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1.
Cementitious materials will be used for the construction of the engineered barrier of the planned repositories for radioactive waste in Switzerland. Superplasticizers (SPs) are commonly used to improve the workability of concretes and, along with a set accelerator (Acc), to produce shotcrete. In this study the influence of a polycarboxylate- (PCE) and a polynaphthalene-sulphonate-based (PNS) SP on the hydration process, mineral composition and the sorption behaviour of metal cations has been investigated using an ordinary Portland cement (OPC), a low-alkali cement mix (LAC) consisting of CEM III-type cement and nanosilica, and a shotcrete-type cement mix (ESDRED) consisting of a CEM I-type cement and silica fume prepared in the presence of an alkali-free set accelerator.Both the PCE and PNS SP do not significantly influence the amount and quantity of hydrates formed during hydration. The concentration of both SPs decreased rapidly in the early stage of the hydration process for all cements due to sorption onto cement phases. After 28 days of hydration and longer, the concentration of the PNS SP in the pore fluids of all cements was generally lower than that of the PCE SP, indicating stronger uptake of the PNS SP. The formate present in the Acc sorbs only weakly onto the cement phases, which led to higher aqueous concentration of organics in the ESDRED cement than in OPC and LAC.Sorption experiments with 63Ni, 152Eu and 228Th on a cation exchange resin indicate that, at concentrations above 0.1 g L−1, the two SPs could reduce sorption of metal cations. Thermodynamic modelling further indicates that radionuclide complexation by formate at the concentration level in Acc can be excluded, suggesting that the apparent effect of Acc in the sorption measurements on the resin can be attributed to colloids formed owing to the high concentrations of Al and S in Acc. Sorption studies with the same radionuclides on hardened cement paste (HCP) in the presence of concrete admixture solutions and pore fluids squeezed from cement pastes further revealed no significant effect on sorption by either the concrete admixtures or their degradation products that were potentially present in the pore fluids. This finding suggests that the investigated concrete admixtures (PNS, PCE, Acc) and their degradation products have no significant impact on radionuclide mobilisation.  相似文献   

2.
The kinetics of radionuclide desorption from bentonite colloids and subsequent sorption onto fracture filling material can influence colloid-facilitated radionuclide migration in ground water. To shed light on the significance of these issues batch-type experiments using a cocktail of strong and weak sorbing radionuclides as well as FEBEX bentonite colloids in the presence of fracture filling material from Grimsel (Switzerland) under Grimsel ground water conditions have been conducted. Results show that tri- and tetravalent radionuclides, 232Th(IV), 242Pu(IV) and 243Am(III) are clearly colloid associated in contrast to 233U(VI), 237Np(V) and 99Tc(VII). Concentrations of colloid-borne 232Th(IV), 242Pu(IV) and 243Am(III) decrease after ∼100 h showing desorption from bentonite colloids while 233U(VI) and 99Tc(VII) concentrations remain constant over the entire experimental time of 7500 h thus showing no interaction either to colloids or to the fracture filling material. 232Th(IV) and 242Pu(IV) data yield a slower dissociation from colloids compared to 243Am(III) indicating stronger RN–colloid interaction. In the case of 237Np(V), a decrease in concentration after ∼300 h is observed which can be explained either by slow reduction to Np(IV) and subsequent sorption to mineral surfaces in accordance with the evolution of pe/pH and/or by a slow sorption onto the fracture filling material. No influence of the different fracture filling material size fractions (0.25–0.5 mm, 0.5–1 mm and 1–2 mm) can be observed implying reaction independence of the mineral surface area and mineralogical composition. The driving force of the observed metal ion desorption from colloids is binding to fracture filling material surfaces being in excess of the available colloid surface area (76:1, 55:1 and 44:1 for the 0.25–0.5 mm, 0.5–1 mm and 1–2 mm size fraction of the FFM, respectively).  相似文献   

