开挖损伤区(EDZ)对核素迁移的影响

随着核电的发展,核电站产生的乏燃料处理处置受到了大众的高度关注.通过对嬗变法、稀释法和隔离法等方法的综合对比,目前,高放废物深地质处置是国内外公认处理核废物的最佳办法.在处置库建造阶段,由于开挖使得围岩中应力重新分布,围岩发生扰动,岩体内部原生裂隙出现扩展、连通,产生新生的微裂隙,岩体的渗透系数变大,这种区域即为开挖损...     

Migration processes in a model deep repository for the disposal of intermediate-level nuclear waste

It is difficult to conceive of radionuclides escaping from a repository by any means other than migration in groundwater. Simple models of the repository are constructed and various migration processes are identified and assessed, according to the flow speed of water through the repository. Diffusion in static water and advection in fast flows are considered separately initially, but later we examine the effect of slow flows in which both these processes contribute to the removal of radionuclides. Concentration profiles across the repository, fluxes of nuclides and total losses are obtained from the analysis. We investigate the time scales necessary for the steady state to be achieved in the repository and conclude that flow speed is roughly inversely proportional to this time scale, i.e. faster flows establish a steady state sooner than slow ones. We also assess the sensitivity of the results to the physical properties of the components of the repository.     

高放废物地质处置库预选场址水文地质研究进展

深地质处置是目前国际上普遍接受的高放废物最终处置方案。对于这种处置方案而言，最有可能使处置库系统中放射性核素释放并进入生物圈的机制是地下水的作用。本文阐述了这种地下水的作用，包括地下水与工程屏障的相互作用、地下水在地质屏障中的核素迁移作用及核素滞留作用；介绍了处置库场址评价中水文地质研究的国际进展和动向；重点介绍了我国高放废物处置库预选场址水文地质研究进程和概况。     

Analytical Models for Migration of Radionuclides in Geologic Sorbing Media

In safety evaluation of nuclear waste repository systems, the analysis on migration of radionuclides in geologic media is important. In previous analytical models called Band- Release, Step-Release and Impulse-Release for decay chains, nuclide-specific release rate and time-dependence of release rate can not be taken into consideration. In this paper, two new release models are proposed. In Preferential-Release model, both of nuclide-specific release rate and time-dependence of leach are taken into account. Exponential-Release model is a special case of Preferential-Release model, in which no preferentiality in leach is assumed. The obtained analytical solutions for these new models are applicable even to four or more member decay chains and are as simple as those for the previous models.

As an example of the application of the new models, the discharge rates of the four nuclides in the decay chain of ²³⁸Pu→²³⁴U→ ²³⁰Th→²³⁶Ra→ into a surface water body are calculated as a function of time after the start of leach and path length from the repository. By using the result, heavy hazard due to ²³⁸Pu at short distance and ²²⁶Ra at long distance are also discussed briefly.     

Development of compartment models with Markov-chain processes for radionuclide transport in repository region

This paper presents a new radionuclide transport model for performance assessment and design of a geologic repository for high-level radioactive waste. The model uses compartmentalization of a model space and a Markov-chain process to describe the transport. The model space is divided into an array of compartments, among which a transition probability matrix describes radionuclide transport. While similar to the finite-difference method, it has several advantages such as flexibility to include various types of transport processes and reactions due to probabilistic interpretation, and higher-order accuracy resulting from direct formulation in a discrete-time frame.We demonstrated application of this model with a hypothetical repository in porous rock formation. First we calculated a three-dimensional steady-state heterogeneous groundwater flow field numerically by the finite-element method. The transition probability matrix was constructed based on the flow field and hydraulic dispersion coefficient. The present approach has been found to be effective in modeling radionuclide transport at a repository scale while taking into account the effects of change in hydraulic properties on the repository performance. Numerical exploration results indicate that engineered barrier configuration and material degradation have substantial effects on radionuclide release from the repository.     

