Pilot field experiment with canister-embedding clay under simulated repository conditions

Precompacted annular clay blocks surrounding a casing with a heater were emplaced in a borehole with 20 cm diameter and 2.8 m depth in granite. The heater was in operation for 8 months and raised the temperature of the clay to 176°C close to the casing. At the extraction the clay was completely water-saturated close to the rock and it had experienced an increased water content also close to the hot casing.

Mineralogical changes of the Ca-smectite clay were indicated by a slight increase in CEC and by an XRD-based conclusion that the content of low-charge smectite increased, while the initial content of kaolinite was reduced.     

A coupled non-isothermal reactive transport model for long-term geochemical evolution of a HLW repository in clay

A numerical model of coupled saturated/unsaturated water flow, heat transfer and multi-component reactive solute transport is presented to evaluate the long-term geochemical evolution in bentonite, concrete and clay formation for a potential geological radioactive waste repository. Changes in formation porosity caused by mineral dissolution/precipitation reactions are taken into account. Simulations were carried out with a general-purpose multicomponent reactive transport code, CORE^2D V4. Numerical results show that pH in the bentonite porewater can vary from neutral to up to 13 over a time scale of 1 Ma although dissolution of silica minerals and precipitation of secondary calcium silicate hydrate (CSH) minerals in bentonite buffer the effect of the hyperalkaline plume. Mineral precipitation reduces the volume of pore space in bentonite close to the bentonite–concrete interface due to the precipitation of CSH minerals. Model results indicate that bentonite porosity decreases less than 25%. The hyperalkaline plume from the concrete only extends to a distance of 0.7 m in the clay formation over the time range of 1 Ma.     

Reactive transport modelling of the long-term interactions of corrosion products and compacted bentonite in a HLW repository in granite: Uncertainties and relevance for performance assessment

Here we present a long-term nonisothermal reactive transport model for the interactions of the corrosion products of a carbon-steel canister and the compacted bentonite of the engineered barrier of a high-level radioactive waste repository in granite. Canister corrosion causes an increase in the pH and the concentration of dissolved Fe²⁺ of the bentonite porewater. Iron precipitates as magnetite and siderite and sorbs via cation exchange and surface complexation on weak sites. Magnetite precipitation reduces significantly the porosity of the bentonite near the canister. The thickness of the zone of reduced porosity is 7 cm at t = 1 Ma. This thickness increases significantly when the dependence of the corrosion rate on the chemical conditions is considered and decreases 3 cm when smectite dissolution and analcime precipitation are taken into account. Model results are not significantly sensitive to the thermal transient and the effect of temperature on the corrosion rate. The conclusions of our simulations are consistent for the most part with those reported by others for engineered barrier systems at similar chemical conditions.     

Investigation of Kinnekulle K-bentonite aimed at assessing the long-term stability of bentonites under repository conditions

Müller-Vonmoos, M., Kahr, G., Bucher, F. and Madsen, F.T., 1990. Investigation of Kinnekulle K. bentonite aimed at assessing the long-term stability of bentonites under repository conditions. Eng. Geol., 28: 269–280.

The mineral formula, the content of fixed potassium and the average interlayer charge of the montmorillonite–illite mixed layer part of four Ordovician K-bentonite samples from Kinnekulle (Sweden) were investigated. The content of fixed potassium was found to decrease from 84% to 50% with increasing depth and distance from the Permian basalt intrusion about 100 m above the bentonite beds. The interlayer charge of the mixed layer is smectitic, i.e. below 0.5 charges per formula unit. The swelling pressures of air-dried, gently grinded and uniaxially compressed specimens with dry densities between 1.5 Mg/m³ and 2 Mg/m³ range from 1 N/mm² to 15 N/mm². This is about 20% of the swelling pressure values of identically treated Ca-montmorillonite. Following ultrasonic treatment, air-drying, gently grinding and compaction, increased swelling pressures of of about 70% of those Ca-montmorillonite were obtained. The reason for this increase in swelling pressure was investigated by dispersion experiments and determinations of water vapour adsorption isotherms of ultrasonically treated and untreated material.     

