Laser depth profiling studies of helium diffusion in Durango fluorapatite

Ultraviolet lasers coupled with sensitive mass spectrometers provide a useful way to measure laboratory-induced noble gas diffusion profiles in minerals, thus enabling the calculation of diffusion parameters. We illustrate this laser ablation depth profiling (LADP) technique for a previously well-studied mineral-isotopic system: ⁴He in Durango fluorapatite. LADP studies were conducted on oriented, polished slabs from a single crystal that were heated under vacuum to a variety of temperatures between 300 and 450 °C for variable times. The resolved ⁴He profiles exhibited error-function loss as predicted by previous bulk ⁴He diffusion studies. All of the slabs, regardless of crystallographic orientation, yielded modeled diffusivities that are statistically co-linear on an Arrhenius diagram, suggesting no diffusional anisotropy of ⁴He in this material. The data indicate an activation energy of 142.2 ± 5.0 (2σ) kJ/mol and diffusivity at infinite temperature - reported as ln(D₀) - of −4.71 ± 0.94 (2σ) m²/s. These values imply a bulk closure temperature for ⁴He in Durango fluorapatite of 74 °C for a 50 μm radius grain, infinite cylinder geometry, and a cooling rate of 10 °C/Myr.     

Lead isotope evidence for recent uranium mobility in geological formations of Brazil: implications for radioactive waste disposal

Lead–lead isotope data from whole rock samples are used to investigate the recent (last few million years) mobility of U and Th. The method is based on the comparison of the calculated present day U and Th concentrations required to yield the Pb isotope composition in the samples with the actual present day concentrations of U and Th obtained by direct measurement. The geological formations studied include the Neoproterozoic carbonate sediments of the Bambuı́ Group, Archean/Paleoproterozoic granite–greenstone terrain of the Contendas–Mirante Complex and a Proterozoic ortho-gneisses hosting U deposit in Lagoa Real. All these formations are in the São Francisco Craton, Brazil. The data show high U mobility in the carbonate sediments and in the deformed ortho-gneisses set in a ductile shear zone. Infiltration of groundwater through fault zones seems to have facilitated the U mobility. The Pb isotope approach is a useful technique complementing U-series disequilibrium studies and may be included for site characterization studies for radioactive waste disposal.     

Subterranean microorganisms and radioactive waste disposal in Sweden

In 1987, microbiology became a part of the Swedish scientific program for the safe disposal of high level nuclear waste (HLW). The goal of the microbiology program is to understand how subterranean microorganisms will interact with the performance of a future HLW repository. The Swedish research program on subterranean microbiology has mainly been performed at two sites in granitic rock aquifers at depths ranging from 70 m down to 1240 m, the Stripa research mine in the middle of Sweden and the Äspö Hard Rock Laboratory (HRL) situated on the south eastern coast of Sweden. Some work has also been performed in cooperation with other national or international research groups in Sweden, Canada and at the natural analogue sites Oklo in Gabon and Maqarin in Jordan. The following conclusions are drawn. There is a very high probability of the existence of a deep subterranean biosphere in granitic rock. The documented presence of a deep biosphere implies that relevant microbial reactions should be included in the performance assessment for a HLW repository. A HLW repository will be situated in a subterranean biosphere that is independent of solar energy and photosynthetically produced organic carbon. The ultimate limitation for an active microbial life will be the availability of hydrogen as energy source over time, and hydrogen has indeed been found in most deep groundwaters. Sulphide producing microorganisms are active in environments typical for a Swedish HLW repository, and the potential for microbial corrosion of the copper canisters must be considered. The bentonite buffer around the copper canisters will be a hostile environment for most microbes due to the combination of radiation, heat and low water availability. Discrete microbial species can cope with each of these constraints, and it is theoretically possible that sulphide producing microbes may be active inside a buffer, although the experiments conducted thus far have shown the opposite. Microorganisms have the capability to enzymatically recombine radiolysis oxidants formed by radiation of water. It has earlier been concluded that the migration of radionuclides due to sorption on microorganisms can be neglected. The influence of microbially produced complexing agents remains to be studied at realistic conditions in deep groundwater. Microorganisms have been found in natural alkaline groundwaters, but it could not be conclusively demonstrated that they were in situ viable and growing, rather than just transported there from neutral groundwater. A possible hypothesis based on the obtained results from investigations of natural alkaline groundwaters is that fresh concrete may be a bit too extreme for active life even for the most adaptable microbe – but this remains to be demonstrated.     

