Numerical study of the THM effects on the near-field safety of a hypothetical nuclear waste repository—BMT1 of the DECOVALEX III project. Part 2: Effects of THM coupling in continuous and homogeneous rocks

An evaluation of the importance of the thermo-hydro-mechanical couplings (THM) on the performance assessment of a deep underground radioactive waste repository has been made as a part of the international DECOVALEX III project. It is a numerical study that simulates a generic repository configuration in the near field in a continuous and homogeneous hard rock. A periodic repository configuration comprises a single vertical borehole, containing a canister surrounded by an over-pack and a bentonite layer, and the backfilled upper portion of the gallery. The thermo-hydro-mechanical evolution of the whole configuration is simulated over a period of 100 years. The importance of the rock mass's intrinsic permeability has been investigated through scoping calculations with three values: 10⁻¹⁷, 10⁻¹⁸ and 10⁻¹⁹ m². Comparison of the results predicted by fully coupled THM analysis as well as partially coupled TH, TM and HM analyses, in terms of several predefined indicators of importance for performance assessment, enables us to identify the effects of the different combinations of couplings, which play a crucial role with respect to safety issues. The results demonstrate that temperature is hardly affected by the couplings. In contrast, the influence of the couplings on the mechanical stresses is considerable.     

核废料处置库近场热-水-力耦合性状

采用高放射性核废料处置库模型试验,以核废料处置库近场的膨润土及岩石为研究对象,建立轴对称模型,选用适当的热、水、力本构方程,运用有限元软件code-bright对核废料处置库关闭后处置库近场的温度场、渗流场、应力场进行考虑TH, HM, TM部分耦合与THM完全耦合的数值模拟分析。得到处置库关闭后近场膨润土及岩石内温度、液体饱和度、应力的变化规律及不同耦合对这些性状影响的敏感度。结果可为核废料处置库的规划、设计以及数值分析时耦合类型的选择提供参考。     

黏土岩温度–渗流–应力耦合作用的长期力学特性研究若干进展

核废料地质处置库长期处于复杂的耦合环境之中,其安全、稳定、高效的建设与运营需要考虑温度场(T)、渗流场(H)、应力场(M)的长期耦合作用。围绕黏土岩多场耦合作用下的长期力学特性,首先总结了国内外在黏土岩温度–渗流–应力耦合条件下长期力学试验研究成果与不足,主要包括HM与THM耦合长期作用对黏土岩渗透特性的影响、黏土岩TM与THM耦合长期力学特性。其次从微观唯相的角度,论述了温度对黏土岩微观结构特征的影响机制。基于上述认识,提出黏土岩THM耦合蠕变力学模型及其适用性。同时,介绍了笔者团队围绕黏土岩THM耦合长期力学特性方面的相关研究成果,结果表明:BoomClay的长期力学特性与温度、孔压、应力和各向异性等因素相关。最后,提出黏土岩THM耦合研究未来应重点关注的方向。     

Double structure THM analyses of a heating test in a fractured tuff incorporating intrinsic permeability variations

This paper presents thermo-hydro-mechanical (THM) analyses simulating the Drift Scale Test (DST) performed at Yucca Mountain. A double structure approach based on two superimposed domains is adopted. Intrinsic permeability changes with deformations imply full THM coupling. Temperatures and gas permeabilities were measured during 4 years and are used to validate the model. Measured gas permeability variations show patterns that are successfully explained by the model calculations. These gas permeability variations may be attributed to thermo-hydraulic effects, and also to mechanical effects. Different cases of intrinsic permeability variations have been considered in the model and their influence on the calculated temperatures, degree of saturations and gas permeabilities are presented. Volumetric deformation, in contraction or dilatancy, implies changes in the aperture of rock fractures that in turn lead to changes in intrinsic permeability. Dilatancy, caused by shear stresses, increases intrinsic permeability. Consideration of this factor contributes significantly to improve the agreement of calculated gas permeability with the measured values obtained during the DST experiment.     

