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A discussion of thermo–hydro–mechanical (THM) processes associated with nuclear waste repositories
International Journal of Rock Mechanics and Mining Sciences 37 (2000) 397±402
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A discussion of thermo±hydro±mechanical (THM) processes associated with nuclear waste repositories C.-F. Tsang a,*, O. Stephansson b, J.A. Hudson c a
Earth Sciences Division, Ernest Lawrence Berkeley National Laboratory, Berkeley, California, USA b Department of Engineering Geology, Royal Institute of Technology, Stockholm, Sweden c Rock Engineering Consultants, UK Accepted 7 October 1999
Abstract The design of a nuclear waste repository involves modeling a complex system of physical mechanisms that operate over a long period of time. The perturbations caused by both the engineering excavation and the waste heat have to be modeled, taking into account the short and long term thermo±hydro±mechanical (THM) processes. In this paper, we discuss the approach philosophy and we comment on heterogeneity, multi-stage data needs and modeling phases. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Time periods of repository evolution
The emplacement of a nuclear waste repository in a geologic formation underground causes a major perturbation of the geologic system. The perturbation is two-fold: (a) a large number of tunnels covering an area of several square kilometers are excavated in the formation, and (b) the stored nuclear waste releases heat, initially of the order of tens of thousands of kilowatts per square kilometer, into the formation. Such a perturbation causes a number of coupled thermo± hydro±mechanical (THM) processes around the repository. The present paper provides a discussion of these THM processes. It is based on a review of a number of reports and books, including the proceedings of an EC conference [5], a literature review by US NRC [9], an edited book [14], a special issue of the International Journal of Rock Mechanics and Mining Sciences [6], as well as a number of individual papers and articles [2,4,7,8,16,17].
For our discussion of issues related to a waste repository, it is useful to group the THM processes into several time periods rather than according to the spatial distance from the near ®eld to the far ®eld of the repository. The US NRC [9] de®ned three temporal stages according to the US regulation 10CFR760:
* Corresponding author. Tel.: +1-510-486-6726; fax: +1-510-4865686. E-mail address: [email protected] (C.F. Tsang).
1. the operation stage (up to permanent closure); 2. the containment stage (from permanent closure to 300±1000 years, probably after the heat pulse has peaked); and 3. the isolation stage (for long-term isolation up to the nominal 10,000+ years). The AEC of Japan de®ned the stages [1] as: 1. construction stage, covering coupled eects during excavation; 2. operation stage, covering period of emplacement of an engineering barrier system; 3. enclosure stage, covering back®lling and closing of the repository; and 4. post-closure stage.
1365-1609/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 5 - 1 6 0 9 ( 9 9 ) 0 0 1 1 4 - 8
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C.-F. Tsang et al. / International Journal of Rock Mechanics and Mining Sciences 37 (2000) 397±402
However, for our consideration of coupled THM processes, we emphasize the earlier time periods, which are when the most signi®cant eects of the processes occur. This is similar to the AECJ de®nition, except that the third stage of the enclosure period is incorporated into the second stage of the operation period Ð since the THM processes are similar during these two stages. Thus, in the following discussion, we group them according to (1) excavation stage, (2) operation stage, and (3) postclosure stage.
3. Excavation stage During the excavation stage, no radioactive waste is yet emplaced to provide thermal input, and the thermal eects of normal geothermal gradients and tunnel ventilation are expected to be small. Then a number of coupled MH processes are expected to occur. Here we have used MH, rather than HM, indicating that the direction of coupling at this stage is mostly from mechanical to hydrologic eects. They are brie¯y described as follows. 1. The excavation of the repository causes a major perturbation of the rock formation by the creation of a large cavity. The impact depends on the initial stress ®eld around the system and on the nature of the excavation method and the repository design. It is not easy to determine the in situ stress ®eld of a region, especially in the presence of fracture sets forming a network, which could well be anisotropic and may not even follow a regular ellipsoidal angular distribution. The excavation will concentrate stress changes around the cavity, which in turn will change the local fracture apertures and permeability. In general, the question is how to determine the anisotropic change in hydraulic conductivity around the repository cavity. Since a repository will probably be excavated in stages, then how does the ®rst stage change the stress ®eld and how do we calculate the MH eect of excavation at a later stage, which is within a dierent stress ®eld environment? 2. The excavation also represents a relatively sudden event and hence the normal and shear stress across nearby fractures may change in a short time, producing sudden aperture changes. This may cause the pore pressure to rise quickly before the water has a chance to move and equilibrate. This has been noticed as a result of calculations, but no experiment has yet been designed to detect it. Such a transient coupled MH eect may cause local failures, as well as local hydraulic conductivity changes. 3. The cavity at this early stage will be ventilated. This
means that water will be taken out of the system and the pore pressure near the repository will be much reduced from the original condition, which may induce fracture closure because of the increase of eective stress (an HM coupling). 4. Reduction of pore pressure near the repository cavity may also result in degassing of the pore water. Gases that were in solution in water under pressure will be released. Thus, the ¯ow in the rock surrounding the cavity will be two-phase ¯ow (i.e., both water and gas are present), so that water permeability is greatly reduced due to gas interference represented by the relative permeability function.