Long-term prediction for seismic hazard for radioactive waste disposal

We consider possible approaches to the long-term prediction for seismic hazard in relation to the practical need for the safety of geological disposal of long-lived radioactive waste. The required period of prediction significantly exceeds the one reflected in the set of maps of General Seismic Zoning of the territory of the Russian Federation (GSZ-97). The first geological repository in Russia is planned to be set up in the Nizhnii Kan granite massif in the Krasnoyarsk Krai. This region is an intraplate territory with a relatively high seismic activity. We summarize the analysis of the known empirical generalizations and theoretical principles underlying the seismic hazard prediction. Real seismic events constantly violate forward-looking statements even for relatively short periods of time. These and other arguments suggest that the hypothesis of stationarity of the seismic regime, which is the basis of long-term prediction today, has limited and uncertain applicability in time. Intraplate earthquake prediction is especially uncertain because of the uncertainty in the factor responsible for generating tectonic stresses in these regions. The short horizon of the prediction, based on statistical methods, can be attributed to the nonlinearity of seismic geodynamic processes. Fundamental laws of tectonic processes should be used as the scientific basis for long-term predictions for seismic hazard at the sites chosen for geological disposal of long-lived radioactive waste. These processes can be reflected in models for the migration of the seismically active boundaries of lithospheric plates and the occurrence of seismic activity in intraplate regions.     

System reliability analysis for near-surface radioactive waste disposal facilities

The low- and intermediate-level radioactive wastes are generally disposed in near-surface disposal facility (NSDF). The NSDF is composed of engineered barriers. The probabilistic safety assessment model has been developed to analyse the performance of NSDF. The endpoints of the assessment are the concentration of the radionuclide in the groundwater and the corresponding dose rate of the radionuclide. The barrier system can have multiple failure modes but practically the possible failure modes are failure of top cover, failure of waste container, degradation of waste form and failure of bottom cover, which are usually considered as independent failure events. Through a sensitivity analysis, the most critical parameters affecting the design reliability for failure criteria are identified as the groundwater velocity and distribution coefficient. The study shows that for the NSDF considered, there is a high degree of dependence between the failure modes, and demonstrates the probability of the simultaneous occurrence of failures. Thus, the need to consider the system reliability in the NSDF is highlighted. The study also advocates the use of optimisation techniques to evaluate the probability of failure, which provides a better estimate of the probability of failure, as validated from the results obtained from Monte Carlo simulations.     

高放废物地质处置库选址中的水文地质调查

本文根据国内外研究经验,讨论了高放废物地质处置库选址中水文地质调查的目的、过程、方法和作用,同时对水文地质参数的获取进行了阐述,旨在为今后研究工作提供参考。     

Minimal alteration of montmorillonite following long-term interaction with natural alkaline groundwater: Implications for geological disposal of radioactive waste

