Symposium – Scientific Basis for Nuclear Waste management XXIV
Research Article
Grimsel 2000 – Status of International Projects at the Grimsel Test Site (GTS)
- W. Kickmaier, W. R. Alexander, S. Vomvoris, I.G. McKinley
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- 21 March 2011, 893
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During 17 years of cooperation, the Grimsel underground test site in the Swiss Alps has become established as a major center for study of the deep geological environment. The present Phase V of operation involves collaboration of 15 organizations from 9 countries.
The 7 major projects currently running can be divided into 3 areas:
- Confirmation of fundamental understanding and testing of models of processes identified to be significant in integrated performance assessment
- Demonstration and optimization of site characterization technology
- Demonstration of the technology for constructing and operating a deep repository in an efficient and quality assured manner.
This sub-division of projects is somewhat simplistic as, wherever possible, large, long- running experiments are designed to achieve a number of goals. The paper will provide a summary of some projects running at Grimsel, an overview of the rationale behind the experimental concepts and a perspective on possible future work.
Development and Analysis of Scenarios for a Permanent Repository for Radioactive Wastes in Salt Rock
- S. Keller
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- 21 March 2011, 901
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Scenarios were developed and analyzed as part of a long-term safety analysis of a permanent repository for radioactive wastes. A procedure is described by which features, events, and processes (FEP) at a specific site can be evaluated for the long-term safety assessment of a permanent repository for radioactive waste. These FEPs can be weighted according to selected criteria and combined to scenarios using a conceptual model of the site.
Application of Supplementary Safety Indicators for H12 Performance Assessment
- A. Takasu, M. Naito, H. Umeki, S. Masuda
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- 21 March 2011, 907
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The safety of a geological disposal system can be assessed using defined indicators such as risk and dose level, which directly measure radiological effects on human beings. Since the safety assessment of a geological disposal system covers a very long period of time, there are inherent uncertainties involved in assumptions about future human activities and environmental conditions, which increase the longer the time frame becomes.
To allow for some of these uncertainties, it has been proposed to supplement assessment results based on dose levels (or equivalent risks) by other independent indicators, without relying on assumptions made in the biosphere model. Safety indicators such as nuclide concentration, flux and radiotoxicity can usefully supplement dose calculations in the evaluation of overall system performance. In particular, they may be more indicative of the isolation capability of a disposal system and the potential risks of radioactive waste disposal over the very long timescales of safety assessment.
In the H12 study, which is the project aimed at establishing a technical basis for HLW disposal in Japan, supplementary safety indicators have been applied to increase confidence in the safety assessment. The nuclide concentrations evaluated in the geosphere and biosphere in the H12 repository system were compared with measurements of naturally occurring nuclides. The comparison indicated that the concentration of radionuclides released from the repository would be several orders of magnitude lower than that of natural radionuclides. This suggests that application of supplementary safety indicators could increase the reliability of long-term safety by more rigorous measurements of concentrations of naturally occurring radionuclides in the characterization phase at a specific site, which will be compared with concentrations of relevant nuclides predicted by site-specific performance assessment models.
An Overview of the Crystal Chemistry, Durability, and Radiation Damage Effects of Natural Pyrochlore
- G.R. Lumpkin, R.C. Ewing, C.T. Williams, A.N. Mariano
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- 21 March 2011, 921
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Numerous studies of pyrochlore group minerals have been completed over the previous 15 years, providing researchers in the field of nuclear waste disposal with a large body of data relevant to the behavior of these minerals in natural systems. The information obtained from studies of natural pyrochlore is applicable to the formulation of diverse waste form compositions and provides data for the assessment of the long-term behavior. Although resistant to dissolution, pyrochlore is subject to chemical alteration by ion exchange with hydrothermal fluids and low temperature ground water; however, Th and U are generally immobile and are retained in the structure. X-ray diffraction, TEM, EXAFS-XANES, and other techniques have been employed in studies of radiation damage. These studies reveal the classic sequence of damage microstructures with increasing dose and provide details about the structure of the amorphous state. Furthermore, the radiation damage studies are now complemented by data relating to the thermal histories of some of the host rocks.
