膨润土在核废料处置环境中的膨胀衰减规律研究

利用Phreeqc软件模拟了核废料处置库中高压实膨润土中发生的化学反应,建立了K+、Ca2+、Na+、Mg2+ 4种阳离子的离子交换和矿物溶解沉淀的一维运移反应模型。模拟了核废料处置环境中,膨润土化学组分在不同流体环境下随时间和空间的变化规律,验证了膨胀性的衰减主要是由Na基蒙脱石转变为Ca基蒙脱石引起。结合膨润土化学组分变化结果,对膨润土膨胀性衰减程度进行了评估。得到1万年后海水和地下水环境下的高庙子膨润土膨胀性衰减系数分别达到0.83和0.92,MX80膨润土达到0.79和0.90。     

太和地区膨润土的成分特征及开发应用

湖北太和地区膨润土中蒙脱石成分的测试和加工特性试验研究,认为该区蒙脱石属八面体内富镁低铁的切托型,这种膨润土适合于制活性白土、制备干燥剂的开发利用,也适合于制备无机凝胶和以极性小分子或无机阳离子作为改性试剂。     

Hydromechanical Behaviour of Rock-Bentonite Interfaces Under Compression

Summary Interfaces between geomaterials may be critical for the long term confinement of the engineered barriers of nuclear waste disposals, particularly if there is water flow. Hydromechanical compression tests have been performed on rock-bentonite interfaces representing the contact between a host rock (toarcian argillite) and an engineered barrier within a nuclear waste repository. The results show that there is no major influence of the bentonite fraction or the nature of the additive as long as the additive is inert (sand or crushed rock): all the interfaces are closed for low values of normal stress (about 4 MPa). On the other hand, the hydromechanical behaviour of the interfaces changes when a high fraction of cement is used. Moreover, it has been shown that bentonite is very sensitive to hydraulic erosion, producing flow channels within the interface zone. A numerical study confirms the importance of erosion for the hydromechanical behaviour of the interface. Authors’ address: Olivier Buzzi, Laboratoire Sols, Solides, Structures, Université Joseph Fourier, BP 53, 38041 Grenoble, Cedex 9, France     

膨润土膨胀变形的分形模型

Xu等[1] 提出基于膨润土表面分形模型的膨胀变形计算方法 Vw／Vm=kPDs-3(式1),并得到了Tsukinuno膨润土、Wyoming膨润土和MX-80膨润土的膨胀变形和膨胀压力(统称膨胀变形)的膨胀试验数据的验证。当时由于条件限制,存在两个问题：(1)膨润土的表面分维一般采用氮吸附法测量,由膨胀变形试验数据反算得到的膨润土表面分维没有得到氮吸附试验数据的验证; (2)在归一化吸水体积与压力的分形模型(式1)中,K是膨润土的物性常数,没有给出理论表达式,只是经验系数。本文采用氮吸附方法测量了高庙子Na基膨润土、高庙子Ca基膨润土、商用膨润土和Tsukinuno膨润土的表面分维,根据式(1)由表面分维计算膨胀变形,并与膨胀试验数据比较,验证膨胀变形的分形模型; 根据双电层理论,导出用双电层参数表示式(1)中K值的理论表达式,用于膨胀变形的计算,并与膨胀变形试验结果进行比较验证。     

Na and HTO diffusion in compacted bentonite: Effect of surface chemistry and related texture

In underground repository concepts for radioactive waste, bentonite is studied as a reference swelling material to be used as an engineered barrier. Under the changing geochemical conditions prevailing within the barrier (saturation with the fluid coming from the host formation, diffusion of various chemical plumes caused by the degradation of some constituents of the barrier-system, etc.), the surface chemistry of the clay particles could evolve. This work aims to characterize the effects of these changes on (i) the microstructure of compacted bentonite samples and (ii) the diffusion properties of HTO and Na in these samples.For this purpose, bentonite sets were equilibrated with different solutions: NaCl, CaCl₂, CsCl solutions as well as an artificial clayey porewater solution. The microstructure of the different samples was characterized by HRTEM and XRD, in a water saturated state. In parallel, effective diffusion coefficients of both HTO and ²²Na were measured for the different samples. The density of the bentonite in the diffusion tests and in the HRTEM observations was set at 1.6 Mg m⁻³.From the microstructural observations and the results of diffusion tests, it is deduced that one key parameter is the occurrence of a gel phase in the material, which is found to depend strongly on the bentonite set: the gel phase dominates in Na-bentonite, while it is lacking in Cs-bentonite. The HTO diffusion coefficients are found to be lower in the samples with high gel phase content. Sodium diffusion does not follow the same trend: when compared with HTO, Na diffuses faster when the gel phase content is high. The latter result could indicate that the “accelerated diffusion mechanism” of cations, already mentioned in the literature, is enhanced in clayey materials that contain a gel phase.     