Bentonite is one of the more safety-critical components of the engineered barrier system in the disposal concepts developed for many types of radioactive waste. Bentonite is utilised because of its favourable properties which include plasticity, swelling capacity, colloid filtration, low hydraulic conductivity, high retardation of key radionuclides and stability in geological environments of relevance to waste disposal. However, bentonite is unstable under the highly alkaline conditions induced by Ordinary Portland Cement (OPC: initial porewater pH > 13) and this has driven interest in using low alkali cements (initial porewater pH9-11) as an alternative to OPC. To build a robust safety case for a repository for radioactive wastes, it is important to have supporting natural analogue data to confirm understanding of the likely long-term performance of bentonite in these lower alkali conditions. In Cyprus, the presence of natural bentonite in association with natural alkaline groundwater permits the zones of potential bentonite/alkaline water reaction to be studied as an analogy of the potential reaction between low alkali cement leachates and the bentonite buffer in the repository. Here, the results indicate that a cation diffusion front has moved some metres into the bentonite whereas the bentonite reaction front is restricted to a few millimetres into the clay. This reaction front shows minimal reaction of the bentonite (volumetrically, less than 1% of the bentonite), with production of a palygorskite secondary phase following reaction of the primary smectites over time periods of 10⁵–10⁶ years.     

高放废物深地质处置：回顾与展望

文章对我国高放废物地质处置研究的历史进行了回顾,并对未来发展进行了展望。我国的高放废物深地质处置研究开发从1985年开始,迄今为止可初步分为3个阶段：①起步和跟踪研究阶段（1985～1998）;②逐步发展阶段（1999～2005）;③政府规划指导阶段（2006至今）。20多年来,我国在国家法律法规、战略规划、选址、工程屏障、核素迁移研究等方面取得了显著进展。我国已经提出在2020年前建成地下实验室、21世纪中叶建成高放废物处置库的目标。研究开发和处置库工程建设分成3个阶段：试验室研究开发和处置库选址阶段（2006～2020）;地下现场试验阶段（2021-2040）和处置库建设阶段（2041～本世纪中叶）。经过全国筛选对比,已初步选定甘肃北山地区为重点预选区,系统的场址评价工作正在进行。已确定采用膨润土作为处置库的回填材料,并初步确定内蒙古高庙子膨润土为我国高放废物处置库的首选缓冲回填材料。工程设计、核素迁移研究和安全评价也取得了一定进展。1999年起与国际原子能机构开展了3期高放废物地质处置技术合作项目,对提高我国的技术水平起到了积极作用。20多年的研究开发工作为我国在21世纪完成高放废物地质处置任务奠定了基础。     

Subseabed disposal of radioactive waste: effects of consolidational fluid flow

Subseabed disposal of radioactive waste applies a multiple-barrier concept with the sediment being the most important barrier for preventing a release of nuclides into the biosphere. While many investigations have been carried out to analyze the risk potential in this type of disposal, the effects of sediment consolidation and associated fluid flow have not fully been taken into consideration. Here, possible effects of consolidational fluid flow in the penetrator disposal option and possible consequences to the transport of nuclides in the sediment are analyzed. Results of numerical experiments demonstrate that consolidation contributes to the transport of radioactive nuclides released from containers buried in the sediment and to the release of nuclides at the sediment-water interface. Both depend on geological conditions and to a large extent on possible alterations of hydraulic conductivity i of the sediment in the vicinity of the entry path of a penetrator.Symbols c concentration ml m^–3 - c _a concentration of adsorbed solute mg kg^–1 (relative to dry weight of sorbing substance) - c _in solute concentration of source q mg m^–3 - c ₀ initial concentration mg m^–3 - ID dispersion tensorm ²s^–1 - ID ^* diffusion tensor m²s^–1 - D coefficient of dispersion m²s^–1 - d ₀ coefficient of molecular diffusion m²s^–1 - d coefficient of effective diffusion m²s^–1 - g gravity m²s^–1 - h piezometric pressure m - k hydraulic conductivity m²s^–1 - m mass kg - p pressure Pa - q source/sink m³s^–1 - S ₀ specific surface m²m^–3 - t time s - v velocity m s^–1 - x, z cartesian coordinates m - compressibiliy of sediment m²N^–1 - _L longitudinal dispersivity m - effective porosity (decimal fraction) - density kg m^–3 - _s density of sediment kg m^–3 - _w density of water kg m^–3 - decay constant per s - kinematic viscosity m²s^–1     

Principles of geomechanical safety assessment for radioactive waste disposal in salt structures

Today, a large amount of knowledge is available concerning various sites of potential high active waste (HAW) repositories in salt media. Domal Zechstein salt formations have been examined at several sites in Germany. Extensive R&D work was initiated in the former Asse Salt Mine in order to explore the suitability of salt for waste isolation by laboratory tests, theoretical studies and in-situ tests with test results forming a technological base for future repository development.