Alteration of Uranium-Rich Microlite
- R. Gieré, E. C. Buck, R. Guggenheim, D. Mathys, E. Reusser, J. Marques
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- 21 March 2011, 935
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Microlite, a Ta-rich member of the pyrochlore group, occurs in 440 Ma old lithium pegmatites in Mozambique and exhibits a pronounced growth zoning, with a U-free core surrounded by a U-rich rim (UO2 ≤ 17 wt%). Subsequent to the uplift of the host rock, microlite was subjected to intense low-temperature alteration during which Na, Ca and F were leached from the microlite crystals. This alteration, resulting from exposure to tropical conditions, also led to localized redistribution of radiogenic Pb (formation of plumbomicrolite) and to hydration of microlite, but U remained immobile. The low-temperature alteration effects are only observed in the U-rich rim, which is characterized by abundant microfractures. As demonstrated by electron diffraction images and powder X-ray patterns, the U-rich rim is largely metamict. Our investigation illustrates the importance of natural analogues in evaluating the durability of pyrochlore-based nuclear waste-form materials over geologic time.
Alteration Features in Natural Zirconolite from Carbonatites
- C.T. Williams, A.G. Bulakh, R. Gieré, G.R. Lumpkin, A.N. Mariano
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- 21 March 2011, 945
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In nature, zirconolite occurs as an accessory mineral in many different rock types, but the majority of reported occurrences are from carbonatites (magmatic carbonates) of geological age varying from a few million years to 2 billion years old. Within these 19 carbonatite occurrences, of which 15 have been studied in some detail, zirconolite displays varying degrees of alteration in six samples. This alteration ranges from incipient minor effects to major corrosion, recrystallization and complete replacement by secondary phases. The degree of alteration broadly correlates with either the age, or actinide content of the zirconolite (or both), and thus the extent and degree of metamictization. Changes in zirconolite composition with alteration include an increase in hydration (H2O), Si, Ba and Pb (possibly radiogenic in origin), and a decrease primarily in Ca and Fe. Th can be remobilized, and of the rare earth elements (REE), there is evidence that the heavy-REE are mobilized more readily than the light-REE. Using backscattered electron images and electron microprobe analyses, this study documents and illustrates the range of alteration features observed in zirconolite from several carbonatites, in terms of both compositional and textural changes, and provides some physico-chemical information on the fluids responsible for the alteration.
Galena Crystallization and the Origin of Sulfur in the Oklo and Bangombé Natural Reactors: The Effects of a ca. 900 Ma Thermal Event
- Lena Zetterström, Torbjörn Sunde
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- 21 March 2011, 953
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Galena in the ca. 1950 Ma old natural fossil fission reactors in Gabon crystallized sometime between 980 Ma and 750 Ma during a period of regional extension and the intrusion of a dolerite dyke swarm. The S isotopic composition of galena, containing radiogenic Pb from uraninite, gives information about the origin of the S. Results from ion microprobe analyses of galena from the reactor zones indicate that S mainly originates from the surrounding sediment. Galena in a thin, altered dolerite dyke also contains non-magmatic S. The presented data gives no positive evidence for the involvement of magmatic S during the ca. 900 Ma galena crystallisation, however, the possibility cannot be ruled out.
Isotopic Fractionation of U in Rocks Reflecting Redox Conditions around a Groundwater Flow Route
- Juhani Suksi, Kari Rasilainen
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- 21 March 2011, 961
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Abstract; Low 234U/238U activity ratios observed in rock and mineral samples were scrutinized. U isotope fractionation leading to 234U depletion (234U/238U<1) in rocks appears to be linked to changes in redox conditions. The fractionation takes place as selective chemical release dominates over direct physical μ recoil. This preferential 234U release depends on the valence contrast between the U isotopes, 238U occurring in +4 form and ingrown 234U, due to oxidizing microenvironment, in +6 form. Observed U isotopic fractionation combined with other uranium series disequilibrium measurements provides a tool for locating redox fronts formed as a result low temperature rock-groundwater interaction.
