混合型缓冲回填材料非饱和特性试验研究

发布时间：2018-05-18 11:45

本文选题：高放废物地质处置 + 缓冲回填材料　；参考：《兰州大学》2013年博士论文

【摘要】：高放废物是一种含有强放射性核素的特殊废物,它具有毒性大、核素半衰期长、发热量大的特点,需要与人类生存环境长期可靠的隔离。高放废物的处置难度极大,目前认为最可行的处置方案是将高放废物在地下深埋,称为深部地质处置法。深埋高放废物的地下工程称为地质处置库。基于“多重屏障体系”概念设计的地下处置库中,缓冲回填材料是阻滞高放废物中的放射性核素向人类生存环境迁移的最主要工程屏障。近二十多年来,我国就缓冲回填材料的选择进行了大量的研究工作。高压实的钠基膨润土具有优越的吸附性能、膨胀自愈性能以及防渗性能,被确定为我国理想的缓冲回填材料。但是,纯膨润土材料热传导性低、可施工性差以及造价昂贵。较差的热传导性不利于纯膨润土将核素衰变产生的热量及时的散发到处置库围岩,导致缓冲层温度超过100℃,而液态水汽化会产生过大的水汽压力会破坏缓冲层结构；温度过高还可能改变膨润土性质以及产生干缩裂缝。纯膨润土塑性过高,加水制样过程中“团粒化”及不均匀湿化现象非常明显,土的可调理性差,高压实的膨润土材料不易获取。膨润土中添加一定量的石英砂,材料干密度提高,既能满足吸附性能、膨胀自愈性能与防渗性能,又能改善热传导性能、提高力学强度、降低长期蠕移风险、优化回填设计与施工性能和减少成本。开发膨润土-石英砂混合物作为混合型缓冲回填材料,是本研究组的创新研究方向,也是世界高放废物地质处置领域缓冲回填材料研究的主流方向。本课题组研究的根本目的是确定最优掺砂率,使膨润土-砂混合物的吸附性能、膨胀自愈性能、防渗性能、力学性能、热传导性能、可施工性能以及长期稳定性等尽最大可能地同时得到满足。处置库缓冲层处于非饱和状态的时间通常超过1000年。在非饱和状态下,高压实的缓冲回填材料会吸收围岩渗出的地下水,产生膨胀应力,材料的物理参数也会改变。缓冲回填材料在吸／失水过程中,土中的吸力将发生改变。同时,缓冲回填材料的饱和度、地下水中的溶质以及周围温度场可能导致缓冲回填材料的导水性发生改变。因此,本研究的具体目标是从非饱和性质着手,研究掺砂率、干密度、孔隙溶质以及温度对膨润土-砂混合物非饱和指标参数的影响,为最终的掺砂率优化提供基础数据。本文选取高庙子(GMZ)膨润土为主料,添加不同比例的石英砂辅料,开发出混合型缓冲回填材料的4种室内压制方法。采用变压实能静力压实法制样,通过土水特征曲线测试试验和浸水试验,对材料的持水特性与非饱和导水性质进行研究。试样设计干密度范围为1.50～1.90g/cm3,掺砂率范围为0%～50%;孔隙液为模拟北山地下水(NaCl与Na2S04按质量比2：1配制),固体总溶解度分别是0.0、6.0和12.0g/L(pH=7.1)；试验温度为20,40和60℃。通过分析上述因素的影响,揭示了材料非饱和特性变化的实质。所得试验结果如下：通过室内对缓冲回填材料的搅拌-湿化-养护-压制过程研究,提出了可行的小尺度制样方法。采用分步搅拌法可将石英砂颗粒与膨润土均匀混合。喷雾法湿化混合材料可以有效地避免团聚体的产生。采用变压实能静力击实法卸载时会发生微量回弹。干密度越大,回弹现象越明显。另外,靠近压实端的干密度大于试样干密度平均值。最后,开发出4种压制方法：恒定击实能动力击实法、变击实能动力击实法、恒定压实能静力压实法以及变压实能静力压实法。其中,变压实能静力压实法能够满足各种参数组合,并且压制密度范围最大。在相同吸力条件下,侧限饱和混合物对应的含水率小于自由饱和混合物对应的含水率值。侧限饱和混合物的持水性能随掺砂率或干密度增大而变差,随溶质浓度增大而增强,基本不受温度影响;自由饱和混合物的持水性能随掺砂率增大而变差,干密度对其无明显影响。本论文优选出了混合物渗透吸力的最佳计算方法。将膨润土-砂混合物归为“混合土”,使用有效含水率对混合物的土水特征曲线进行修正,发现侧限饱和混合物的持水性能主要受有效粘土密度控制,而自由饱和混合物的持水性能主要受掺砂率控制。脱湿试验结束后对试样的扫描电镜观察发现,侧限饱和试样脱湿后收缩干裂现象不明显,自由饱和试样脱湿后普遍发生收缩干裂现象,干缩裂隙主要发生在石英砂颗粒边界附近,干裂程度随掺砂率增大有加重趋势。干密度减小或掺砂率增大,材料的有效粘土密度减小,有效孔隙率增大,最终导致材料的导水率增大,抗渗性能减弱。渗入液中Na+浓度从0g/L增至12g/L,渗入液中溶质会对扩散层中的Na+起排斥作用,从而使扩散层厚度变薄,最终导致缓冲回填材料的导水率增大,抗渗性能减弱。温度从20℃升至40℃,渗入液粘滞系数减小,导致材料的导水率增大,抗渗性能减弱。试验过程中,材料吸水膨胀产生应力。渗入端应力值发展快于渗出端。干密度越大,掺砂率越小,材料应力值越大。试验结束后,试样中部靠近渗入端的干密度有所下降,渗出端的干密度有所提高,试样十密度原有的不均匀分布有所缓和。说明在围限条件下,混合物吸水膨胀变形有使内部密度向均一化调整的趋势。通过研究不同掺砂率膨润土-砂混合物的压实性质、水理性质、膨胀性质、渗透性质和力学性质等,评价掺砂率不同时膨润土-砂混合物的各种基本性能指标,包括渗透系数、粘聚力、内摩擦角、膨胀应力、膨胀应变、液限和塑限。最后,以缓冲回填材料的长期稳定性为前提,采用模糊集中法对各因素进行综合判断,确定我国混合型缓冲回填材料的最优配比,即掺砂率为20%～30%,干密度为1.60～1.80g/cm3。
[Abstract]:High radioactive waste is a kind of special waste containing strong radioactive nuclide. It has the characteristics of large toxicity, long nuclide half-life and high calorific value. It needs long-term and reliable isolation from human living environment. The disposal of high radioactive waste is very difficult. At present, it is considered that the most feasible disposal scheme is to bury the high radioactive waste in the deep underground, which is called deep geological disposal. The underground engineering, which is deeply buried in high level waste, is called the geological repository. In the underground repository based on the concept of "multiple barrier system", the buffer backfill material is the most important engineering barrier to block the migration of radioactive nuclides from the high level radioactive waste to the human living environment.
