多边界条件下缓冲/回填材料的膨胀特性

高放废物处置库中缓冲/回填材料的膨胀特性是评价其屏障性能的关键因素。受试验仪器等条件的限制,以往的膨胀试验基本都是在恒体积和恒应力这两种极端边界条件下开展的,无法有效反映处置库中的复杂应力–应变环境。研发了一套新型的多边界条件膨胀仪,集成了恒体积、恒应力和柔性等刚度3种边界条件。利用该膨胀仪对中国高庙子(GMZ)缓冲/回填材料开展了一系列膨胀试验,分析了边界条件对试样膨胀特性的影响。结果表明:研发的多边界条件膨胀仪具有操作简单、性能稳定等特点,能有效地模拟各种边界条件;边界条件对试样膨胀指标具有重要影响,对应的最终膨胀力关系为:恒体积柔性等刚度恒应力边界条件,最终膨胀率关系为:恒应力柔性等刚度恒体积边界条件;膨胀平衡极限(SEL)曲线可作为评价土体在复杂边界条件下水化时的最终体变和膨胀力的参考。研究结果对进一步认识土体的膨胀特性和指导高放废物处置库中缓冲/回填材料的设计具有一定参考意义。     

高放废物地质处置库缓冲/回填材料的气体渗透问题研究进展

简要回顾和总结了国内外学者对处置库中的气体形成与渗透机制,注气试验装置与试验方法,以及气体渗透特性等方面的研究成果和最新进展。研究表明,深地质处置库运营过程中会由于金属无氧腐蚀等产生以氢气为主的多组分气体,气体在缓冲/回填材料中主要以对流–扩散渗流、黏性–毛细管两相流、局部膨胀通道渗流和沿宏观裂缝渗流等4种机制渗透。为此,各国学者先后研发了恒体积渗透装置、K0渗透装置、等向应力渗透装置和三轴渗透装置等注气试验装置,采用控制注气流量速率法或控制注气压力法开展了大量注气试验,发现缓冲/回填材料的气体渗透率和特征压力等渗透特性均与其物理性质和边界条件等诸多因素有关。考虑到处置库实际运营工况的复杂性,多场(热–水–化–力)多相(固–液–气)耦合条件下的缓冲/回填材料的气体渗透特性研究将是今后的一个重要工作方向。     

缓冲/回填材料——膨润土颗粒及其混合物研究进展

膨润土颗粒是一种用于填充高放废物地质处置库中各种施工接缝和空隙的缓冲/回填材料。从膨润土颗粒的制备方法、填充技术与堆积性质、热传导特性、水力特性、结构演化规律及力学特性等6个方面,全面回顾和总结了近年来对膨润土颗粒的研究成果与最新进展,并分别指出了各方面值得进一步深入研究的几个课题。研究表明,膨润土颗粒可由多种方法制备,也可采用多种技术填充到处置库中,其堆积干密度和均匀性与充填技术、级配、堆积方式等因素有关,其热传导系数与干密度、含水率和温度等因素有关,其水力–力学特性与级配、干密度及温度等因素有关。通水水化或降低吸力过程中,颗粒混合物由初始松散结构逐渐转变为胶结融合结构,及至水化饱和后基本达到宏观上的均一化结构,但微观层次的均一化过程仍将持续漫长的时间。考虑到处置库实际运营工况的复杂性,科学高效的颗粒混合物填充技术、多场(热–水–化–力)耦合条件下的颗粒混合物水力–力学特性及结构演化规律是今后值得深入探索的研究方向。     

接缝对高压实膨润土工程屏障自封闭性能的影响

 以高庙子膨润土为研究对象,采用自主研制的多功能膨胀–渗透仪,对不同初始干密度的试样进行不同接缝宽度条件下的膨胀力和渗透试验,并对水化后的膨润土进行水银孔隙率定试验,从微观结构层面分析接缝对膨润土渗透性的影响机制。结果表明：接缝的存在将导致膨润土的膨胀力减小、渗透性增大,从而降低膨润土在处置库中的自封闭性能;在水化过程中,接缝的存在会引起膨润土内部孔隙大小和数量的变化,宏观表现为膨润土渗透性增大。     