Field and Laboratory Examination of Uranium Microcrystallization and Its Role in Uranium Transport
- Takashi Murakami, Toshihiko Ohnuki, Hiroshi Isobe, Tsutomu Sato
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- 21 March 2011, 971
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Adsorption is believed to be a dominant mechanism of uranium distribution between solid and solution, and thus, to play a major role in uranium transport. Because iron oxides and hydroxides are abundant at the Earth's surface and are great adsorbents of uranium, we have examined natural rocks that contain iron minerals along with uranium, and also carried out Fe-U coprecipitation and aging experiments to find how uranium is distributed between Fe minerals. Transmission and scanning electron microscopy reveals that microcrystals (10-50 nm) of metatorbernite (Cu(UO2)2(PO4)28H2O) are scattered within nodules consisting of fine-grained (2-50 nm) goethite and hematite, where the ground water is undersaturated with respect to metatorbernite, for the natural rocks from the Koongarra ore deposit, Australia. The microscopy also reveals that microcrystals (a few nm) of dehydrated schoepite ((UO2)O0.25(OH)1.5) are formed among fine-grained hematite after aging coprecipitated Fe-U in the laboratory, and the solution is undersaturated with respect to schoepite. The beam size of microscopes is found to be important for the chemical analysis of such microcrystals. We detect a strong signal of uranium for a beam size < 40 nm; whereas a weak uranium signal is obtained for a beam size > 150 nm. Our results indicate that such a weak uranium signal should not be taken as a result of homogeneously distributed uranium over goethite and hematite surfaces by, for instance, adsorption. The micrcrystallization observed in both the field and laboratory suggests that fine grained uranyl minerals play a major role in uranium transport and migration.
Geochemistry of Hydrothermal Veins Containing Zirconolite and Betafite at Adamello, Italy
- R. Gieré, G.R. Lumpkin, C.T. Williams, K.L. Smith, T.E. Payne, P.J. Mcglinn, K.P. Hart, F. Oberli
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- 21 March 2011, 979
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Hydrothermal veins containing zirconolite and betafite occur in dolomite marbles at the contact with a tonalite intrusion of the Adamello batholith, Italy. The veins display four distinct mineral zones and are highly enriched in Th, U, Ti, Zr, Nb, and rare earth elements (REE) relative to their host rock. Wide ranges in concentration of these elements and distinct inter- element fractionation trends exist across the four vein zones. The behavior of Th closely reflects that of P, Ti, Nb, and heavy REE, but was distinct from that of U, Zr and light REE (La to Sm). The presence and composition of refractory minerals such as zirconolite, betafite, uraninite, thorianite, baddeleyite, rutile, allanite, and aeschynite provide evidence for the transport of Ti, Zr, Nb, REE, and actinides by a fluid, which was further characterized by a low pH and high contents of F, P, Cl and H2S. Thermodynamic analysis of these veins indicates that they were formed at 550-600°C, 200 MPa total pressure, and from a fluid derived from the tonalite. Mineralogical and textural observations suggest that crystallization of the refractory minerals was in part induced by precipitation of fluorapatite and sulfides. The geochemical data further indicate that precipitation of light REE was induced by the fluid/wall-rock interaction which led to a significant dilution of the fluid by CO2. The studied veins provide an example of high- temperature transport of actinides and REE in the Earth's crust and show that these elements can be precipitated from a fluid and subsequently immobilized by zirconolite and betafite, two important actinide hosts in ceramic nuclear waste forms.
