Hydro-thermal performance of unsaturated bentonite-sand buffer material

Several series of one-dimensional heat and moisture flow tests were performed to examine the moisture and temperature distributions in the buffer material compacted to a dry density of 1.67 Mg m⁻³ and water content of 17.7%. In all tests, water was allowed to infiltrate into a horizontal soil column from one end under a constant hydrostatic head of 276 kPa. Also the specimens were heated from the other end by the heater to a constant temperature.

It is experimentally demonstrated that the moisture moves from both ends toward the mid part of the soil column due to both thermal gradient from one end and hydraulic gradient from the other end. It was observed that, in spite of no overall volume change, local volume change occurs in the system. The measured temperatures along the length of the specimen indicate that temperature distributions stabilize within a short period of time. The time required for the temperature to stabilize decreases as the heater skin temperature increases.

The diffusivity parameters are calculated using the measured moisture and temperature profiles combined with the finite difference method. Powell's optimization algorithm was used to determine the material parameters. Good agreements between experimentally measured and calibrated volumetric water content shows that the diffusion parameters can be expressed in a linear function of the volumetric water content and temperature.     

膨润土-砂的膨胀特性与蒙脱石质量比率

在高放废物(HLW)地质处置的工程屏障体系中,使用以膨润土为主料的混合型缓冲回填材料,是阻截放射性核素向地下水环境迁移的最主要的包封设施。膨润土中添加一定量的石英砂,既能优化回填设计与施工性能,又能满足对放射性核素的有效拦截。本文综合分析了初始含水量、干密度、膨润土含量、压力等对压实膨润土-砂混合物膨胀性能指标的影响,提出"蒙脱石的质量比率"这一指标,将干密度和膨润土含量归一为蒙脱石质量比率指标,得出混合材料膨胀力随蒙脱石质量比率增长呈指数增长的规律,从而可以实现对膨胀力的定量预测。     

高效废物处置库的混合型缓冲回填材料压缩特性研究

高放废物处置库中缓冲回填材料需要预先压缩到一定密实度,其压缩特性对处置库的安全设计、施工及运营有重要影响。目前,国内初步考虑将膨润土-砂混合物作为缓冲回填材料的首选。采用WG型单杠杆固结仪,对不同干密度、掺砂率的膨润土-砂混合物进行侧限压缩试验研究。试验结果表明：随着干密度的增大,压缩系数呈二次曲线减小;随着掺砂率的增大,压缩系数呈非线性增大。通过引入有效黏土密度和有效含水率的概念,对不同干密度、掺砂率的膨润土-砂混合物的压缩系数进行解释,并根据试验数据得到了回归的数学模型,试图为缓冲回填材料的配比优化研究提供相关依据。     

混合型缓冲回填材料膨胀变形试验研究

选用高庙子钠基膨润土(GMZ001)为缓冲回填材料的主料,添加不同比例的石英砂,对掺砂率分别为0%、10%、20%、30%、40%和50%的膨润土-砂混合物压实试样进行室内膨胀变形试验。结果表明：膨胀率随时间呈指数增长;掺砂率一定时,最大膨胀率随初始干密度的增大而线性增长;初始干密度一定时,最大膨胀率与掺砂率呈二次函数关系。引入有效黏土密度的概念对最大膨胀率进行计算,建立了任意掺砂率和初始干密度条件下膨润土-砂混合物最大膨胀率归一化模型,为混合型缓冲回填材料在高放废物地质处置中膨胀行为的预测与控制提供了依据。     

Coupled solution of heat and moisture flow in unsaturated clay barriers in a repository geometry

An analytical solution in the Laplace transform domain is obtained for the transient heat and moisture transport in an unsaturated clay buffer with a geometry simulating repository conditions. A numerical inversion scheme based on Crump's method is used to obtain the time‐domain solution. The coupled effect of thermally driven moisture transport is especially investigated because of its importance to alter the flow field in low‐permeability buffers. The practical background is based on the case of an engineering bentonite barrier placed in a drift excavated in rock in the context of underground disposal of high‐level radioactive waste. Parametric study has been performed to assess the effects of dimensionless geometry and material parameters on flow field. Despite the simplified assumptions required in order to obtain analytical expressions, the results incorporate the main mechanisms involved in the coupled thermo‐hydraulic (T–H) problem, and they may be eventually used for validation purposes. Copyright © 2006 John Wiley & Sons, Ltd.     

