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[en] A coupled thermal, hydrologic and mechanical (THM) analysis is conducted to evaluate the impact of coupled THM processes on the performance of a potential nuclear waste repository at Yucca Mountain, Nevada. The analysis considers changes in rock mass porosity, permeability, and capillary pressure caused by rock deformations during drift excavation, as well as those caused by thermo-mechanically induced rock deformations after emplacement of the heat-generating waste. The analysis consists of a detailed calibration of coupled hydraulic-mechanical rock mass properties against field experiments, followed by a prediction of the coupled thermal, hydrologic, and mechanical behavior around a potential repository drift. For the particular problem studied and parameters used, the analysis indicates that the stress-induced permeability changes will be within one order of magnitude and that these permeability changes do not significantly impact the overall flow pattern around the repository drift
[en] This paper presents results from a coupled thermal, hydrological and mechanical analysis of thermally-induced permeability changes during heating and cooling of fractured volcanic rock at the Drift Scale Test at Yucca Mountain, Nevada. The analysis extends the previous analysis of the four-year heating phase to include newly available data from the subsequent four year cooling phase. The new analysis of the cooling phase shows that the measured changes in fracture permeability follows that of a thermo-hydro-elastic model on average, but at several locations the measured permeability indicates (inelastic) irreversible behavior. At the end of the cooling phase, the air-permeability had decreased at some locations (to as low as 0.2 of initial), whereas it had increased at other locations (to as high as 1.8 of initial). Our analysis shows that such irreversible changes in fracture permeability are consistent with either inelastic fracture shear dilation (where permeability increased) or inelastic fracture surface asperity shortening (where permeability decreased). These data are important for bounding model predictions of potential thermally-induced changes in rock-mass permeability at a future repository at Yucca Mountain
[en] In this report, we scrutinize the work by the Swedish Nuclear Fuel and Waste Management Company (SKB) related to coupled thermal, hydrological and mechanical (THM) processes within the SR-Can project. SR-Can is SKB's preliminary assessment of long-term safety for a KBS-3 nuclear waste repository, and is a preparation stage for the SR-Site assessment, the report that will be used in SKB's application for a final repository. We scrutinize SKB's work related to THM processes through review and detailed analysis, using an independent modeling tool. The modeling tool is applied to analyze coupled THM processes at the two candidate sites, Forsmark and Laxemar, using data defined in SKB's site description models for respective sites. In this report, we first provide a brief overview of SKB's work related to analysis of the evolution of coupled THM processes as presented in SRCan, as well as supporting documents. In this overview we also identify issues and assumptions that we then analyze using our modeling tool. The overview and subsequent independent model analysis addresses issues related to near-field behavior, such as buffer resaturation and the evolution of the excavation-disturbed zone, as well as far-field behavior, such as stress induced changes in hydrologic properties. Based on the review and modeling conducted in this report, we conclude by identifying a number of areas of weaknesses, where we believe further work and clarifications are needed. Some of the most important ones are summarized below: 1) We found that SKB's calculation of peak temperature might not have been conducted for the most conservative case of extreme drying of the buffer under dry rock conditions and an unexpectedly high thermal diffusion coefficient. Our alternative analysis indicates that temperatures close to 100 might be achieved under unfavorable (and perhaps unexpected) conditions in which the buffer is dried to below 20% near the canister. We believe SKB should conduct further analyses to show that such extreme drying of the buffer to below 20% could not occur, or that such drying would not result in a peak temperature higher than 100 deg C. 2) We found that SKB's estimates for the time of full resaturation of the buffer might be underestimated, because the analysis is based on models assuming nearby water feeding conditions. Moreover, SKB's analysis does not consider the potential impact and uncertainties regarding water-retention properties of the rock mass and the potential impact of ventilation-induced drying during the operational phase is not addressed. SKB's estimated time to full resaturation is valid for an assumed distance to water feeding boundary of 12 m and for one single assumed retention curve of the rock. We believe SKB should provide additional analyses to show that the assumed distance to the water-feeding boundary is reasonable and conduct additional sensitivity analyses on water-retention properties and ventilation effects. 3) We found that SKB's reliance on the backfill as an important source for water supply to resaturate the buffer, in the case of extremely dry rock conditions, may be unjustified. If a bentonite-rock mixture (30/70) is used, the buffer may be resaturated by water supply from the backfill, but then the saturation in the backfill would decrease, preventing it from swelling and thereby keeping it from fulfilling an important safety function indicator criterion. If Friedland Clay is used as backfill, its capillary suction at emplacement would be higher than that of the buffer, and therefore water would be sucked from the buffer into the backfill, effectively keeping the buffer dry. We believe SKB should conduct further studies or reconsider the backfill design, to assure buffer resaturation from the backfill in the case of extremely dry rock conditions. 