4. Operation stage During this stage the nuclear waste and buer/back®ll or liner materials have been emplaced. Thus, at this stage, there is thermal input and a series of THM processes occur. 1. The heat from the nuclear waste will heat up the buer/back®ll and the rock. This will occur over several decades in the dierent repository rooms as they are successively ®lled with the waste. Thermally induced stresses will be created around the repository, which may change the hydraulic conductivity. The TM eects have been relatively well studied and much experience has been gained in their modelling and observation. 2. What is interesting here is the heating up of the multiple media system, the waste canister, back®ll materials (e.g., bentonite), and then the surrounding rock. These all have dierent expansion coecients. How they move and compress each other and how the interfaces between them behave may cause a signi®cant change in the hydraulic properties of these interfaces. 3. Since water and rock have dierent thermal expansivity, thermal input may also cause signi®cant pore pressure changes. This is particularly the case for repositories in clay where the original hydraulic conductivity is low and much of the water is in closed pores. The thermal input will then cause changes in eective stress, possibly giving rise to local failures and hence increased local hydraulic conductivity. 4. For bentonite back®ll, the incoming water from the rock will increase its saturation and cause it to swell. The imposition of the swelling pressure and the capability of the bentonite to ®ll in gaps and fractures will change the local hydrologic properties and hence the water ¯ow paths. 5. It is not obvious how to assess the behaviour of the gases, which could be either air from the open cav-
C.-F. Tsang et al. / International Journal of Rock Mechanics and Mining Sciences 37 (2000) 397±402
ity migrating into the rock or gas from degassing of the water during the ®rst stage. They will move into the system under buoyancy or temperature gradients; and then may redissolve into water or form gas pockets that expand with temperature. Their presence will change signi®cantly the local permeability to water ¯ow, as represented by the relative permeability function. 6. The heat will induce convective ¯ow in the rock, and, as in the case of the Yucca Mountain site in the US, the local temperature near a waste canister may be high, and signi®cant vaporization will occur. The water vapour will move away from the repository and condense at cooler regions of the rock. This process forms a complex hydrologic system that requires a fully multiphase code for its analysis. How this system will aect the ®eld of eective stress and thus the mechanical condition is an open question. 7. During this period the repository also may be kept open to allow for the retrievability option. Thus, the system is ventilated and heat/moisture taken out of it. This, coupled with thermal evaporation, will cause a dehydration in the near ®eld.
399
into 10,000 years. One example is that the thermal expansion distribution may open fractures below the repository and close fractures above it, thus creating an under-pressure region below. The possible occurrence of this coupled TMH eect should be evaluated by modelling and be detected and studied by a repository monitoring program. In general, mechanical deformation during the resaturation and repressurization is an irreversible coupled THM process. It is not expected that the system will return to pre-excavation conditions because of mechanical hysteresis. Similarly, the thermal strain during the cycle of temperature build-up and decrease may also be an irreversible process. The role of gases during this stage, in terms of their ¯ow and dissolution in water, is also an open question. Yet, hopefully, any eect at this later stage would be moderate in magnitude and in time rate. Other coupled processes have been envisaged away from this `base case scenario'. These include chemical eects, seismically induced HM, glacially induced THM, erosion, asteroid/meteorite induced THM.
6. Remarks on heterogeneity 5. Post-closure stage This is the stage after permanent closure of the repository. The repository is now sealed and thus there is no ventilation or escape of moisture. The repository cavity is then resaturated (if below the water table) and repressurized to its original hydrostatic pressure corresponding to its depth. During this stage, a number of important changes take place. 1. Thermally, the temperature builds up to a certain level and then decreases. The temperature peak of the heating±cooling cycle is reached after 15±100 years near the waste canister, but may take 200± 1000 years to attain in the far ®eld. The exact temporal and spatial distribution of the heating cycle depends on the waste inventory and repository design. 2. This is also the period when the hydraulic pressure is rebuilt in the back®lled and sealed repository opening. Thermally induced ¯ow or convection (TH) depends on thermal energy imparted to the water, which will last much longer than the temperature pulse. The convective velocity could peak at around 10,000 years. 3. In the same way, the thermal mechanical (TM) eect is also dependent on thermal energy imparted to the rock and is not directly dependent on temperature. Thus coupled TM processes will also last
Along with the above discussions of coupled THM processes, two important and dicult scienti®c problems need to be considered. These are discussed in this and the next sub-sections. The ®rst is the question of heterogeneity. Though much research has been done on this subject in recent years, it is still a wide open question. We do not yet know clearly and de®nitely how to characterize heterogeneity at a potential repository site, or how to design and perform a reasonable set of ®eld measurements to obtain the basic heterogeneity parameters. Because of this, heterogeneity is a critical uncertainty that pervades all modelling input, the modelling sensitivities and output, and may well be one of the factors limiting the con®dence level of THM predictions. The subject is important in the sense that THM models may be required to support a PA safety case for regulatory approval. A special class of heterogeneity is the fracture network in the rock. This is well known in crystalline rocks. There is a suggestion that there are also fractures in clay formations (unless the clay is very plastic) and that fractures may be induced during dehydration of clay as well [4]. All the coupled THM processes mentioned above will be present in fractured porous systems. In other words, both fractures (with their high permeability and low stiness) and the porous block need to be considered. In hydrothermal coupling it is now well known that it is the presence of both