Thermo-hydro-mechanical behavior of clay rock for deep geological disposal of high-level radioactive waste

In the context of deep geological disposal of radioactive waste in clay formations, the thermo-hydromechanical(THM) behavior of the indurated Callovo-Oxfordian and Opalinus clay rocks has been extensively investigated in our laboratory under repository relevant conditions:(1) rock stress covering the range from the lithostatic state to redistributed levels after excavation;(2) variation of the humidity in the openings due to ventilation as well as hydraulic drained and undrained boundary conditions;(3)gas generation from corrosion of metallic components within repositories; and(4) thermal loading from high-level radioactive waste up to the designed maximum temperature of 90℃ and even beyond to150℃. Various important aspects concerning the long-term barrier functions of the clay host rocks have been studied:(1) fundamental concept for effective stress in the porous clay-water system;(2) stressdriven deformation and damage as well as resulting permeability changes;(3) moisture influences on mechanical properties;(4) self-sealing of fractures under mechanical load and swelling/slaking of clay minerals upon water uptake;(5) gas migration in fractured and resealed claystones; and(6) thermal impact on the hydro-mechanical behavior and properties. Major findings from the investigations are summarized in this paper.     

Coupled T–H–M issues relating to radioactive waste repository design and performance

In this paper, coupled thermo-hydro-mechanical (THM) issues relating to nuclear waste repository design and performance are reviewed. Concise statements, that were developed from DECOVALEX discussions, on the current state-of-knowledge are presented. Section 1 describes the THM background and the interface with performance assessment (PA). The role of THM issues in the overall repository design context is amplified in Section 2, which includes a review of the processes in terms of repository excavation, operation and post-closure stages. It is important to understand the overall context, the detailed THM issues, the associated modelling and how these issues will be resolved in the wider framework. Also, because uncoupled and coupled numerical codes have been used for this subject, there is discussion in Section 3 on the nature of the codes and how the content of the codes can be audited. To what extent does a particular code capture the essence of the problem in hand? Consideration is also given to the associated question of code selection and the future of numerical codes. The state-of-knowledge statements are presented in Section 4 under 11 headings which follow the repository design sequence. The overview conclusion is that “A predictive THM capability is required to support repository design because precedent practice information is insufficient. Many aspects of THM processes and modelling are now well understood and there is a variety of numerical codes available to provide solutions for different host rock and repository conditions. However, modelling all the THM mechanisms in space and time is extremely complex and simplifications will have to be made — if only because it is not possible to obtain all the necessary detailed supporting information. Therefore, an important step is to clarify the THM modelling requirement within the PA context. This will help to indicate the complexity of THM modelling required and hence the models, mechanisms, type of computing, supporting data, laboratory and in situ testing, etc. required. An associated transparent and open audit trail should be developed.” We also include comments from reviewers and highlight four outstanding issues which are currently being studied in the DECOVALEX III programme.     

比利时高放废物处置库设计及与基岩和工程屏障体系的热-水-力性状的相关研究

在比利时，泥岩中地质处置是高放废物最终处置的首选，处置库在高放废物与生物圈之间的多重屏障基础上设计的，而Boom泥岩作为基岩的研究已有20多年历史，1980年比利时做出重大决定，建立名为HADES的地下研究机构，以研究Boom泥岩在地下223m处的力学性质，并调查和论证处置的可行性，为处置库屏障（天然和人工）提供可靠数据，在HADES的众多现场试验中很多试验用来对基岩和工程屏障体系（包括封口和回填的可行性）热-水-力性状进行研究，包括CACTUS，ATLAS，BACCHUS和RESEAL等项目。自1995年以来，研究开发计划向大型和示范性试验方向发展。最主要成果是运用工业技术建立地下研究设施（竖井和井巷）可行性得到了验证，且这种工业技术给研究提供一个较好机会，便于进一步认识基岩泥岩（CLIPEX方案）的水-力性状及了解隧道开挖工程（SELFRAC课题）对挖掘破坏区的影响，另一个重大成果是成功地实现对一种称为“OPHELE”的预掉膨润土（人工屏障材料）加热和水化地面大型试验。下一步工作内容包括实现大尺寸现场加热器试验（PRACLAY试验），此试验预计于2006年开始，并可持续10a之久，据此，首先简要描述比利时高效废物处置库设计，然后回顾Boom泥岩和工程屏障体系的热-水-力性状相关试验，最后介绍下一步大规模PRACLAY试验。     

