculated with the Klinkenberg empirical equation. The gas threshold pressure and gas permeability ranged from 30 to 40 bar and from 1017 to 1018 m2, respectively. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction As of 2012, 23 nuclear power reactors are in operation in Korea. To service those reactors, a Low- and Intermediate-Level Radioactive Waste (LILW) disposal facility is under construction in Korea. The LILW disposal facility in Korea uses underground disposal with a concrete silo and will be capable of disposing 100,000 LILW drums (Fig. 1) when the construction is completed. The concrete silo is being built 80 m below sea level. The concrete silo will be saturated with ground water after the disposal facility is finally closed. Then, over time, gases could be generated by various mechanisms including metal corrosion, microbial degradation of organic matter, and radiolysis (Park et al., 2010). These generated gases could travel to the upper part of the concrete silo and start to accumulate. Since the accumulation of gases could increase the internal pressure of the concrete silo, the integrity of the engineered barrier (i.e. the concrete silo) could be damaged. In order to prevent an increase in internal pressure, it is necessary to design and develop an effective gas ventilation system for concrete silos used in such disposal facilities. Fig. 2 illustrates the concept of a gas transport pathway in a concrete silo. Several countries also have developed similar LILW underground disposal facilities. In Sweden, the SFR 1 LILW disposal facility is in operation at Forsmark, and it is scheduled to be closed in ⇑ Corresponding author. Tel.: +82 31 8005 6010; fax: +82 31 8005 6014. E-mail address: [email protected] (J. Kim). 0306-4549/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2012.12.012
2040. The SFR 1 consists of a silo and several horizontal rock vaults (SKB, 2008). The silo is located 60 m below sea level and is made of reinforced concrete with a height of 53 m, a diameter of 28 m and a thickness of 0.8 m. The space between the waste packages are filled with porous concrete grout and the top of the concrete cylinder is sealed with a 1 m thick concrete lid after emplacement of the waste packages. The concrete lid has evacuation pipes that are filled with sand: therefore, gases generated in the concrete cylinder can escape through the pipes. A 100 mm thick layer of sand and a 1.5 m thick sand/bentonite mixture are located above the lid. The remaining part of the silo above the lid is backfilled with sand or gravel and connected to concrete plugs (SKB, 2001). After the facility finally closes, the generated gases will be able to travel to the top of the silo through sand-filled pipes preventing the concrete silo from becoming overpressurized. When the pressure of the gases cumulated above the lid exceed certain pressure, they can be discharged into the surrounding host rock, or into the concrete plugs. In Finland, the VLJ disposal facility has been operating at the Olkiluoto site since 1992. The VLJ disposal facility is located 60–100 m below the surface and consists of two silos, an access tunnel and shaft. The intermediate- and low-level waste drums are packaged into concrete containers separately. Then, they are disposed of in a reinforced concrete silo and shotcrete silo, respectively. A 25 cm thick concrete lid is installed on top of the silo, and a dome-shaped cover is placed above the lid. The cover has a gas discharge path, which is called the Gas Lock on the top. After the
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J. Kim et al. / Annals of Nuclear Energy 55 (2013) 1–8
Fig. 1. Concept of the concrete silo in Korea.
facility finally closes, the generated gases will be able to gather at the top of the dome and then discharge through the Gas Lock. The void between the lid and the cover is backfilled with crushed rock after the facility closes (Vieno et al., 1998). Switzerland decided to use cavern disposal for its LILW disposal facility. It will be located underground in Opalinus Clay 300–400 m below the surface. The disposal facility consists of seven emplacement caverns 11.0 m in width, 13.2 m in height and about 200 m in length. The wall of the cavern is sprayed with a concrete lining and cementitious mortar is used to fill the space between the disposal containers. Each emplacement cavern is linked to an operation tunnel through a branch tunnel. NAGRA (National Cooperative for the Disposal of Radioactive Waste in Switzerland) introduced the concept of an Engineered Gas Transport System (EGTS) consisting of a seal and plug in the tunnels. The seal is made of a compacted mixture of sand/bentonite or a compacted bentonite and concrete block. Generated gases can move into the operation tunnel through the EGTS and then discharge into the surrounding host
rock (NAGRA et al., 2008). The disposal facility in Switzerland does not include a silo, but the concept of an EGTS can be used to plan the migration of gas through an operation tunnel. Research on gas migration in underground disposal facilities has been carried out by many organizations. NAGRA has performed various experimental projects at the Grimsel Test Site and the Mont Terri Rock Laboratory. The GMT (Gas Migration Test in an Engineered Barrier System (EBS) and Geosphere) is one of the projects that evaluated the function of a gas migration system in a repository and the gas migration model in an EBS and a geosphere, and demonstrated an EBS in a disposal facility (Yamamoto, 2010). A large-scale cavern was excavated in the shear zone and an engineered barrier was replicated using a concrete silo, a sand/bentonite mixture, and water saturated backfill on the top which also included a plug. After the gas injection test, the EBS was excavated again, and the pathways of the gas and water, the performance of the concrete silo, and the characteristics of the sand/bentonite mixture were all evaluated. In addition, the researchers did
Fig. 2. Concept of the gas transport pathway in the silo.