地下水含盐量抑制缓冲砌块接缝的愈合效果

高放废物地质处置库在长期运营过程中,压实膨润土砌块水化膨胀,砌块之间的接缝逐渐封闭。地下水中的盐分会限制膨润土的膨胀量,进而抑制接缝体系的自愈合程度。以高庙子膨润土为研究对象,开展渗透试验、热传导试验及微观结构试验,评估渗透溶液含盐量对混合型缓冲砌块接缝愈合效果的影响。压制圆饼状含接缝试样,接缝中充填颗粒膨润土,以甘肃北山地下水模拟溶液开展刚性壁渗透试验,随后测试接缝附近的热传导系数和微观结构,与蒸馏水试验结果进行对比分析。试验结果表明,入渗液为蒸馏水时,接缝体系的水力传导系数最小。随着入渗液总溶解性固体浓度TDS上升到10 g/L,水力传导系数持续增大超过一个数量级,即含盐量越高,对接缝体系有效愈合的抑制效应越明显。渗透试验之后,试样接缝带内部干密度有所提高,非接缝部位的干密度有所降低,证明接缝愈合是通过膨润土向接缝低应力空间膨胀实现的。与蒸馏水相比,入渗液盐分浓度的升高,试样接缝带干密度的提高幅度减小,说明盐分抑制了接缝体系的愈合潜能。随着入渗液浓度增加,热传导系数总体呈微弱的减小趋势,而且接缝带内部的热传导系数方差增大,即接缝空间的热稳定性降低。     

OPHELIE回填混合物的热–水–力耦合特征

在以比利时高放废物处置参考设计方案的OPHELIE地面大型试验的框架为背景下，进行了一套完整的固结试验项目以获取回填材料的热–水–力反应特征。缓冲材料是预先经高压压实的土块，其主要成分为60%的FoCa泥岩，35%的砂以及5%的石墨(干燥重量)。除了对温度和吸力控制设备的简要描述之外，还将阐述有代表性的试验结果，包括持水特性、不同应力、不同吸力、不同温度下的压缩性、膨胀性能，以及在吸湿、吸热条件下的塌陷现象。     

膨胀土–EPS缓冲层–挡墙体系侧压力的模拟和计算

采用ABAQUS有限元热力耦合模块,以热膨胀类比膨胀土增湿膨胀,对膨胀土–EPS缓冲层–挡墙体系进行了数值模拟,研究了EPS缓冲层与墙体和膨胀土间的界面摩擦、墙后膨胀土的宽度、EPS块体间隙等因素对EPS缓冲层减压性能的影响。结果表明:(1)膨胀土–EPS缓冲层之间、挡墙–EPS缓冲层之间的界面摩擦力使作用在挡墙上的侧压力产生重分布,进而显著影响挡墙的倾覆力矩,但对水平推力影响很小;(2)墙后膨胀土宽度越宽,作用在膨胀土–EPS缓冲层–挡墙上的侧压力越大。当膨胀土宽度超过挡墙高度的2倍时,墙后侧压力不再明显增加;(3)组成EPS缓冲层的EPS块体间的间隙不影响挡墙上的侧压力分布。结合数值模拟结果,对膨胀土–EPS缓冲层–挡墙体系的工作机理进行了分析,提出了挡墙侧压力的计算模型。以重力式挡墙为例,说明了该计算模型在膨胀土–EPS缓冲层–挡墙体系设计中的应用,为EPS用于膨胀土挡墙缓冲减压的工程设计提供参考。     