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fractures and porous matrix that promotes the heat pipe eect. In THM, similar special eects may occur. Furthermore, there can be strong heterogeneity even in the porous medium itself, and it is not extraordinary to ®nd, for example, variations of hydraulic conductivity of one or two orders of magnitude or more in the porous medium. In the general case, heterogeneity means dierent thermohydromechanical properties at dierent points in space. Modelling the THM processes in a heterogeneous system is still very much an open problem. Work is being done in at least three directions for the case of fractured media. The ®rst is the development of an eective continuum model by homogenization of the fracture network [12,15]. In this approach, the eects of the fractures in a calculational element are approximated by the behaviour of an eective anisotropic continuum block. As shown by Stietel et al. [15], the results are quite inaccurate when tested against detailed numerical results. The second is the approach being taken by Noorishad and others [11] who are developing a 3D heterogeneous model of ubiquitous joints, allowing preferred elastic/plastic directions for the elements. The basis for both of these approaches is not yet established; in other words, one may question whether such an equivalence exists. Perhaps the equivalent continuum blocks are valid for a limited range of fracture parameters and for a limited set of stress and hydraulic conditions. The third direction is to employ a detailed fracture network model and let each network fracture deform (change in aperture) according to variations in the local pore pressures, gradient, and in situ stress conditions. The diculties with this approach include both the need for neglecting the role of the matrix between fractures in coupled processes and for neglecting the interference between nearby fractures, particularly the way in which the presence and behaviour of one fracture aects the local in situ stress ®eld seen by a neighbouring fracture. We anticipate that, as this ®eld develops, additional methods will be developed to model coupled eects in heterogeneous rocks. It may be found that a completely dierent approach is needed to deal with heterogeneity for performance assessment of a nuclear waste repository. 7. Remarks on multiple-stage data needs Data needs for each stage of the repository development and assessment should be built up from the previous stage. The discussions below give a general approach and some special remarks on data needed for each stage. Hopefully this will stimulate careful thinking in the design of testing and monitoring plans.
It is useful to note that each stage of repository development represents a major perturbation of the hydromechanical conditions of the rock formation. Responses to these perturbations can be utilised to characterize the system, and hence they need to be monitored in detail, both spatially and temporally. Before the excavation starts, information is needed on the geologic structures and stress distributions. The former includes the geometry of faults and joints, and the presence of geologic domains, as well as eective boundaries of the region to be studied. The stress distributions may require borehole measurements such as hydrofracturing and pro®ling. Injection and withdrawal pressure transient tests also need to be done. Of particular importance are the hydraulic and mechanical conditions at the chosen boundaries. In general, many of these data and information details are not easy to obtain. Some may have to be left as uncertain and estimated later through calibration. The Nirex work at Sella®eld in preparation for the RCF Shaft sinking is an example of this kind of eort [10]. After the excavation stage, the impact of the excavation on rock hydromechanical behaviour is very useful to further characterize and understand the site. Deformations of the cavity pro®le, especially near fractures that intercept the cavity, are useful for estimating the mechanical condition near those locations. Distribution of water emergence in the freshly excavated surfaces and changes in pore pressure distribution also will give information on major ¯ow paths and how they are aected by mechanical changes because of the excavation. Some eorts along these lines have been made, for example, in the Kamaishi experiment [13]. However that study was on a smaller scale, with the test pit being 1.7 m in diameter and 5 m in depth. Careful monitoring design needs to be made prior to the excavation. During the operation stage, assuming that the physical and chemical properties of the back®ll materials are known from laboratory testing, monitoring of rock changes and hydromechanical conditions at the rock± back®ll interface may be made at a number of representative locations. The objective of this monitoring program is to understand the rock responses with thermal input from the waste and under the swelling pressure of the material back®ll (e.g., bentonite). The swelling is in general non-uniform. An example of this kind of eort is the FEBEX experiment [3]. During the post-closure stage, we do not expect drastic or abrupt changes in thermohydromechanical conditions. But, as explained in a previous section, signi®cant changes involving a temperature rise-and-fall cycle, as well as TH and TM cycles with a much longer duration, occur. Certain types of remote geophysical monitoring of these changes may still be possible. One school of thought is that we should abandon
C.-F. Tsang et al. / International Journal of Rock Mechanics and Mining Sciences 37 (2000) 397±402
the site after establishing monuments to warn future generations not to disturb the underground system. However, an alternative view is that monitoring should still be continued to detect these changes, and to ensure they are within model expectation ranges. .