裂隙水流-传热对高放废物处置库近场温度影响的三维离散元分析

裂隙水流-传热是高放废物处置库行为的重要影响因素。为研究裂隙水流-传热对高放废物处置库近场温度的影响,采用3DEC离散元软件计算分析了完整岩体模型和裂隙岩体水流模型对处置库近场温度分布和演变的影响。计算分析表明:由于裂隙水流的吸热降温作用,裂隙岩体模型的废物罐表面膨润土温度低于完整岩体模型的废物罐表面膨润土温度,并缩短了达到稳态所需要的时间;裂隙水流上游区域废物罐表面膨润土温度显著低于裂隙水流下游区域废物罐表面膨润土温度;在设定条件下,裂隙岩体模型的废物罐表面膨润土最高温度约为完整岩体模型废物罐表面膨润土最高温度的75%,裂隙水流速度从0.2mm/s增大到0.5mm/s,废物罐表面膨润土最高温度降低约4%。     

集成缓冲材料对锶的长期阻滞效应

缓冲材料作为高放废物处置库中多重屏障体系的最后一道人工屏障,其对放射性核素的阻滞性能将直接影响到处置库的长期稳定性和安全性。以具有低渗透性和良好的膨胀自愈性的膨润土作为集成缓冲材料的基材,以沸石和黄铁矿作为矿物添加剂,三者按照质量比为63∶27∶10均匀混合构成集成缓冲材料B7ZP,并采用恒定源扩散实验分析了锶在干密度为1.70g/cm~3试样中的扩散特性,结果表明,B7ZP缓冲材料对锶具有良好的阻滞性能,其表观扩散系数为3.30×10~(-12) m~2/s。同时,以多孔介质污染物迁移理论为依据,建立了锶在集成缓冲材料B7ZP中迁移的对流-弥散-吸附多场耦合方程,并应用Matlab软件分析了不同的时间尺度、渗流速率、表观扩散系数和阻滞因子等因素下集成缓冲材料B7ZP对锶的长期阻滞性能,为高放废物处置库的缓冲材料设计和长期阻滞性能评价提供科学依据。     

Formation of filtration fields close to near-surface radwaste repositories

Information is presented about the formation of filtration fields in sections of near-surface radwaste repositories when the properties of the surrounding rock are uniformly isotropic. It is noted that the technogenic ground flow parameters can depend on the water permeability of the repositories and their dimensions in a plane. Comparing different filtration fields, an attempt is made to determine the state of the insulating properties of a repository from the character of the change of the ground water level near the boundary of the repository. Criteria are given for evaluating the integrity of the engineered barriers of the repository. The conditions for uniformly isotropic properties of surrounding rock are given for three scenarios in which the engineered barriers of the repository are impermeable to water, are destroyed, or have lost their protective properties. It is noted that a change of the filtration field, just as the geochemical and radiochemical conditions in the surrounding rock, is one of the main indicators for determining the integrity of a repository. Translated from Atomnaya énergiya, Vol. 105, No. 6, pp. 334–338, December, 2008.     

高放废物地质处置黏土岩处置库围岩研究现状

世界上很多国家都对处置库的可能围岩进行了详细研究。通过对比,认为花岗岩、黏土岩、岩盐比较适合作为处置库围岩,而黏土岩由于具有自封闭性、渗透率低等其他岩石类型不可比拟的优点,因而将黏土岩作为高放废物地质处置库围岩越来越受到各国的关注。文章同时介绍了瑞士、法国、比利时等国家在黏土岩中所进行的大量研究,均认为在黏土岩中处置高放废物和乏燃料是安全的。文章还对黏土岩处置库概念设计、黏土岩处置库围岩地下实验室研究,以及我国开展黏土岩处置库研究的意义等进行了综述。     

Long-term prediction of the migration of radionuclides from solid radwaste storage sites at the Siberian Chemical Works

The results of a calculation of the migration of radionuclides from a repository for solid radioactive wastes from the reactor plant of the Siberian Chemical Works, performed using a method recommended by IAEA with the AMBER 4.4 computer program and by finding an analytical solution, are presented. Three scenarios for migration of radionuclides from the repository are described and estimates of their specific activity in the uppermost water layer and the results on an analysis of the sensitivity of models to a change in the input parameters are presented. __________ Translated from Atomnaya énergiya, Vol. 103, No. 2, pp. 134–136, August, 2007.     

Handling of low-level wastes at the Siberian Chemical Works

Safety codes require that open surface disposal sites for liquid low-level wastes at radiochemical works be closed down. At the Siberian Chemical Works, the first operation was to stop the flow of wastes into the pulp repository. The presence of burial sites predetermined the choice of precisely this method as an alternative for dumping wastes into open disposal sites. The results of a study of the physicochemical characteristics of the wastes flowing into the pulp repository PKh-1,2 from four plants at the Works made it possible to develop a unified scheme for preparing them for disposal — wastes are put into a deep repository and maximum use is made of the individual properties of reprocessed wastes, which decreases the consumption of additional reagents substantially, i.e., it decreases the cost of the preparation process. The scheme developed has successfully passed commercial prototype tests and is now in the design stage. __________ Translated from Atomnaya énergiya, Vol. 103, No. 2, pp. 112–115, August, 2007.     