In the past more than 20 years, a lot of research work has been done on the selection of cushioning backfill materials in China. The high pressure solid bentonite has superior adsorption properties, expansion self healing properties and anti-seepage properties, which has been determined as the ideal buffer backfilling material in China. However, the thermal conductivity of pure bentonite is low, the construction property is poor and the cost is expensive. The poor thermal conductivity is not conducive to the timely emission of the heat generated by the nuclide decay to the surrounding rock of the repository, resulting in the temperature of the buffer layer exceeding 100 degrees C, and the excessive water vapor pressure in liquid water vaporization will destroy the structure of the buffer layer; high temperature may also change the properties of bentonite and produce shrinkage cracks. The phenomenon of "pelletization" and inhomogeneous wetting in the process of water adding is very obvious, and the reunion of the soil is poor and the high pressure bentonite is not easy to obtain. A certain amount of quartz sand is added to the bentonite, and the dry density of the material is improved, which can not only meet the adsorption properties, but also improve the self healing property and the permeability, and improve the heat conduction performance. High mechanical strength, reducing long-term creep risk, optimizing backfilling design and construction performance and reducing cost. Developing bentonite quartz sand mixture as mixed buffer backfill material is the research direction of this research group, and also the mainstream direction of research on buffer backfill materials in the field of high level waste geological disposal.
The fundamental purpose of this research group is to determine the optimum sand mixing rate, so that the adsorption property of bentonite sand mixture, the self healing properties, the permeability, the mechanical properties, the thermal conductivity, the constructability and the long-term stability are at the same time. The time of the buffer layer in the non saturated state is usually more than 100. For 0 years. In unsaturated state, the cushioning backfill material of high pressure can absorb ground water from the surrounding rock, produce expansion stress, and change the physical parameters of the material. In the process of suction / water loss, the suction of the soil will change. At the same time, the saturation of the backfill material, the solute in the groundwater and the ambient temperature field will be buffered. Therefore, the specific goal of this study is to study the effects of sand mixing rate, dry density, pore solute and temperature on the unsaturated index parameters of bentonite sand mixture from unsaturated properties, and provide basic data for the final optimization of sand mixing rate.