缓冲回填材料砌块接缝密封及愈合研究

高放废物地质处置多重屏障体系中,缓冲回填材料砌块接缝构成潜在的"水力缺陷"和强度薄弱部位。室内试验制备压实膨润土模拟接缝试样,设置接缝宽度为2 mm,采用膨润土干粉、膨润土泥浆、微粒膨润土3种材料对接缝进行密封。将含接缝试样进行柔性壁渗透试验,之后开展热传导试验,评价砌缝不同密封形式愈合效果的差别。自然堆填密度和自由膨胀率试验得到,颗粒粒组为0.25~0.5 mm,且颗粒率为70%时,微粒膨润土达到最佳组构。试验数据表明,不同密封处理的含接缝试样的水力传导系数范围为(2.51~4.94)×10-10 cm/s,其水力传导系数Rg由小到大的顺序为Rg=70%微粒砌缝、Rg=90%微粒砌缝、Rg=50%微粒砌缝、泥浆砌缝、Rg=30%微粒砌缝和干粉砌缝,与接缝密封材料初始填充干密度成负相关。沿着接缝方向,各试样的平均热传导系数范围为1.43~1.63 W/(m·k),其中Rg=70%微粒膨润土砌缝的导热性能最好。垂直接缝方向,各试样的热传导系数基本呈现出接缝愈合带略小于接缝两侧的趋势。研究证实,膨润土干粉、膨润土泥浆和微粒膨润土3种形式的密封材料中,Rg=70%微粒膨润土是优良的密封材料,密封后的接缝愈合效果最佳。随着试样不断饱水膨胀,接缝带干密度提高,接缝结构面逐渐愈合消失,愈合带水力传导性质和热传导性质与砌块材料逐渐趋于一致。     

高压实高庙子膨润土GMZ01的膨胀力特征

 高放废物深地质处置中，缓冲/回填材料系统起着工程屏障、水力学屏障、化学屏障、传导和散失放射性废物衰变热等重要作用，是高放废物地质处置库长期安全性和稳定性的有效保障。前人研究表明，膨润土是理想的缓冲/回填材料。在归纳总结压实膨润土膨胀力的室内试验研究成果的基础上，采用恒体积试验法研究高压实高庙子膨润土GMZ01的膨胀力特性，该膨润土已经被确定为我国高放废物地质处置库首选缓冲材料。结果表明，高压实高庙子膨润土GMZ01膨胀力随时间的变化曲线是一条渐近线，而时间/膨胀力与时间之间存在很好的线性关系；膨胀力发展过程曲线与吸水量曲线具有明显的阶段性特征；高压实高庙子膨润土的膨胀力和干密度之间存在指数关系，干密度是影响膨胀力的一项重要的因素。所取得的膨胀力特性成果，对于高庙子膨润土膨胀性能的正确判定具有非常重要的意义。     

新型对拉GFRP螺栓止水性能试验分析

概述对拉螺栓应用发展现状,提出将GFRP材料用作对拉螺栓杆体。对48个传统及新型对拉螺栓圆台柱体抗渗试件的界面渗透性能进行了试验研究,对比分析不同对拉螺栓形式对粘结界面渗透性能的影响;对粘结界面的渗流特性和层面力学参数,如等效水力隙宽、渗透系数等进行了分析。结果表明,对拉螺栓与混凝土粘结界面是渗水的主要通道,止水片可以提高粘结界面的抗渗性能,新型对拉GFRP止水螺栓具有较好的止水性能。在粘结界面渗水高度和渗透时间测试的基础上,根据渗透压力和渗透时间,推导了粘结界面的等效水力隙宽及渗透系数的计算公式,粘结界面的渗流基本上符合层流渗流。不同的对拉螺栓与混凝土之间的粘结界面渗透系数和等效水力隙宽存在较大差异。     