Numerical analysis of THM coupling of a deeply buried roadway passing through composite strata and dense faults in a coal mine

Because of the reduction in shallow-buried coal resources, increasingly coal mines in China extend to deep resources. The coupling of thermal-hydro–mechanical (THM) fields is much more complicated in deep strata, especially when the roadway passes through composite strata and dense faults. High ground pressures, high ground temperature and high seepage pressure are most important features of deep strata. In order to investigate the THM coupling mechanism of deeply buried roadways, this article presents the geological features of deep strata and analyzes the interactions among thermal, hydraulic and mechanical fields considering the water-weakening effect of the surrounding rock. Finally, the THM coupling process of a deeply buried roadway passing through composite strata and dense faults is simulated. The deformation characteristics and distribution of the stress field are described in the results.     

Thermally induced mechanical and permeability changes around a nuclear waste repository—a far-field study based on equivalent properties determined by a discrete approach

A numerical investigation is conducted on the impacts of the thermal loading history on the evolution of mechanical response and permeability field of a fractured rock mass containing a hypothetical nuclear waste repository. The geological data are extracted from the site investigation results at Sellafield, England.A combined methodology of discrete and continuum approaches is presented. The results of a series of simulations based on the DFN–DEM (discrete fracture network–distinct element method) approach provide the mechanical and hydraulic properties of fractured rock masses, and their stress-dependencies. These properties are calculated on a representative scale that depends on fracture network characteristics and constitutive models of intact rock and fractures. In the present study, data indicate that the large scale domain can be divided into four regions with different property sets corresponding to the depth. The results derived by the DFN–DEM approach are then passed on to a large-scale analysis of the far-field problem for the equivalent continuum analysis.The large-scale far-field analysis is conducted using a FEM code, ROCMAS for coupled thermo-mechanical process. The results show that the thermal stresses of fractured rock masses vary significantly with mechanical properties determined at the representative scale. Vertical heaving and horizontal tensile displacement are observed above the repository. Observed stress and displacement fields also shows significant dependency on how the mechanical properties are characterized. The permeability changes induced by the thermal loading show that it generally decreases close to the repository. However, change of permeability is small, i.e., a factor of two, and thermally induced dilation of fracture was not observed. Note that the repository excavation effects were not considered in the study.The work presented in this paper is the result of efforts on a benchmark test (BMT2) within the international co-operative projects DECOVALEX III and BENCHPAR.     

岩石物理力学性质对断裂附近地应力场的影响

应用离散元方法模拟研究了断裂两侧岩石的物理力学性质(包括岩石内摩擦角、弹性模量、泊松比、粘聚力等)对其附近地应力场的影响。这些因素对断裂附近的地应力场均有不同程度的影响，它们的改变均能使主应力方位和量值发生变化。而且，岩石物理力学性质的影响与断裂力学性质、边界条件等的影响是互相联系的。分别研究了单一断裂和复合断裂模型，在复合断裂模型中，断裂两侧岩石的弹性模量对应力变化的影响最大。     

Coupled analysis of flow, stress and damage (FSD) in rock failure

Rock is a heterogeneous geological material that contains natural weakness of various scales. When rock is subjected to mechanical loading, these pre-existing weaknesses can close, open, grow or induce new fractures, which can in turn change the structure of the rock and alter its fluid flow properties. Experimental results provide strong evidence that rock permeability is not a constant, but a function of stresses and stress-induced damage. A flow-stress-damage (FSD) coupling model for heterogeneous rocks that takes into account the growth of existing fractures and the formation of new fractures is proposed herein. Implemented with the Rock Failure Process Analysis code (F-RFPA^2D), this FSD model is used to investigate the behaviour of fluid flow and damage evolution, and their coupling action, in samples that are subjected to both hydraulic and biaxial compressive loadings. The modeling results suggest that the nature of fluid flow in rocks varies from material to material, and strongly depends upon the heterogeneity of the rocks.     

