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  • 1.
    André, Magnus
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Malmström, Maria E.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Determining sorption coefficients in intact rock using an electrical potential gradient as a driving force for migration2006In: Scientific Basis for Nuclear Waste Management XXIX / [ed] VanIseghem, P, WARRENDALE, PA: MATERIALS RESEARCH SOCIETY , 2006, Vol. 932, p. 975-982Conference paper (Refereed)
    Abstract [en]

    The transport of radionuclides in indigenous rock is greatly affected by the sorption of cations in the porous rock matrix. For the determination of sorption coefficients, batch experiments have traditionally been used. For these experiments, the rock sample is crushed into fine particles to reduce the experimental time. However, this procedure increases the specific surface area of the sample and the new surfaces created could have different sorption qualities than the naturally occurring surfaces, which may impair the results of sorption coefficient determinations. A new method for determining sorption coefficients in intact rock is being developed, using electromigration as a means to speed up the transport process, thereby allowing for faster equilibration between the rock sample and the tracer solution. Here, we report results from preliminary experiments, using cesium as a sorbing tracer, showing a consistent difference between sorption coefficients obtained using electromigration methods on intact rock samples and traditional batch experiments on crushed samples.

  • 2.
    André, Magnus
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Malmström, Maria
    KTH, School of Industrial Engineering and Management (ITM), Industrial Ecology.
    Neretnieks, Ivar
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Determination of sorption properties of intact rock samples: New methods based on electromigration2009In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 103, no 3-4, p. 71-81Article in journal (Refereed)
    Abstract [en]

    Two new methods for determining sorption coefficients in large rock samples have been developed. The methods use electromigration as a means to speed up the transport process, allowing for fast equilibration between rock sample and tracer solution. An electrical potential gradient acts as a driving force for transport in addition to the concentration gradient and forces the cations through the rock sample towards the cathode. The electrical potential gradient induces both electromigration and electroosmotic flow with a resulting solute transport that is large compared to diffusive fluxes. In one of the methods, the solute is driven through the sample and collected at the cutlet side. In the other, simpler method, the rock sample is equilibrated by circulating the solute through the sample. The equilibration of rock samples, up to 5 cm in length, with an aqueous solution has been accomplished within days to months. Experiments using cesium as a sorbing tracer yield results consistent with considerably more time demanding in-diffusion experiments. These methods give lower distribution coefficients than those obtained using traditional batch experiments with crushed rock. (C) 2008 Elsevier B.V. All rights reserved.

  • 3.
    André, Magnus
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Malmström, Maria
    KTH, School of Industrial Engineering and Management (ITM), Industrial Ecology.
    Neretnieks, Ivar
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Rapid surface area determination of crystalline rock using impedance spectroscopyManuscript (preprint) (Other academic)
  • 4.
    André, Magnus
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Malmström, Maria
    KTH, School of Industrial Engineering and Management (ITM), Industrial Ecology.
    Neretnieks, Ivar
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Specific surface area determinations on intact drillcores and evaluation of extrapolation methods for rock matrix surfaces2009In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 110, no 1-2, p. 1-8Article in journal (Refereed)
    Abstract [en]

    Permanent storage of spent nuclear fuel in crystalline bedrock is investigated in several countries. For this storage scenario, the host rock is the third and final barrier for radionuclide migration. Sorption reactions in the crystalline rock matrix have strong retardative effects on the transport of radionuclides. To assess the barrier properties of the host rock it is important to have sorption data representative of the undisturbed host rock conditions. Sorption data is in the majority of reported cases determined using crushed rock. Crushing has been shown to increase a rock samples sorption capacity by creating additional surfaces. There are several problems with such an extrapolation. In studies where this problem is addressed, simple models relating the specific surface area to the particle size are used to extrapolate experimental data to a value representative of the host rock conditions. In this article, we report and compare surface area data of five size fractions of crushed granite and of 100 mm long drillcores as determined by the Brunauer Emmet Teller (BET)-method using N-2-gas. Special sample holders that could hold large specimen were developed for the BET measurements. Surface area data on rock samples as large as the drillcore has not previously been published. An analysis of this data show that the extrapolated value for intact rock obtained from measurements on crushed material was larger than the determined specific surface area of the drillcores, in some cases with more than 1000%. Our results show that the use of data from crushed material and current models to extrapolate specific surface areas for host rock conditions can lead to over estimation interpretations of sorption ability. The shortcomings of the extrapolation model are discussed and possible explanations for the deviation from experimental data are proposed.

  • 5.
    André, Magnus
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivar
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Malmström, Maria
    KTH, School of Industrial Engineering and Management (ITM), Industrial Ecology.
    Measuring sorption coefficients and BET surface areas on intact drillcore and crushed granite samples2008In: Radiochimica Acta, ISSN 0033-8230, E-ISSN 2193-3405, Vol. 96, no 9-11, p. 673-677Article in journal (Refereed)
    Abstract [en]

    In general sorption coefficients, K-d cat ion exchange capacity, CEC, and BET surface areas are measured on crushed rock samples because it is very time consuming to measure K-d and CEC on larger rock pieces as it takes a long time for the sorbing species to penetrate into and equilibrate a large sample. Also conventional sample holders for BET measurements are too small to hold a large sample. We have manufactured large sample holders for BET measurements and modified the equipment so that it is possible to measure BET surface areas on samples with 50 mm diameter and LIP to 100 mm length. Results are presented for intact pieces and compared to results on crushed material from the same drillcore. For K-d and CEC measurements we have developed a technique and equipment by which ions can be made to rapidly intrude into and equilibrate the internal surfaces of the same size samples as mentioned above. The method is based on electro-migration where the sample is placed between two vessels one with an anode and other with a cathode. The electric potential gradient drives the ions into and through the sample very much faster than molecular diffusion does. With Cs as the sorbing ion a few weeks were sufficient to equilibrate the 50 mm long sample. In previous diffusion experiments it took more than a year to equilibrate a 15 mm thick sample. A special mixing technique eliminates the development of low and high PH in the electrode compartments. K-d results from measurements on an intact drillcore are presented and comparison is made with results obtained on crushed material from the same bore core. The results from the sorption experiments are compared with the results from the BET surface area determinations in an attempt to evaluate the use of the BET surface area as a proxy for sorption behaviour.