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computers simulation with various codes such as CODE_BRIGHT, GETFLOW, MHERLIN, ROCKFLOW and TOUGH2 in parallel with the GMT. NAGRA has performed the GAST (Gas-permeable Seal Test) project at GTS since 2010 (NAGRA, 2010). The objectives of the GAST are to maintain the radionuclide retention capacity of the EBS and to improve the gas transport capacity of the backfill. The EBS is also called the EGTS, and it is composed of mortar and a sand/bentonite mixture. A numerical simulation was also performed in order to verify the performance of the EGTS, and the results showed that the gases are discharged effectively through the EGTS. A large column experiment was conducted by NAGRA in 1998 to research gas transport in backfills (Mayer et al., 1998). The length and diameter of the column were about 1 m and 0.3 m, respectively. The column element was filled with mortar as a backfill material, and a sample cell with plug materials was connected to the top of the column. As the gas was injected, the change in pressure, saturation, and discharging gas flow were observed. The FORGE (Fate of Repository Gases) project is a panEuropean project with many international organizations, and the key issues of the project are the generation and migration of repository gases. As part of the FORGE project, CIEMAT conducted a test on the gas permeability of concrete (Villar et al., 2010). Two different experimental methods, a steady state method and a non-steady state method, were used. Results of both methods showed that the permeability of the concrete specimens has a range of 1017–1016 m2. In Sweden, SKB carried out LASGIT (Large-Scale Gas Injection Test) at Äspö HRL (Hard Rock Laboratory) (Cuss et al., 2010). A full-scale deposition hole was drilled into the gallery floor, and a gas injection test was done in order to obtain new information on the mechanism of gas flow in buffered bentonite. In addition, France ANDRA (Agence nationale pour la gestion des dechets radioactifs) conducted a Gas Threshold Pressure Test (GTPT) at Bure URL in 2004 to understand gas migration in the Callovian–Oxfordian layer (Senger et al., 2006). The test was done in situ in a deposition hole, and the results showed that the gas threshold pressure of the Callovian–Oxfordian layer was about 9.5 MPa. In 1997, an experimental study on the gas permeability of concrete depending on its degree of saturation was done in France. The apparent permeability was calculated with the Hagen-Poiseuille equation, and then, the intrinsic permeability was calculated using the Klinkenberg equation (Abbas et al., 1999). With the goal of designing gas ventilation systems, simulations for gas generation and gas migration are essential in order to calculate the specifications for a gas ventilation system. A study on gas generation was done by the Korea Radioactive Waste Management Corporation (KRMC) and FNC Technology in 2009 that included modeling of gas generation and a large-scale in situ experiment. In addition, a study on the migration of gas was started by the KRMC and FNC Technology in 2011. The goal of the gas migration study was to design a gas ventilation system for the Korean LILW disposal facility by performing mock-up experiments and computer modeling. Prior to performing the computer modeling, properties of the silo’s concrete such as the gas threshold pressure and gas permeability should be obtained in advance. In order to measure the gas threshold pressure and gas permeability of the silo’s concrete, experimental measurements were carried out by FNC Technology and KRMC in 2012.
2. Theoretical background 2.1. Mechanism of gas migration in a porous medium The migration of gases through a low-permeable porous medium is governed by the hydraulic and mechanical properties of the medium, by the gas pressure in the upstream region, and by
the hydro-mechanical state of the medium. There are four basic mechanisms for the migration of a gas in a porous medium as follows (NAGRA et al., 2008): – – – –
Advective–diffusive transport of a gas dissolved in porewater. Visco-capillary two-phase flow. Dilatancy-controlled gas flow. Gas transport along macroscopic tensile fractures.
The advective–diffusive transport of a gas dissolved in porewater is governed by three laws: Darcy’s law, Fick’s law and Henry’s law. Darcy’s law states the velocity of a fluid passing through a porous medium is proportional to the head loss and inversely proportional to the path length. Eq. (1) gives the relationship as follows (NDA, 2010):
Q ¼ KA
dh k Pb Pa ¼ A ; dl l L
ð1Þ
where Q is the total discharge; K is the hydraulic conductivity; A is the cross-sectional area to flow; h is the head; l and L are the length of porous media; k is the permeability; l is the viscosity; and Pb and Pa are the pressure at outlet and inlet, respectively. Fick’s law states the flow velocity of diffusive material per unit area is proportional to the concentration gradient under a steady state condition shown by Eq. (2) (Domenico and Schwartz, 1990):
J ¼ Dd gradðCÞ;
ð2Þ
where J is the diffusional flux; Dd is the effective diffusion coefficient; C is the concentration of species. Henry’s law states the partial pressure of an ideal gas in the gas phase is proportional to the concentration of the gas in the aqueous phase shown in Eq. (3) (Domenico and Schwartz, 1990):
KH ¼
ðgasÞaq ; Pgas
ð3Þ
where KH is the Henry’s law constant; Pgas is the partial pressure of the gas in the gas phase; and (gas)aq is the molar concentration of the gas in solution. Porewater in the pores of a porous medium is displaced by gas due to the viscous and capillary forces. Visco-capillary two-phase flow is explained by the multiphase version of Darcy’s law shown in Eq. (4) (Bear, 1972):
v b ¼ k
krb qb ðrqb qb gÞ; lb
ð4Þ
where vb is the Darcy velocity in phase b; k is the intrinsic permeability; krb is the relative permeability to phase b; lb is the viscosity; pb is the fluid pressure in the phase and capillary pressure; and g is the gravitational acceleration. The gas entry pressure pae is the controlling factor for the twophase flow characteristics of a porous medium. The gas entry pressure is also known as the capillary threshold pressure, which is defined as the difference between the gas pressure and water pressure necessary to supersede the porewater from a fully saturated porous medium. If the pressure becomes larger than gas entry pressure, the gas migration is governed by the intrinsic permeability, the relative permeability and the water retention curve. Dilatancy-controlled gas flow is the primary mechanism for argillaceous media, which have a low tensile strength. The argillaceous media cannot bear gas pressures greater than the minimum principal stress applied to the rock mass for long-term periods. Thus, microfractures are expected to form in the rock mass before the minimum principal stress is achieved. If the shear stress is larger than the maximum stress or the stress state is ideal for rock
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deformation, gas flow through microfractures could occur. Gas flow through microfractures causes increases in the pore space and intrinsic permeability, and a change in the relationship between the capillary pressure and saturation. Gas transport along macroscopic tensile fractures is formed when the gas pressure is greater than the sum of the tensile strength and the minimum principal stress of the rock mass. Gas flow along macroscopic tensile fractures is considered a singlephase flow. The gas flow will be stopped when the gas pressure in the fracture becomes less than the minimum principal stress. If the rock has a low tensile strength, a macroscopic fracture forms only when the gas pressure increases rapidly. Usually, since disposal facilities have a sufficiently large capacity for gas storage, a LILW disposal facility would not experience this phenomenon. In addition, gas generation will become smaller over time such that gases can be discharged sufficiently through the host rock and with the engineered gas transport system. The gas threshold pressure is given as the sum of the gas entry pressure and the liquid pressure. The gas entry pressure is a key parameter in capillary pressure functions such as van Genuchten and Brooks-Corey. Results of previous studies have shown that the gas entry pressure and intrinsic gas permeability have a linear relationship on a logarithmic scale (Senger et al., 2006). 2.2. Gas permeability The flow rate through a porous medium is governed by permeability. Darcy’s law defines the coefficient of permeability. The gas permeability of a porous medium at a certain pressure is calculated with the Hagen-Poiseuille equation for a compressible fluid shown by Eq. (5) (Abbas et al., 1999):