Modeling coupled THM processes of geological porous media with multiphase flow: Theory and validation against laboratory and field scale experiments

A FEM model for analysis of fully coupled multiphase flow, thermal transport and stress/deformation in geological porous media was developed based on the momentum, mass and energy conservation laws of the continuum mechanics and the averaging approach of the mixture theory over a three phase (solid–liquid–gas) system. Six processes (i.e. stress–strain, water flow, gas flow, vapor flow, heat transport and porosity evolution processes) and their coupling effects are considered, which not only makes the problem well-defined, but renders the governing PDEs closed, complete, compact and compatible. Displacements, pore water pressure, pore gas pressure, pore vapor pressure, temperature and porosity are selected as basic unknowns. The physical phenomena such as phase transition, gas solubility in liquid, thermo-osmosis, moisture transfer and moisture swelling are modeled. As a result, the relative humidity and other related variables in porous media can be evaluated on a sounder physical basis. A three dimensional computer code, THYME3D, was developed, with eight degrees of freedom at each node. The laboratory CEA Mock-up test and the field scale FEBEX benchmark test on bentonite performance assessment for underground nuclear waste repositories were used to validate the numerical model and the software. The coupled THM behaviors of the bentonite barriers were satisfactorily simulated, and the effects and impacts of the governing equations, constitutive relations and property parameters on the coupled THM processes were understood in terms of more straightforward interpretation of physical processes at microscopic scale of the porous media. The work developed enables further in-depth research on fully coupled THM or THMC processes in porous media.     

Effect of salt of various concentrations on liquid limit,and hydraulic conductivity of different soil-bentonite mixtures

Effect of the various concentrations of NaCl and CaCl₂ on the four different soil-bentonite mixtures has been evaluated. The results show that the liquid limit of the mixtures decreases with an increase in the salt concentration. Liquid limit decreased significantly with an increase in CaCl₂ concentration from 0 to 0.1 N. However, a further increase in the concentration did not produce any significant decrease in liquid limit. A quite opposite trend was observed for the NaCl solution. An increase in NaCl concentration from 0 to 0.1 N did not produce any major decrease in the liquid limit, but a further increase in concentration from 0.1 to 1 N decreased the liquid limit significantly. Consolidation tests were carried out on the mixtures to evaluate the effect of mineralogical composition of the bentonite on the hydraulic conductivity (k) of the mixture in the presence of various salts concentrations. The k for any mixtures was found to be decreasing with decrease in the salt concentration. At relatively low concentration, Ca²⁺ had more effect on the k in comparison to the same concentration of Na⁺. However, at 1 N of NaCl and CaCl₂ almost an equal value of k was observed. A comparison of the performance of four bentonites showed that the mixture with bentonite having highest exchangeable sodium percentage (ESP) exhibited the lowest k when permeated with de-ionized (DI) water, however, k increased with an increase in the salt concentration. Similarly, mixture with a bentonite of lower ESP exhibited a higher k with DI water but with the increase in the salt concentration alteration in the k, compared to all other mixtures, was relatively less.     

膨润土深加工的研究进展

由于膨润土具优良的物化性能、丰富的资源、低廉的价格,使得国内外有关科技人员对其深加工的方法和技术进行了大量的研究。在膨润土的提纯、钙基膨润土的钠化和活性白土的制备、各种有机膨润土的制备、膨润土凝胶的制备、用膨润土制备白炭黑等各种膨润土深加工的方法和技术,以及目前国际上研究的热点:柱撑粘土材料的制备和有机-无机纳米复合材料的制备等方面都有一定的进展。柱撑粘土是一种具有很高催化裂化活性的类分子筛新型固体酸催化材料,也是一种优良的新型环保材料。有机-无机纳米复合材料（纳米粘土）既具有粘土矿物的优良强度和尺寸稳定性,又具有聚合物断裂性能、可加工性和介电性能。膨润土的深加工技术开辟了非金属矿综合利用的新途径。     

Parametric studies on confinement of radionuclides in the excavated damaged zone due to bentonite type and temperature change

A parametric study is presented in this paper in order to examine the potential of the extruding bentonite into a fracture in the EDZ to confine radionuclides. Radionuclide migration of cesium and neptunium were studied at elevated temperatures and for a sodium- and calcium-type bentonite. Parameter values were obtained based on empirical studies for hydraulic conductivity, molecular diffusion, and sorption. Results indicate extrusion speed is affected by temperature changes. Elevated temperatures also affect radionuclide migration. For Cs, migration is enhanced due to decreasing sorption, while Np migration is inhibited due to increasing sorption. The potential to confine radionuclides is favorable, and the choice of bentonite does not seem to affect radionuclide confinement in the extruding region.