Partitioning of Actinides, Rare Earth Elements, and Other Trace Elements In Titanium-Rich Veins From Adamello, Italy
- Gregory R. Lumpkin, Reto Gieré, T.E. Payne, P.J. McGlinn, K.P. Hart
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- 21 March 2011, 989
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Extensive mineralogical and chemical studies have been carried out on the Ti-rich hydrothermal veins emplaced within the contact aureole of the Adamello batholith. In addition to other actinide and rare earth element host phases, the veins contain both zirconolite and betafite and provide information relevant to ceramic wasteforms designed for the disposal of actinide-rich nuclear wastes. In this paper, we describe the results of element partitioning studies based on dissolution experiments using 9M HCl. Generally, the acid-resistant minerals include allanite, baddeleyite, betafite, chalcopyrite, geikielite, titanite, spinel, and zirconolite. We also found that the major silicate minerals forsterite, phlogopite, and titanian clinohumite and the sulfide mineral pyrrhotite are partially dissolved by the acid treatment, whereas calcite and apatite are highly soluble (as expected). In particular, the distributions of Th and U between the acid-resistant and acid-soluble fractions indicate that they partition mainly between zirconolite, titanite, betafite, and apatite. However, there is a considerable increase in the amounts of Zr, Nb, Th, and U released in certain actinide-rich samples that may result from enhanced dissolution following radiation damage.
Surveying for Migration Pathways in the Granitic Rock Using Nuclear Track Detectors, Autoradiography and Digital Image Analysis as an Aid To Construct the Basis for Heterogeneous Diffusion Modeling
- M. Kemppainen, E. Oila, M. Siitari-Kauppi, P. Sardini, K-H. Hellmuth
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- 21 March 2011, 999
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Radioelement migration within a rock matrix under natural long-term conditions is a complex process controlled by various parameters. Pure physical parameters such as porosity, hydraulic conductivity and diffusivity are usually sufficient to describe transport in well-defined laboratory systems. In natural rock matrices transport is influenced by physical pore properties such as pore size distribution, connectivity, tortuosity, constrictivity and petrological and chemical nature and charge on the fluid-rock interface. The overall characterization of heterogeneous rock structures is needed for the accurate heterogeneous diffusion modeling.
Here we describe a method for the detection of μ-particles from uranium in cm-scale rock samples based on the analysis of the tracks formed in organic polymer, CR-39. On the other hand the uranium tracks were compared with the migration pathways and porosity distribution produced with the 14C-polymethylmethacrylate impregnation method (14C-PMMA). For analyzing mineral specific uranium occurrence and porosities the staining methods were used to produce the mineral map of the rock sample. Digital image analysis techniques were applied to the different cm-scale pictures of rock samples. Scanning electron microscopy and energy dispersive X-ray analysis (SEM/EDS) were performed in order both to study the pore apertures of grain boundaries and fissures in greater detail and to detect the uranium phases.
The high uranium content was found to be congruent with the porous mineral phases; altered plagioclase and biotite grains, and the intra- and intergranular fissures detected with the 14C-PMMA technique. Plenty of microfractures transsecting potassium feldspar and quartz grains were filled with calcite together with precipitated uranium.
Remedial Action for Radioactive Waste Rock Piles in China
- Chen Zhangru, Jin Yuanxin, Song Lanying, Wu Qingyan
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- 21 March 2011, 1009
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The Chinese government has conducted remedial action for radioactive waste rock piles since 1990. The radioactive waste rocks produced in the course of geological exploration of uranium deposits are widely distributed over several hundreds of locations in more than 20 provinces in China.
The following remedial actions for radioactive waste rock piles have been undertaken in China:
1. Protecting dams for stabilizing the piles have been built. A total length of about 50,000 m of these dams has been completed.
2. Soil layers have been emplaced on the radioactive waste rock piles. The focus of the action is to eliminate the radiation harmful to public health. A total area of 750,000 square meters of soil ahs been placed on the piles in China.