kapp ¼
2QPatm Ll 2
AðP
P2atm Þ
ð5Þ
;
where kapp is the apparent gas permeability (m2); Q is the flow rate of discharging gas (m3/sec); A is the cross-sectional area (m2); L is the height of the specimen (m); l is the viscosity of the gas (N s/m2); P is the absolute pressure of the upstream region (N/m2); and Patm is the atmospheric pressure (N/m2). The gas permeability of a porous medium (e.g. concrete) varies with the gas pressure. Thus, the gas permeability is different because of measurements with an incompressible fluid in a specific medium. The Klinkenberg empirical equation (Eq. (6)) represents this effect as follows (Harris et al., 2000):
b ; k ¼ k1 1 þ pm
ð6Þ
where k is the gas permeability at pressure p; k1 is the gas permeability at infinite pressure; b is the Klinkenberg coefficient; and pm is the mean pressure of the applied pressure and the atmospheric pressure. Therefore, as the average pressure applied on a medium becomes larger, the gas permeability becomes smaller. The gas permeability at infinite pressure is equivalent to that of the incompressible fluid. The terms k1 and k are also called the intrinsic permeability (kint) and apparent permeability (kapp) respectively. The intrinsic permeability, kint, is irrelevant to the pressure
applied on a medium, but it is the unique property of a porous network. 3. Materials 3.1. Concrete specimens The specimens were cylindrical pieces of concrete 150 mm in diameter and 50 mm in height. The composition of the specimens is presented in Table 1. The concrete specimens were cured for 28 days in water. A cured specimen is shown in Fig. 3. 3.2. Experimental apparatus The experimental apparatus consisted of a pressure vessel, gas tank, pressure regulators, flow regulator, flow meters, differential pressure gauge, and valves. Nitrogen gas was used as the gas. The general layout of the apparatus is shown in Fig. 4. The pressure vessel consisted of an inlet/outlet nozzle, Viton sleeve, confining area and cover. The nozzle had a groove with a radial shape, so that the gas could be distributed uniformly onto the surface of the specimen. The confining area was filled with the gas, applying force onto the Viton sleeve. As a result, the Viton sleeve adheres to the side area of the cylindrical specimen so that the injected gas cannot move along the gap between the side area of the specimen and the Viton sleeve. Fig. 5 shows the inside of the pressure vessel. 4. Methods 4.1. Gas threshold pressure First, a fully saturated specimen was placed into the pressure vessel. Then, nitrogen was injected with a uniform flow rate through the flow regulator and upstream nozzle. Based on the experiment by NAGRA (Mayer et al., 1998), injection rates of 1.5 and 3.0 mL/min were used in this study. The pressure of the downstream region was kept at atmospheric pressure. As the gas was injected, the pressure of the upstream region became greater and the gas began to infiltrate into the specimen. As time passed, the gas moved along the pore system of the concrete specimen, and started to be discharged into the downstream region at a specific pressure. After the discharge occurred, the rate of increase of the pressure started to decrease and reached a point which is called the gas threshold pressure. 4.2. Gas permeability 4.2.1. Preprocessing of the specimen A total of six specimens were used for the measurements. All the specimens were weighed at 100% saturation. Then, the specimens were dried in a 100 °C oven until the mass of the specimens did not change any further. When the mass of the specimens remained constant over 24 h, it was assumed that the specimens were completely dried (0% saturation). 4.2.2. Measurement of the gas permeability A dried specimen was inserted into the vessel, and nitrogen was injected until the pressure of the upstream region reached a
Table 1 Composition of the concrete specimen. Source of aggregate
Mix type
W/(C + F) (%)
S/a (%)
Daejong and jinae area
E-1-40
40.0
38.4
Proportion (kg/m3) Water
Cement
Fly-ash
3= 00
Coarser
Finer
WRA
AEA
206
412
103
961
357
241
2.3193
0.1287
4
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J. Kim et al. / Annals of Nuclear Energy 55 (2013) 1–8
Fig. 3. The concrete specimen.
Fig. 5. The pressure vessel.
specific pressure. The pressure of the upstream region remained constant for each case of the measurements. After the gas was discharged from the downstream region, sufficient time was allowed to achieve a steady state flow. By measuring the steady flow rate of the discharged gas and using the Hagen-Poiseuille equation (Eq. (5)), the permeability at a specific pressure and saturation (the apparent permeability) could be calculated (Monlouis-Bonnaire et al., 2004). Nitrogen at 20 °C with a viscosity of 1.7568 105 N s/m2 at a mole fraction of 1.0 was used for the viscosity, l (Cole and Wakeham, 1985). The apparent gas permeability was measured at three different pressures (2, 3 and 4 bar), and the relationship function between 1/Pm (Pm: mean pressure, Patm2 þP) and kapp was calculated from the linear approximation. Using the Klinkenberg
equation (Eq. (6)), the intrinsic permeability kint could be calculated from the y-intercept of the linear function (Abbas et al., 1999). 5. Results and discussion 5.1. Gas threshold pressure Experimental measurements for the gas threshold pressure were done with fully saturated concrete specimens and repeated two times for each gas injection rate (1.5 and 3.0 mL/min). For the 1.5 mL/min cases, the results were 39.38 and 33.62 bar, respectively, with an average of 36.50 bar. The results of the 3.0 mL/min cases were 39.85 and 36.43 bar, respectively, with an average of 38.14 bar. These results are presented in Table 2 and shown in
Fig. 4. Layout of the experimental system.
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Table 2 Results of gas threshold pressure. Injected gas flow rate (mL/min)
Case no.
Gas threshold pressure (bar)
Average gas threshold pressure (bar)
1.5
1 2
39.38 33.62
36.50 ± 4.07
3.0
1 2
39.85 36.43
38.14 ± 2.42
Fig. 6. Result of gas threshold pressure at 1.5 mL/min (case no. 1). Black line and red line (dotted) stand for differential pressure and discharged gas flow rate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Result of gas threshold pressure at 3.0 mL/min (case no. 1). Black line and red line (dotted) stand for differential pressure and discharged gas flow rate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. Result of gas threshold pressure at 1.5 mL/min (case no. 2). Black line and red line (dotted) stand for differential pressure and discharged gas flow rate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Result of gas threshold pressure at 3.0 mL/min (case no. 2). Black line and red line (dotted) stand for differential pressure and discharged gas flow rate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Figs. 6–9. At the lower injection rate, the rate of increase for the differential pressure in the upstream region was low; thus, there was sufficient time for the gas to migrate into the pore system of the concrete specimen. Therefore, the gas could penetrate into the specimen at a relatively lower pressure. This could be one reason why the average result of the 1.5 mL/min case was lower than that of the 3.0 mL/min case. In this respect, a realistic rate of gas generation in a silo is an essential parameter in order to estimate a precise gas threshold pressure. In the early stage of the measurement, the differential pressure between the upstream and downstream regions increased linearly. However, the increase rate of the differential pressure decreased when approaching the gas threshold pressure. This means that the discharge of gas increased gradually
instead of being explosively released all at once, when the gas reached the downstream region through the specimen. The standard deviations of the results were 4.07 and 2.42 bar for the 1.5 and 3.0 mL/min cases, respectively. Since the concrete specimens were a porous medium, the pore network structures of each specimen varied greatly. Therefore, the results had considerably large standard deviations. This can be seen in the results for the measurements of pore sizes in the concrete specimens. Fig. 10 shows the pore size distribution of the concrete specimens. 5.2. Gas permeability Measurements for the gas permeability were done with completely dried concrete specimens and repeated two times for each
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Fig. 10. Pore size distribution of the concrete specimen.