  • 6.
    Arjmand, Mehdi
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Liu, Longcheng
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Exergetic efficiency of high-temperature-lift chemical heat pump (CHP) based on CaO/CO2 and CaO/H2O working pairs2013In: International journal of energy research (Print), ISSN 0363-907X, E-ISSN 1099-114X, Vol. 37, no 9, p. 1122-1131Article in journal (Refereed)
    Abstract [en]

    The use of reversible chemical reactions in recuperation of heat has gained significant interest due to higher magnitude of reaction heat compared to that of the latent or sensible heat. To implement chemical reactions for upgrading heat, a chemical heat pump (CHP) may be used. A CHP uses a reversible chemical reaction where the forward and the reverse reactions take place at two different temperatures, thus allowing heat to be upgraded or degraded depending on the mode of operation. In this work, an exergetic efficiency model for a CHP operating in the temperature-level amplification mode has been developed. The first law and the exergetic efficiencies are compared for two working pairs, namely, CaO/CO2 and CaO/H2O for high-temperature high-lift CHPs. The exergetic efficiency increases for both working pairs with increase in task, TH, decrease in heat source, TM, and increase in condenser, TL, temperatures. It is also observed that the difference in reaction enthalpies and specific heats of the involving reactants affects the extent of increase or decrease in the exergetic efficiency of the CHP operating for temperature-level amplification.

  • 7. Bond, A. E.
    et al.
    Bruský, I.
    Chittenden, N.
    Feng, X. -T
    Kolditz, O.
    Lang, P.
    Lu, R.
    McDermott, C.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Pan, P. -Z
    Šembera, J.
    Shao, H.
    Yasuhara, H.
    Zheng, H.
    Development of approaches for modelling coupled thermal–hydraulic–mechanical–chemical processes in single granite fracture experiments2016In: Environmental Earth Sciences, ISSN 1866-6280, E-ISSN 1866-6299, Vol. 75, no 19, article id 1313Article in journal (Refereed)
    Abstract [en]

    The geological formation immediately surrounding a nuclear waste disposal facility has the potential to undergo a complex set of physical and chemical processes starting from construction and continuing many years after closure. The DECOVALEX project (DEvelopment of COupled models and their VALidation against EXperiments) was established and maintained by a variety of waste management organizations, regulators and research organizations to help improve capabilities in experimental interpretation, numerical modelling and blind prediction of complex coupled systems. In the present round of DECOVALEX (D-2015), one component of Task C1 has considered the detailed experimental work of Yasuhara et al. (Appl Geochem 26:2074–2088, 2011), wherein three natural fractures in Mizunami granite are subject to variable fluid flows, mechanical confining pressure and different applied temperatures. This paper presents a synthesis of the completed work of six separate research teams, building on work considering a single synthetic fracture in novaculite. A range of approaches are presented including full geochemical reactive transport modelling and 2D and 3D high-resolution coupled thermo–hydro–mechanical–chemical (THMC) models. The work shows that reasonable fits can be obtained to the experimental data using a variety of approaches, but considerable uncertainty remains as to the relative importance of competing process sets. The work also illustrates that a good understanding of fracture topography, interaction with the granite matrix, a good understanding of the geochemistry and the associated multi-scale THMC process behaviours is a necessary pre-cursor to considering predictive models of such a system.

  • 8. Bond, A. E.
    et al.
    Chittenden, N.
    Fedors, R.
    Lang, P.
    McDermott, C.
    Neretnieks, Ivars
    KTH.
    Pan, P. Z.
    Šembera, J.
    Bruský, I.
    Watanabe, N.
    Lu, R.
    Yasuhara, H.
    Coupled THMC modelling of single fractures in novaculite and granite2018In: 2nd International Discrete Fracture Network Engineering Conference, DFNE 2018, American Rock Mechanics Association (ARMA), 2018Conference paper (Refereed)
    Abstract [en]

    The host rock immediately surrounding a nuclear waste repository has the potential to undergo a complex set of physical and chemical processes starting from construction of the facility and continuing until many years after closure. Understanding the relevant processes of fracture evolution may be key to supporting the attendant safety arguments for such a facility. Experimental work has been examined wherein artificial fractures in novaculite and granite are subject to a mechanical confining pressure, variable fluid flows and different applied temperatures. This paper presents a synthesis of the work of seven separate research teams. A range of approaches are summarized including detailed thermal-hydrological-mechanical-chemical (THMC) models and homogenized ‘single compartment’ models of the fracture; the latter with a view to larger network or effective continuum models. The competing roles of aqueous geochemistry, pressure solution, stress corrosion and pure mechanics were found to be significant in the reproduction of the experimental observations. The results of the work show that while good, physically plausible representations of the experiment can be obtained, there is considerable uncertainty in the relative importance of the various processes, and that the parameterization of these processes can be closely linked to the physical interpretation of the fracture surface topography.