Characteristics of Strength for Hydraulic Fracturing of Buffer Material

The fundamental characteristics of strength for the hydraulic fracturing of highly compacted bentonite were studied. Firstly, the constant pressurize rate tests were carried out for the material having various specifications. Secondly, the cyclic pressurize test was carried out to examine the self-sealing function as a buffer material. Thirdly, the constant pressure test was carried out to observe the change in strength during seepage. The observed phenomena were analytically examined. As a result, it was found that the strength for hydraulic fracturing of the buffer material increased with the increase of initial dry density, decrease of sand-mixture ratio and decrease of water content. The swelling pressure of the buffer material worked as a constraint stress for the strength for hydraulic fracturing. The fracture made by hydraulic fracturing was fixed through the supply of water. However a long period of low-pressure supply was needed to recover the strength at the failed parts. While the tensile failure was dominant, the specimen having a low dry density might be failed initially by the shear failure. When the water content became large during seepage, the strength for hydraulic fracturing reduced.     

膨润土防水材料的防水机理

以钠基膨润土为原料,选用合适的匹配材料和添加剂加工制作的膨润土防水板、膨润土胶泥和膨润土止水条是利用钠基膨润土遇水膨胀性而以水止水、自封结构和防水基面上的小裂缝,并给防水基面施加膨胀压力,保证防水效果。     

陆相页岩CO2压裂裂缝起裂和扩展特征试验研究

富含黏土矿物的陆相页岩气储层在传统水基压裂过程中易出现基质黏土吸水膨胀现象,降低页岩双重孔隙介质渗透特性,使得页岩气开采遇到一定的阻碍。因此,针对高水敏性页岩气储层,采用CO₂等无水压裂技术的特性研究显得愈发重要。采用延长陆相页岩和致密砂岩试样开展CO₂与清水室内压裂对比试验,揭示了CO₂压裂起裂及扩展特征。实验结果表明：CO₂压裂起裂压力远低于清水压裂,平均值仅为后者的60%;页岩丰富软弱层理面削弱最大主应力对主裂缝扩展的影响,导致沿层理面产生剪切裂缝;低黏度CO₂的高渗滤导致孔隙压力升高,并降低基质缺陷及弱面处的有效应力,有利于裂缝尖端沿着试样内部缺陷薄弱区域扩展,从而形成粗糙裂隙表面。可见,CO₂无水压裂技术适用于超低渗透性页岩气储层改造,研究结果可为水敏性陆相页岩气的现场压裂施工与参数优化提供支撑。     

Constitutive modeling for compacted bentonite buffer materials as unsaturated and saturated porous media

Bentonite has remarkable swelling characteristics and low permeability that enhance the stability of the buffer materials in repositories for the geological disposal of radioactive waste. It is necessary to apply reliable numerical simulation techniques to assess the safety and mechanical stability of repositories over a long period of time. Having a constitutive model that can describe the mechanical behavior of bentonite is key to such numerical simulations. This paper proposes an elasto-plastic constitutive model to describe the changes in the mechanical properties of bentonite due to saturation in the progress of a repository becoming submerged under groundwater. In the proposed model, the swelling index is formulated as a function of the degree-of-saturation to express not only the swelling behavior, but also the dependency of the degree-of-saturation on the dilatancy characteristics. The montmorillonite content is used as a material parameter to determine the normal consolidation line. The experimental results of swelling volume and swelling pressure tests in previous literature are shown to have been satisfactorily predicted by the proposed model.     

The effects of technological voids on the hydro-mechanical behaviour of compacted bentonite–sand mixture

Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. A good understanding of their hydro-mechanical behaviour is essential to ensure disposal safety. In this study, a mixture of MX80 bentonite and sand was characterised in the laboratory in terms of water retention property, swelling pressure, compressibility and hydraulic conductivity. The effects of the technological voids or the voids inside the soil were investigated. The technological voids are referred to as the macro-pores related to different interfaces involving the buffer material, whereas the voids inside the soil are referred to as common macro-pores within the compacted bentonite/sand mixture. The results obtained show that at high suction, the amount of water absorbed in the soil depends solely on suction, whereas at low suction it depends on both suction and the bentonite void ratio. There is a unique relationship between the swelling pressure and the bentonite void ratio, regardless of the sample nature (homogeneous or not) and the sand fraction. However, at the same bentonite void ratio, a higher hydraulic conductivity was obtained on the samples with technological voids. The effect of sand fraction was evidenced in the mechanical yield behaviour: at the same bentonite void ratio, the bentonite–sand mixture yielded at a higher pre-consolidation stress.     