Table 3 Results of gas permeability. Saturation 0%
Applied pressure (bar) 2 3 4
Case no. 1 2 1 2 1 2
Discharged gas flow rate (mL/min) 18 36 36 48 51 57
applied pressure (2, 3 and 4 bar). The discharged gas flow rates at 2 bar were 18 and 36 mL/min, respectively. Thus, the kapp were 3.75 1017 and 7.51 1017 m2, respectively, which were calculated with Eq. (6). The average kapp was 5.63 1017 m2 at 2 bar. At 3 bar, the discharged gas flow rates were 36 and 48 mL/min, and the kapp were 4.01 1017 and 5.34 1017 m2, respectively. The average kapp was 4.68 1017 m2 at 3 bar. When P was 4 bar, the discharged gas flow rates were 51 and 57 mL/min, and the kapp were 3.55 1017 and 3.97 1017 m2, respectively. The average kapp was 3.76 1017 m2 at 4 bar. These results are presented in Table 3 and the approximated linear function is plotted in Fig. 8. From Eq. (7), the intrinsic permeability kint at 0% saturation was 1.04 1018 m2. Similar to the results of the gas threshold pressure, the deviation of the results between each specimen was significantly large. This was caused by the various pore network structures of each specimen as mentioned above. Fig. 11 shows that as the applied pressure becomes greater, the kapp becomes smaller.
Fig. 11. Result of gas permeability.
kapp (m2)
Average kapp (m2) 17
3.75 10 7.51 1017 4.01 1017 5.34 1017 3.55 1017 3.97 1017
5.63 10
17
± 2.66 10
kint (m2) 17
1.04 1018
4.68 1017 ± 9.40 1018 3.76 1017 ± 2.97 1018
6. Conclusion The Korean LILW disposal facility is currently under construction. Construction will be finished in June 2014, and gas relatedsafety of the disposal facility need to be readdressed at each stage of repository project until the final repository closure. The disposal facility will be saturated with ground water after it finally closes, and gases will be generated due to various phenomena. To prevent the overpressurization of the engineered barrier, a gas ventilation system is essential. To design a gas ventilation system, the characteristic properties of the silo’s concrete such as the gas threshold pressure, gas permeability and gas entry pressure should be measured and estimated in advance. In this study, the gas threshold pressure and gas permeability for the concrete silo at the LILW disposal facility in Korea were calculated using concrete specimens that had the same composition as the concrete used to construct the silo. Concrete specimens with a diameter of 15 cm and a height of 5 cm were fabricated and cured for 28 days. First, the gas threshold pressure was measured with fully saturated concrete specimens by injecting the gas at a constant flow rate. The average results of each case were 36.50 and 38.14 bar for 1.5 and 3.0 mL/min, respectively. The standard deviations of the results were considerably large since the concrete specimens had a wide distribution of pore size (i.e. from few nanometers to hundreds of micrometers). Since the silo is a huge concrete structure, it is expected that the pore structure of the silo will be very diverse depending on its location. In addition, the gas threshold pressure and the pressure at which gas begins to be discharged will vary depending on the location. Hence, the highest value of the gas threshold pressure should be used to design a gas ventilation system to be on the conservative side. Second, the discharged gas flow rates at certain applied pressures were measured with totally dried specimens, and then the gas permeability was calculated with the Hagen-Poiseuille and
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Klinkenberg equations. The deviation of the results for the discharged gas flow rates was significantly large also because of the various pore sizes and structures of the concrete. The apparent permeability for each applied pressure was calculated, and then, the intrinsic permeability at 0% saturation was calculated as 1.04 1018 m2. Since the relative permeability at 0% saturation equals unity, the absolute gas permeability of the concrete silo at the Korean LILW disposal facility was calculated as 1.04 1018 m2. This value can be used as the permeability parameter in the computer modeling of a gas ventilation system. The standard deviations of each result are great. More realistic results can be obtained by repeating the measurements using more specimens. In addition, the gas entry pressure and relative gas permeability at different saturations of the silo’s concrete will be measured in future studies. Acknowledgements This work was supported by the Radioactive Waste Management of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (20111720100010). References Abbas, A., Carcasses, M., Ollivier, J.-P., 1999. Gas permeability of concrete in relation to its degree of saturation. Mater. Struct. 32, 3–8. Bear, J., 1972. Dynamics of Fluids in Porous Media. American Elsevier Publ., New York.
Cole, W.A., Wakeham, W.A., 1985. The viscosity of nitrogen, oxygen, and their binary mixtures in the limit of zero density. J. Phys. Chem. Ref. Data 14, 209– 226. Cuss, R.J., Harrington, J. F., Noy, D.J., 2010, Large scale gas injection test (Lasgit) performed at the Äspö Hard Rock Laboratory-summary report 2008, SKB technical, report TR-10-38. Domenico, P.A., Schwartz, F.W., 1990. Physical and Chemical Hydrogeology. John Wiley & Sons, New York. Harris, A.W., Atkinson, A., Claisse, P.A., 2000. Transport of gases in concrete barriers. Waste Manage. 12, 155–178. Mayer, G., Moetsch, H.A., Wittmann, F.H., 1998. Large-scale Experiment for Water and Gas Transport in Cementitious Backfill Materials (Phase 1) COLEX I, NAGRA technical, report 98-03. Monlouis-Bonnaire, J.P., Verdier, J., Perrin, B., 2004. Prediction of the relative permeability to gas flow of cement-based materials. Cem. Concr. Res. 34, 737– 744. NAGRA, 2008. Effects of Post-Disposal Gas Generation in a Repository for Low- and Intermediate-Level Waste, Sited in the Opalinus Clay of Northern Switzerland, NAGRA technical report 08-07. NAGRA, 2010. Gas-Permeable Seal Test (GAST) at Grimsel Test – Experiment Concept, Aktennotiz AN /10-388. NDA, 2010. Geological disposal – gas status report, NDA report no. NDA/RWMD/ 037. Park, J.B., Lee, S.J., Kim, S.H., Kim, J.Y., 2010. Analysis on operation cases of largescale gas generation experiment for LILW disposal in overseas. J. Korean Radioactive Waste Soc. 8, 229–238 (in Korean). Senger, R., Enachescu, C., Doe, T., Distinguin, M., Delay, J., Frieg, B., 2006. Design and analysis of a gas threshold pressure test in a low-permeability clay formation at Andra’s Underground Research Laboratory, Bure (France), Proceedings, TOUGH Symposium 2006. SKB, 2001. PROJECT SAFE – Gas related processes in SFR, SKB report R-01-11. SKB, 2008. Safety analysis SFR 1 – long-term safety, SKB report R-08-130. Vieno, T., Nordman, H., 1998. VLJ repository safety analysis, report TVO-1/98. Villar, M.V., Martin, P.L., Romero, F.J., 2010. Results of the tests on concrete (Part 1), FORGE, report D3.6. Yamamoto, S., 2010. Overview of Gas Migration Test (GMT) at Grimsel Test Site, Technical Meeting for Gas Generation Research at KRMC.