  • 9. Bond, Alexander E.
    et al.
    Brusky, Ivan
    Cao, Tianqing
    Chittenden, Neil
    Fedors, Randall
    Feng, Xia-Ting
    Gwo, Jin-Ping
    Kolditz, Olaf
    Lang, Philipp
    McDermott, Christopher
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Pan, Peng-Zhi
    Sembera, Jan
    Shao, Hua
    Watanabe, Nori
    Yasuhara, Hide
    Zheng, Hong
    A synthesis of approaches for modelling coupled thermal-hydraulic-mechanical-chemical processes in a single novaculite fracture experiment2017In: Environmental Earth Sciences, ISSN 1866-6280, E-ISSN 1866-6299, Vol. 76, no 1, article id 12Article in journal (Refereed)
    Abstract [en]

    The geological formation immediately surrounding a nuclear waste disposal facility has the potential to undergo a complex set of physical and chemical processes starting from construction and continuing many years after closure. The DECOVALEX project (DEvelopment of COupled models and their VALidation against EXperiments) was established and maintained by a variety of waste management organisations, regulators and research organisations to help improve capabilities in experimental interpretation, numerical modelling and blind prediction of complex coupled systems. In the present round of DECOVALEX (D-2015), one component of Task C1 has considered the detailed experimental work of Yasuhara et al. (Earth Planet Sci Lett 244: 186-200, 2006), wherein a single artificial fracture in novaculite (micro-or crypto-crystalline quartz) is subject to variable fluid flows, mechanical confining pressure and different applied temperatures. This paper presents a synthesis of the completed work of six separate research teams. A range of approaches are presented including 2D and 3D high-resolution coupled thermo-hydro-mechanical-chemical models. The results of the work show that while good, physically plausible representations of the experiment can be obtained using a range of approaches, there is considerable uncertainty in the relative importance of the various processes, and that the parameterisation of these processes can be closely linked to the interpretation of the fracture surface topography at different spatial scales.

  • 10. Bozkurt, S.
    et al.
    Lucisano, M.
    Moreno, Luis
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Peat as a potential analogue for the long-term evolution in landfills2001In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 53, no 02-jan, p. 95-147Article, review/survey (Refereed)
    Abstract [en]

    A survey of the existing studies on peat and its decomposition processes is presented with the aim to characterise the long-term behaviour of peat accumulating systems. The chemical and physical characteristics of peat together with its accumulation and decay processes have been analysed. Peat is an acidic mixture of dead and decomposed. mainly vegetable, matter formed in boggy areas; it is the youngest and least altered component of the combustible rocks and is characterised by the lowest content of fixed carbon and the highest content of volatile constituents. Peat is formed by degeneration processes under exclusion of atmospheric oxygen by the action of water; the speed of formation depends upon the climatic and environmental conditions. In most peatlands two layers can be characterised: the aerobic acrotelm and the anaerobic catotelm, their relative importance being controlled mainly by the position of the water table. In the acrotelm the aerobic processes are responsible for the loss of up to 90% of the original mass, Degeneration in the acidic and anaerobic catotelm is still imperfectly characterised even though the catotelm is the real site of peat accumulation. Most of the recent literature considers peat as composed of easily degradable compounds, e.g. polysaccharides. and recalcitrant matter (lignin and complex aromatics). The lone-term destiny of peat has not been sufficiently characterised: although in a large majority of cases it seems probable that peat decomposes completely (even though slowly) provided that it is given a sufficiently long residence rime in the catotelm, some cases can still be interpreted as examples of simple accumulation. The rates of influx of oxygen and hence the degradation of organic matter into both saturated and partially saturated peat have been estimated. The depletion rate is about 4500 g m(-1) year(-1) for partially saturated peat. The average depletion rate of the peat for this case will then be such that it will take on the order of 5 to 50 years to degrade half of the organics in a 10 cm partially saturated layer. For the water-saturated case the depletion rate varies between 8 and 12 g m(-2) year(-1), which is considerably lower than in the partially saturated region. The models used to analyse the field and laboratory data on generation, diffusion and emission of methane and carbon dioxide indicate that laboratory data and field observations agree reasonably well. It is suggested that peat-accumulating ecosystems may be valuable natural analogues for the study of the long-term destiny of industrial and municipal solid wastes. Accurate studies of active mires together with an ad hoc review of the existing literature give valuable insights in this problem. Peatlands might then be considered as organic waste deposition experiments lasting up to several thousands years.

  • 11. Bozkurt, S.
    et al.
    Moreno, Luis
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Long-term processes in waste deposits2000In: Science of the Total Environment, ISSN 0048-9697, E-ISSN 1879-1026, Vol. 250, no 03-jan, p. 101-121Article in journal (Refereed)
    Abstract [en]

    A conceptual model, which is a unitary and continuous description of the overall processes in waste deposits, has been developed. In the model the most important processes governing the long-term fate of organic matter in landfills and the transport and retention of toxic metals are included. With the model as a base, a number of scenarios with different levels of complexity have been defined and studied in order to carry out long-term assessments of the chemical evolution in waste deposits for industrial and municipal solid waste containing much organic matter and the leaching of toxic metals. The focus of the modelling has been to quantify the important processes occurring after the methane production phase has ceased, i.e. during the humic phase. The scenarios include the main mechanisms based on various transport processes as well as different landfill constructions, e.g. binding capacities of sulfides and humic substances. They also include transport mechanisms by which the reactant oxygen can intrude into a deposit, sorption capacities of hydrous ferric oxides, and pH-buffering reactions, etc. Scoping calculations have shown that the binding capacity of humic substances is sufficient to bind all toxic metals (Cd, Cr, Pb, Zn and Hg). In addition, the humics could also bind a smaller part of Ca, Fe and Al, provided much of the organic waste remain as humic substances. Sulfides on the other hand can bind approximately twice the amount of all toxic metals. The binding capacity of hydrous ferric oxides, which can be formed by oxidation reactions during the humic phase, is estimated to be three times the total content of metals that can sorb on hydrous ferric oxides. In the studied landfill the pi-I-buffering capacity, primarily represented by calcite, is estimated to be 1 mol/kg dry waste. Quantifications indicate that the alkalinity of the wastes is high enough to buffer the acidity produced by the oxidation of sulfides and by the degradation of organic matter, as well as that added by acid precipitation. Therefore, the main conclusion is that higher remobilisation rates of heavy metals due to lowering of pH are not expected for many thousands of years.

