高放废物地质处置库中非饱和缓冲层的热-水-力耦合数值模拟

作为高放废物地质处置库中的缓冲/回填材料,膨润土在核废料放出的热量和周围天然岩体中地下水共同作用下,其热-水-力耦合作用效果对处置库的稳定性、安全性有着极为重要的影响。以非饱和土多场耦合理论为基础,基于ABAQUS有限元分析平台,建立二维轴对称模型,对高放废物地质处置库中膨润土缓冲层的热-水-力耦合过程开展了有限元模拟,给出了处置库近场缓冲层中不同位置的温度、饱和度、竖向应力及位移、孔隙水压力及吸力的分布及变化规律,并重点分析了温度场的影响,通过与已有研究成果的对比,证实了所计算结果的合理性。     

核废料处置库膨润土膨胀变形研究

膨润土因其优良的膨胀性,常被用于核废料处置库的土障材料。因为核废料处置库会受到温度、渗流、力学、化学等多场耦合作用,膨润土在复杂环境下的的膨胀特性也会受到影响。本文建立了一个核废料处置库的多场耦合模型,模拟了核废料处置库中膨润土土障的温度、饱和度、应力和位移情况,对复杂环境下的膨润土土障的安全性能进行了评价。     

核废料处置概念库近场热-水-应力耦合二维有限元模拟

建立了分析饱和–非饱和介质中热–水–应力耦合现象的应力平衡方程、水连续性方程、能量守恒方程和弹塑性矩阵,并使用所开发的有限元程序对一个核废料处置概念库近场的热–水–应力耦合过程进行了数值模拟,考察了缓冲层及岩体中若干点的温度、饱和度、孔隙水压力及水平方向正应力随时间的变化情况,并将部分结果与国外类似软件的计算数据作了对比,看到二者的规律在定性上有某种一致性,但定量上仍存在一定差别,笔者试着分析了原因并得出了一定的认识。     

核废料地质处置近场热-水-应力-迁移耦合二维有限元分析

引入渗透迁移方程,将所开发的饱和-非饱和孔隙介质中热-水-应力耦合弹塑性模型及其二维有限元程序进行了扩展和改进,使之可以同步地对温度场、渗流场、应力场和放射性核素浓度场的变化进行解析。然后针对一个假想的核废料处置库算例,考察了在热-水-应力-迁移耦合作用条件下近场温度、孔隙水压力及放射性核素浓度的分布及变化。     

不同场热-水-应力耦合过程二维有限元分析

从建立应力平衡方程、水连续性方程、能量守恒方程和弹塑性矩阵入手,提出了一个饱和-非饱和孔隙介质中的热-水-应力耦合模型和开发了相应的有限元程序.为验证本耦合模型及程序的可靠性和重点探讨不同场的热-水-应力耦合过程,以高放射性核废料地质处置的Prototype Repository Project(PRP)原位试验为模拟对象,对一个处置试验坑道近场进行了水单场、热-水耦合、水-应力耦合、热-应力耦合和热-水-应力耦合条件下的数值分析,考察了工程屏障及围岩中的温度、饱和度及应力的变化、分布情况,并得出了一定的认识.     

核废料处置库近场岩石热应力半解析方法研究

针对地下核废物处置库近场环境为饱和裂隙岩体时的情况,提出了一种由分布热源、饱和稀疏裂隙和岩石构成的简化概念模型,根据Goodier提出的热弹性位移势,采用拉普拉斯变换、格林函数法和数值积分等方法,计算求解概念模型中处置库近场岩石的温变热应力。并以实际算例分析分布热源作用下稀疏裂隙岩体受三维水流-传热过程影响的热应力特征以及裂隙水流速和分布热源间距对裂隙岩体热应力分布的影响。结果表明,裂隙水的流动传热作用将处置库近场岩石的热量向下游传递,从而降低了近场岩石的温度,减小了近场岩石中的热应力,裂隙水流速越快,对近场岩石中热应力的影响越明显;热源间距越小时,处置库近场岩石的热应力越大;当热源间距小于一定值时,不同热源间传热作用的叠加将使处置库近场岩石的热应力峰值急剧增大。     