3.
Natural colloids have a potential role in facilitating the transport of radionuclides in groundwater. To assess the role of mobile colloidal phases in radionuclide transport, characterization of colloids for size, size distribution, zeta potential, surface charge and elemental composition is required. Groundwater samples were collected from 12 borewells in the study area and were characterized with respect to physicochemical parameters. Water quality parameters such as pH, temperature, specific conductance, TDS and dissolved O2 were measured in situ. Based on salinity, two groundwater types were identified in the study area: (1) freshwater type and (2) brackish type. Laboratory and field-scale turbidity measurements in addition to quantitative analysis of major ions were carried out. It was observed that the colloid concentrations are reduced at higher salinity. Zetasizer, particle size analyzer, scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction techniques were used to characterize the colloidal particles in groundwater. Colloids were present in all the groundwater samples, the concentration varying between 0.05 and 6 mg/L. Colloid concentration was greater in wells with low ionic strength and the number of colloidal particles varied between 3 × 109 and 4 × 1011 particles/L. The average colloid size varied between 200 and 350 nm for various samples. The zeta potential of the colloidal particles varied between −25.5 and –34.0 mV. SEM analysis of colloidal particles revealed the presence of clays particularly kaolinite and that the mineral composition of colloids reflects the mineralogy of the aquifer. For proper risk assessment and remediation studies, the role of colloids in radionuclide transport assumes greater importance. This study highlights the need for, and relevance of, detailed colloid characterization to assess its role in the migration of radionuclides from near surface disposal facilities.  相似文献   

4.
Predicting the long-term safety and performance of a repository for intermediate- to high-level radioactive wastes requires the evaluation of various scenarios which may influence the integrity of the repository system. One such scenario, radionuclide transport, is described, and some of the geochemical processes which serve to enhance or retard transport are illustrated with examples selected from natural analogue or natural system studies. These studies, as distinct from laboratory simulations, help considerably in understanding how near- and far-field radionuclide transport mechanisms work over geological timescales (thousands to hundreds of thousands of years), more in line with the anticipated lifespan of a repository.

Processes addressed include: stability of the spent fuel UO2 matrix; bentonite backfill as a barrier to radionuclide diffusion; retention of radionuclides by absorption on fracture minerals; transport and diffusion of radionuclides controlled by interconnected bedrock porosity; geochemical influence and behaviour of repository construction materials on radionuclide transport.  相似文献   


5.
《Applied Geochemistry》1988,3(5):535-548
Large-volume groundwater samples were collected at the Nevada Test Site from within a nuclear detonation cavity and from approximately 300 m outside the cavity. The samples were filtered and ultrafiltered, and the filtrates and various particle size fractions were analyzed for chemical composition and radionuclide activity. In samples from both locations, approximately 100% of the transition element (Mn, Co) and lanthanide (Ce, Eu) radionuclides were associated with colloids. Their presence outside the cavity indicates transport in the colloidal form. Distribution coefficients calculated for Ru, Sb, and Cs nuclides from both field sample locations indicate equilibrium partitioning on the 0.05-0.003 μm colloids. Calculation of transport efficiencies relative to colloid mass concentrations and dissolved neutral or anionic nuclides indicates that both the cations and the radiolabelled colloids appear to experience capture by or exchange with immobile aquifer surfaces.  相似文献   

6.
《Applied Geochemistry》1991,6(5):565-574
Natural colloids (1–450 nm) and suspended particles (>450 nm) were characterized in groundwaters of the Whiteshell Research Area of southern Manitoba to evaluate their potential role in radionuclide transport through fractured granite. Data on particle concentrations, size distributions, compositions and natural radionuclide content were collected to predict radionuclide formation and to provide a database for future colloid migration studies. The concentrations of colloids between 10 and 450 nm ranged between 0.04 and 1 mg/l. The concentrations of suspended particles, which require higher groundwater velocities for transport, varied from 0.04 to 14 mg/l. Colloid (10–450 nm) concentrations as low as these observed in Whiteshell Research Area groundwater would have a minimal effect on radionuclide transport, assuming that radionuclide sorption on colloids is reversible. If radiocolloid formation is not reversible, and radionuclide-containing colloids cannot sorb onto fracture walls, the importance of natural colloids in radionuclide transport will depend upon particle migration properties.  相似文献   