3. Radioactive waste rock piles have been vegetated. Aesthetic shaping of the landscape in the exploratory district of the uranium deposits is our expectant goal. A total area of about 560,000 square meters of vegetation has been placed on the soil covering the piles.
Through these remedial actions, the environmental situation has been extensively improved in the remedial districts. The individual annual effective dose equivalent is less than 1 Sv/a. The radon emission rate is less than 20 pCi/m2.s. The gamma ray external exposure rate has been greatly reduced.
Influence of Colloids in Experimental Solubility Studies of Ni in Natural and Synthetic Aqueous Media
- Ulla Vuorinen, Torbjörn Carlsson, Tommi Kekki
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- 21 March 2011, 1015
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The estimation of nickel solubility values for performance assessment is impaired by the lack of experimentally verified solubility values in conditions relevant to actual nuclear waste disposal sites. For this reason, experimental nickel solubility studies were conducted in conditions relevant to a possible Finnish disposal site, Olkiluoto. Using an initial nickel concentration of 10−3 M, nickel solubility was approached from over-saturation in three aqueous media – natural fresh groundwater, synthetic saline groundwater, and cement-conditioned groundwater with added ferrous iron and sulfide. In addition to the nickel solubility values, nickel association with the particle and colloidal phases was measured. Almost 100%of the initial nickel was found associated with the separated particle phase in the natural groundwater and the cement-conditioned groundwater, and in only the natural groundwater about 0.04%or 0.4%with the separated colloidal fraction depending on the initially added amount of ferrous iron and sulfide. In the synthetic saline groundwater, all the nickel was found in the soluble fraction except for a minor fraction sorbed onto the walls of the sample bottle. Colloid-bound nickel did not have any practical influence on the solubility values obtained. The inverse behavior of Ni solubility and sorption in the natural groundwater compared to that in the other two aqueous phases studied was suspected to be caused by the unidentified yellow color of the groundwater for which indications of both possible organic and inorganic nature was obtained. Based on the measured results, the presence of sulfide and ferrous iron would decrease the amount of mobile nickel, both soluble and colloidal, in non-saline natural and cement-conditioned groundwater, whereas in saline groundwater nickel remains in the mobile phases.
Insights to Repository Performance through Study of a Nuclear Test Site
- D.K. Smith, A.B. Kersting, J.L. Thompson, D.L. Finnegan
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- 21 March 2011, 1023
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Underground nuclear test sites offer an unprecedented opportunity to evaluate processes relevant to high-level waste repository performance in the absence of engineered barriers. Radionuclide migration programs at the Nevada Test Site represent a twenty-five year systematic investigation of the diverse radiologic source terms residual from weapons testing and the evolution of the hydrologic source term which comprises those radionuclides dissolved in or otherwise available for transport by groundwater. The Nevada Test Site shares actinide source terms, correlative geology, an identical tectonic setting, similar climate, and a thick unsaturated zone with the adjacent potential Yucca Mountain high-level waste repository and provides a natural laboratory to assess long-term radionuclide transport in the near field. Analog studies may ultimately help validate predictions of radionuclide transport from the potential Yucca Mountain repository.
Building Confidence in Radionuclide Transport Models for Fractured Rock: The Nagra/JNC Radionuclide Retardation Programme
- K. Ota, W.R. Alexander, P.A. Smith, A. Möri, B. Frieg, U. Frick, H. Umeki, K. Amano, M.M. Cowper, J.A. Berry
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- 21 March 2011, 1033
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The joint Nagra/JNC Radionuclide Retardation Programme has now been ongoing for 15 years with the main aim of direct testing of radionuclide transport models in as realistic a manner as possible. A large programme of field, laboratory and natural analogue studies has been carried out at the Grimsel Test Site in the central Swiss Alps and the Kamaishi In Situ Test Site in north-east Japan. The understanding and modelling of both the processes and the structures influencing radionuclide transport/retardation in fractured host rocks have matured as has the experimental technology, which has contributed to develop confidence in the applicability of the underlying research models in a repository performance assessment. In this paper, the successes and set-backs of this programme are discussed as is the general approach to the thorough testing of the process models and of model assumptions. In addition, a set of key findings is presented, involving discussions on the enhancement of confidence through the program.