高放废物处置北山预选区地表岩体结构与岩体质量相关性及其应用研究

岩体工程质量是影响高放废物地质处置（HLW）工程长期稳定性和安全性的关键因素,也是处置库场址比选的重要依据。在地表岩体节理调查统计的基础上,采用RMR(relative metabolic rate)分级方案,探讨北山高放废物处置预选区内以花岗岩为主体围岩的岩体质量与岩体结构的对应特征,建立结构面间距D与RMR岩体质量总评分值之间的定量关系,对北山预选区芨芨槽地表岩体质量分布进行研究。研究结果表明,RMR与D值的点群分布具有扇形发散的正相关特征,随D值的增大,RMR亦呈增长趋势,但低级序岩体结构与岩体质量比高级序岩体表现出更好的对应关系。研究认为,选用RMR与D的点群中心线对应的线性方程评价花岗岩候选区的岩体质量可达到快速评价岩体质量、查明各级岩体空间分布特征的目的,同时也便于对不同候选场区进行大范围岩体质量对比。分析结果显示,北山芨芨槽预选地段BS16钻孔周边3 km2范围内主要以Ⅰ、Ⅱ级岩体为主,岩体质量总体较好且分布相对较为均匀。     

Constitutive modeling of cyclically loaded clays with entrapped gas bubbles under undrained and unexhausted conditions

Acta Geotechnica - Fine-grained gassy marine sediments are widely distributed around the world, and the cyclic behavior of this type of sediments has a great influence on the foundations of...     

Pyrite surface interaction with selected organic aqueous species under anoxic conditions

The interaction between low-molecular weight organic compounds and pyrite under anoxic conditions has been studied using a combination of electrophoresis and batch sorption experiments. The results suggest that acetate, carbamide, ethylamine, formamide, purine, D-ribose, and adenine, as well as the amino acids alanine, cysteine and glycine, interact within the electrophoretic shearplane of the pyrite surface. The observed surface interaction between the negatively charged surface of pyrite and the organic aqueous species takes place regardless of the formal charge of the aqueous species of interest. This indicates that the interaction of organic molecules with pyrite surfaces under anoxic conditions is dictated by interactions with specific surface sites (thiol or iron surface sites) rather than electrostatic forces. Dissolved metals typically enhance the interaction of the organics species. This enhancement is either due to an alteration in the distribution of thiol and iron groups on the pyrite surface or by the formation of ternary surface complexes.     

The preg-robbing of gold and silver by clays during cyanidation under agitation and heap leaching conditions

Laboratory scale leaching tests were carried out in different experimental conditions, with monitoring of gold and silver recovery, the concentration of cyanide used in leaching, solution pH, and precious metal adsorption onto clay. Evidence for the adsorption or re-precipitation of gold from cyanide solutions onto clay minerals is conclusive and indicates that adsorption contributes to the total gold loss from conventional cyanidation. The loss is related to the type of clay, and appears to be affected by the extent of time cyanidation. It was observed that silver adsorption capacity was above 80% and that the degree of adsorption for gold oscillated between 1.68% and 7.49%, depending on the type and characteristics of the evaluated clay.     

干湿循环作用下高低液限黏土防渗性能对比研究

针对低、中、高3种干密度的低液限和高液限压实黏土,开展经历干湿循环过程的渗透系数和孔隙结构变化特征对比研究。结果表明：经3次干湿循环后,相同液限条件下,高压实黏土渗透系数增加比例高于低压实黏土;相同干密度条件下,高液限黏土渗透系数增加比例高于低液限黏土。干湿循环过程中,压实黏土孔隙结构损伤对渗透系数影响随着压实度和液限的提高而加大,而裂隙的发育对渗透系数影响随着压实度的增加和液限的降低而降低。干湿循环过程中低液限压实黏土试样只收缩不开裂,而高液限黏土裂隙发育明显,小尺寸渗透试样无法完全反映裂隙发育对渗透系数的影响,其渗透系数的变化更多是孔隙结构的变化所致,建议通过现场渗透试验或室内大尺寸渗透试验对干湿循环作用下不同液限黏土渗透系数的差异作进一步研究。     