8. Comments on modelling phases and scaling Three modelling phases should be carried out, corresponding to the three stages of repository development and based on data collected before the start of each stage. As the data and information build up for successive stages, so does the degree of sophistication of the model for successive phases. Each modelling phase should include: 1. data evaluation and calibration of model structure/ parameters; 2. modelling studies, including sensitivity study and uncertainty evaluation; and 3. predictive modelling of the next stage. To evaluate and understand the ®eld observations and monitoring data at each stage, it is necessary to compare with predictive results of the models from the previous phase. Further, calibrations are particularly important to prevent error buildup with successive phases of the modelling exercise. The degree of success with which this can be done depends on the design and execution of the monitoring and testing program at each repository stage. It is to be noted that the period of the actual excavation of the repository represents an opportunity to obtain data at the scale of the repository and at the magnitude of its mechanical impact. It is important to think carefully about how to design a data-gathering strategy for that period and set it in place. The repository may otherwise be excavated without adequate preparation, and consequently proper data are not obtained for THM evaluation. The problem is that once this unique opportunity to obtain data on largescale THM responses is overlooked, it will be dicult to gain it again.
Acknowledgements We would like to take this opportunity to acknowledge the inspiration and insight that the authors received from Neville G. W. Cook over the years through his papers, presentations and informal discussions. The ®rst author speci®cally expresses gratitude and indebtedness to Neville for his suggestion some 20 years ago to explore the coupled processes research
401
and for his friendship and encouragement all through Neville's years in Berkeley. The paper has bene®ted greatly by the comments from a large number of reviewers through the International DECOVALEX project, particularly those of Johan Andersson, Antonio Gens, Aimo Hautojaevi, Ghislain de Marsily, Alain Millard, Leslie Knight and Piet Zuidema. We are most grateful for their thoughtful suggestions. Work is supported jointly by the DECOVALEX project, Swedish Nuclear Power Inspectorate (SKI) and US Department of Energy, Oce of Basic Energy Sciences, Engineering and Geosciences Division, through Contract number DE-AC0376SF00098.
References [1] AECJ. Guidelines on research and development relating to geological disposal of high-level radioactive waste in Japan. Advisory Committee on Nuclear Fuel Cycle Backend Policy, Atomic Energy Commission of Japan, November 1996. [2] Come B. A review of thermal, mechanical and hydrogeological properties of hard, fractured rocks. In: Haijtink B, editor. Proceedings of Workshop on Testing and Modelling of Thermal, Mechanical and Hydrogeological Properties of Host Rocks for Deep Geological Disposal of Radioactive Waste, European Commission Report EUR 16219 EN, 1995. [3] ENRESA. Test plan for FEBEX experiment. Spain: Empresa Nacional de Residuos Radiactivos SA, 1996. [4] Gera F. Radioactive waste disposal in clays, a review of site characterization, thermo±hydro±mechanical eects and repository design. In: Haijtink B, editor. Proceedings of Workshop on Testing and Modelling of Thermal, Mechanical and Hydrogeological Properties of Host Rocks for Deep Geological Disposal of Radioactive Waste, European Commission Report EUR 16219 EN, 1995. [5] Haijtink B. Testing and modelling of thermal, mechanical and hydrogeological properties of host rocks for deep geological disposal of radioactive waste. In: Proceedings of the Brussels Workshop, European Commission Report EUR 16219, 1995. p. 298. [6] IJRMMS. Special issue: thermo±hydro±mechanical coupling in rock mechanics. In: Stephansson O, guest editor, International Journal of Rock Mechanics and Mining Sciences, Vol. 32, No. 5 July, 1995. [7] Jing L, Tsang C-F, Stephansson O. DECOVALEX Ð An international co-operative research project on mathematical models of coupled THM processes for safety analysis of radioactive waste repositories. In Special Issue on THM Coupling in Rock Mechanics. Int J Rock Mech & Min Sci 1995;32(5):387±98. [8] Langer M. Review of R&D requirements on THM properties of salt. In: Haijtink B, editor. Proceedings of Workshop on Testing and Modelling of Thermal, Mechanical and Hydrogeological Properties of Host Rocks for Deep Geological Disposal of Radioactive Waste, European Commission Report EUR 16219 EN, 1995. [9] Manteufel RD, Ahola MP, Turner DR, Chowdhury AH. A literature review of coupled thermal-hydrologic-mechanical-chemical processes pertinent to the proposed high-level nuclear waste repository at Yucca Mountain. US Nuclear Regulatory Commission Report NUREG/CR-6021, Center for Nuclear
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[10]
[11]
[12]
[13]
C.-F. Tsang et al. / International Journal of Rock Mechanics and Mining Sciences 37 (2000) 397±402 Waste Regulatory Analyses Report CNWRA 92±011, July, 1993. Nirex. DECOVALEX II Project, de®nition for test case 1. Stockholm: Sella®eld DECOVALEX Secretariat, Royal Institute of Technology, 1997. Noorishad J, Tsang CF. Coupled thermohydroelasticity phenomena in variably saturated fractured porous rocks ± formulation and numerical solution. In: Stephansson O, Jing L, Tsang C-F, editors. Mathematical and experimental studies of coupled thermo±hydro±mechanical processes of fractured media. Elsevier Science, p. 93±134. Oda M. An equivalent continuum model for coupled stress and ¯uid ¯ow analysis in jointed rock masses. Water Resources Research 1986;22:1845±56. PNC. DECOVALEX II project, de®nition for test case 2,
[14]
[15]
[16] [17]
Kamaishi. Stockholm: DECOVALEX Secretariat, Royal Institute of Technology, 1997. Stephansson O, Jing L, Tsang CF. Coupled thermo±hydro± mechanical processes of fractured media. In: Developments in geotechnical engineering 79. Amsterdam: Elsevier Science BV, 1996. Stietel A, Millard A, Treille E, Vuillod E, Thoravel A, Ababou R. Continuum representation of coupled hydromechanic processes of fractured media. In: Stephansson O, Jing L, Tsang CF, editors. Coupled thermal±hydro±mechanical processes of fractured medium. Developments in Geological Engineering, vol. 79, Amsterdam: Elsevier Science BV, 1996. Tsang CF. Coupled processes associated with nuclear waste repositories. San Diego: Academic Press, 1987. Tsang CF. Coupled hydromechanical-thermochemical processes in rock fractures. Review of Geophysics 1991;29(4):537±51.