  • 12. Bozkurt, S.
    et al.
    Sifvert, M.
    Moreno, Luis
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    The long-term evolution of and transport processes in a self-sustained final cover on waste deposits2001In: Science of the Total Environment, ISSN 0048-9697, E-ISSN 1879-1026, Vol. 271, no 03-jan, p. 145-168Article in journal (Refereed)
    Abstract [en]

    A new principle for confinement of waste based on a self-sustained seal is presented. The top cover is considered to consist of two main layers; an organic carbon rich surface layer that is able to support vegetation and an inorganic layer beneath it. The function of the cover is to mitigate oxidation and acidification of landfilled waste and hence the release of toxic metals. It is suggested that forest soil formation and soil development could prove to be valuable information sources for the study of the long-term behaviour of a final cover on waste deposits. Since the cover is expected to develop in northern temperate climate the focus is on Spodosol soil. A number of simulations of the long-term behaviour of the final self-sustained landfill cover are made, including the rates of influx of oxygen into the cover. A cover having a large portion of organic matter compared with a cover with no organics can considerably decrease the oxygen concentration and thus the influx of oxygen into a landfill. The calculated oxygen intrusion rate for the former case is of the order of 0.05 kg m(-2) year(-1). Degradation of the organics produces acids. Our simulations indicate that the pH-buffering capacity of the mineral layer, represented by calcite and primary rock minerals, will last for many thousands of years.

  • 13. Crawford, J.
    et al.
    Moreno, Luis
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Determination of the flow-wetted surface in fractured media2003In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 61, no 04-jan, p. 361-369Article in journal (Refereed)
    Abstract [en]

    Diffusion and sorption in the rock matrix are important retardation mechanisms for radionuclide transport in fractured media. For the conditions existing in a deep repository in crystalline rock, interaction with the rock matrix is controlled by the water flowrate in the fractures and the surface area in contact with the flowing water (the so-called flow-wetted surface (FWS)). The flow-wetted surface may be determined from the frequency of open fractures intersecting a borehole. The choice of packer distance used in these hydraulic measurements is crucial, however, since several open fractures may be found in one packer interval. The use of a packer distance that is too large may result in a considerable underestimation of the flow-wetted surface. This is especially important in zones with a high frequency of open fractures (fracture zones) where a small packer distance is a fundamental requirement. A large volume of hydraulic data has been compiled in Sweden from measurements using quite small packer distances. Over the last decade, the most common packer distance used for the hydraulic tests has been 3 m, although some new measurements using a shorter packer distance have also been performed. In several cases, the resolution of these measurements has been less than 0.5 m. All these data have been analysed in detail. From these data, the flow-wetted surface has been calculated and compared with the flow-wetted surface estimated in earlier studies. The results show the importance of using a small packer distance for carrying out borehole transmissivity measurements.

  • 14. Crawford, J.
    et al.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Simulation of the redox buffer depletion rate in landfills of combustion residue waste materials2001In: Water, Air and Soil Pollution, ISSN 0049-6979, E-ISSN 1573-2932, Vol. 128, no 04-mar, p. 223-242Article in journal (Refereed)
    Abstract [en]

    Simulations have been made of the redox buffer depletion rate in landfills of combustion residue wastes. The model is based upon a one-dimensional mass balance where oxygen transport results from a combination of advective and diffusive processes. A simple, generalised representation of redox buffering properties is used where dissolved oxygen reacts in accordance with a first order rate law. Using certain simplifying assumptions, it is possible to obtain an analytical solution for the time required for complete depletion of the redox buffering capacity of landfilled solid wastes. The results indicate the overwhelming importance that the effective diffusivity of oxygen in the landfill has upon the rate of redox buffer depletion. The results also show under which conditions transport effects may be expected to dominate the leaching processes and determine an upper limit for the depletion rate.

  • 15.
    Crawford, James
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Moreno, Luis
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Harmonisation of site characterisation and performance assessment modelling - The relative importance of surface sorption and matrix interaction phenomena2004In: Scientific Basis For Nuclear Waste Management XXVII / [ed] Oversby, VM; Werme, LO, 2004, Vol. 807, p. 551-556Conference paper (Refereed)
    Abstract [en]

    In the context of geological repositories for nuclear waste disposal, the goal of site characterisation (SC) is to obtain input data that can later be used as a basis for performance assessment calculations (PA). Performance assessment is required to give an indication as to whether the repository will behave as intended over the geological timescales relevant for risk analysis. Processes that may be important in a PA setting for constraining radionuclide transport may not necessarily be dominating, or indeed may not even be observable during SC investigations. In this paper it is shown that the migration of sorbing tracers is governed largely by surface sorption phenomena in typical SC-type field experiments whereas in a PA scenario, matrix interaction instead can be expected to play an overwhelmingly important role. This study uses data and settings that are representative for fractured rocks in Sweden, but the method used and the conclusions may have more general applicability.