Revisiting the Advection-Dispersion Model Testing an Alternative
- Ivars Neretnieks
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- 21 March 2011, 1043
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Some of the basic assumptions of the Advection-Dispersion model, AD-model, are revisited. That model assumes a continuous mixing along the flowpath similar to Fickian diffusion. This implies that there is a constant dispersion length irrespective of observation distance. This is contrary to most field observations. The properties of an alternative model based on the assumption that individual water packages can retain their identity over long distances are investigated. The latter model is called the Multi-Channel model, MCh-model. Inherent in the latter model is that if the waters in the different pathways are collected and mixed, the “dispersion length” is proportional to observation distance.
Using diffusion theory it is investigated over which distances or contact times, adjacent water packages will keep their identity. It is found that for a contact time of 10 hours, two streams, each wider than 6 mm, that flow side by side, will not have lost their identity. For 1000 hours contact time the minimum width is 6 cm.
The MCh and AD-models were found to have very similar Residence Time Distributions, RTD, for Peclet numbers larger than 3. A generalized relation between flowrate and residence time is developed, including the so-called cubic law and constant aperture assumptions. Using the generalized relation, surprisingly it is found that for a system that has the same average flow volume and average flowrate the form of the RTD curves are the same irrespective of the form of the relation.
Both models are also compared for a system where there is strong interaction of the solute with the rock matrix. In this case it is assumed that the solute can diffuse into and out of the fracture walls and also to sorb on the micro-fractures of the matrix. The so-called Flow Wetted Surface, FWS, between the flowing water in the fracture and the rock is a key entity in such systems. It is found that the AD-model predicts much later arrivals and lower concentrations than does the MCh-model for strongly sorbing solutes.
The conditions are explored for when in-filling particles in the fracture will not be equilibrated but will act as if there was seemingly a much larger FWS. It is found that for strongly sorbing tracers, relatively small particles can act in this way for systems and conditions that are typical of many tracer tests.
Conditions for when uptake into stagnant zones of water in the fracture itself could be important are also explored. It is found that this mechanism can be quite important in fractures with large apertures.
The assumption that the tracer residence time found by cautiously injecting a small stream of traced water represents the residence time in the whole fracture is explored. It is found that the traced stream can potentially sample a much larger fraction of the fracture than the ratio between the traced flowrate and the total pumped flowrate. In some recent field experiments the visually observed fracture apertures indicate that this may well be the case and possibly a more than two times larger fraction of the fracture is sampled than the flow rate ratio would indicate. This of course has an impact on the Flow Wetted Surface the traced stream contacts.
The MCh-model was used to simulate some recent tracer tests in a single fracture at the ÄSPÖ Hard rock laboratory in Sweden. Non-sorbing tracers, HTO and Uranin were used to determine the mean residence time and its variance. Laboratory data on diffusion and sorption properties were used to “predict” the RTD of the sorbing tracers. It was found that diffusion into stagnant zones of water in the fracture could be 10 to 300 times larger than the uptake into the rock matrix in these experiments. This was also found to give good agreement with the experiments for the non sorbing and weakly sorbing tracers. For the strongly sorbing tracers it was necessary to invoke a 2-6 times stronger interaction with the material in the stagnant zones than expected. Some reasons for such behavior are explored.
The transmissivity variance found from the five bore-holes intersecting the fracture where the tracer experiments were made were used to determine the exponent in the generalized relation between aperture and flowrate. The equivalent to the exponent 3 in the cubic law was found to be 1.3-1.6.