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A discussion of thermo±hydro±mechanical (THM) processes associated with nuclear waste repositories C.-F. Tsang a,*, O. Stephansson b, J.A. Hudson c a
Earth Sciences Division, Ernest Lawrence Berkeley National Laboratory, Berkeley, California, USA b Department of Engineering Geology, Royal Institute of Technology, Stockholm, Sweden c Rock Engineering Consultants, UK Accepted 7 October 1999
Abstract The design of a nuclear waste repository involves modeling a complex system of physical mechanisms that operate over a long period of time. The perturbations caused by both the engineering excavation and the waste heat have to be modeled, taking into account the short and long term thermo±hydro±mechanical (THM) processes. In this paper, we discuss the approach philosophy and we comment on heterogeneity, multi-stage data needs and modeling phases. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Time periods of repository evolution
The emplacement of a nuclear waste repository in a geologic formation underground causes a major perturbation of the geologic system. The perturbation is two-fold: (a) a large number of tunnels covering an area of several square kilometers are excavated in the formation, and (b) the stored nuclear waste releases heat, initially of the order of tens of thousands of kilowatts per square kilometer, into the formation. Such a perturbation causes a number of coupled thermo± hydro±mechanical (THM) processes around the repository. The present paper provides a discussion of these THM processes. It is based on a review of a number of reports and books, including the proceedings of an EC conference [5], a literature review by US NRC [9], an edited book [14], a special issue of the International Journal of Rock Mechanics and Mining Sciences [6], as well as a number of individual papers and articles [2,4,7,8,16,17].
For our discussion of issues related to a waste repository, it is useful to group the THM processes into several time periods rather than according to the spatial distance from the near ®eld to the far ®eld of the repository. The US NRC [9] de®ned three temporal stages according to the US regulation 10CFR760:
* Corresponding author. Tel.: +1-510-486-6726; fax: +1-510-4865686. E-mail address: [email protected] (C.F. Tsang).
1. the operation stage (up to permanent closure); 2. the containment stage (from permanent closure to 300±1000 years, probably after the heat pulse has peaked); and 3. the isolation stage (for long-term isolation up to the nominal 10,000+ years). The AEC of Japan de®ned the stages [1] as: 1. construction stage, covering coupled eects during excavation; 2. operation stage, covering period of emplacement of an engineering barrier system; 3. enclosure stage, covering back®lling and closing of the repository; and 4. post-closure stage.
1365-1609/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 5 - 1 6 0 9 ( 9 9 ) 0 0 1 1 4 - 8
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C.-F. Tsang et al. / International Journal of Rock Mechanics and Mining Sciences 37 (2000) 397±402
However, for our consideration of coupled THM processes, we emphasize the earlier time periods, which are when the most signi®cant eects of the processes occur. This is similar to the AECJ de®nition, except that the third stage of the enclosure period is incorporated into the second stage of the operation period Ð since the THM processes are similar during these two stages. Thus, in the following discussion, we group them according to (1) excavation stage, (2) operation stage, and (3) postclosure stage.