Resulting from the inhomogeneity of salt structures the demanded safety of a permanent repository for radioactive wastes can be demonstrated only by a specific site analysis in which the entire system, “the geological situation, the repository, and the form and amount of the wastes” and their interrelationships are taken into consideration.

The site analysis has three essential tasks: (1) Assessment of the thermomechanical load capacity of the host rock, so that deposition strategies can be determined for the site; (2) Determination of the safe dimensions of the mine (e.g. stability of the caverns and safety of the operations); and (3) Evaluation of the barriers and the long-term safety analysis for the authorization procedure.

The geotechnical stability analysis is a critical part of the safety assessment. Engineering–geological study of the site, laboratory and in situ-experiments, geomechanical modelling, and numerical static calculations comprise such an analysis.

Within a scenario analysis — according to the multi-barrier principle, the geological setting is checked to be able to contribute significantly to the waste isolation over long periods. The assessment of the integrity of the geological barrier can only be performed by making calculations with geomechanical and hydrogeological models. The proper idealization of the host rock in a computational model is the basis of a realistic calculation of stress distribution and excavation damage effects. The determination of water permeability along discontinuities is necessary in order to evaluate the barrier efficiency of each host rock.

In this paper some important processes for the performance assessment are described, namely creep and fracturing, permeability and infiltration, and halokinesis and subrosion.

For the future, the role and contributions of geoscientific and rock mechanics work within the safety assessment issues (e.g. geomechanical safety indicators) must be identified in greater detail, e.g. considerations of geomechanical natural analogy for calibration of constitutive laws.     

Eastern Altantic fracture zones as potential disposal sites for radioactive waste

Disposal of radioactive waste in the sea floor of fracture zones associated with the flanks of the Mid-Atlantic Ridge may be a satisfactory alternative to land disposal. Effective physiographic, sedimentary, chemical, and oceanographic barriers exist in these aseismic deep canyons, especially in the eastern Atlantic. In addition, the major producers of radioactive wastes are likely to be near the Atlantic Ocean. If such a disposal strategy is adopted, intensive study of the sedimentologic and oceanographic properties of oceanic fracture zones will be necessary.     

Glass-iron-clay interactions in a radioactive waste geological disposal: An integrated laboratory-scale experiment

Glass-iron-clay setups were reacted at 90 °C for 6-18 months to investigate the coupled interactions between glass alteration, Fe corrosion and clay transformation. The reacted interfaces were probed at the microscopic level using complementary characterization methods (scanning electron microscopy coupled with energy-dispersive X-ray analysis, micro-Raman spectroscopy, micro X-ray diffraction, micro X-ray fluorescence spectroscopy, and micro X-ray absorption near-edge structure spectroscopy). The 10-μm thick Fe foil was fully corroded within 10 months, exposing glass to the pore solution. Iron corrosion led to the formation of a layer containing mostly magnetite, siderite and Fe-rich phyllosilicates with one tetrahedral and one octahedral sheet (TO) or two tetrahedral and one octahedral (TOT) sheet per layer. The clay in contact with this corrosion layer was enriched in siderite (FeCO₃). Glass alteration resulted in the formation of a gel layer whose thickness increased with reaction time (from 20 μm after 6 months to 80 μm after 18 months) and a thin layer of secondary precipitates that concentrated lanthanides, P, and Mo. Assuming conservative behavior of Zr, the Si molar concentration in the gel is about 57% that in the glass. Glass dissolution remained at a rate close to the initial dissolution rate r₀. The data are consistent with glass dissolution sustained by the uptake of dissolved Si and charge-compensating cations on secondary (corrosion) products, thus maintaining the gel porosity open and facilitating the leaching of easily soluble elements.     