混合型缓冲回填材料非饱和水分扩散试验研究

设计了专门的浸水试验装置,测定了膨润土-石英砂混合物在非膨胀条件下吸水引起的水分分布及膨胀应力变化,研究了混合型缓冲回填材料非饱和水力学性质。试验结果表明,干密度约1.70 g/cmP3、掺砂率为30%的GMZ001膨润土-砂混合物的水分扩散系数与含水率的关系呈U型变化,即随着含水率增加,汽态水分扩散系数减小,液态水分扩散系数增大。根据理论推导建立非饱和导水率估算方程。计算发现,混合物的非饱和导水率随含水率的增大而增大。其中,汽态水分非饱和导水率先增大后略有减小,液态水非饱和导水率始终增大。试验发现,试样吸水后干密度趋于均匀化,试样内部不同位置的应力发展与水分迁移过程密切相关：渗入端的应力在入渗初期增长很快,随后减慢;渗出端的应力持续稳定增长;入渗96 h后,两端应力趋于一致。估算出的水分扩散系数及非饱和导水率,可用于评价混合型缓冲回填材料阻隔核素迁移安全性。     

A model for moisture and heat flow in fractured unsaturated porous media

The present study investigates propagation of a cohesive crack in non‐isothermal unsaturated porous medium under mode I conditions. Basic points of skeleton deformation, moisture, and heat transfer for unsaturated porous medium are presented. Boundary conditions on the crack surface that consist of mechanical interaction of the crack and the porous medium, water, and heat flows through the crack are taken into consideration. For spatial discretization, the extended finite element method is used. This method uses enriched shape functions in addition to ordinary shape functions for approximation of displacement, pressure, and temperature fields. The Heaviside step function and the distance function are exploited as enrichment functions for representing the crack surfaces displacement and the discontinuous vertical gradients of the pressure and temperature fields along the crack, respectively. For temporal discretization, backward finite difference scheme is applied. Problems solved from the literature show the validity of the model as well as the dependency of structural response on the material properties and loading. Copyright © 2016 John Wiley & Sons, Ltd.     

Heat-induced moisture movement within a clay-based sealing material

The bentonite-sand buffer material, which isolates the nuclear-fuel waste containers from the host rock mass, is a vital component of the multiple engineered barrier scheme advocated by the Canadian Nuclear Fuel Waste Management Program. This paper presents results of laboratory investigations which examine the influence of the container surface temperatures on the moisture migration within a compacted buffer in a borehole environment.     

An analytical technique for evaluation of coupled heat and mass flow coefficients in unsaturated soil

This study was done to evaluate a phenomenological approach for determining the coupled heat and water diffusion parameters. This technique requires the use of both theory and experimental data. The transport equation was solved analytically using nondimensional analysis. The calculated diffusion parameters of a compacted sand–bentonite-based material are presented. The agreement between the back calculated values and the experimental volumetric water content profiles was good. Practical application of the technique is also discussed.     

Coupled thermo-hydro-mechanical modelling of bentonite buffer material

Mechanistic model development and numerical analyses were carried out on coupled thermo-hydraulic-mechanical processes in bentonite-based buffer material for the geological disposal of high-level radioactive waste with small-scale laboratory experiments and a full-scale test. The mechanism of water movement in compacted bentonite was identified by applying theoretical equations to the experimental results. The application clearly explained the observed results of the temperature dependence of the hydraulic conductivity in the saturated state and the water diffusivity in the unsaturated state for the compacted bentonite and the dry density dependence of the diffusivity. The full-scale coupled test, BIG-BEN, was conducted at PNC (Power Reactor and Nuclear Fuel Development Corporation) Tokai Works. The results of the numerical analyses for the full-scale test which are based on the present knowledge of coupled processes and our small-scale experiments were in good agreement with the measured results except for mechanical phenomena. Copyright © 1999 John Wiley & Sons, Ltd.     