多因素耦合作用下缓冲材料对铀的长期阻滞效应数值模拟北大核心CSCD

近场环境条件下核素在缓冲材料中的迁移扩散受控于温度场、渗流场、膨胀应力场和化学吸附场的耦合作用,其对核素的阻滞特性将影响到核素随地下水向处置库围岩迁移并返回生物圈的能力,开展多因素耦合作用下缓冲材料对铀的长期阻滞效应研究,对地质处置库的长期安全性评价具有重要的意义。本研究基于混合物理论、连续介质理论、质量守恒、动量守恒、能量守恒及溶质扩散的Fick定律,推导出饱和缓冲材料中核素迁移扩散的热-水-力-化耦合控制方程,并借助于COMSOL Multiphysics软件的直接全耦合求解优势,以自主研制的缓冲材料长期阻滞性能Mock-up实验装置为几何模型,采用内置接口和添加热-水-力-化耦合控制方程中的耦合项作为源项相结合方式,实现了多物理场耦合作用下铀在饱和缓冲材料中迁移扩散行为的直接耦合分析,其长期阻滞特性数值模拟结果表明:初期阶段铀在缓冲材料中迁移扩散较缓慢,迁移距离随时间增幅在1 m左右;中后期阶段,随缓冲材料对铀的吸附容量逐渐趋于饱和后,其迁移距离较初期阶段增加更为明显,迁移距离随时间增幅为3 m左右。多因素耦合下核素在饱和缓冲材料中迁移扩散的热-水-力-化耦合控制方程构建、求解及长期阻滞性能模拟研究的方法,能够为我国高放废物深地质处置库地下实验室开展1∶1工程尺度的工程屏障设计与安全性能评价提供技术参考。     

核素迁移延迟系数对环境评价结果的影响

环境影响报告书是实施环境影响评价制度的一项重要内容。本文讨论了核素在地质介质迁移研究中所用的延迟系数对下游计算点最大浓度计算结果的影响。实验结果表明：延迟系数并非常数;而计算结果显示：延迟系数对下游计算点的最大浓度计算结果有较大的影响。如果这些问题得不到足够的注意和应有的解决,将可能极大地影响评价结果的保守性,从而影响计算结果的可信性。文中指出,核素迁移模式中所应用的延迟系数应该由实测得到,而不要用某些公式由分配系数推导,以免对浓度计算,进而对后果计算带来较大的不保守性。     

A GoldSim model for the safety assessment of an HLW repository

An assessment program for the evaluation of a high-level waste (HLW) repository has been developed by utilizing GoldSim, by which nuclide transports in the near- and far-field of a repository as well as a transport through a biosphere under various natural and manmade disruptive events affecting a nuclide release could be modeled and evaluated. To demonstrate its usability, three illustrative cases including the influence of a groundwater flow pattern through canisters associated with a flowing groundwater through fractures, and the possible disruptive events caused by an accidental human intrusion or an earthquake have been investigated and illustrated for a hypothetical Korean HLW repository. Even though all the parameter values applied to a hypothetical repository are assumed without any real base, the illustrative cases are very informative especially when seeing the result of the probabilistic calculation with the groundwater flow patterns which is turned out to be possibly important for nuclide release and further transport in and around the repository system.     

Thermal aspects of radioactive waste burial in hard rock

One of the possible methods that has been considered for the disposal of radioactive waste is deep burial in stable rock formations. This paper reviews recent work on modelling the way in which the heat emitted by the decaying radionuclides in the waste could affect this disposal option, emphasizing both the effects on depository design and on migration by flowing groundwater. It focuses particularly on research in the U.K. into the feasibility of burying high-level waste in fractured crystalline rock. After introductory sections on the characteristics of the waste and rock, there are three major sections on the temperature field in the surrounding rock, the stresses generated in the rock, and the groundwater flow.     