热-力耦合条件下高放废物处置室间距研究

根据我国高放废物地质处置研究开发规划指南、北山预选区实测地质环境参数和处置库围岩和缓冲材料实测性能参数,采用有限差分计算程序FLAC2D模拟不同处置室间距时高放废物处置库热-力耦合条件下的温度场、应力场和位移场规律。经多方案比较,处置室中线间距为2倍处置室直径时,缓冲材料中的最高温度在95℃左右;由于高放废物释热,引起岩体膨胀,产生热应力,在处置室下角部产生应力集中,但均小于处置库围岩的长期强度;处置库围岩产生的膨胀位移,均在其弹性应变范围内,不会引起处置库围岩破坏。该方案可作为北山预选区建造高放废物处置库的参考依据。     

中国高放废物地质处置:现状和展望

本文阐述了中国的高放废物处置政策,回顾了中国高放废物地质处置在选址和场址评价、缓冲回填材料等方面的进展,提出了中国高放废物地质处置的初步规划,指出我国计划在2015年确定地下实验室场址.还展望了国际合作前景.     

我国高放废物深地质处置战略规划探讨

本文探讨我国高放废物地质处置的战略规划，提出我国高放废物处置库的开发可参考采用“三步曲”式的技术路线，即处置库选址和场址评价一特定场址地下实验室研究一处置库建设。处置库的选址和场址评价工作可与地下实验室研究的相关工作结合。以2040年前后建成处置库为目标，把工作划分为4个阶段，即选址和场址评价阶段、场址确认和地下实验室建设阶段、现场实验和示范处置阶段及处置库建设阶段，规划出各阶段的工作目标和工作内容。论证各项工作内容之间的逻辑关系，指出选址和场址评价是基础、基础研究和地下实验室研究是支撑、性能评价是“指挥棒”、设计并建造出符合标准的处置库是目标。将选址工作划分为预选地段对比、预选场址对比和场址确认3个阶段。按此规划设想，我国将在2015年以前确定处置库和地下实验室的场址，并开始建造地下实验室。2025年左右建成地下实验室，2040年建成处置库。     

包气带黄土环境研究与放射性废物处置

黄土中的粘粒含量较高、比表面积较大、吸附性较强、阳离子交换容量也较高,这些因素对迟滞放射性核素的迁移有利。而且黄土的非饱和渗透系数K(θ)远远低于它的饱水渗透系数K_(?)。使得放射性核素在黄土中的迁移很慢。因此,包气带黄土作为中、低放射性废物处置库的环境屏障是可行的。     

高放废物地质处置新场岩体三维地质模型构建与应用

我国高放废物地质处置经过30多年研究,已初步确定新场为地下实验室推荐场址。开展新场岩体三维地质建模研究,一方面能够充分利用已有资料准确地表达各种地质现象,直观再现地质单元的空间展布及其相互关系;另一方面可以挖掘隐含的地质信息,方便地质分析和工程决策,这在地下实验室研究阶段非常重要。在对新场已有资料综合分析和解译的基础上,建立了岩体的三维地质模型,直观地再现了新场岩体地质环境特征;并基于地质建模结果,开展了钻孔设计等工程应用,取得了较好的效果。本研究可为我国高放废物地质处置后续工作中地质分析与工程设计提供有益参考和技术支持。     

高放废物地质处置元数据设计与编辑模块开发

在对元数据的含义、作用、组成及对高放领域研究的数据内容和特点分析的基础上,对元数据编辑模块的设计进行了详细描述,并在VS2005平台下应用C#语言进行了模块的实际开发,最终实现了便于进行数据共享的XML格式存储功能。元数据编辑模块的应用将促进我国高放废物地质处置信息化工作的进一步深入。