  • 16.
    Crawford, James
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Epistemic and aleatory uncertainty in recommended, generic rock K-d values used in performance assessment studies2006In: Scientific Basis for Nuclear Waste Management XXIX, WARRENDALE, PA: MATERIALS RESEARCH SOCIETY , 2006, Vol. 932, p. 251-258Conference paper (Refereed)
    Abstract [en]

    Over the past decade or so there has been an explosion in the number of sorption modelling approaches and applications of sorption modelling for understanding and predicting solute transport in natural systems. The most widely used and simplest of all models, however, is that employing a constant distribution coefficient (K-d) relating the sorbed concentration of a solute on a mineral surface and its aqueous concentration. There are a number of reasons why a constant partitioning coefficient is attractive to environmental modellers for predicting radionuclide retardation, and in spite of all the shortcomings and pitfalls associated with such an approach, it remains the leitmotif of most performance assessment transport modelling. This paper examines the scientific basis underpinning the K-d-approach and its broad defensibility in a performance assessment framework. It also examines sources of epistemic and aleatory uncertainty that undermine confidence in K-d-values reported in the open literature. The paper focuses particularly upon the use of so-called "generic" data for generalised rock types that may not necessarily capture the full material property characteristics of site-specific materials. From the examination of recent literature data, it appears that there are still a number of outstanding issues concerning interpretation of experimental laboratory data that need to be considered in greater detail before concluding that the recommended values used in performance assessments are indeed conservative.

  • 17. Hudson, John A.
    et al.
    Bäckström, Ann
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
    Rutqvist, J.
    Jing, Lanru
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
    Backers, T.
    Chijimatsu, M.
    Christiansson, R.
    Feng, X. T.
    Kobayashi, A.
    Koyama, Tomofumi
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
    Lee, H. S.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Pan, P. Z.
    Rinne, M.
    Shen, B. T.
    Characterising and modelling the excavation damaged zone in crystalline rock in the context of radioactive waste disposal2009In: Environmental Geology, ISSN 0943-0105, E-ISSN 1432-0495, Vol. 57, no 6, p. 1275-1297Article in journal (Refereed)
    Abstract [en]

    This paper describes current knowledge about the nature of and potential for thermo-hydro-mechanical-chemical modelling of the excavation damaged zone (EDZ) around the excavations for an underground radioactive waste repository. In the first part of the paper, the disturbances associated with excavation are explained, together with reviews of Workshops that have been held on the subject. In the second part of the paper, the results of a DECOVALEX [DEmonstration of COupled models and their VALidation against EXperiment: research funded by an international consortium of radioactive waste regulators and implementers (http://www.decovalex.com)] research programme on modelling the EDZ are presented. Four research teams used four different models to simulate the complete stress-strain curve for Avro granite from the Swedish A"spo Hard Rock Laboratory. Subsequent research extended the work to computer simulation of the evolution of the repository using a 'wall-block model' and a 'near-field model'. This included assessing the evolution of stress, failure and permeability and time-dependent effects during repository evolution. As discussed, all the computer models are well suited to sensitivity studies for evaluating the influence of their respective supporting parameters on the complete stress-strain curve for rock and for modelling the EDZ.

  • 18. Koyama, T.
    et al.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Jing, Lanru
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
    A numerical study on differences in using Navier-Stokes and Reynolds equations for modeling the fluid flow and particle transport in single rock fractures with shear2008In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 45, no 7, p. 1082-1101Article in journal (Refereed)
    Abstract [en]

    The study on fluid flow and transport processes of rock fractures in most practical applications involves two fundamental issues: the validity of Reynolds equation for fluid flow (as most often assumed) and the effects of shear displacements on the magnitudes and anisotropy of the fluid flow velocity field. The reason for such concerns is that the impact of the surface roughness of rock fractures is still an unresolved challenging issue. The later has been systematically investigated with results showing that shear displacement plays a dominant role on evolutions of fluid velocity fields, for both magnitudes and anisotropy, but the former has not received examinations in details due to the numerical complexities involving solution of the Navier-Stokes (NS) equations and the representations of fracture geometry during shear. The objective of this paper aims to solve this problem through a FEM modeling effort. Applying the COMSOL Multiphysics code (FEM) and assuming a 2D problem, we consider the coupled hydromechanical effect of fracture geometry change due to shear on fluid flow (velocity patterns) and particle transport (streamline/velocity dispersion), using measured topographical data of natural rock fracture surfaces. The fluid flow in the vertical 2D cross-sections of single rock fractures was simulated by solving both the Navier-Stokes and the Reynolds equation, and the particle transport was predicted by the streamline particle tracking method with calculated flow velocity fields (vectors) from the flow simulations, obtaining results such as flow velocity profiles, total flow rates, particle travel time, breakthrough curves and the Peclet number, Pe, respectively. The results obtained using NS and Reynolds equations were compared to illustrate the degree of the validity of the Reynolds equation for general applications in practice since the later is mush more computationally efficient for solving large-scale problems. The flow simulation results show that both the total flow rate and the flow velocity fields in a rough rock fracture predicted by the NS equation were quite different from those predicted by the Reynolds equation. The results show that a roughly 5-10% overestimation on the flow rate is produced when the Reynolds equation is used, and the ideal parabolic velocity profiles defined by the local cubic law, when Reynolds equation is used, is no longer valid, especially when the roughness feature of the fracture surfaces changes with shear. These deviations of flow rate and flow velocity profiles across the fracture aperture have a significant impact on the particle transport behavior and the associated properties, such as the travel time and Peclet number. The deviations increase with increasing flow velocity and become more significant when fracture aperture geometry changes with shear.

  • 19.
    Koyama, Tomofumi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
    Vilarrasa, Victor
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
    Jing, Lanru
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Engineering Geology and Geophysics.
    Shear-induced flow channels and its effect on the particle transport in a single rock fractureIn: Hydrogeology Journal, ISSN 1431-2174, E-ISSN 1435-0157Article in journal (Other academic)
    Abstract [en]

    The effect of mechanical shearing on fluid flow anisotropy and particle transport in rough rock fractures was investigated using numerical modeling. Two opposite surfaces of a rock fracture of 194x194 mm in size were laser scanned to generate their respective digital profiles. Fluid flow through the fracture was simulated using a finite element code that solves the Reynolds equation, while incremental relative movement of the upper surface was maintained numerically to simulate a shearing process without normal loading. The motion of solute particles during shearing was studied using a simple particle-tracking code. It was found that shearing introduces anisotropy in both fluid transmissivity and particle motion, with a greatly increased flow rate and particle travel velocity in the direction perpendicular to the direction of shear. This finding has an important impact in the interpretation of the results of coupled hydro-mechanical and tracer transport experiments of hydraulic and transport properies of rock fractures.