4) We found that SKB's geomechanical analysis of the potential for rock-mass failure correctly identifies a high potential for spalling failure around the deposition holes at both Laxemar and Forsmark. However, a strong potential for tensile failure in the rock wall of tunnels and its consequence forming a continuous damaged zone along the tunnels is not identified. Moreover, SR-Can does not address the possibility of long-term time-dependent degradation of rock-strength parameters. SKB's assumption that the long-term strength is equal to the relatively short-term strength observed in in situ experiments might not be sufficiently conservative. We believe SKB needs to address the issue of time-dependence in the mechanical parameters as a part of their safety assessment. 5) We found that SKB correctly identifies possible stress-induced changes in permeability near excavations, as well as thermal-mechanically induced change in the far-field permeability. However, SKB analysis does not consider the possibility of large-scale shear reactivation in the far field. Many fractures at the site might already critically stressed for shear. During the thermal period, shear stresses around the repository will increase. We believe that SKB needs to evaluate potential permeability changes due to such shear reactivation and their importance for radionuclide transport. Modeling results developed by the SKB and in this report involve application of complex coupled-processes modeling. An independent analysis using a different model simulator than SKB, is necessary for an in-depth check of SKB's results, to identify issues that might have been overlooked, to test assumptions, and to evaluate how sensitive their results are to such assumptions. The results presented in this report are related to SR-Can, but should also be considered by the SKB when defining their work scope on coupled THM processes for the upcoming SR-Site assessment. Thus, further site-specific analyses on these important aspects for the performance assessment of the future Swedish deep geological disposal of spent nuclear fuel should be conducted
[en] As a result of the termination of the Yucca Mountain Project, the United States Department of Energy (DOE) has started to explore various alternative avenues for the disposition of used nuclear fuel and nuclear waste. The overall scope of the investigation includes temporary storage, transportation issues, permanent disposal, various nuclear fuel types, processing alternatives, and resulting waste streams. Although geologic disposal is not the only alternative, it is still the leading candidate for permanent disposal. The realm of geologic disposal also offers a range of geologic environments that may be considered, among those clay shale formations. Figure 1-1 presents the distribution of clay/shale formations within the USA. Clay rock/shale has been considered as potential host rock for geological disposal of high-level nuclear waste throughout the world, because of its low permeability, low diffusion coefficient, high retention capacity for radionuclides, and capability to self-seal fractures induced by tunnel excavation. For example, Callovo-Oxfordian argillites at the Bure site, France (Fouche et al., 2004), Toarcian argillites at the Tournemire site, France (Patriarche et al., 2004), Opalinus clay at the Mont Terri site, Switzerland (Meier et al., 2000), and Boom clay at Mol site, Belgium (Barnichon et al., 2005) have all been under intensive scientific investigations (at both field and laboratory scales) for understanding a variety of rock properties and their relations with flow and transport processes associated with geological disposal of nuclear waste. Clay/shale formations may be generally classified as indurated and plastic clays (Tsang et al., 2005). The latter (including Boom clay) is a softer material without high cohesion; its deformation is dominantly plastic. For both clay rocks, coupled thermal, hydrological, mechanical and chemical (THMC) processes are expected to have a significant impact on the long-term safety of a clay repository. For example, the excavation-damaged zone (EDZ) near repository tunnels can modify local permeability (resulting from induced fractures), potentially leading to less confinement capability (Tsang et al., 2005). Because of clay's swelling and shrinkage behavior (depending on whether the clay is in imbibition or drainage processes), fracture properties in the EDZ are quite dynamic and evolve over time as hydromechanical conditions change. To understand and model the coupled processes and their impact on repository performance is critical for the defensible performance assessment of a clay repository. Within the Natural Barrier System (NBS) group of the Used Fuel Disposition (UFD) Campaign at DOE's Office of Nuclear Energy, LBNL's research activities have focused on understanding and modeling such coupled processes. LBNL provided a report in this April on literature survey of studies on coupled processes in clay repositories and identification of technical issues and knowledge gaps (Tsang et al., 2010). This report will document other LBNL research activities within the natural system work package, including the development of constitutive relationships for elastic deformation of clay rock (Section 2), a THM modeling study (Section 3) and a THC modeling study (Section 4). The purpose of the THM and THC modeling studies is to demonstrate the current modeling capabilities in dealing with coupled processes in a potential clay repository. In Section 5, we discuss potential future R and D work based on the identified knowledge gaps. The linkage between these activities and related FEPs is presented in Section 6.