3. Excavation stage During the excavation stage, no radioactive waste is yet emplaced to provide thermal input, and the thermal eects of normal geothermal gradients and tunnel ventilation are expected to be small. Then a number of coupled MH processes are expected to occur. Here we have used MH, rather than HM, indicating that the direction of coupling at this stage is mostly from mechanical to hydrologic eects. They are brie¯y described as follows. 1. The excavation of the repository causes a major perturbation of the rock formation by the creation of a large cavity. The impact depends on the initial stress ®eld around the system and on the nature of the excavation method and the repository design. It is not easy to determine the in situ stress ®eld of a region, especially in the presence of fracture sets forming a network, which could well be anisotropic and may not even follow a regular ellipsoidal angular distribution. The excavation will concentrate stress changes around the cavity, which in turn will change the local fracture apertures and permeability. In general, the question is how to determine the anisotropic change in hydraulic conductivity around the repository cavity. Since a repository will probably be excavated in stages, then how does the ®rst stage change the stress ®eld and how do we calculate the MH eect of excavation at a later stage, which is within a dierent stress ®eld environment? 2. The excavation also represents a relatively sudden event and hence the normal and shear stress across nearby fractures may change in a short time, producing sudden aperture changes. This may cause the pore pressure to rise quickly before the water has a chance to move and equilibrate. This has been noticed as a result of calculations, but no experiment has yet been designed to detect it. Such a transient coupled MH eect may cause local failures, as well as local hydraulic conductivity changes. 3. The cavity at this early stage will be ventilated. This
means that water will be taken out of the system and the pore pressure near the repository will be much reduced from the original condition, which may induce fracture closure because of the increase of eective stress (an HM coupling). 4. Reduction of pore pressure near the repository cavity may also result in degassing of the pore water. Gases that were in solution in water under pressure will be released. Thus, the ¯ow in the rock surrounding the cavity will be two-phase ¯ow (i.e., both water and gas are present), so that water permeability is greatly reduced due to gas interference represented by the relative permeability function.
4. Operation stage During this stage the nuclear waste and buer/back®ll or liner materials have been emplaced. Thus, at this stage, there is thermal input and a series of THM processes occur. 1. The heat from the nuclear waste will heat up the buer/back®ll and the rock. This will occur over several decades in the dierent repository rooms as they are successively ®lled with the waste. Thermally induced stresses will be created around the repository, which may change the hydraulic conductivity. The TM eects have been relatively well studied and much experience has been gained in their modelling and observation. 2. What is interesting here is the heating up of the multiple media system, the waste canister, back®ll materials (e.g., bentonite), and then the surrounding rock. These all have dierent expansion coecients. How they move and compress each other and how the interfaces between them behave may cause a signi®cant change in the hydraulic properties of these interfaces. 3. Since water and rock have dierent thermal expansivity, thermal input may also cause signi®cant pore pressure changes. This is particularly the case for repositories in clay where the original hydraulic conductivity is low and much of the water is in closed pores. The thermal input will then cause changes in eective stress, possibly giving rise to local failures and hence increased local hydraulic conductivity. 4. For bentonite back®ll, the incoming water from the rock will increase its saturation and cause it to swell. The imposition of the swelling pressure and the capability of the bentonite to ®ll in gaps and fractures will change the local hydrologic properties and hence the water ¯ow paths. 5. It is not obvious how to assess the behaviour of the gases, which could be either air from the open cav-
C.-F. Tsang et al. / International Journal of Rock Mechanics and Mining Sciences 37 (2000) 397±402
ity migrating into the rock or gas from degassing of the water during the ®rst stage. They will move into the system under buoyancy or temperature gradients; and then may redissolve into water or form gas pockets that expand with temperature. Their presence will change signi®cantly the local permeability to water ¯ow, as represented by the relative permeability function. 6. The heat will induce convective ¯ow in the rock, and, as in the case of the Yucca Mountain site in the US, the local temperature near a waste canister may be high, and signi®cant vaporization will occur. The water vapour will move away from the repository and condense at cooler regions of the rock. This process forms a complex hydrologic system that requires a fully multiphase code for its analysis. How this system will aect the ®eld of eective stress and thus the mechanical condition is an open question. 7. During this period the repository also may be kept open to allow for the retrievability option. Thus, the system is ventilated and heat/moisture taken out of it. This, coupled with thermal evaporation, will cause a dehydration in the near ®eld.
399
into 10,000 years. One example is that the thermal expansion distribution may open fractures below the repository and close fractures above it, thus creating an under-pressure region below. The possible occurrence of this coupled TMH eect should be evaluated by modelling and be detected and studied by a repository monitoring program. In general, mechanical deformation during the resaturation and repressurization is an irreversible coupled THM process. It is not expected that the system will return to pre-excavation conditions because of mechanical hysteresis. Similarly, the thermal strain during the cycle of temperature build-up and decrease may also be an irreversible process. The role of gases during this stage, in terms of their ¯ow and dissolution in water, is also an open question. Yet, hopefully, any eect at this later stage would be moderate in magnitude and in time rate. Other coupled processes have been envisaged away from this `base case scenario'. These include chemical eects, seismically induced HM, glacially induced THM, erosion, asteroid/meteorite induced THM.