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Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Gas threshold pressure and gas permeability of silo concrete specimens for a low- and intermediate-level waste disposal facility in Korea Juyub Kim a,⇑, Juyoul Kim a, Haeryong Jung b, Jae-Chul Ha b, Eun-Hee Kim c a
FNC Technology, 46 Tabsil-ro, Giheung-gu, Yongin 446-902, Republic of Korea Korea Radioactive Waste Management Corporation, 1045 Daedeok-Daero, Yuseong-gu, Daejeon 305-353, Republic of Korea c Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 28 September 2012 Received in revised form 3 December 2012 Accepted 5 December 2012 Available online 11 January 2013 Keywords: Gas threshold pressure Gas permeability Gas migration Concrete silo LILW disposal
a b s t r a c t In order to simulate gas migration at an underground disposal facility, the characteristics of a medium such as the gas threshold pressure and gas permeability need to be measured in advance. In this study, the gas threshold pressure and gas permeability of silo concrete specimens for a Korean LILW (Low- and Intermediate-Level Waste) disposal facility were measured. The concrete specimens had the same composition as the concrete used in the construction of the silo. The gas threshold pressure was measured by injecting a constant gas flow into cross sections of the specimens. To measure the gas permeability, selected pressures were applied to the specimens and the apparent permeability was calculated using the Hagen-Poiseuille equation. The intrinsic permeability was calculated with the Klinkenberg empirical equation. The gas threshold pressure and gas permeability ranged from 30 to 40 bar and from 1017 to 1018 m2, respectively. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction As of 2012, 23 nuclear power reactors are in operation in Korea. To service those reactors, a Low- and Intermediate-Level Radioactive Waste (LILW) disposal facility is under construction in Korea. The LILW disposal facility in Korea uses underground disposal with a concrete silo and will be capable of disposing 100,000 LILW drums (Fig. 1) when the construction is completed. The concrete silo is being built 80 m below sea level. The concrete silo will be saturated with ground water after the disposal facility is finally closed. Then, over time, gases could be generated by various mechanisms including metal corrosion, microbial degradation of organic matter, and radiolysis (Park et al., 2010). These generated gases could travel to the upper part of the concrete silo and start to accumulate. Since the accumulation of gases could increase the internal pressure of the concrete silo, the integrity of the engineered barrier (i.e. the concrete silo) could be damaged. In order to prevent an increase in internal pressure, it is necessary to design and develop an effective gas ventilation system for concrete silos used in such disposal facilities. Fig. 2 illustrates the concept of a gas transport pathway in a concrete silo. Several countries also have developed similar LILW underground disposal facilities. In Sweden, the SFR 1 LILW disposal facility is in operation at Forsmark, and it is scheduled to be closed in ⇑ Corresponding author. Tel.: +82 31 8005 6010; fax: +82 31 8005 6014. E-mail address: [email protected] (J. Kim). 0306-4549/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2012.12.012
2040. The SFR 1 consists of a silo and several horizontal rock vaults (SKB, 2008). The silo is located 60 m below sea level and is made of reinforced concrete with a height of 53 m, a diameter of 28 m and a thickness of 0.8 m. The space between the waste packages are filled with porous concrete grout and the top of the concrete cylinder is sealed with a 1 m thick concrete lid after emplacement of the waste packages. The concrete lid has evacuation pipes that are filled with sand: therefore, gases generated in the concrete cylinder can escape through the pipes. A 100 mm thick layer of sand and a 1.5 m thick sand/bentonite mixture are located above the lid. The remaining part of the silo above the lid is backfilled with sand or gravel and connected to concrete plugs (SKB, 2001). After the facility finally closes, the generated gases will be able to travel to the top of the silo through sand-filled pipes preventing the concrete silo from becoming overpressurized. When the pressure of the gases cumulated above the lid exceed certain pressure, they can be discharged into the surrounding host rock, or into the concrete plugs. In Finland, the VLJ disposal facility has been operating at the Olkiluoto site since 1992. The VLJ disposal facility is located 60–100 m below the surface and consists of two silos, an access tunnel and shaft. The intermediate- and low-level waste drums are packaged into concrete containers separately. Then, they are disposed of in a reinforced concrete silo and shotcrete silo, respectively. A 25 cm thick concrete lid is installed on top of the silo, and a dome-shaped cover is placed above the lid. The cover has a gas discharge path, which is called the Gas Lock on the top. After the
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Fig. 1. Concept of the concrete silo in Korea.
facility finally closes, the generated gases will be able to gather at the top of the dome and then discharge through the Gas Lock. The void between the lid and the cover is backfilled with crushed rock after the facility closes (Vieno et al., 1998). Switzerland decided to use cavern disposal for its LILW disposal facility. It will be located underground in Opalinus Clay 300–400 m below the surface. The disposal facility consists of seven emplacement caverns 11.0 m in width, 13.2 m in height and about 200 m in length. The wall of the cavern is sprayed with a concrete lining and cementitious mortar is used to fill the space between the disposal containers. Each emplacement cavern is linked to an operation tunnel through a branch tunnel. NAGRA (National Cooperative for the Disposal of Radioactive Waste in Switzerland) introduced the concept of an Engineered Gas Transport System (EGTS) consisting of a seal and plug in the tunnels. The seal is made of a compacted mixture of sand/bentonite or a compacted bentonite and concrete block. Generated gases can move into the operation tunnel through the EGTS and then discharge into the surrounding host
rock (NAGRA et al., 2008). The disposal facility in Switzerland does not include a silo, but the concept of an EGTS can be used to plan the migration of gas through an operation tunnel. Research on gas migration in underground disposal facilities has been carried out by many organizations. NAGRA has performed various experimental projects at the Grimsel Test Site and the Mont Terri Rock Laboratory. The GMT (Gas Migration Test in an Engineered Barrier System (EBS) and Geosphere) is one of the projects that evaluated the function of a gas migration system in a repository and the gas migration model in an EBS and a geosphere, and demonstrated an EBS in a disposal facility (Yamamoto, 2010). A large-scale cavern was excavated in the shear zone and an engineered barrier was replicated using a concrete silo, a sand/bentonite mixture, and water saturated backfill on the top which also included a plug. After the gas injection test, the EBS was excavated again, and the pathways of the gas and water, the performance of the concrete silo, and the characteristics of the sand/bentonite mixture were all evaluated. In addition, the researchers did
Fig. 2. Concept of the gas transport pathway in the silo.