7.
The fluid transfer of radionuclides in the geologic medium is considered under conditions when radionuclides are contained in fluids not only as solutes but also as colloids. The effect of colloidal transport of radionuclides on the rate of spreading of radioactive contamination in an underground medium is estimated, with assessment of this effect in mathematical models describing the transport of radionuclides by subsurface water. For this purpose, the exchange of radionuclides between subsurface water, colloid, and an immobile solid phase is considered, taking into account the precipitation of colloidal particles on both the immobile solid phase and other colloidal particles and their recurrent mobilization into the liquid phase. It is noted that, in real colloidal transfer, the heterogeneity of the geologic medium and colloidal particles in subsurface water is of great importance. The known models of colloidal transfer of radionuclides are evaluated on the basis of the analysis performed.  相似文献   

8.
The producers of nuclear waste, within all countries exploring options, including Canada, have determined the long-term solution to be a deep geological repository. In the Canadian concept, within the deep geologic repository a number of clay-based barriers will separate the containers from the surrounding geosphere. Following placement the surrounding groundwater will infiltrate into the repository. In order to analyze the performance of the repository under very complex conditions, accurate material properties are required. The chemistry of the host rock is an important aspect as the behaviour of clay-based barrier materials could be affected by the saturating saline groundwater. This paper investigates the saturated mechanical behaviour of light backfill (composed of 50 % silica sand and 50 % Na-bentonite clay) and dense backfill (composed of 70 % crushed granite, 25 % glacial lake clay and 5 % Na-bentonite clay) and the quantifying the effect of pore fluid chemistry on the strength and compressibility behaviour of the materials. The results indicate that light backfill behaviour is strongly influenced by its pore fluid chemistry while dense backfill shows limited effects. The material parameters of light backfill and dense backfill are interpreted for input into numerical simulations. These results and interpretation enrich the understanding of the mechanical response of light and dense backfill, two components of the sealing system of the Canadian deep geologic repository.  相似文献   

9.
《Applied Geochemistry》1999,14(4):433-445
Coprecipitation of radionuclides with secondary solids is currently neglected in safety assessments for radioactive waste repositories, although this process is thought to be important in limiting radionuclide solution concentrations. This paper provides a systematic review of laboratory data on metal coprecipitation with calcite, presented in the form of phenomenological partition coefficients. The aim of this investigation is to provide a consistent set of parameter values for the quantitative modelling of radionuclide coprecipitation with calcite, which will be the dominant alteration product in cementitious repositories accessed by carbonate-rich groundwater.From the data reviewed, empirical correlations have been derived that relate experimentally determined partition coefficients (λMe) to measurable chemical properties of the coprecipitated metals (ionic radii and sorption parameters of the incorporated trace metals, solubility products of the pure metal carbonates). These correlations have then been used to predict the partition coefficients of radionuclides for which no laboratory data exist.Such predictions indicate that the actinides will partition strongly into calcite under reducing conditions (λMe ∼200–1000 for trivalent, λMe ∼20–200 for tetravalent actinides). Nickel(II) incorporation will be moderate (λMe ∼1–10), while incorporation of ions like U(VI), Cs(I), Sr(II) and Ra(II) in calcite will be weak (λMe<1).In spite of substantial uncertainties, the estimated partition coefficients are sufficiently accurate to allow a semi-quantitative evaluation of the effect of radionuclide coprecipitation with calcite in limiting radionuclide solution concentrations in well characterised repository environments.  相似文献   

10.
高放废物地质处置研究中的矿物学问题   总被引：2，自引：0，他引：2  
在高放废物地质处置研究中的一些矿物学问题应引起矿物学家的注意 。高放废物地 质处置库的缓冲/回填材料是选择以钠质蒙脱石为主要成分的钠基膨润土,还是以钙质蒙脱 石为主的钙基膨润土?怎样选择对99Tc、129I有较好吸附能力的硒汞矿、脆 硫锑铅矿和辉锑矿的替代物来作缓冲/回填材料中的添加剂,以能阻滞99Tc和129 I的迁移?沸石对某些放射性核素的吸附特征也需进一步研究。  相似文献   