石墨−膨润土缓冲材料的最优配置方法

核废料封存到处置库后还会继续释放衰变热,需要加快热量向周边围岩消散,确保处置库处于安全运行状态。而提高缓冲层的导热性能成为解决该问题的突破口。将天然石墨粉掺入到钠基膨润土中,以期配置成导热速度快、隔离能力强的缓冲层回填材料。通过开展石墨?膨润土混合物的自由膨胀率、恒体积膨胀力、饱和渗透系数和导热系数等试验,研究石墨掺入率（Rg分别为5%、10%、15%、20%、30%、40%）和不同粒径（297、150、74、44 μm）对其水?力?热性能的影响规律。结果表明,掺入石墨后显著提高了膨润土的导热性能,但其提升幅度受石墨掺入率、初始含水率和初始干密度等因素影响。综合分析石墨?膨润土混合物的水?力?热性能参数,发现最优石墨掺入率处于15%～20%（质量比）范围内;相同石墨掺入率下石墨粒径为150 μm或74 μm时混合物的水?力性能最优。石墨?膨润土混合物压实后的孔隙分布显示,相同石墨掺入率下石墨粒径过大或者过小都易形成大孔隙。究其原因,天然鳞片状石墨呈扁平状,大部分膨润土颗（团）粒小于石墨,与石墨属于点?面接触方式。尤其是压实程度不高时,膨润土颗（团）粒和石墨接触面处存在大量的孔隙;而且石墨属于憎水性材料,对水分子的拖拽力弱,即使膨润土吸水膨胀后,水分也容易从石墨薄片表面处通过。     

混合型缓冲材料砌块渗透性及其各向异性研究

缓冲材料砌块具备良好的防渗性能,是保证高放废物处置库封闭性的关键。缓冲材料砌块经单轴加载挤压成型,块体尺寸大,测试与评价其渗透性能的均匀性和各向异性,对缓冲屏障设计具有重要价值。研究制备不同干密度的混合型缓冲材料砌块,定向切割获得砌块内部不同位置、不同方向的小试样,开展刚性壁和柔性壁渗透试验,综合评价了混合型缓冲砌块不同方向的水力传导系数。研究结果表明：砌块干密度越大,砌块的水力传导系数越低。所有砌块的水力传导系数均处于10?10 cm/s数量级,沿砌块z、?、r方向的水力传导系数基本相等,表明砌块渗透性具有良好的均匀性和各向同性,且满足高放缓冲材料防渗要求。两种试验方法获得的渗透结果差异较小,柔性壁渗透条件下砌块产生10%～20%的体积膨胀,导致其水力传导系数略大于刚性壁下的平行试样。引入CT试验,分析了渗透前后砌块的微观特性变化。试验证实,在饱和渗透之后,以CT值显示的砌块密度分布趋于均匀,砌块内部存在的微裂隙愈合消失。     

高放废物地质处置库预选缓冲材料压缩性能研究

缓冲材料是高放废物深地质处置多重屏障系统中非常重要的一道人工屏障，内蒙古高庙子钠基膨润土被确定为我国高放废物深地质处置缓冲材料的首选基料。通过研究该钠基膨润土（GMZ01）在不同含水量条件下的压实成型性、不同压实干密度和不同含水量压实样品的无侧限压缩性能，认为：含水量为15％的高庙子天然钠基膨润土压实成型性最好，压实干密度越大，无侧限抗压强度越大；在无侧限压缩实验时，含水量为15％的高密度压实膨润土样品比含水量为10％和20％的样品的抗压强度大，变形小。     

Heat-induced moisture transport in the vicinity of a spherical heat source

The present paper develops an analytical approach to the problem of heat-induced moisture movement in the vicinity of a spherical heat source embedded in an undeformable, moist porous solid of infinite extent. A transient-state distribution of temperature within the infinite medium is assumed to induce the moisture transport process. The numerical results, presented in the paper, illustrate the influence of the moisture transport characteristics on the time-dependent distribution of moisture within the porous medium. © 1998 John Wiley & Sons, Ltd.     

Green's functions for a fully coupled thermoporoelastic material

The Green's functions for a set of fully coupled linear equations governing thermal consolidation of a homogeneous isotropic material are presented. The Green's functions are evaluated for one, two and three spatial dimensions, and in the Laplace and real-time domains. The Green's functions are employed in a boundary integral analysis of a simple problem to demonstrate the influence of full coupling on the field quantities.     