Discussion on the Use of Matrix Diffusion Model after a Multidisciplinary Study of a Granitic Boulder Sample
- N. Marcos, M. Siitari-Kauppi, J. Suksi, K. Rasilainen, R. Finch, K. -H. Hellmuth
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- 21 March 2011, 1053
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This paper aims to discuss the applicability of the classical matrix diffusion model against the integrated body of new data obtained by different methodologies on several samples of three granite boulders. The matrix diffusion model was tested against observations from the upper (most weathered in contact with air) and lower (fresh in contact with the ground) part of a boulder block. A U(VI) enrichment up to nearly 300 ppm (compared to about 10 ppm background concentration) mostly as uranophane was observed in the zone between the weathered and fresh rock. U-series disequilibrium studies indicated that most of U has been accumulated recently, about 10 000 years ago [1].
High interconnected porosity (total porosity of > 1% and up to about 5.5% in altered minerals) characterizes the weathered zone (upper part), whereas the maximum porosity values in the fresh zone (lower part) of the rock are about 0.4 – 0.6%. Stable isotope studies δ18O and δ2H confirm that the mineralogical changes observed in the weathered upper part are due to old hydrothermal events. That is, the alteration is much older than uranium accumulation. Mössbauer spectroscopy showed that the Fe(III) content of the biotites from the upper to the lower part decreases from 30% in the weathered zone to 17% in the fresh rock, thus indicating possible redox control for the observed U precipitation. Fission track studies showed that secondary U(VI) also occurs within minerals grains (especially plagioclase) in the upper part.
Mathematical simulations indicate that matrix diffusion alone is not enough to reconstruct the past U accumulation. The simulated concentrations derived from U concentration in pore water multiplied by Kd are clearly too small, indicating apparent insufficiency of the Kd approach. However, even with only matrix diffusion, the simulations roughly reconstruct the observation that U levels are clearly higher in the upper part of the boulder than in the lower part.
Brownian Simulation of Matrix Diffusion
- Sofie Andersson, Allan T. Emrén
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- 21 March 2011, 1065
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The commonly used approach in dealing with matrix diffusion is to assign an effective diffusion constant for the radionuclide in the rock matrix. The idea behind this approach is that, on a scale much larger than the pore size, the irregularities tend to cancel out. Although it might look plausible at first sight, this approach has been questioned both for theoretical and experimental reasons.
Here, Brownian simulation has been used to investigate the transport of dissolved material in a rock matrix modeled as a system of pores with a wide variability in size and shape. The Boltzmann distribution is used locally, although the system globally is far from equilibrium.
The simulation consists of two main parts. First, the model rock is formed by precipitation of irregular mineral grains from a liquid phase. As the grains grow, they tend to form a mostly solid piece of rock.
In the second part of the simulation, a dissolved species is introduced at one side of the rock and allowed to diffuse through its pore system. It is found that no apparent diffusion constant, D, can explain the properties of the system. Rather, D is found to be a function of both distance and time.
Surface Conductivity and Diffusion Models - Comparison and Evaluation -
- Y. Ohlsson, K. Arnerdal, I. Neretnieks
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- 21 March 2011, 1073
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The interest for studying the mobility of near surface cat-ions in rock and clay pores has increased during the last 3-4 years. Several researchers have worked experimentally with liquid phase diffusion experiments and with electrical conductivity measurements, and on developing models describing the phenomenon. Our own measurements have shown that surface mobility can contribute substantially to ionic transport in crystalline rock. Some recently proposed models for surface mobility are discussed.
Part of the problem in comparing different surface transport models lies within the different definitions of what the diffuse layer and the Stern layer really comprise. There are also differences in what is actually considered to be adsorbed ions and what part of these ions that can be considered mobile. We attempt to reconcile some of the different approaches by describing some very simplified concepts upon which all the models are based. This permits us to discuss the different views within one framework. Experimental results interpreted using the various models are discussed in the context of the simplified framework.