真三轴条件下的岩石细观损伤力学模型

本文提出了真三轴条件下的岩石细观损伤力学模型，建立了岩石的损伤演化方程，给出了损伤柔度的求解公式。数值分析表明中主应力对岩石应力－应变关系有明显的影响，一般表现为随中主应力的增加，最大主应力方向的变形减小，最小主应力方向的变形增大；但当最大主应力很大时，大的中主应力反而使最大主应力方向的变形增加，最小主应力方向的变形减小。     

A comparative study of stress influence on fracture apertures in fragmented rocks

This study compares the calculated fracture apertures in a fragmented rock layer under different stress scenarios using two different approaches. Approach 1 is a simplified method using a two-dimensional (2D) mapping of the fracture network and projects the far-field stresses to individual fractures, and calculates the dilation, normal and shear displacements using experimental stiffnesses available in the literature. Approach 2 employs a three-dimensional (3D) finite element method (FEM) for the mechanical analysis of the fragmented rock layer considering the interaction with the neighbouring rock layers, frictional interfaces between the rock blocks, stress variations within the fragmented rock layer, and displacements, rotations and deformations of rock blocks. After calculating the fracture apertures using either of the approaches, the permeability of the fragmented rock layer is calculated by running flow simulations using the updated fracture apertures. The comparison between the results demonstrates an example of the inaccuracies that may exist in methods that use simplified assumptions such as 2D modelling, ignoring the block rotations and displacements, projected far-field stresses on fractures, and the stress variations within the rock layer. It is found that for the cases considered here, the permeability results based on apertures obtained from the simplified approach could be 40 times different from the results from apertures calculated using a full mechanical approach. Hence, 3D mechanical modelling implementing realistic boundary conditions, while considering the displacements and rotations of rock blocks, is suggested for the calculation of apertures in fragmented rocks.     

Simulations of THM processes in buffer-rock barriers of high-level waste disposal in an argillaceous formation

The main objective of this paper is to investigate and analyse the thermo-hydro-mechanical (THM) coupling phenomena and their influences on the repository safety. In this paper, the high-level waste (HLW) disposal concept in drifts in clay formation with backfilled bentonite buffer is represented numerically using the CODE_BRIGHT developed by the Technical University of Catalonia in Barcelona. The parameters of clay and bentonite used in the simulation are determined by laboratory and in situ experiments. The calculation results are presented to show the hydro-mechanical (HM) processes during the operation phase and the THM processes in the after-closure phase. According to the simulation results, the most probable critical processes for the disposal project have been represented and analyzed. The work also provides an input for additional development regarding the design, assessment and validation of the HLW disposal concept.     

核废料处置库近场岩石热应力半解析方法研究

针对地下核废物处置库近场环境为饱和裂隙岩体时的情况,提出了一种由分布热源、饱和稀疏裂隙和岩石构成的简化概念模型,根据Goodier提出的热弹性位移势,采用拉普拉斯变换、格林函数法和数值积分等方法,计算求解概念模型中处置库近场岩石的温变热应力。并以实际算例分析分布热源作用下稀疏裂隙岩体受三维水流-传热过程影响的热应力特征以及裂隙水流速和分布热源间距对裂隙岩体热应力分布的影响。结果表明,裂隙水的流动传热作用将处置库近场岩石的热量向下游传递,从而降低了近场岩石的温度,减小了近场岩石中的热应力,裂隙水流速越快,对近场岩石中热应力的影响越明显;热源间距越小时,处置库近场岩石的热应力越大;当热源间距小于一定值时,不同热源间传热作用的叠加将使处置库近场岩石的热应力峰值急剧增大。     

Modification of thermo-elasto-viscoplastic model for soft rock and its application to THM analysis of heating tests