Transient heat flow in a stratified medium

The equation of heat conduction is solved for a horizontally stratified medium initially at constant temperature and subject to a step-function surface temperature change. The problem is solved by Laplace transformation and by applying a concept of further stratifications of the medium into unitary layers of constant ratio of thickness to the square root of thermal diffusivity. The solution takes the form of an infinite sum of complementary error functions, with coefficients given by recursion relations, is suitable for numerical applications and offers an attractive alternative to harmonic and quasi transient approaches in calculating the penetration of transient surface temperature variations into a layered medium. Use of the theorem of superposition yields a general expression for an arbitrary surface temperature function. The utility of the theory is illustrated by modelling examples of palaeoclimatically induced subsurface temperature and heat flow perturbations.     

Modelling coupled heat and contaminant transport in groundwater

This paper presents the results of a research programme conducted on the geotechnical centrifuge at The University of Western Australia to investigate coupled heat and contaminant transport in the soil surrounding a buried waste source. The phenomena which govern heat and contaminant transport through porous media are discussed, the principles of geotechnical centrifuge modelling are outlined, and relevant scaling laws that govern the relationship between a centrifuge model and the prototype, with respect to the problem of coupled waste transport, are presented. A model test, simulating two-dimensional migration from a buried heat and contaminant source, is described, and the results from four model tests are presented. The experimental data show that hydraulic instability is responsible for the transport of contaminant in the soil around the source and that the mode of instability is determined by the magnitude of the effective Rayleigh number.     

Modelling transient heat and moisture transfer in unsaturated soil using a parallel computing approach

A parallel numerical model, employing a finite difference explicit scheme for the analysis of coupled heat and moisture transfer in unsaturated soil, is employed to simulate a laboratory experiment of heating of medium sand. The model, written in a two-dimensional polar co-ordinate formulation, is programmed in the concurrent language Occam and executed on a parallel computing network of transputers. Parallelization is adopted as a means of overcoming computing difficulties, which limited numerical solutions to those at steady state, to enable transient behaviour to be simulated. The parallel algorithm was found to be very efficient, enabling a full solution of transient behaviour to be obtained. An investigation of the ability of the model to accurately simulate the complex, interrelated coupled nature of both two-dimensional transient and steady-state behaviour yielded very good correlation between experimental and numerical results. It can therefore be concluded that overall the results obtained provide confidence in the validity of the approach proposed.     

温度对土壤水分运动基本参数的影响

为探讨土壤温度变化对土壤中水分运动过程和运动参数影响机理,在室内实验的基础上结合理论探讨,对不同温度条件下实验土壤的水分特征曲线、导水率、扩散率和比水容量等土壤水分运动基本参数的温度效应进行了研究分析.结果表明:土壤温度对土壤水分性质及土壤结构性质影响显著,二者共同作用使得土壤水分运动过程发生改变,且其影响效应可通过土...     

Three-dimensional heat,moisture and air transfer in unsaturated soils

A new three-dimensional numerical model of coupled heat, moisture and air transfer in unsaturated soil is presented. In particular, the model accommodates moisture transfer in the form of liquid and vapour flow and heat transfer arising from conduction, convection and latent heat of vaporization. The bulk flow of dry air and the movement of air in a dissolved state are also included. The theoretical basis of the model, the finite element solution of the spatial terms and finite difference solution of the temporal terms are briefly presented. Attention is focused on the verification of the new numerical solution. This is achieved via comparisons with independent solutions of heat, moisture and air transfer in an unsaturated soil. The physical problem considered includes the highly non-linear hydraulic properties of sand. Thermal conductivity is also included as a function of soil moisture content. Excellent correlation of results is shown thus providing confidence in the new model. The new model is also applied to a number of test cases which illustrate the need for the development of a model which can fully include three-dimensional behaviour. In particular, three applications are presented each increasing in complexity. The first application illustrates three-dimensional heat transfer. This particular application is verified against existing commercial finite element software. Subsequent applications serve to illustrate how the coupled processes of heat moisture and air transfer combine to yield three-dimensional problems even within a simple geometric domain. Visualization of three-dimensional results is also addressed. © 1998 by John Wiley & Sons, Ltd.