Interface shear behaviour of tunnel backfill materials in a deep-rock nuclear waste repository in Finland

The need for environmental protection and safety in facilities dedicated for the final safe disposal of spent nuclear fuel is paramount. Highly engineered multi-barriers are widely used in such waste containment facilities in order to provide a tight seal for the waste they contain. In Finland, several research studies have been conducted to investigate the feasibility of the final safe disposal of spent nuclear fuel in crystalline bedrock by incorporating the KBS-3V multi-barrier repository concept. As the saturation of the tunnels in a repository progresses, the pre-compressed bentonite buffer may swell and generate very high swelling pressure in the range of 7–15 MPa. Such high swelling pressure can cause the upheaval and the compression of the tunnel backfill that would eventually decrease the density of the buffer. For various reasons, the current KBS-3V design suggests that the saturated density of the buffer should be maintained within a narrow range of 1950–2050 kg/m³ at all times. As the swelling of the buffer directly influences the saturated density of the buffer, it must be controlled by designing a tunnel backfill that possesses an adequate amount of interface shear strength to sustain any additional pressure that is exerted by the swelling of the buffer. This study presents the findings of a series of direct shear box tests conducted on various tunnel backfill interfaces. Additionally, different types of rock profiles were also tested with the selected backfill materials. Based on the results, it was observed that the interface shear behaviour of different backfill-rock interfaces varied significantly with the surface roughness of the rock, while clay blocks resulted in similar shear behaviour with all the backfill materials.     

Sealing performance of compacted block joints backfilled with bentonite paste or a particle-powder mixture

Technology gaps are inevitable parts of the buffer barriers in the high-level radioactive waste (HLW) repositories. The mechanical properties, permeability and water-solute-radionuclide migration of buffer barrier are different when considering the gaps. The present study was designed to investigate the sealing effect of joint between the compacted blocks with two types of sealing materials, a bentonite paste (P) and a bentonite particles and powder mixture (P/P) both from the macro- and micro-aspects.A series of laboratory test was conducted to study the hydro-mechanical properties of bentonite blocks after backfilling the joint between blocks. Both the intake and outflow water volume, and the pressure on radial and axial direction were monitored during the hydration. Following the permeability test, the joint’s sealing was evaluated by a final dry density, water content distribution and microstructure evolution. The results showed that the axial and radial pressure of blocks backfilled with bentonite P and bentonite P/P mixture were higher than that of blocks with a blank joint. Pressure preferentially increased in the low-stress areas (the joint areas), contributing to sealing of the joint. The hydraulic conductivities for backfilled bentonite blocks were almost on the same order of magnitude as to intact bentonite blocks, revealing a clear sealed hydraulic behavior. The final dry density decreased in the block area and increased in the joint area after hydration, forming a transition zone with a certain width. A smaller difference was found between the block and joint area in the backfilled block. The mercury intrusion porosimetry (MIP) test and scanning electron microscope (SEM) test showed that backfill decreased the numbers of inter-aggregates pores ranged between 8 and 15 μm compared with that in the blank joint. The comprehensive data showed that a joint backfilled with a bentonite P/P mixture has a higher swelling capacity, lower permeability, and denser structure compared with joint backfilled with bentonite P. The results suggested that joint backfilling improved the integrity of the blocks and enhanced the swelling property and impermeability compared with that of the non-backfilled blocks. Both bentonite P and bentonite P/P has satisfactory sealing performance of compacted block joints, with better effect of the later.