11.
在野外调查的基础上, 研究不同钠吸附比(SAR)条件下滨海含水层胶体释放规律, 采集天津平原区代表性含水层土样, 采用室内批实验和土柱实验, 研究胶体释放浓度和释放量的动态特征, 识别不同含水介质胶体释放的差异性, 同时分析胶体颗粒释放过程中含水介质渗透性变异特征.结果表明, 在NaCl和CaCl2混合溶液系统中, 含水层含水介质胶体颗粒的释放与地下水溶液所含阳离子价态有关, Ca2+含量越高, SAR越小, 胶体越不容易释放出来, Ca2+对胶体释放的抑制作用越强烈.土柱实验表明不同吸附比情况下粉砂和中砂胶体释放浓度均很小; 当地下水中SAR为0和8时粉土胶体释放很少, 当溶液的SAR为∞时胶体有明显的释放, 释放浓度随孔隙体积数的增加而增加, 在52个孔隙体积数瞬时达到峰值73.76 mg/L, 然后缓慢减少, 最后累积释放量达0.13 mg/g.胶体释放过程中, 粉土、粉砂和中砂渗透性损失分别为98.2%、86.7%和95.9%;含水介质渗透性的降低主要是由于释放的胶体颗粒在空间上重新分布造成的.   相似文献   

12.
袁俊航  隆威 《探矿工程》2015,42(8):71-74
以28天最优配方激发酸性矿渣粉为胶凝材料进行试验,分析了质量浓度、灰砂比、减水剂掺量、搅拌时间4个因素对砂浆流动度、失水率、干缩率、抗压强度的影响规律,并且针对灰砂比这一因素将水泥与矿渣粉胶凝材料浆液性能进行了对比。最后选取了合适的比例进行正交试验来优化各因素组合,使砂浆综合性能达到最优。  相似文献   

13.
Reliable thermodynamic models assessing the interaction of radionuclides with cementitious materials are important in connection with long-term predictions of the safe disposal of radioactive waste in cement-based repositories. In this study, a geochemical model of U(VI) interaction with calcium silicate hydrates (C–S–H phases), the main component of hardened cement paste (HCP), has been developed. Uranium(VI) sorption isotherms on C–S–H phases of different Ca:Si ratios (C:S) and structural data from spectroscopic studies provided the indispensable set of experimental data required for the model development. This information suggested that U(VI) is neither adsorbed nor incorporated in the Ca–O octahedral layers of the C–S–H structure, but rather is located in the interlayer, similar to Ca2+ and other cations. With a view to the high recrystallisation rates and the cryptocrystalline ‘gel-like’ structure of the C–S–H phases, these observations indicated a U(VI) uptake driven by the formation of a solid solution.  相似文献   

14.
《Applied Geochemistry》2005,20(11):2082-2096
Calcite is an important component of many potential host rocks currently under consideration for the disposal of radioactive wastes. Even in the chemically disturbed zone formed around a cementitious repository, this mineral remains largely unaffected by the hyper-alkaline waters migrating out of the near field. Thus, due to its abundance and geochemical stability, calcite could play an important role in the retardation of radionuclides released from a repository for nuclear wastes. Actinides are an important class of elements present in almost all radioactive waste streams, and for this reason, investigations of their retention behaviour under representative chemical conditions are particularly relevant to assessing safe disposal in the long term. Organic ligands originating from the degradation of cellulosic materials in the repository or present as cement additives could possibly reduce the retardation of tri- and tetravalent actinides due to the formation of stable metal–ligand complexes in solution. In this study, isosaccharinic acid (ISA) and gluconic acid (GLU) have been taken as representatives of cellulose degradation products and concrete admixtures, respectively. Batch-type sorption experiments have been conducted to investigate the effect of ISA and GLU on the retardation of 152Eu, 241Am and 228Th by calcite. 152Eu and 241Am are representatives of the trivalent lanthanides and actinides, respectively, and 228Th is a representative of the tetravalent actinides.High ISA and GLU concentrations in solution were found to significantly affect the sorption of the radionuclides. Rd values for Eu(III) and Am(III) decreased significantly at ISA concentrations above 10−5 M and at GLU concentrations above 10−7 M. The critical concentration limits were similar for Th(IV), that is 2 × 10−5 M in the case of ISA and 10−6 M in the case of GLU. The effects of ISA and GLU on the immobilisation of Eu(III), Am(III) and Th(IV) were interpreted in terms of complex formation in solution. In the case of Eu(III) and Am(III) in ACW, the metal–ligand complexes were found to have a 1:1 stoichiometry. Complexation constants of these aqueous ISA and GLU complexes with Eu(III) and Am(III) were determined and discussed in connection with the presently unclear situation concerning the stability constant of the Eu(OH)30 species. In the case of Th(IV) in ACW, it was assumed that a Th(IV)–ISA–Ca complex and a Th(IV)–GLU–Ca complex are formed, both having a 1:2:1 stoichiometry. The complexation constants of these complexes were determined and compared with the literature data.Assuming maximum concentrations for ISA and GLU in the pore water of the disturbed zone of a cementitious repository based on representative near-field conditions ([ISA]aq = 3 × 10−6 M, [GLU]aq = 10−7 M), it is predicted that the formation of aqueous ISA and GLU complexes would not significantly affect Eu(III), Am(III) and Th(IV) sorption on calcite.  相似文献   