[en] Over the past decade SKI supported a collaborative research effort between Lawrence Berkeley National Laboratory (LBNL) and the Royal Institute of Technology (KTH) to provide SKI with independent codes and model experience for investigation of SKB's work on EBS. The emphasis is on coupled thermo-hydro-mechanical (THM) processes in the bentonite buffer and surrounding rock mass. Two numerical models are adapted and utilized for the studies of EBS. The first one is ROCMAS, which is a finite element code for three-dimensional analysis of coupled THM processes of unsaturated/saturation porous and fractured geological media. A version of this code has been tailor-made for a rigorous analysis of EBS including bentonite swelling and rock interaction. The second code is TOUGHFLAC, which has the capability of solving coupled THM problems under multi-phase flow conditions. The multi-phase flow capability means that it can used for studying migration of gas and its interaction with the liquid in more detail. The TOUGH-FLAC code utilizing two established codes; TOUGH2 for TH analysis and FLAC3D code for rock and soil mechanics analysis. Both ROCMAS and TOUGH-FLAC are currently applied to various problems within the DECOVALEX project. Participation in the DECOVALEX project has been extremely valuable for developing and strengthening SKI's ability of an independent examination of SKB's works on EBS. Thus, SKI currently has available these two numerical models that have the same capabilities as SKB's numerical models, and in certain aspects far exceeds the capabilities of SKB's numerical models. This is important because SKI needs not only to investigate what SKB do but also investigate what they are not doing, thus providing them with insightful comments and guidelines. Maybe the most important gain of the DECOVALEX project is the experience in solving realistic problems related to EBS, which provides experience and depths of knowledge for the future analysis of a real site. This is extremely valuable because the most important asset is not the numerical model itself (provided that it meets minimum requirements), but the knowledge how to adapt and apply the numerical model correctly for the specific problem
[en] Geological repositories for disposal of high-level nuclear wastes generally rely on a multi-barrier system to isolate radioactive wastes from the biosphere. The multi-barrier system typically consists of a natural barrier system, including repository host rock and its surrounding subsurface environment, and an engineering barrier system (EBS). EBS represents the man-made, engineered materials placed within a repository, including the waste form, waste canisters, buffer materials, backfill and seals (OECD, 2003). EBS plays a significant role in the containment and long-term retardation of radionuclide release. EBS is involved in complex thermal, hydrogeological, mechanical, chemical and biological processes, such as heat release due to radionuclide decay, multiphase flow (including gas release due to canister corrosion), swelling of buffer materials, radionuclide diffusive transport, waste dissolution and chemical reactions. All these processes are related to each other. An in-depth understanding of these coupled processes is critical for the performance assessment (PA) for EBS and the entire repository. Within the EBS group of Used Fuel Disposition (UFD) Campaign, LBNL is currently focused on (1) thermal-hydraulic-mechanical-chemical (THMC) processes in buffer materials (bentonite) and (2) diffusive transport in EBS associated with clay host rock, with a long-term goal to develop a full understanding of (and needed modeling capabilities to simulate) impacts of coupled processes on radionuclide transport in different components of EBS, as well as the interaction between near-field host rock (e.g., clay) and EBS and how they effect radionuclide release. This final report documents the progress that LBNL has made in its focus areas. Specifically, Section 2 summarizes progress on literature review for THMC processes and reactive-diffusive radionuclide transport in bentonite. The literature review provides a picture of the state-of-the-art of the relevant research areas addressed by LBNL. Section 3 documents the current modeling tools, available at LBNL, for the EBS study. This may be important for identifying future modeling activities within the EBS group with these current capabilities and needs for future EBS modeling development. Section 4 presents the results of geomechanical modeling using the Barcelona Basic Model (BBM) constitutive relationship for thermo-elasto-plastic media such as bentonite and an update on reactive-diffusive transport modeling approaches through bentonite in the EBS. Section 5 discusses identified knowledge gaps and technical issues as well as short- and long-term R and D plans.