膨润土-砂混合物缓冲层热传导性试验研究

由于高放废物长期衰变放热,因此缓冲材料的热性能参数对处置库的稳定和安全运营具有重要意义。膨润土-砂混合物中石英砂的掺入可有效改善其热传导性能。本研究采用中粒石英砂作辅料,利用Hot Disk TPS2500s热常数分析仪对不同干密度、掺砂率和含水率试样进行测量,分析不同参数对导热性能的影响;采用自行设计的装置对按比例缩小后的试样进行不同热源温度下的热传导模拟试验,并对缓冲层热-力耦合过程进行数值模拟分析,得到了缓冲层温度、应力和位移的变化及分布情况,对比分析不同热源幅值对其分布影响的敏感程度。研究结果表明,导热系数随着干密度、掺砂率增大而增大,且含水率越高增幅越明显;热源温度越高,缓冲层温度、应力及位移的变化越大,且热-力耦合条件下,径向比轴向变化相对较大。热源温度较低时处置库更为稳定。     

热-水-应力耦合弹塑性二维有限元程序研制

通过从已有的应力平衡方程、水连续性方程、能量守恒方程和弹塑性矩阵入手,使用Galerkin方法。将各控制方程分别在空间域和时间域进行离散,初步开发出了一个用于分析饱和岩土介质中热-水-应力耦合弹塑性问题的二维有限元程序。通过一维固结问题的有限元解与太沙基解析解的对比,以及对一个简化的核废料地下处置的热-水-应力耦合问题的数值计算,验证了本程序的正确性。     

膨润土-砂混合型缓冲/回填材料研究现状与展望

高放废物处置中,膨润土-砂混合型缓冲/回填材料力学性质的研究对高放废物地下处置库的建设具有重要意义。在总结国内外缓冲/回填材料研究现状和成果基础上,该文认为试验仪器的研制、开发添加不同材料的混合材料、考虑高温-高压-高吸力条件的试验研究、模型试验、热-水-力-化耦合模型的建立和数值分析是今后一段时间内混合型缓冲/回填材料研究的重点。     

缓冲材料热-水-力耦合模型试验研究

高放废物深地质处置采用多重屏障设计体系,缓冲材料是位于废物罐和围岩之间的一道重要的人工屏障。在放射性衰变热、地下水入侵和围岩应力等作用下,缓冲材料经历复杂的热-水-力耦合过程,评价其长期性能对高放废物地质处置库的稳定运行至关重要。缓冲材料模型试验是研究膨润土在多场耦合环境下性能变化的重要途径。中型实验台架是大型实验台架(China-Mock-up)的重要补充,用来模拟与大型实验台架边界相同的环境下,即热量和水分别从缓冲材料的不同侧向另一侧传递和渗透的条件下膨润土的行为特征。通过实时监测缓冲材料在长期加热和加水条件下的温度、相对湿度、力学等特征参数,揭示了在热-水-力耦合条件下缓冲材料的性能变化规律,同时对台架进行了拆解,对拆解样品的含水量、干密度和微观结构等进行了分析测试,研究结果可为高放废物处置库缓冲材料的工程设计提供参数支持。     

A numerical study of THM effects on the near-field safety of a hypothetical nuclear waste repository—BMT1 of the DECOVALEX III project. Part 3: Effects of THM coupling in sparsely fractured rocks

As a part of the international DECOVALEX III project, and the European BENCHPAR project, the impact of thermal–hydrological–mechanical (THM) couplings on the performance of a bentonite-back-filled nuclear waste repository in near-field crystalline rocks is evaluated in a Bench-Mark Test problem (BMT1) and the results are presented in a series of three companion papers in this issue. This is the third paper with focuses on the effects of THM processes at a repository located in a sparsely fractured rock. Several independent coupled THM analyses presented in this paper show that THM couplings have the most significant impact on the mechanical stress evolution, which is important for repository design, construction and post-closure monitoring considerations. The results show that the stress evolution in the bentonite-back-filled excavations and the surrounding rock depends on the post-closure evolution of both fields of temperature and fluid pressure. It is further shown that the time required to full resaturation may play an important role for the mechanical integrity of the repository drifts. In this sense, the presence of hydraulically conducting fractures in the near-field rock might actually improve the mechanical performance of the repository. Hydraulically conducting fractures in the near-field rocks enhances the water supply to the buffers/back-fills, which promotes a more timely process of resaturation and development of swelling pressures in the back-fill, thus provides timely confining stress and support to the rock walls. In one particular case simulated in this study, it was shown that failure in the drift walls could be prevented if the compressive stresses in back-fill were fully developed within 50 yr, which is when thermally induced rock strain begins to create high differential (failure-prone) stresses in the near-field rocks.     