  • 20. Larsson, Martin
    et al.
    Oden, Magnus
    Niemi, Auli
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Tsang, Chin-Fu
    A new approach to account for fracture aperture variability when modeling solute transport in fracture networks2013In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 49, no 4, p. 2241-2252Article in journal (Refereed)
    Abstract [en]

    A simple yet effective method is presented to include the effects of fracture aperture variability into the modeling of solute transport in fracture networks with matrix diffusion and linear sorption. Variable apertures cause different degrees of flow channeling, which in turn influence the contact area available for these retarding processes. Our approach is based on the concept of specific flow-wetted surface (sFWS), which is the fraction of the contact area over the total fracture surface area. Larsson et al. (2012) studied the relationship between sFWS and the standard deviation sigma ln K of the conductivity distribution over the fracture plane. Here an approach is presented to incorporate this into a fracture network model. With this model, solute transport through fracture networks is then analyzed. The cases of S=0 and S=1 correspond to those of no matrix diffusion and full matrix diffusion, respectively. In between, a sFWS breakpoint value can be defined, above which the median solute arrival time is proportional to the square of sFWS. For values below the critical sFWS (more channeled cases), the change is much slower, converging to that of no matrix diffusion. Results also indicate that details of assigning sFWS values for individual fractures in a network are not crucial; results of tracer transport are essentially identical to a case where all fractures have the mean sigma ln K (or corresponding mean sFWS) value. This is obviously due to the averaging effect of the network.

  • 21. Liapis, A. I.
    et al.
    Grimes, B. A.
    Lacki, K.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Modeling and analysis of the dynamic behavior of mechanisms that result in the development of inner radial humps in the concentration of a single adsorbate in the adsorbed phase of porous adsorbent particles observed in confocal scanning laser microscopy experiments: diffusional mass transfer and adsorption in the presence of an electrical double layer2001In: Journal of Chromatography A, ISSN 0021-9673, E-ISSN 1873-3778, Vol. 921, no 2, p. 135-145Article in journal (Refereed)
    Abstract [en]

    A theoretical model for adsorption of a single charged adsorbate that accounts for the presence of an electrical double layer in the pores of adsorbent particles is constructed and solved. The dynamic behavior of the mechanisms of the model can result in the development of inner radial humps (concentration rings) in the concentration of a single charged analyte (adsorbate) in the adsorbed phase of porous adsorbent particles. The results of the present work demonstrate the implication of the concept regarding the effect of the presence of an electrical double layer in the pores of adsorbent particles and the induced interactions between the electrostatic potential distribution and the mechanisms of mass transport of the species by diffusion, electrophoretic migration, and adsorption. Furthermore, the mechanisms of the model could explain qualitatively the development of the concentration ring (hump) observed in confocal scanning laser microscopy experiments.

  • 22. Liu, J. S.
    et al.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Effect of water radiolysis caused by dispersed radionuclides on oxidative dissolution of spent fuel in a final repository2001In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 135, no 2, p. 154-161Article in journal (Refereed)
    Abstract [en]

    When released out of a canister, the radionuclides originally incorporated in the spent fuel can still deposit radiation energy (even more efficiently) into the pore water, cause water radiolysis, and produce oxidants in the buffering material. This phenomenon is termed secondary water radiolysis. The oxidants thus produced can possibly diffuse back to oxidize the spent fuel and to increase the oxidative dissolution rare of the fuel, The effect of the secondary water radiolysis has been identified and preliminarily addressed by a mass-balance model. To explore whether the effect is significant on spent-fuel dissolution, the upper-boundary limit of the effect has been set up by considering a scenario that is very unlikely to occur. Several extreme assumptions have been made: First, the canister fails completely 10(3) yr after deposition; second, the sl,ent fuel is oxidized instantaneously; and third, the radionuclides considered are those that dominantly contribute to radiolysis between 10(3) to 10(5) yr. With these assumptions, the spent-fuel dissolution rate can be increased dramatically if 10% or more of the oxidants produced by the secondary water radiolysis diffuse back to oxidize the spent fuel. It thus indicates that the effect of the secondary water radiolysis could be significant with some extreme assumptions. With more realistic assumptions, the effect could possibly become minimal. The subject is worth further investigation.

  • 23. Liu, J. S.
    et al.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Study of the consequences of secondary water radiolysis surrounding a defective canister2003In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 142, no 3, p. 294-305Article in journal (Refereed)
    Abstract [en]

    In the concept of deep geological disposal of spent nuclear fuel, a chemically reducing environment in the near field of a repository is favorable for retaining the radionuclides in the fuel. Water radiolysis can possibly change a reducing environment in the near field to an oxidizing environment. In this paper, the consequences of secondary water radiolysis, caused by radionuclides released from the spent nuclear fuel and dispersed in the bentonite buffer surrounding a canister, have been studied. The canister is assumed to be initially defective with a hole of a few millimeters on its wall. The small hole will considerably restrict the transport of oxidants through the canister wall and the release of radionuclides to the outside of the canister. The spent fuel dissolution is assumed to be controlled by chemical kinetics at rates extrapolated from experimental studies. Two cases are considered. In the first case it is assumed that secondary phases of radionuclides [such as amorphous Pu(OH)(4) and AmOHCO3] do not precipitate inside the canister. The model results show that a relatively large domain of the near field can be oxidized by the oxidants of secondary radiolysis. In the second case it is assumed that secondary phases of radionuclides precipitate inside the canister, and the radionuclide concentration within the canister is controlled by its respective solubility limit. The amount of radionuclides released out of the canister will then be limited by the solubility of the secondary phases. The effect of the secondary radiolysis outside the canister on the rate of spent fuel oxidation inside a defective canister will be quite limited and can be neglected for any practical purposes in this case.