[en] This paper presents an international, multiple-code, simulation study of coupled thermal, hydrological, and mechanical (THM) processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement drifts. Simulations were conducted considering two types of repository settings: (a) open emplacement drifts in relatively shallow unsaturated volcanic rock, and (b) backfilled emplacement drifts in deeper saturated crystalline rock. The results showed that for the two assumed repository settings, the dominant mechanism of changes in rock permeability was thermal-mechanically-induced closure (reduced aperture) of vertical fractures, caused by thermal stress resulting from repository-wide heating of the rock mass. The magnitude of thermal-mechanically-induced changes in permeability was more substantial in the case of an emplacement drift located in a relatively shallow, low-stress environment where the rock is more compliant, allowing more substantial fracture closure during thermal stressing. However, in both of the assumed repository settings in this study, the thermal-mechanically-induced changes in permeability caused relatively small changes in the flow field, with most changes occurring in the vicinity of the emplacement drifts
[en] As part of the ongoing international DECOVALEX project, four research teams used five different models to simulate coupled thermal, hydrological, and mechanical (THM) processes near waste emplacement drifts of geological nuclear waste repositories. The simulations were conducted for two generic repository types, one with open and the other with back-filled repository drifts, under higher and lower postclosure temperatures, respectively. In the completed first model inception phase of the project, a good agreement was achieved between the research teams in calculating THM responses for both repository types, although some disagreement in hydrological responses is currently being resolved. In particular, good agreement in the basic thermal-mechanical responses was achieved for both repository types, even though some teams used relatively simplified thermal-elastic heat-conduction models that neglected complex near-field thermal-hydrological processes. The good agreement between the complex and simplified process models indicates that the basic thermal-mechanical responses can be predicted with a relatively high confidence level
[en] This report presents the results of an international, multiple-code, benchmark test (BMT) simulation study of coupled thermal, hydrological, mechanical, and chemical (THMC) processes in the excavation disturbed zone (EDZ) around an emplacement drift of a hypothetical nuclear waste repository. The study focuses on mechanical responses and long-term chemo-mechanical effects that may lead to time-dependent changes in the mechanical and hydrological properties of the EDZ. Five research teams participated in this BMT using a wide range of modeling approaches, including boundary element, finite element; finite difference, particle mechanics, and cellular automaton methods. An important part of the BMT was to investigate how these widely different approaches could be adapted and developed to model the evolution of the EDZ and to include time-dependent processes to model the complex coupled THMC processes at various scales and times around an emplacement tunnel. Thus, this BMT was not a strictly defined problem for code-to-code comparison, but rather was designed to promote innovative model developments towards simulation of chemo-mechanical interactions, with the future goal of fully coupled THMC modeling. The initial pre-excavation conditions were represented by the values of in situ stresses, temperature and fluid pressure at a depth of 500 m in crystalline rocks derived from the Aespoe Hard Rock Laboratory, Sweden. For detailed modelling of the failure processes and an analysis of the evolution and extent of the EDZ, a fracture network geometry based on investigations at Aespoe Hard Rock Laboratory, Sweden were included in the study. The post-closure environment is simulated using a time-varying temperature, fluid pressure, and boundary stress. The time-varying temperature, fluid pressure, and boundary stresses are extracted from separate simulation results using the full drift-scale models of DECOVALEX-THMC, Task D. The results demonstrated how widely different modeling approaches can be adapted to simulate the evolution of the EDZ around a heat-releasing nuclear waste emplacement tunnel in fractured rock. Each modeling approach has its special capabilities for studying different aspects of the EDZ evolution at different scales. The results also show that, during the excavation of the drift, the permeability will increase as a result of the general unloading of fractures