Coupled thermo-hydro-mechanical analysis of a heater test in fractured rock and bentonite at Kamaishi Mine — comparison of field results to predictions of four finite element codes

Four computer codes were applied for a prediction of coupled thermo-hydro-mechanical responses during an in situ heater experiment which simulates a nuclear waste deposition hole with a waste over-pack and bentonite buffer, surrounded by fractured rock. The elevated temperature in the heater surroundings, which was maintained at 100°C for 8.5 months, generated substantial heat-driven moisture flow and swelling in the clay buffer, and thermal expansion of the surrounding fractured rock. Predicted system responses — including temperature, moisture content, fluid pressure, stress and displacement — were compared to measurements at 70 sensors located both in the clay buffer and the near-field rock. An overall good agreement between modeling and measured results indicates that most thermo-hydro-mechanical responses are fairly well represented by the coupled numerical models. Uncertainties occur for modeling of hydromechanical behavior of the swelling clay buffer at low saturation, modeling of near-field heterogeneous mechanical behavior of the low-stressed fractured rock, and modeling of the rock–buffer interface.     

Modelling of thermal rock mass properties at the potential sites of a Swedish nuclear waste repository

A repository for spent nuclear fuel will generate heat. For the SKB KBS-3 repository with some 6000 waste canisters positioned at 500 m depth in crystalline rock, the layout together with the thermal properties of the host rock will control the temperature evolution in the near-field and in the far-field. The canister spacing will be determined by thermal design criteria, which place limits on the maximum temperature that can be allowed in the bentonite buffer surrounding the canisters. Since the rock volume available for deposition may be limited, there may also be a need for optimizing the layout of the repository.In this paper the simulation model used to define canister spacing guidelines for SKB's repository layout is presented and discussed. Heat transport properties of the rock are shown to be key factors for the repository layout. It is shown how results from measurements of temperatures at different points in the interior of KBS-3 type deposition holes in the Prototype Repository in Äspö HRL, southern Sweden, are used to find values of buffer and canister heat transport parameters to be used as input in the model. A brief summary of the data and model uncertainties is given.The methodology employed for thermal modelling in the SKB site investigations is presented. Thermal properties are mainly measured in the laboratory or calculated from mineral composition. Both determination methods are applied to small volumes (cm scale), but thermal conductivity values are required at a scale relevant for the canisters. A methodology for upscaling has been established. In order to describe the thermal properties of large volumes of rock the site is divided into a number of rock domains, each with a characteristic geology.     

遍有节理岩体的双重孔隙–裂隙介质热–水–应力耦合模型及有限元分析

 建立一种饱和–非饱和遍有节理岩体的双重孔隙–裂隙介质热–水–应力耦合模型,其特点是应力场和温度场是单一的,但具有不同的孔隙渗流场和裂隙渗流场,以及可考虑裂隙的组数、间距、方向、连通率和刚度对本构关系的影响,并研制出相应的二维有限元程序。针对一个假定的高放废物地质处置库,就岩体为双重介质和单重介质2种工况进行数值分析,考察缓冲层和岩体中的温度、孔隙水压力、饱和度、地下水流速和主应力的变化、分布情况。结果显示,地下水由双重介质进入缓冲层中要快得多,2种工况的计算域中温度差别不大,但缓冲层及附近部位的主应力大小及分布有显著不同,单重介质的应力集中程度要大。     

Numerical study of the THM effects on the near-field safety of a hypothetical nuclear waste repository—BMT1 of the DECOVALEX III project. Part 2: Effects of THM coupling in continuous and homogeneous rocks