  • 24.
    Liu, Jinsong
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Coupled transport/reaction modelling of copper canister corrosion aided by microbial processes2004In: Radiochimica Acta, ISSN 0033-8230, E-ISSN 2193-3405, Vol. 92, no 11-sep, p. 849-854Article in journal (Refereed)
    Abstract [en]

    Copper canister corrosion is an important issue in the concept of a nuclear fuel repository. Previous studies indicate that the oxygen-free copper canister could hold its integrity for more than 100 000 years in the repository environment. Microbial processes may reduce sulphate to sulphide and considerably increase the amount of sulphide available for corrosion. In this paper. a coupled transport/reaction model is developed to account for the transport of chemical species produced by microbial processes. The corroding agents like sulphide would come not only from the intruding groundwater, but also from the reduction of sulphate near the canister. The reaction of sulphate-reducing bacteria and the transport of sulphide in the bentonite buffer is included in the model. The local depth of copper canister corrosion is calculated by the model.

  • 25.
    Liu, Longcheng
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    A Dynamic Force Balance Model for Colloidal Expansion and Its DLVO-Based Application2009In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 25, no 2, p. 679-687Article in journal (Refereed)
    Abstract [en]

    A force balance model that describes the dynamic expansion of colloidal bentonite gels/sols is presented. The colloidal particles are assumed to consist of one or several thin sheets with the other dimensions much larger than their thickness. The forces considered include van der Waals force, diffuse double layer force, thermal force giving rise to Brownian motion, gravity, as well as friction force. The model results in an expression resembling the instationary diffusion equation but with an immensely variable diffusivity. This diffusivity is strongly influenced by the concentration of counterions as well as by the particle concentration in the colloid gel/sol. The properties of the model are explored and discussed, exemplified by the upward expansion of an originally highly compacted bentonite tablet in a test tube. Examples are presented for a number of cases with ionic concentrations varying between very dilute waters up to several molar of counterions. The volume fraction of particles ranges from 40% to very dilute sols.

  • 26.
    Liu, Longcheng
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    A Novel Approach to Determine the Critical Coagulation Concentration of a Colloidal Dispersion with Plate-like Particles2009In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 25, no 2, p. 688-697Article in journal (Refereed)
    Abstract [en]

    The critical coagulation concentration (ccc) of counterions is commonly described by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory on the basis of a static force balance. It can, however, also be estimated from a kinetic point of view by studying the process of colloidal coagulation, or from a dynamic point of view by considering colloidal transport in nonequilibrium systems where other processes such as diffusion and the influence of gravity come into play. In particular, in a test tube where colloidal expansion takes place, the ccc can be interpreted as the electrolyte concentration below which expansion of colloids would always lead to full access to the entire volume of the test tube and above which a sharp boundary is established between a colloidal gel and pure water. On the basis of this perception and the dynamic force balance model that we developed to describe colloidal expansion in a test tube, accounting for the effects of particle diffusion and gravity in contrast to the DLVO theory, we propose an alternative way to assess the ccc of counterions. We also derive an approximate expression for the case of homointeraction at constant charge for montmorillonite. The estimated ccc values agree quite well with those observed experimentally for both Na+ and Ca2+ counterions for montmorillonite dispersions, at pH similar to 6.5. This is in contrast to the DLVO theory, which overpredicts the ccc by about 2 orders of magnitude. In addition, the detailed analyses suggest that the ccc of counterions decreases with increasing surface area and with the thickness of the particles. For montmorillonite, the ccc is nearly independent of the surface charge density of the particles for the range of typical charge densities.

  • 27.
    Liu, Longcheng
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    A coupled model for oxidative dissolution of spent fuel and transport of radionuclides from an initially defective canister2001In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 135, p. 273-285Article in journal (Refereed)
    Abstract [en]

    An earlier model for oxidative dissolution of spent fuel was developed by including the release behavior of actinides from the fuel surface and the barrier effect of Zircaloy claddings. The aim here is to explore the possibility and consequences of precipitation in the water film around the fuel pellets due to solubility, and transport limitations of nuclides. The model has been applied in the performance assessment of a damaged canister under natural repository conditions, by coupling to a redox-front-based model for transport of nuclides. The simulation results identify? that the time of penetration of the canister, the size of the damage, and the initial free volume of the fuel rods are important factors that dominate the dissolution behavior of the fuel matrix and thus the transport behavior of actinides in the nearfield of a repository.

  • 28.
    Liu, Longcheng
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    A Model for Radionuclide Release from a Defective CanisterArticle in journal (Other academic)
  • 29.
    Liu, Longcheng
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    A reactive transport model for oxidative dissolution of spent fuel and release of nuclides within a defective canister2002In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 137, p. 228-240Article in journal (Refereed)
    Abstract [en]

    In this study, we develop a mechanism-based model to take into account most of the important processes that may influence the dissolution behavior of spent fuel and subsequently the release behavior of nuclides within a defective canister in a final repository for high-level nuclear waste. The model is, in essence, a redox-controlled reactive transport model that provides a description of the mass transport of multiple species involved in both local equilibrium and kinetically controlled reactions in the system. The complexity of the kinetics of the various redox reactions involved and the requirement of the long-term prediction, however, make numerical implementation of the fully coupled model computationally inefficient. A series of scoping calculations was performed to highlight the local characteristics and behaviors of the system, and to provide a basis for refinement of the reactive transport model. The results indicate that the rapid buildup of hydrogen within the system is mainly attributed to corrosion of the cast-iron insert that primarily occurs under anaerobic conditions, rather than to radiolysis of water. The system that is rapidly in equilibrium with 50 bar hydrogen would then keep pH constant throughout the system. In addition, simulations suggest that reduction of dissolved hexavalent uranium by ferrous iron adsorbed onto the corrosion products and by dissolved H-2 are the most important mechanisms to retard the release of uranium out of the canister. More importantly, it is found that the pseudo stationary state approximation may well be applied to the system. This greatly simplifies the numerical implementation of the reactive transport model.