parallel to the drift wall and increased shear stress on fractures near the crown of the tunnel. The results also show that progressively increasing differential stresses near the crown of the emplacement tunnel during the first 100 years may cause additional failure and permeability changes. However, the time dependency may play only a small role during the first 100 years of loading, which is a relatively short time for chetnically mediated processes. Without the heat load, the maximum principal stress is about 60 MPa (of the order of 20% of the small-scale peak strength), which may be too low to induce significant time-dependent mechanical changes. On the other hand, predictions of chemically mediated time-dependent mechanical changes over a 100,000-year period are still very uncertain, but could be conservatively bounded. Such bounds would be better constrained using fully coupled THMC models, considering critical reaction rates. Although several promising attempts were made to model coupled THMC processes in this study, including the chemically mediated time-dependent processes, fully coupled THMC models are still lacking, and more experimental data to support such models are needed
[en] This report is intended as a Guidance Document explaining current knowledge about the nature of and potential for thermo-hydro-mechanical-chemical modelling of the Excavation Damaged Zone (EDZ) around the excavations for an underground radioactive waste repository. In Part 1 of the report, the disturbances associated with excavation are explained together with reviews of Workshops that have been held on the subject. The manifold aspects of the EDZ are discussed under the three headings of the rock mass response to tunnelling, the influence of the excavation method, and EDZ characterisation methods. The evolution of the repository over thousands of years is qualitatively described via the subjects of time-dependent effects and stress redistribution, thermal effects, hydrogeological effects, and chemical effects. The first part of the document concludes with a discussion of uncertainties and limitations in measuring and characterising the EDZ. In Part 2 of the report, the results of a DECOVALEX research programme on modelling the EDZ are presented. Initially, the research involved the physical testing and computer modelling of the failure behaviour of a rock specimen in uniaxial compression. Four research teams used four different models to simulate the complete stress-strain curve for Aevroe granite from the Swedish Aespoe Hard Rock Laboratory. Conclusions are then made concerning the overall capabilities of these models. Subsequent research extended the work to computer simulation of the evolution of the repository using a 'wall block model' and a 'near-field model'. This included assessing the evolution of stress, failure and permeability and time dependent effects during repository evolution. In both sets of computer modelling, the types of codes used were an Elasto-Plastic Cellular Automaton (EPCA), the FRACOD boundary element (BEM) code with discrete fracture propagation, the THAMES finite element (FEM) code, and PFC, a distinct element particle flow code. In the later work studying the evolution of the repository, the TOUGH-FLAC simulator using the finite difference method (FDM) and ROCMAS, a finite element (FEM) code, were also used. Additionally, in the later work, EDZ fracture data from the Aespoe HRL were used to simulate the fracture network in the EDZ. The conclusions are that physical testing of granitic specimens under dry conditions, saturated with formation and saline waters, showed that the presence of pore water affected the elastic modulus, compressive strength, and geometry of the complete stress-strain curve. All the computer models were suitable for sensitivity studies to evaluate the influence of their respective supporting parameters on the complete stress-strain curve for rock, thus demonstrating the value of numerical models as a research and engineering design tool. We have also demonstrated how widely different modelling approaches can be adapted to simulate the evolution of EDZ around a heat-releasing nuclear waste emplacement tunnel in fractured rock. To make any reasonable assessment of the EDZ evolution, the above models need to be supplied with proper input data. If the models are properly calibrated and validated after excavation, then we can probably make a reasonable estimate of how the EDZ will progress during the heating period after emplacement. Although several promising attempts were made to model coupled THMC processes in this study, including chemically mediated time-dependent processes, fully coupled THMC models are still lacking, and more experimental data to support such models are needed