An evaluation of the importance of the thermo-hydro-mechanical couplings (THM) on the performance assessment of a deep underground radioactive waste repository has been made as a part of the international DECOVALEX III project. It is a numerical study that simulates a generic repository configuration in the near field in a continuous and homogeneous hard rock. A periodic repository configuration comprises a single vertical borehole, containing a canister surrounded by an over-pack and a bentonite layer, and the backfilled upper portion of the gallery. The thermo-hydro-mechanical evolution of the whole configuration is simulated over a period of 100 years. The importance of the rock mass's intrinsic permeability has been investigated through scoping calculations with three values: 10⁻¹⁷, 10⁻¹⁸ and 10⁻¹⁹ m². Comparison of the results predicted by fully coupled THM analysis as well as partially coupled TH, TM and HM analyses, in terms of several predefined indicators of importance for performance assessment, enables us to identify the effects of the different combinations of couplings, which play a crucial role with respect to safety issues. The results demonstrate that temperature is hardly affected by the couplings. In contrast, the influence of the couplings on the mechanical stresses is considerable.     

盐溶液中膨润土峰值剪切强度的计算方法

膨润土作为高放废物地质处置库的工程屏障材料,在高放废物储存罐与围岩间起缓冲作用,与地下围岩裂隙水接触,膨润土的剪切强度产生变化,危及核废物处置库缓冲层的安全。因此,计算盐溶液中膨润土的剪切强度对保障核废物处置库缓冲层安全具有十分重要的意义。利用双电层理论,从微观角度定性地解释了盐溶液浓度对膨润土峰值剪切强度影响的机理;根据膨润土在盐溶液中的应力平衡,提出考虑盐溶液的渗透吸力影响的修正有效应力概念,反映盐溶液浓度对膨润土强度的影响;基于膨润土表面的分形模型,导出与渗透吸力对应的修正有效应力的表达式,盐溶液的渗透吸力采用修正的Debye-Hückel公式计算;根据Mohr-Coulomb准则,膨润土在盐溶液中的峰值剪切强度(■)由修正有效应力(p~e)表示为统一线性关系:■。     

高放废物地质处置缓冲回填材料研究进展综述

随着核能的发展,不可避免要产生大量高放射性废物。对于高放废物的处置国际上普遍接受的方法是深部地质处置。膨润土及其含砂混合料以其优异的性能被认为是高放废物地质处置库中理想的缓冲/回填材料。该文从仪器设备的研制、本构模型、膨胀特性、模型实验及原位试验、热-力-水耦合过程的理论研究及其数值模拟等方面简要总结了膨润土及其含砂混合料作为缓冲/回填材料的一些国际研究进展。作者期待国家相关部门能加大经费投入和支持力度,推动我国这一领域的研究走向全面和深入,为核能事业的发展保驾护航。     

三点弯曲下热处理北山花岗岩的断裂特性研究

针对我国未来高放核废料地质处置等重大工程实际需要,开展热处理后岩石的断裂破坏行为研究具有重要意义。以我国未来可能的核废料储库选址地北山花岗岩为研究对象,通过带加载SEM高温试验系统对经过热处理后的花岗岩进行三点弯曲破坏试验,在25 ℃～100 ℃范围内,北山花岗岩以脆性破坏为主。在低应力作用下,裂纹扩展主要受到应力集中及多种矿物力学行为及它们之间的黏结力作用大小的影响,最终裂纹的萌生主要发生在矿物颗粒之间较弱的胶结面上,裂纹初始扩展角主要由这些矿物颗粒之间夹角来决定,因此裂纹初始扩展角通常与水平面有个夹角;随着荷载继续增加后,裂纹会逐渐沿着水平方向扩展,这个主要方向的矿物承受了最大的弯曲拉应力,而岩石的抗拉强度通常很低。室温到100 ℃的范围内,花岗岩平均断裂韧性几乎不发生变化,稍微的波动可认为是花岗岩的非均值性所造成的。通过数字散斑相关计算方法实现岩石细观尺度变形全场测量。测量结果表明,各个矿物颗粒表面发生较为复杂的变形,多个矿物颗粒界面两边发生完全相反方向的位移,这表明界面两边颗粒受到拉应力作用。尽管在初始阶段,变形主要发生在弯曲拉应力最大界面,但最终破坏可能发生在另外地方,这时的破坏主要受到非均匀的弱面所控制。可见,细观尺度岩石的破坏受到最大应力状态处与岩石的非均质处变形的综合影响。