可膨胀波纹管水力膨胀力学特性研究

可膨胀波纹管技术主要用于解决复杂深孔井下事故频发、套管层次受限等技术难题。管串水力膨胀是该技术应用的关键环节,为了研究可膨胀波纹管水力膨胀中的力学特性,提高井下事故处理的成功率,本文采用有限元数值模拟与室内试验相结合的方式共同研究波纹管的水力膨胀过程。通过多次波纹管水力膨胀室内试验,对比有限元理论分析结果,得出二者对于波纹管水力膨胀力学特性曲线基本一致。可膨胀波纹管的弹性形变压力为0～2.5 MPa,管串压力处于3～9 MPa时截面关键点位移速率较大,可膨胀波纹管的水力膨胀压力为14 MPa,膨胀后波纹管截面尺寸外径最大均值Xmax 为136.98 mm,外径最小均值Xmin为104.58 mm,达到了波纹管机械膨胀对截面尺寸的要求,为波纹管野外试验及产业化应用提供了重要的理论依据与技术支撑。     

Hydrological triggering conditions of landslides in varved clays in the French Alps

In this paper the meteorological and hydrological conditions are analyzed which trigger shallow and deeper landslides in glacio-lacustrine deposits (varved or laminated clays) in the French Alps. The hydrological system of these landslides consists of a colluvial cover which supplies water into the fissures of the underlying varved clays. From these fissures water can infiltrate more or less horizontally into the silt layers of the varved clays. A combined reservoirs model was used to simulate the water fluctuations in the colluvial cover and the fissures. Both the water level in the fissures and the residence time of water in the fissures are mainly controlled by the amount of water storage in the colluvial cover. Simulations over the last 25 years show that almost each year the fissures are completely filled with water for several months a year.

Infiltration experiments in the field show that infiltration into the varved clays occurs mainly by horizontal infiltration into the silt laminae. Calculated infiltration rates from these fissures into the silt layers show that the mean yearly residence time of water in the fissures is not sufficient to fully saturate the clay mass each year. It is therefore concluded that the triggering of the landslide movements is mainly controlled by the development of positive pore water pressures in the fissure system and that the rise of pore water pressures induced by the matrix system of the varved clays only plays a minor role. The calculations also show that drainage of the colluvial cover is a very efficient measure to stabilize the deeper landslides.     

Application of performance data from evaporite mines to salt nuclear waste repository design

Summary Data on the performance of operating mines in rock salt and potash from United States and British sources is reviewed. A design validation scheme for nuclear waste repositories is proposed, based on back analysis of the response of operating evaporite mines.     

动荷载作用下结构性软土微结构变化的分形研究

为探讨天津滨海新区典型结构性软土的动力特性,采用动三轴试验和压汞试验,对交通荷载作用下结构性软土微观结构变化进行了研究。取原状土和动荷载作用后的土样进行压汞测试,用分形理论对压汞测试数据进行分析,探讨孔隙分布特征,提出孔径划分的方法,给出了卓越频率与土体孔隙变化的规律。研究结果表明：在动荷载作用后,天津滨海新区典型软土的孔隙分布发生改变,可分为3种类型;利用Menger海绵模型研究交通荷载作用下结构性软土的孔隙分布特性是可行的;用孔隙分维数可实现对卓越频率的预测以及路基土体变形行为的机制分析。     

考虑非Darcy渗流时循环荷载下饱和黏土一维固结分析

考虑到土体处于超固结状态下的压缩性一般要比正常固结状态下的低,引入描述非Darcy渗流的Hansbo方程,修正了Terzaghi饱和黏土一维固结方程,并将其推广到低频循环荷载作用的情况。采用有限体积法对该方程进行了求解,并讨论了非Darcy渗流参数、循环荷载周期以及超固结状态下土体压缩性对固结进程的影响。计算结果表明,在矩形波载作用下,按孔压定义和按变形定义的固结度都随时间增长而震荡增加,且按孔压定义的固结度的震荡幅度明显大于按变形定义的固结度。另外,非Darcy渗流减缓了地基的沉降速率,且循环荷载周期越短,或超固结状态下压缩性越高,地基的沉降速率就越慢。     