6. Remarks on heterogeneity 5. Post-closure stage This is the stage after permanent closure of the repository. The repository is now sealed and thus there is no ventilation or escape of moisture. The repository cavity is then resaturated (if below the water table) and repressurized to its original hydrostatic pressure corresponding to its depth. During this stage, a number of important changes take place. 1. Thermally, the temperature builds up to a certain level and then decreases. The temperature peak of the heating±cooling cycle is reached after 15±100 years near the waste canister, but may take 200± 1000 years to attain in the far ®eld. The exact temporal and spatial distribution of the heating cycle depends on the waste inventory and repository design. 2. This is also the period when the hydraulic pressure is rebuilt in the back®lled and sealed repository opening. Thermally induced ¯ow or convection (TH) depends on thermal energy imparted to the water, which will last much longer than the temperature pulse. The convective velocity could peak at around 10,000 years. 3. In the same way, the thermal mechanical (TM) eect is also dependent on thermal energy imparted to the rock and is not directly dependent on temperature. Thus coupled TM processes will also last
Along with the above discussions of coupled THM processes, two important and dicult scienti®c problems need to be considered. These are discussed in this and the next sub-sections. The ®rst is the question of heterogeneity. Though much research has been done on this subject in recent years, it is still a wide open question. We do not yet know clearly and de®nitely how to characterize heterogeneity at a potential repository site, or how to design and perform a reasonable set of ®eld measurements to obtain the basic heterogeneity parameters. Because of this, heterogeneity is a critical uncertainty that pervades all modelling input, the modelling sensitivities and output, and may well be one of the factors limiting the con®dence level of THM predictions. The subject is important in the sense that THM models may be required to support a PA safety case for regulatory approval. A special class of heterogeneity is the fracture network in the rock. This is well known in crystalline rocks. There is a suggestion that there are also fractures in clay formations (unless the clay is very plastic) and that fractures may be induced during dehydration of clay as well [4]. All the coupled THM processes mentioned above will be present in fractured porous systems. In other words, both fractures (with their high permeability and low stiness) and the porous block need to be considered. In hydrothermal coupling it is now well known that it is the presence of both
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fractures and porous matrix that promotes the heat pipe eect. In THM, similar special eects may occur. Furthermore, there can be strong heterogeneity even in the porous medium itself, and it is not extraordinary to ®nd, for example, variations of hydraulic conductivity of one or two orders of magnitude or more in the porous medium. In the general case, heterogeneity means dierent thermohydromechanical properties at dierent points in space. Modelling the THM processes in a heterogeneous system is still very much an open problem. Work is being done in at least three directions for the case of fractured media. The ®rst is the development of an eective continuum model by homogenization of the fracture network [12,15]. In this approach, the eects of the fractures in a calculational element are approximated by the behaviour of an eective anisotropic continuum block. As shown by Stietel et al. [15], the results are quite inaccurate when tested against detailed numerical results. The second is the approach being taken by Noorishad and others [11] who are developing a 3D heterogeneous model of ubiquitous joints, allowing preferred elastic/plastic directions for the elements. The basis for both of these approaches is not yet established; in other words, one may question whether such an equivalence exists. Perhaps the equivalent continuum blocks are valid for a limited range of fracture parameters and for a limited set of stress and hydraulic conditions. The third direction is to employ a detailed fracture network model and let each network fracture deform (change in aperture) according to variations in the local pore pressures, gradient, and in situ stress conditions. The diculties with this approach include both the need for neglecting the role of the matrix between fractures in coupled processes and for neglecting the interference between nearby fractures, particularly the way in which the presence and behaviour of one fracture aects the local in situ stress ®eld seen by a neighbouring fracture. We anticipate that, as this ®eld develops, additional methods will be developed to model coupled eects in heterogeneous rocks. It may be found that a completely dierent approach is needed to deal with heterogeneity for performance assessment of a nuclear waste repository. 7. Remarks on multiple-stage data needs Data needs for each stage of the repository development and assessment should be built up from the previous stage. The discussions below give a general approach and some special remarks on data needed for each stage. Hopefully this will stimulate careful thinking in the design of testing and monitoring plans.
It is useful to note that each stage of repository development represents a major perturbation of the hydromechanical conditions of the rock formation. Responses to these perturbations can be utilised to characterize the system, and hence they need to be monitored in detail, both spatially and temporally. Before the excavation starts, information is needed on the geologic structures and stress distributions. The former includes the geometry of faults and joints, and the presence of geologic domains, as well as eective boundaries of the region to be studied. The stress distributions may require borehole measurements such as hydrofracturing and pro®ling. Injection and withdrawal pressure transient tests also need to be done. Of particular importance are the hydraulic and mechanical conditions at the chosen boundaries. In general, many of these data and information details are not easy to obtain. Some may have to be left as uncertain and estimated later through calibration. The Nirex work at Sella®eld in preparation for the RCF Shaft sinking is an example of this kind of eort [10]. After the excavation stage, the impact of the excavation on rock hydromechanical behaviour is very useful to further characterize and understand the site. Deformations of the cavity pro®le, especially near fractures that intercept the cavity, are useful for estimating the mechanical condition near those locations. Distribution of water emergence in the freshly excavated surfaces and changes in pore pressure distribution also will give information on major ¯ow paths and how they are aected by mechanical changes because of the excavation. Some eorts along these lines have been made, for example, in the Kamaishi experiment [13]. However that study was on a smaller scale, with the test pit being 1.7 m in diameter and 5 m in depth. Careful monitoring design needs to be made prior to the excavation. During the operation stage, assuming that the physical and chemical properties of the back®ll materials are known from laboratory testing, monitoring of rock changes and hydromechanical conditions at the rock± back®ll interface may be made at a number of representative locations. The objective of this monitoring program is to understand the rock responses with thermal input from the waste and under the swelling pressure of the material back®ll (e.g., bentonite). The swelling is in general non-uniform. An example of this kind of eort is the FEBEX experiment [3]. During the post-closure stage, we do not expect drastic or abrupt changes in thermohydromechanical conditions. But, as explained in a previous section, signi®cant changes involving a temperature rise-and-fall cycle, as well as TH and TM cycles with a much longer duration, occur. Certain types of remote geophysical monitoring of these changes may still be possible. One school of thought is that we should abandon
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the site after establishing monuments to warn future generations not to disturb the underground system. However, an alternative view is that monitoring should still be continued to detect these changes, and to ensure they are within model expectation ranges. .