15.
《Applied Geochemistry》1993,8(6):605-616
The Cigar Lake U deposit is located in northern Saskatchewan in the eastern part of the Athabasca Sandstone Basin, and consists of a high-grade ore body (up to 55% U) located at a depth of ∼430 m. As part of a study to evaluate the analog features of this deposit with respect to a disposal vault for waste nuclear fuel, colloids (1–450 nm) and suspended particles (450nm) in groundwater have been investigated to evaluate their effect on element transport through the U deposit. Tangential-flow ultrafiltration was used to concentrate particles from 501 groundwater samples in order to characterize the size distribution, concentration, composition and natural radionuclide content of particles in representative parts of the U deposit. Although Cigar Lake groundwaters contain particles in all sizes ranging from 10 nm to slightly larger than 20 μm, most samples contained a relatively high concentration of colloids in the 100–400 nm size range. Particle compositions are similar to the composition of minerals in the sandstones and ore body, suggesting that particles in groundwater are generated by the erosion of fracture-lining minerals. As a result, particle concentrations in groundwater are affected by the integrity of the host rock. In some piezometers the high initial concentrations of suspended particles, which may have been drilling artifacts, decreased during the collection of the first 350 1. Although colloid concentrations fluctuated during sampling, there are no indications that these concentrations will be permanently reduced by continued groundwater pumping. The observed colloid and suspended particle concentrations in the deep groundwaters are too low to have a significant impact on radionuclide migration, provided that radionuclide sorption is reversible. If radionuclides are irreversibly sorbed to particles they cannot sorb to the host rock and their migration can only be evaluated with an understanding of particle mobility. The data for dissolved and particulate U, Th and Ra were used to calculate field-derived distribution ratios (Rd) between particles and groundwater. The wide range of observed Rd values indicates that these radionuclides in particulate form are not in equilibrium with groundwater. U-series isotope data indicated that most of the U and Ra on particles was derived from groundwater. Some particles could have retained their U for as long as 8000 a. The U and Ra contents of particles in the ore and surrounding clay zones are significantly higher than in particles from sandstone, suggesting that the clay has been an effective barrier to particle migration.  相似文献   

16.
An experimental approach to the characterisation of the complex, multi-scale pore geometry in cement-stabilised soils is presented, in which the pore size distribution inclusively spans at least six orders of magnitude from ~3 nm up to >3 mm. These most likely result from the combined effects of granular inter-particle packing, clay/cement clothing and bridging effects, cement hydration and clay/cement pozzolanic reactions, and alteration of larger pore geometries as a result of solid mass mobilisation and transport following capillary wetting/drying regimes. Experimental data are presented and were obtained through a combination of X-ray computed tomography, mercury intrusion porosimetry and N2 physisorption supported by ‘wet mode’ environmental scanning electron microscopy. Data strongly suggest that macropore/capillary pore size distribution, mean pore size, sorptivity and transport coefficients are a function of particle size distribution (when compaction energy is constant). Mesopore size distribution, which dominates hygric sorption/desorption behaviour, occurs within the clay/cement matrix and also appears to be strongly influenced by the particle size distribution of the granular phase. All other factors being equal, manipulation of granular particle size distribution can be used to engineer the hygric (vapour) and capillary (liquid) potentials and also the fluid transport coefficients of these materials.  相似文献   