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computers simulation with various codes such as CODE_BRIGHT, GETFLOW, MHERLIN, ROCKFLOW and TOUGH2 in parallel with the GMT. NAGRA has performed the GAST (Gas-permeable Seal Test) project at GTS since 2010 (NAGRA, 2010). The objectives of the GAST are to maintain the radionuclide retention capacity of the EBS and to improve the gas transport capacity of the backfill. The EBS is also called the EGTS, and it is composed of mortar and a sand/bentonite mixture. A numerical simulation was also performed in order to verify the performance of the EGTS, and the results showed that the gases are discharged effectively through the EGTS. A large column experiment was conducted by NAGRA in 1998 to research gas transport in backfills (Mayer et al., 1998). The length and diameter of the column were about 1 m and 0.3 m, respectively. The column element was filled with mortar as a backfill material, and a sample cell with plug materials was connected to the top of the column. As the gas was injected, the change in pressure, saturation, and discharging gas flow were observed. The FORGE (Fate of Repository Gases) project is a panEuropean project with many international organizations, and the key issues of the project are the generation and migration of repository gases. As part of the FORGE project, CIEMAT conducted a test on the gas permeability of concrete (Villar et al., 2010). Two different experimental methods, a steady state method and a non-steady state method, were used. Results of both methods showed that the permeability of the concrete specimens has a range of 1017–1016 m2. In Sweden, SKB carried out LASGIT (Large-Scale Gas Injection Test) at Äspö HRL (Hard Rock Laboratory) (Cuss et al., 2010). A full-scale deposition hole was drilled into the gallery floor, and a gas injection test was done in order to obtain new information on the mechanism of gas flow in buffered bentonite. In addition, France ANDRA (Agence nationale pour la gestion des dechets radioactifs) conducted a Gas Threshold Pressure Test (GTPT) at Bure URL in 2004 to understand gas migration in the Callovian–Oxfordian layer (Senger et al., 2006). The test was done in situ in a deposition hole, and the results showed that the gas threshold pressure of the Callovian–Oxfordian layer was about 9.5 MPa. In 1997, an experimental study on the gas permeability of concrete depending on its degree of saturation was done in France. The apparent permeability was calculated with the Hagen-Poiseuille equation, and then, the intrinsic permeability was calculated using the Klinkenberg equation (Abbas et al., 1999). With the goal of designing gas ventilation systems, simulations for gas generation and gas migration are essential in order to calculate the specifications for a gas ventilation system. A study on gas generation was done by the Korea Radioactive Waste Management Corporation (KRMC) and FNC Technology in 2009 that included modeling of gas generation and a large-scale in situ experiment. In addition, a study on the migration of gas was started by the KRMC and FNC Technology in 2011. The goal of the gas migration study was to design a gas ventilation system for the Korean LILW disposal facility by performing mock-up experiments and computer modeling. Prior to performing the computer modeling, properties of the silo’s concrete such as the gas threshold pressure and gas permeability should be obtained in advance. In order to measure the gas threshold pressure and gas permeability of the silo’s concrete, experimental measurements were carried out by FNC Technology and KRMC in 2012.
2. Theoretical background 2.1. Mechanism of gas migration in a porous medium The migration of gases through a low-permeable porous medium is governed by the hydraulic and mechanical properties of the medium, by the gas pressure in the upstream region, and by
the hydro-mechanical state of the medium. There are four basic mechanisms for the migration of a gas in a porous medium as follows (NAGRA et al., 2008): – – – –
Advective–diffusive transport of a gas dissolved in porewater. Visco-capillary two-phase flow. Dilatancy-controlled gas flow. Gas transport along macroscopic tensile fractures.
The advective–diffusive transport of a gas dissolved in porewater is governed by three laws: Darcy’s law, Fick’s law and Henry’s law. Darcy’s law states the velocity of a fluid passing through a porous medium is proportional to the head loss and inversely proportional to the path length. Eq. (1) gives the relationship as follows (NDA, 2010):
Q ¼ KA
dh k Pb Pa ¼ A ; dl l L
ð1Þ
where Q is the total discharge; K is the hydraulic conductivity; A is the cross-sectional area to flow; h is the head; l and L are the length of porous media; k is the permeability; l is the viscosity; and Pb and Pa are the pressure at outlet and inlet, respectively. Fick’s law states the flow velocity of diffusive material per unit area is proportional to the concentration gradient under a steady state condition shown by Eq. (2) (Domenico and Schwartz, 1990):
J ¼ Dd gradðCÞ;
ð2Þ
where J is the diffusional flux; Dd is the effective diffusion coefficient; C is the concentration of species. Henry’s law states the partial pressure of an ideal gas in the gas phase is proportional to the concentration of the gas in the aqueous phase shown in Eq. (3) (Domenico and Schwartz, 1990):
KH ¼
ðgasÞaq ; Pgas
ð3Þ
where KH is the Henry’s law constant; Pgas is the partial pressure of the gas in the gas phase; and (gas)aq is the molar concentration of the gas in solution. Porewater in the pores of a porous medium is displaced by gas due to the viscous and capillary forces. Visco-capillary two-phase flow is explained by the multiphase version of Darcy’s law shown in Eq. (4) (Bear, 1972):
v b ¼ k
krb qb ðrqb qb gÞ; lb
ð4Þ
where vb is the Darcy velocity in phase b; k is the intrinsic permeability; krb is the relative permeability to phase b; lb is the viscosity; pb is the fluid pressure in the phase and capillary pressure; and g is the gravitational acceleration. The gas entry pressure pae is the controlling factor for the twophase flow characteristics of a porous medium. The gas entry pressure is also known as the capillary threshold pressure, which is defined as the difference between the gas pressure and water pressure necessary to supersede the porewater from a fully saturated porous medium. If the pressure becomes larger than gas entry pressure, the gas migration is governed by the intrinsic permeability, the relative permeability and the water retention curve. Dilatancy-controlled gas flow is the primary mechanism for argillaceous media, which have a low tensile strength. The argillaceous media cannot bear gas pressures greater than the minimum principal stress applied to the rock mass for long-term periods. Thus, microfractures are expected to form in the rock mass before the minimum principal stress is achieved. If the shear stress is larger than the maximum stress or the stress state is ideal for rock