In deep geological disposal for high level radioactive waste, one of the most important factors is to study the thermo-hydraulic-mechanical (THM) behavior of the natural barrier, usually a host rock, during the heat process and hydraulic environment change. In this paper, a thermo-elasto-viscoplastic model proposed by Zhang and Zhang (2009) is firstly modified by adopting the t_ij concept (Nakai and Mihara, 1984) so that the influence of intermediate stress can be properly considered. The performance of the modified model is confirmed with drained triaxial compression tests and creep tests on soft sedimentary rock and manmade soft rock under different temperatures. Based on the modified model, a program called ‘SOFT’, using the finite element method (FEM) and the finite difference method (FDM) in the soil–water–heat coupling problem, has been developed to simulate the aforementioned THM behavior of geological materials. In order to verify the applicability of the program, an isotropic element heating test for soft rock under drained condition with different initial overconsolidation ratio (OCR), is firstly simulated by the proposed THM FE-FD program. In the test, the thermal volumetric change was found to be dependent on OCR. The simulated results show that the THM phenomenon observed in the laboratory test can be explained by the proposed numerical method. Meanwhile, a real-scale field heating test reported by Gens et al. (2007) is also simulated by the same THM FE-FD program. The material parameters of the rock involved in the constitutive model are determined based on the element tests for the rock in the laboratory. It is shown that the simulated results agreed well with the test results such as the time change of the temperature, the excess pore water pressure (EPWP) and the heat-induced strain.     

Sealing of fractures in claystone

The sealing behavior of fractures in clay rocks for deep disposal of radioactive waste has been comprehensively investigated at the GRS laboratory. Various sealing experiments were performed on strongly cracked samples of different sizes from the Callovo-Oxfordian argillite and the Opalinus clay under relevant repository conditions. The fractured samples were compacted and flowed through with gas or synthetic pore-water under confining stresses up to 18 MPa and elevated temperatures from 20 °C to 90 °C. Sealing of fractures was quantified by measurements of their closure and permeability. Under the applied thermo-hydro-mechanical (THM) conditions, significant fracture closure and permeability decrease to very low levels of 10^?19 to 10^?21 m² were observed within time periods of months to years. The properties of the resealed claystones are comparable with those of the intact rock mass. All test results suggest high sealing potentials of the studied claystones.     

A discussion of thermo–hydro–mechanical (THM) processes associated with nuclear waste repositories

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.     

Experiments on thermo-hydro-mechanical behaviour of Opalinus Clay at Mont Terri rock laboratory,Switzerland

Repositories for deep geological disposal of radioactive waste rely on multi-barrier systems to isolate waste from the biosphere.A multi-barrier system typically comprises the natural geological barrier provided by the repository host rock e in our case the Opalinus Clay e and an engineered barrier system(EBS).The Swiss repository concept for spent fuel and vitrified high-level waste(HLW)consists of waste canisters,which are emplaced horizontally in the middle of an emplacement gallery and are separated from the gallery wall by granular backfill material(GBM).We describe here a selection of five in-situ experiments where characteristic hydro-mechanical(HM)and thermo-hydro-mechanical(THM)processes have been observed.The first example is a coupled HM and mine-by test where the evolution of the excavation damaged zone(EDZ)was monitored around a gallery in the Opalinus Clay(ED-B experiment).Measurements of pore-water pressures and convergences due to stress redistribution during excavation highlighted the HM behaviour.The same measurements were subsequently carried out in a heater test(HE-D)where we were able to characterise the Opalinus Clay in terms of its THM behaviour.These yielded detailed data to better understand the THM behaviours of the granular backfill and the natural host rock.For a presentation of the Swiss concept for HLW storage,we designed three demonstration experiments that were subsequently implemented in the Mont Terri rock laboratory:(1)the engineered barrier(EB)experiment,(2)the in-situ heater test on key-THM processes and parameters(HE-E)experiment,and(3)the full-scale emplacement(FE)experiment.The first demonstration experiment has been dismantled,but the last two ones are on-going.