17.
袁俊航  隆威 《探矿工程》2015,42(6):71-75
采用工业废弃物——酸性矿渣粉取代水泥作为胶凝材料,以激发酸性矿渣粉活性、提高砂浆综合性能、降低矿山充填成本为目的而进行试验研究,以抗压强度为指标,通过单一激发剂、复合激发剂激发酸性矿渣粉制成胶凝材料的试验,确定了激发酸性矿渣粉活性较好的激发剂,然后选取了4种激发剂进行正交试验,得出了3个不同龄期的最优激发配方。解决了酸性矿渣粉难被激发利用的问题。  相似文献   

18.
Prediction of colloid release and transport as affected by reactive species remains a significant challenge for field applications. In this paper, we report experimental and modeling results of ferrihydrite colloid release under the influence of citrate species. Using a 3-plane surface complexation model, equilibrium constants were obtained for the three proposed inner-sphere complexes by fitting a citrate adsorption isotherm on ferrihydrite at pH 4, and a pH adsorption envelop with 0.64 mM citrate. The constants were used in a reactive transport model for simulating reaction fronts of dissolved species during injection of citrate in ferrihydrite-coated quartz columns. Simulation results show that sorption alone may not adequately describe the breakthrough curves. Inclusions of ferrihydrite dissolution and re-adsorption of Fe(III) improve the prediction of dissolved species transport. Additionally, matrix diffusion may be needed for improved prediction. For the release of colloidal iron oxides it was shown that both oxide dissolution and interfacial repulsion controlled the process during complete breakthrough. However, the peak release of colloids, which occurred during the actual breakthrough of dissolved species, was mainly brought about by electric double layer forces. These particles underwent detachment-deposition-detachment cycles along the flow path, and emerged in the effluent with the major reaction front. To quantitatively predict colloid release, a semi-empirical linear correlation was established, linking the calculated electric potential to experimental colloid release rates. The model may be applied to the prediction and scaling of aquifer remediation studies involved in the injection of organic ligands to mobilize particle bound contaminants.  相似文献   

19.
砂浆孔隙溶液水盐相变特性是研究其水-热-盐耦合模型以及水泥基材料损伤机理的关键理论基础。为探究河西走廊盐渍土地区砂浆孔隙溶液水、盐相变规律,首先利用差示扫描量热法测定水盐相变过程中热流、相变温度等热参数;其次基于热量守恒和水分质量平衡方程,初步实现水盐分离;然后分别计算不同温度下的冰、盐晶和未冻水含量,揭示了低温下砂浆孔隙溶液水盐相变机理;最后结合微观扫描和物质能谱图,明晰了砂浆内部劣化机制。研究结果表明:随着盐浓度增大,砂浆孔隙溶液冻结温度降低、相变时间延后、水盐相变顺序调换及未冻水含量向低温方向移动;相同盐浓度下,氯化钠对砂浆孔隙溶液冻结温度的降幅最大,混合盐次之,硫酸钠最小;砂浆在低温盐侵蚀下遭受物理化学耦合作用,其中氯化钠对砂浆破坏性最大,混合盐次之,硫酸钠劣化相对较轻。  相似文献   

20.
The Swedish Nuclear Fuel and Waste Management Company has recently submitted an application for a license to construct a final repository for spent nuclear fuel, at approximately 500?m depth in crystalline bedrock. Migration pathways through the geosphere barrier are geometrically complex, with segments in fractured rock, deformation zones, backfilled tunnels, and near-surface soils. Several simplifications of these complex migration pathways were used in the assessments of repository performance that supported the license application. Specifically, in the geosphere transport calculations, radionuclide transport in soils and tunnels was neglected, and deformation zones were assumed to have transport characteristics of fractured rock. The effects of these simplifications on the projected performance of the geosphere barrier system are addressed. Geosphere performance is shown to be sensitive to how transport characteristics of deformation zones are conceptualized and incorporated into the model. Incorporation of advective groundwater travel time within backfilled tunnels reduces radiological dose from non-sorbing radionuclides such as I-129, while sorption in near-surface soils reduces radiological doses from sorbing radionuclides such as Ra-226. These results help quantify the degree to which geosphere performance was pessimistically assessed, and provide some guidance on how future studies to reduce uncertainty in geosphere performance may be focused.  相似文献   

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