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deformation, gas flow through microfractures could occur. Gas flow through microfractures causes increases in the pore space and intrinsic permeability, and a change in the relationship between the capillary pressure and saturation. Gas transport along macroscopic tensile fractures is formed when the gas pressure is greater than the sum of the tensile strength and the minimum principal stress of the rock mass. Gas flow along macroscopic tensile fractures is considered a singlephase flow. The gas flow will be stopped when the gas pressure in the fracture becomes less than the minimum principal stress. If the rock has a low tensile strength, a macroscopic fracture forms only when the gas pressure increases rapidly. Usually, since disposal facilities have a sufficiently large capacity for gas storage, a LILW disposal facility would not experience this phenomenon. In addition, gas generation will become smaller over time such that gases can be discharged sufficiently through the host rock and with the engineered gas transport system. The gas threshold pressure is given as the sum of the gas entry pressure and the liquid pressure. The gas entry pressure is a key parameter in capillary pressure functions such as van Genuchten and Brooks-Corey. Results of previous studies have shown that the gas entry pressure and intrinsic gas permeability have a linear relationship on a logarithmic scale (Senger et al., 2006). 2.2. Gas permeability The flow rate through a porous medium is governed by permeability. Darcy’s law defines the coefficient of permeability. The gas permeability of a porous medium at a certain pressure is calculated with the Hagen-Poiseuille equation for a compressible fluid shown by Eq. (5) (Abbas et al., 1999):
kapp ¼
2QPatm Ll 2
AðP
P2atm Þ
ð5Þ
;
where kapp is the apparent gas permeability (m2); Q is the flow rate of discharging gas (m3/sec); A is the cross-sectional area (m2); L is the height of the specimen (m); l is the viscosity of the gas (N s/m2); P is the absolute pressure of the upstream region (N/m2); and Patm is the atmospheric pressure (N/m2). The gas permeability of a porous medium (e.g. concrete) varies with the gas pressure. Thus, the gas permeability is different because of measurements with an incompressible fluid in a specific medium. The Klinkenberg empirical equation (Eq. (6)) represents this effect as follows (Harris et al., 2000):
b ; k ¼ k1 1 þ pm
ð6Þ
where k is the gas permeability at pressure p; k1 is the gas permeability at infinite pressure; b is the Klinkenberg coefficient; and pm is the mean pressure of the applied pressure and the atmospheric pressure. Therefore, as the average pressure applied on a medium becomes larger, the gas permeability becomes smaller. The gas permeability at infinite pressure is equivalent to that of the incompressible fluid. The terms k1 and k are also called the intrinsic permeability (kint) and apparent permeability (kapp) respectively. The intrinsic permeability, kint, is irrelevant to the pressure
applied on a medium, but it is the unique property of a porous network. 3. Materials 3.1. Concrete specimens The specimens were cylindrical pieces of concrete 150 mm in diameter and 50 mm in height. The composition of the specimens is presented in Table 1. The concrete specimens were cured for 28 days in water. A cured specimen is shown in Fig. 3. 3.2. Experimental apparatus The experimental apparatus consisted of a pressure vessel, gas tank, pressure regulators, flow regulator, flow meters, differential pressure gauge, and valves. Nitrogen gas was used as the gas. The general layout of the apparatus is shown in Fig. 4. The pressure vessel consisted of an inlet/outlet nozzle, Viton sleeve, confining area and cover. The nozzle had a groove with a radial shape, so that the gas could be distributed uniformly onto the surface of the specimen. The confining area was filled with the gas, applying force onto the Viton sleeve. As a result, the Viton sleeve adheres to the side area of the cylindrical specimen so that the injected gas cannot move along the gap between the side area of the specimen and the Viton sleeve. Fig. 5 shows the inside of the pressure vessel. 4. Methods 4.1. Gas threshold pressure First, a fully saturated specimen was placed into the pressure vessel. Then, nitrogen was injected with a uniform flow rate through the flow regulator and upstream nozzle. Based on the experiment by NAGRA (Mayer et al., 1998), injection rates of 1.5 and 3.0 mL/min were used in this study. The pressure of the downstream region was kept at atmospheric pressure. As the gas was injected, the pressure of the upstream region became greater and the gas began to infiltrate into the specimen. As time passed, the gas moved along the pore system of the concrete specimen, and started to be discharged into the downstream region at a specific pressure. After the discharge occurred, the rate of increase of the pressure started to decrease and reached a point which is called the gas threshold pressure. 4.2. Gas permeability 4.2.1. Preprocessing of the specimen A total of six specimens were used for the measurements. All the specimens were weighed at 100% saturation. Then, the specimens were dried in a 100 °C oven until the mass of the specimens did not change any further. When the mass of the specimens remained constant over 24 h, it was assumed that the specimens were completely dried (0% saturation). 4.2.2. Measurement of the gas permeability A dried specimen was inserted into the vessel, and nitrogen was injected until the pressure of the upstream region reached a
Table 1 Composition of the concrete specimen. Source of aggregate
Mix type
W/(C + F) (%)
S/a (%)
Daejong and jinae area
E-1-40
40.0
38.4
Proportion (kg/m3) Water
Cement
Fly-ash
3= 00
Coarser
Finer
WRA
AEA
206
412
103
961
357
241
2.3193
0.1287
4
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Fig. 3. The concrete specimen.
Fig. 5. The pressure vessel.
specific pressure. The pressure of the upstream region remained constant for each case of the measurements. After the gas was discharged from the downstream region, sufficient time was allowed to achieve a steady state flow. By measuring the steady flow rate of the discharged gas and using the Hagen-Poiseuille equation (Eq. (5)), the permeability at a specific pressure and saturation (the apparent permeability) could be calculated (Monlouis-Bonnaire et al., 2004). Nitrogen at 20 °C with a viscosity of 1.7568 105 N s/m2 at a mole fraction of 1.0 was used for the viscosity, l (Cole and Wakeham, 1985). The apparent gas permeability was measured at three different pressures (2, 3 and 4 bar), and the relationship function between 1/Pm (Pm: mean pressure, Patm2 þP) and kapp was calculated from the linear approximation. Using the Klinkenberg
equation (Eq. (6)), the intrinsic permeability kint could be calculated from the y-intercept of the linear function (Abbas et al., 1999). 5. Results and discussion 5.1. Gas threshold pressure Experimental measurements for the gas threshold pressure were done with fully saturated concrete specimens and repeated two times for each gas injection rate (1.5 and 3.0 mL/min). For the 1.5 mL/min cases, the results were 39.38 and 33.62 bar, respectively, with an average of 36.50 bar. The results of the 3.0 mL/min cases were 39.85 and 36.43 bar, respectively, with an average of 38.14 bar. These results are presented in Table 2 and shown in
Fig. 4. Layout of the experimental system.