总磷对黏土物理力学性质影响的试验研究

通过室内试验,考察了去离子水及不同浓度总磷溶液对黏土物理力学性质的影响,并通过土体矿物成分含量及微观形貌分析,对总磷溶液与黏土作用机制进行了初步探讨。结果表明,总磷对土体物理力学性质影响明显：黏土塑性指数随着总磷溶液浓度的增大而减小,有效黏聚力先增大后减小,有效内摩擦角则随着溶液浓度的增大而增大;各浸泡条件下,黏土应力-应变关系曲线变化规律基本一致,均呈应变硬化现象,土体剪切峰值随总磷溶液浓度的增大先增大后减小;总磷溶液与土体间相互作用主要包括离子交换作用、胶结作用及微生物分解作用,并同时受到孔隙液介电常数及黏滞性的影响,这些作用通过改变黏土的矿物成分含量,使其微观形貌及孔隙特征发生明显变化,最终导致土体的宏观力学特性改变。     

Impact of iron on nuclear glass alteration in geological repository conditions: A multiscale approach

Interactions between nuclear glass and Fe were investigated in a clayey environment to better understand the mechanisms and driving forces controlling the long-term behavior of high-level waste glass in a geological repository. An integrated experiment involving a Glass–Iron–Clay (GIC) stack was run at a laboratory scale in anoxic conditions for 2 years and the interfaces were characterized by a multiscale approach using scanning electron microscopy coupled with energy dispersive spectroscopy, transmission electron microscopy, Raman microspectroscopy and scanning transmission X-ray microscopy at the SLS Synchrotron. The characterization of glass alteration patterns on cross sections revealed an increase in glass alteration with the Fe content and the proximity between the glass and Fe. The alteration layers are polyphase and stratified with an inner porous gel layer incorporating Fe and an outer layer composed of nanocrystalline Fe-silicates. Several mechanisms which could affect the glass alteration kinetics and the transport properties of the alteration layer are proposed to explain this pattern: (i) consumption of hydrolyzed silica by precipitation of Fe-silicates; (ii) penetration of Fe within the gel porosity probably as precipitates such as Fe oxyhydroxide or Fe-silicates. These new data may imply some consequences when considering the long-term behavior of glass in geological disposal conditions.     

Vertical uplift resistance of circular plate anchors in clays under undrained condition

The vertical uplift resistance of circular plate anchors, embedded horizontally in a clayey stratum whose cohesion increases linearly with depth, has been obtained under undrained ( = 0) condition. The axi-symmetric static limit analysis formulation in combination with finite elements proposed recently by the authors has been employed. The variation of the uplift factor (F_c) with changes in the embedment ratio (H/B) has been computed for several rates of increases of soil cohesion with depth. It is noted that in all the cases, the magnitude of F_c increases continuously with depth up to a certain value of H_cr/B beyond which the uplift factor becomes essentially constant. The proposed static limit analysis formulation is seen to provide acceptable results even for the two other simple chosen axi-symmetric problems.     

Stability of an unsupported vertical circular excavation in clays under undrained condition

By using the axisymmetric finite elements static limit analysis formulation, proposed recently by the authors, the stability numbers (γH/c_o) for an unsupported vertical circular excavation in clays, whose cohesion increases with depth, have been determined under undrained condition; γ = unit weight, H = height of the excavation and c_o = cohesion along ground surface. The results are obtained for various values of H/b and m; where b = the radius of the excavation and m = a non-dimensional parameter which accounts for the rate of the increase of cohesion with depth. The values of the stability numbers increase continuously both with increases in H/b and m. The results obtained in this study compare well with those available in literature.