Physico-chemical characterization of clays at Kalpakkam nuclear plant site towards assessment of radioactive waste disposal

In order to evaluate the safety of a radioactive waste repository in a geological formation, it is necessary to study the prevailing sub surface geology surrounding the disposal facility. The Kalpakkam subsurface was found to comprise of charnockite/hypersthene granite which is overlain by clay, sandy clay and sand. Clays can act as a barrier for groundwater flow and attenuate radionuclide migration. Accordingly, in the present study, clay samples were collected at chosen depths in the subsurface within the study area at Kalpakkam and colloidal clay fractions were separated. The clay colloids after separation were characterized for their size, shape, morphology, elemental composition and zeta potential. Photon correlation spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction technique and zetasizer were used to characterize the colloidal particles. The detailed characterization and analysis has revealed that the major clay mineral present in the subsurface at Kalpakkam is kaolinite. Minor quantities of smectite and illite were also identified in some clay samples. It was observed that the clays present in the study area are having low cation exchange capacity for radionuclides and emphasizes the requirement of proper backfill materials around disposal facilities for retardation of radionuclide migration. The occurrence of clay minerals at depths within the aquifer thickness warrants analysis of groundwater samples for suspended colloidal clay content. These suspended clay particles in groundwater have potential to transport of radionuclides in the ground environment along with groundwater flow depending on the nature of clay colloids and its zeta potential. Above investigations with respect to clay characterization would help the designer in incorporating the adequate safety measures in construction of near-surface disposal facility.     

Nuclide migration under non-equilibrium sorption in a rock matrix surrounding a repository

A theoretical analysis of the migration of nuclides leaching out of a repository is presented. The analysis is applicable to a situation where the sorption process can be described by a first-order reversible reaction model. By means of a parametric study the effects of adsorption and desorption rate constants, groundwater velocity and diffusion coefficients on nuclide migration are elucidated. The results indicate that the migration rate in the surrounding geomedium decreases with increasing adsorption rate constant, and enhances with increasing desorption rate constant.     

Some aspects on the optimum repository plug design with respect to the escape of gas

BfS is in the progress of developing a closure concept for the repository for low and intermediate level radioactive waste in Morsleben (ERAM). In the course of this work, the optimal design of the plug is currently being evaluated with respect to gas escape and the exchange of potentially contaminated brine through the plug. For the sealing to behave well in the long term, it is important that the gas formation processes do not disrupt the plug or enhance the radionuclide release, e.g. by means of excessive pressure build-up. The object has been to study different scenarios for gas and brine transport for two alternative plug concepts, by using the multi-phase flow model TOUGH. Rock convergence due to creep has been included in the modelling. The results of the calculations indicate that the closure concepts restrict he exchange of brine and allow escape of gas; an excavation-damaged zone around tunnels is a potential pathway for gas and brine, and the effect of the rock convergence is small. The results also indicate that a very dense plug results in excessive pressurisation of the repository, whereas a permeable plug results in an increased exchange of brine.     

化学种态分析软件CHEMSPEC最新进展

我国高放废物地质处置研发工作已经步入建造地下实验室阶段。地下实验室建造安全评价和未来的处置库性能评价中均需要关键放射性核素在相应深部地质条件下的扩散和迁移参数,而关键核素的扩散和迁移参数与核素在相应水岩体系中的化学种态密切相关。为满足我国核设施退役治理工作的需要,尤其是我国高放废物地质处置相关安全评价的需要,北京大学核环境化学课题组于2008年开始编写具有完全著作权的化学种态分析软件CHEMSPEC。经过多次修改和完善,目前已经具备了较好的计算功能。本文介绍该软件在表面配合模型、数据库补充和程序优化方面的最新进展,以实例形式介绍该软件的新性能,以期为我国相关实验研究者使用该软件提供参考。     

Ca^(2+)盐溶液在高庙子膨润土中的扩散研究北大核心CSCD

高放废物地质处置库近场地下水可能会对处置库内的屏障体系产生影响,降低处置库的安全稳定。为研究地下水中盐离子在处置库内缓冲回填体系的扩散规律,本文开展了静态无外荷载条件下内蒙古高庙子(GMZ)膨润土在Ca^(2+)盐溶液中自发渗吸的吸附扩散室内试验。从土的微观结构和经典扩散理论对Ca^(2+)在不同干密度和初始饱和度的膨润土试样中的自发扩散规律进行了分析。研究结果表明,在膨润土初始饱和度相同的情况下,试样阻滞系数随其干密度增加而增大,此时Ca^(2+)的扩散能力减弱;当膨润土干密度相同时,随着初始饱和度的增加基质吸力作用减弱,阻滞系数减小,Ca^(2+)的扩散能力减弱。