In this paper, 4 kinds of indoor compaction methods for mixed type cushioning backfill materials were developed by selecting GMZ bentonite as the main material and adding different proportions of quartz sand excipients. The static compaction method of variable compaction energy was used to study the water holding characteristics and unsaturated water conductivity of the material by the soil water characteristic curve test and the immersion test. The dry density range of the sample is 1.50 ~ 1.90g/cm3, and the range of sand mixing rate is 0% ~ 50%. The pore fluid is the simulated North Mountain groundwater (NaCl and Na2S04 are prepared by mass ratio 2:1), the total solid solubility is 0.0,6.0 and 12.0g/L (pH=7.1), the test temperature is 20,40 and 60. By analyzing the influence of the above factors, the unsaturated characteristics of the material are revealed. The results of the experiment are as follows:
Through the study of the stirring humidification curing and pressing process of the cushioning backfill material in the room, a feasible small scale sample preparation method is proposed. The separation of quartz sand particles and bentonite can be evenly mixed by the stepwise mixing method. The spray method wetting mixture can effectively avoid the formation of the aggregate. In addition, the dry density of the dry density is more obvious. In addition, the dry density near the compacted end is greater than the average dry density of the sample. Finally, 4 kinds of compaction methods are developed: constant compaction dynamic force compaction method, dynamic compaction method of variable compaction, static compaction method and static compaction method of variable compaction energy. The force compaction method can meet various parameter combinations and suppress the maximum density range.
Under the same suction condition, the water content of the side limit saturated mixture is less than the water content corresponding to the free saturated mixture. The water holding performance of the side limit saturated mixture decreases with the increase of sand mixing rate or dry density, which increases with the increase of the solute concentration, and is basically not affected by the temperature; the water holding property of the free saturated mixture increases with the sand mixing rate. In this paper, the optimum calculation method of the dry density of the mixture is not obvious. The optimum calculation method for the permeation suction of the mixture is selected. The bentonite sand mixture is classified as a "mixed soil", and the soil water characteristic curve of the mixture is corrected with the effective water content, and the water holding property of the side limit saturated mixture is mainly controlled by the effective clay density. The water holding performance of the free saturated mixture is mainly controlled by the sand mixing rate. After the dehumidification test, the scanning electron microscope observation shows that the shrinkage dry crack is not obvious after dehumidification of the side limit saturated sample. The shrinkage crack is generally occurring after the dehumidification of the free saturated specimen, and the dry shrinkage crack mainly occurs near the boundary of quartz sand particles and the degree of dry cracking. There is a tendency to increase with the increase of sand mixing rate.
The decrease of dry density or the increase of sand content, the effective clay density of the material decreases and the effective porosity increases, which eventually leads to the increase of the water conductivity of the material and the weakening of the impermeability. The concentration of Na+ in the permeable fluid is increased from 0g/L to 12g/L, and the solute in the permeable fluid will repel the Na+ in the diffusion layer, which makes the thickness of the diffusion layer thinner and eventually leads to the buffer backfilling. The water conductivity of the material is increased and the permeability is weakened. The viscosity of the infiltration fluid decreases from 20 to 40, which leads to the increase of water conductivity and the weakening of the impermeability of the material. In the test process, the stress of the material is expanded to produce stress. The stress value of the permeable end develops faster than the exudation end. The greater the dry density, the smaller the sand mixing rate, the greater the stress value of the material. The test knot is greater. After the beam, the dry density near the permeable end of the sample is decreased, the dry density of the exudation end is improved, and the original uneven distribution of the ten density of the specimen is relieved.
By studying the compaction properties of bentonite sand mixture with different sand mixing rates, water properties, swelling properties, permeability and mechanical properties, various basic performance indexes of bentonite sand mixture are evaluated, including permeability coefficient, cohesive force, internal friction angle, expansion stress, expansion strain, liquid limit and plastic limit. The long-term stability of the material is the premise, and the fuzzy concentration method is used to judge the factors comprehensively. The optimal ratio of the mixed buffer backfill materials in China is determined, that is, the sand mixing rate is 20% to 30%, the dry density is 1.60 ~ 1.80g/cm3..
【学位授予单位】：兰州大学
【学位级别】：博士
【学位授予年份】：2013
【分类号】：TU41

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