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Table 2 Results of gas threshold pressure. Injected gas flow rate (mL/min)
Case no.
Gas threshold pressure (bar)
Average gas threshold pressure (bar)
1.5
1 2
39.38 33.62
36.50 ± 4.07
3.0
1 2
39.85 36.43
38.14 ± 2.42
Fig. 6. Result of gas threshold pressure at 1.5 mL/min (case no. 1). Black line and red line (dotted) stand for differential pressure and discharged gas flow rate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Result of gas threshold pressure at 3.0 mL/min (case no. 1). Black line and red line (dotted) stand for differential pressure and discharged gas flow rate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. Result of gas threshold pressure at 1.5 mL/min (case no. 2). Black line and red line (dotted) stand for differential pressure and discharged gas flow rate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Result of gas threshold pressure at 3.0 mL/min (case no. 2). Black line and red line (dotted) stand for differential pressure and discharged gas flow rate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Figs. 6–9. At the lower injection rate, the rate of increase for the differential pressure in the upstream region was low; thus, there was sufficient time for the gas to migrate into the pore system of the concrete specimen. Therefore, the gas could penetrate into the specimen at a relatively lower pressure. This could be one reason why the average result of the 1.5 mL/min case was lower than that of the 3.0 mL/min case. In this respect, a realistic rate of gas generation in a silo is an essential parameter in order to estimate a precise gas threshold pressure. In the early stage of the measurement, the differential pressure between the upstream and downstream regions increased linearly. However, the increase rate of the differential pressure decreased when approaching the gas threshold pressure. This means that the discharge of gas increased gradually
instead of being explosively released all at once, when the gas reached the downstream region through the specimen. The standard deviations of the results were 4.07 and 2.42 bar for the 1.5 and 3.0 mL/min cases, respectively. Since the concrete specimens were a porous medium, the pore network structures of each specimen varied greatly. Therefore, the results had considerably large standard deviations. This can be seen in the results for the measurements of pore sizes in the concrete specimens. Fig. 10 shows the pore size distribution of the concrete specimens. 5.2. Gas permeability Measurements for the gas permeability were done with completely dried concrete specimens and repeated two times for each
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Fig. 10. Pore size distribution of the concrete specimen.
Table 3 Results of gas permeability. Saturation 0%
Applied pressure (bar) 2 3 4
Case no. 1 2 1 2 1 2
Discharged gas flow rate (mL/min) 18 36 36 48 51 57
applied pressure (2, 3 and 4 bar). The discharged gas flow rates at 2 bar were 18 and 36 mL/min, respectively. Thus, the kapp were 3.75 1017 and 7.51 1017 m2, respectively, which were calculated with Eq. (6). The average kapp was 5.63 1017 m2 at 2 bar. At 3 bar, the discharged gas flow rates were 36 and 48 mL/min, and the kapp were 4.01 1017 and 5.34 1017 m2, respectively. The average kapp was 4.68 1017 m2 at 3 bar. When P was 4 bar, the discharged gas flow rates were 51 and 57 mL/min, and the kapp were 3.55 1017 and 3.97 1017 m2, respectively. The average kapp was 3.76 1017 m2 at 4 bar. These results are presented in Table 3 and the approximated linear function is plotted in Fig. 8. From Eq. (7), the intrinsic permeability kint at 0% saturation was 1.04 1018 m2. Similar to the results of the gas threshold pressure, the deviation of the results between each specimen was significantly large. This was caused by the various pore network structures of each specimen as mentioned above. Fig. 11 shows that as the applied pressure becomes greater, the kapp becomes smaller.
Fig. 11. Result of gas permeability.
kapp (m2)
Average kapp (m2) 17
3.75 10 7.51 1017 4.01 1017 5.34 1017 3.55 1017 3.97 1017
5.63 10
17
± 2.66 10
kint (m2) 17
1.04 1018
4.68 1017 ± 9.40 1018 3.76 1017 ± 2.97 1018
6. Conclusion The Korean LILW disposal facility is currently under construction. Construction will be finished in June 2014, and gas relatedsafety of the disposal facility need to be readdressed at each stage of repository project until the final repository closure. The disposal facility will be saturated with ground water after it finally closes, and gases will be generated due to various phenomena. To prevent the overpressurization of the engineered barrier, a gas ventilation system is essential. To design a gas ventilation system, the characteristic properties of the silo’s concrete such as the gas threshold pressure, gas permeability and gas entry pressure should be measured and estimated in advance. In this study, the gas threshold pressure and gas permeability for the concrete silo at the LILW disposal facility in Korea were calculated using concrete specimens that had the same composition as the concrete used to construct the silo. Concrete specimens with a diameter of 15 cm and a height of 5 cm were fabricated and cured for 28 days. First, the gas threshold pressure was measured with fully saturated concrete specimens by injecting the gas at a constant flow rate. The average results of each case were 36.50 and 38.14 bar for 1.5 and 3.0 mL/min, respectively. The standard deviations of the results were considerably large since the concrete specimens had a wide distribution of pore size (i.e. from few nanometers to hundreds of micrometers). Since the silo is a huge concrete structure, it is expected that the pore structure of the silo will be very diverse depending on its location. In addition, the gas threshold pressure and the pressure at which gas begins to be discharged will vary depending on the location. Hence, the highest value of the gas threshold pressure should be used to design a gas ventilation system to be on the conservative side. Second, the discharged gas flow rates at certain applied pressures were measured with totally dried specimens, and then the gas permeability was calculated with the Hagen-Poiseuille and
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Klinkenberg equations. The deviation of the results for the discharged gas flow rates was significantly large also because of the various pore sizes and structures of the concrete. The apparent permeability for each applied pressure was calculated, and then, the intrinsic permeability at 0% saturation was calculated as 1.04 1018 m2. Since the relative permeability at 0% saturation equals unity, the absolute gas permeability of the concrete silo at the Korean LILW disposal facility was calculated as 1.04 1018 m2. This value can be used as the permeability parameter in the computer modeling of a gas ventilation system. The standard deviations of each result are great. More realistic results can be obtained by repeating the measurements using more specimens. In addition, the gas entry pressure and relative gas permeability at different saturations of the silo’s concrete will be measured in future studies. Acknowledgements This work was supported by the Radioactive Waste Management of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (20111720100010). References Abbas, A., Carcasses, M., Ollivier, J.-P., 1999. Gas permeability of concrete in relation to its degree of saturation. Mater. Struct. 32, 3–8. Bear, J., 1972. Dynamics of Fluids in Porous Media. American Elsevier Publ., New York.
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