8. Comments on modelling phases and scaling Three modelling phases should be carried out, corresponding to the three stages of repository development and based on data collected before the start of each stage. As the data and information build up for successive stages, so does the degree of sophistication of the model for successive phases. Each modelling phase should include: 1. data evaluation and calibration of model structure/ parameters; 2. modelling studies, including sensitivity study and uncertainty evaluation; and 3. predictive modelling of the next stage. To evaluate and understand the ®eld observations and monitoring data at each stage, it is necessary to compare with predictive results of the models from the previous phase. Further, calibrations are particularly important to prevent error buildup with successive phases of the modelling exercise. The degree of success with which this can be done depends on the design and execution of the monitoring and testing program at each repository stage. It is to be noted that the period of the actual excavation of the repository represents an opportunity to obtain data at the scale of the repository and at the magnitude of its mechanical impact. It is important to think carefully about how to design a data-gathering strategy for that period and set it in place. The repository may otherwise be excavated without adequate preparation, and consequently proper data are not obtained for THM evaluation. The problem is that once this unique opportunity to obtain data on largescale THM responses is overlooked, it will be dicult to gain it again.
Acknowledgements We would like to take this opportunity to acknowledge the inspiration and insight that the authors received from Neville G. W. Cook over the years through his papers, presentations and informal discussions. The ®rst author speci®cally expresses gratitude and indebtedness to Neville for his suggestion some 20 years ago to explore the coupled processes research
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and for his friendship and encouragement all through Neville's years in Berkeley. The paper has bene®ted greatly by the comments from a large number of reviewers through the International DECOVALEX project, particularly those of Johan Andersson, Antonio Gens, Aimo Hautojaevi, Ghislain de Marsily, Alain Millard, Leslie Knight and Piet Zuidema. We are most grateful for their thoughtful suggestions. Work is supported jointly by the DECOVALEX project, Swedish Nuclear Power Inspectorate (SKI) and US Department of Energy, Oce of Basic Energy Sciences, Engineering and Geosciences Division, through Contract number DE-AC0376SF00098.
References [1] AECJ. Guidelines on research and development relating to geological disposal of high-level radioactive waste in Japan. Advisory Committee on Nuclear Fuel Cycle Backend Policy, Atomic Energy Commission of Japan, November 1996. [2] Come B. A review of thermal, mechanical and hydrogeological properties of hard, fractured rocks. In: Haijtink B, editor. Proceedings of Workshop on Testing and Modelling of Thermal, Mechanical and Hydrogeological Properties of Host Rocks for Deep Geological Disposal of Radioactive Waste, European Commission Report EUR 16219 EN, 1995. [3] ENRESA. Test plan for FEBEX experiment. Spain: Empresa Nacional de Residuos Radiactivos SA, 1996. [4] Gera F. Radioactive waste disposal in clays, a review of site characterization, thermo±hydro±mechanical eects and repository design. In: Haijtink B, editor. Proceedings of Workshop on Testing and Modelling of Thermal, Mechanical and Hydrogeological Properties of Host Rocks for Deep Geological Disposal of Radioactive Waste, European Commission Report EUR 16219 EN, 1995. [5] Haijtink B. Testing and modelling of thermal, mechanical and hydrogeological properties of host rocks for deep geological disposal of radioactive waste. In: Proceedings of the Brussels Workshop, European Commission Report EUR 16219, 1995. p. 298. [6] IJRMMS. Special issue: thermo±hydro±mechanical coupling in rock mechanics. In: Stephansson O, guest editor, International Journal of Rock Mechanics and Mining Sciences, Vol. 32, No. 5 July, 1995. [7] Jing L, Tsang C-F, Stephansson O. DECOVALEX Ð An international co-operative research project on mathematical models of coupled THM processes for safety analysis of radioactive waste repositories. In Special Issue on THM Coupling in Rock Mechanics. Int J Rock Mech & Min Sci 1995;32(5):387±98. [8] Langer M. Review of R&D requirements on THM properties of salt. In: Haijtink B, editor. Proceedings of Workshop on Testing and Modelling of Thermal, Mechanical and Hydrogeological Properties of Host Rocks for Deep Geological Disposal of Radioactive Waste, European Commission Report EUR 16219 EN, 1995. [9] Manteufel RD, Ahola MP, Turner DR, Chowdhury AH. A literature review of coupled thermal-hydrologic-mechanical-chemical processes pertinent to the proposed high-level nuclear waste repository at Yucca Mountain. US Nuclear Regulatory Commission Report NUREG/CR-6021, Center for Nuclear
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