  • 30.
    Liu, Longcheng
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Analysis of fluid flow and solute transport in a fracture intersecting a canister with variable aperture fractures and arbitrary intersection angles2005In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 150, no 2, p. 132-144Article in journal (Refereed)
    Abstract [en]

    A multitude of simulations have been made for different types of rough-walled fractures, by using FEM-LAB((R)), to evaluate the mass transfer to and from water flowing through a fracture with spatially variable apertures and with an arbitrary angle of intersection to a canister that contains spent nuclear fuel. This paper presents and discusses only the results obtained for the Gaussian fractures. The simulations suggest that the intersection angle has only a minor influence on both the volumetric and the equivalent flow rates. The standard deviation of the distribution of the volumetric flow rates of the many realizations increases with increasing roughness and spatial correlation length of the aperture field, and so does that of the equivalent flow rates. The mean of the distribution of the volumetric flow rates is determined, however, solely by the hydraulic aperture, while that of the equivalent flow rates is determined by the mechanical aperture. Based upon the analytical solutions for the parallel plate model, it has been found that the distributions of both the volumetric and the equivalent flow rates are close to the Normal. Thus, two simple expressions can be devised to quantify the stochastic properties of fluid flow and solute transport through spatially variable fractures without making detailed calculations in every fracture intersecting a deposition hole or a tunnel.

  • 31.
    Liu, Longcheng
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Analysis of fluid flow and solute transport through a single fracture with variable apertures intersecting a canister: Comparison between fractal and Gaussian fractures2006In: Physics and Chemistry of the Earth, ISSN 1474-7065, E-ISSN 1873-5193, Vol. 31, no 14-okt, p. 634-639Article in journal (Refereed)
    Abstract [en]

    Canisters with spent nuclear fuel will be deposited in fractured crystalline rock in the Swedish concept for a final repository. The fractures intersect the canister holes at different angles and they have variable apertures and therefore locally varying flowrates. Our previous model with fractures with a constant aperture and a 90 degrees intersection angle is now extended to arbitrary intersection angles and stochastically variable apertures. It is shown that the previous basic model can be simply amended to account for these effects. More importantly, it has been found that the distributions of the volumetric and the equivalent flow rates are all close to the Normal for both fractal and Gaussian fractures, with the mean of the distribution of the volumetric flow rate being determined solely by the hydraulic aperture, and that of the equivalent flow rate being determined by the mechanical aperture. Moreover, the standard deviation of the volumetric flow rates of the many realizations increases with increasing roughness and spatial correlation length of the aperture field, and so does that of the equivalent flow rates. Thus, two simple statistical relations can be developed to describe the stochastic properties of fluid flow and solute transport through a single fracture with spatially variable apertures. This obviates, then, the need to simulate each fracture that intersects a canister in great detail, and allows the use of complex fractures also in very large fracture network models used in performance assessment.

  • 32.
    Liu, Longcheng
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Fluid flow and solute transport though a fracture intersecting a canister - Analytical solutions for the parallel plate model2004In: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXVII / [ed] Oversby, VM; Werme, LO, 2004, Vol. 807, p. 749-754Conference paper (Refereed)
    Abstract [en]

    In this paper, we are concerned with a specific scenario where a large fracture intersects, at its center, a canister that contains spent nuclear fuel. Assuming that a nuclide is free to release from the canister into groundwater flowing through the fracture, a detailed formulation of the volumetric flow rate and the equivalent flow rate are made for the parallel plate model. The formulas proposed have been validated by numerical examinations; they are not only simple in forms but also universal in applications where the flow may be taken normal, inclined or parallel to the axis of the canister. Of great importance, they provide a convenient way to predict the average properties of fluid flow and solute transport through a single fracture with spatially variable apertures.

  • 33.
    Liu, Longcheng
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Homo-interaction between parallel plates at constant charge2008In: Colloids and Surfaces A: Physicochemical and Engineering Aspects, ISSN 0927-7757, E-ISSN 1873-4359, Vol. 317, no 1-3, p. 636-642Article in journal (Refereed)
    Abstract [en]

    Various approximate solutions to the Poisson-Boltzmann (PB) equation have been derived to describe the interaction of electric diffuse double layers adjacent to charged surfaces. However, all these expressions are case-specific and accurate only in limited ranges of particle separations. None can cover the entire range of plate separations and/or surface charge densities generally found in real systems. In this paper, we derive an approximate expression for the force between two parallel similar plates with constant surface charge densities in a symmetrical electrolyte solution, which agrees well with the rather complex exact analytical solution over a wide range of plate separations. The method used is based on the so-called "compression" approach developed previously for the case of low surface charge densities. The results are also in good agreement with "exact" numerical solutions over a wide range where no restriction is actually required on the magnitudes of the surface charge densities, surface potentials or the distance between the plates. Furthermore, an expression for the derivative of the force is also given, which is fairly simple and is very useful in modelling, e.g. colloidal transport problems based on a force balance on particles in a colloidal system. In such cases it is very impractical to use either the exact analytical or numerical solution to the PB,equation.

  • 34.
    Liu, Longcheng
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Sensitivity Analysis of Uranium Solubility under Strongly Oxidizing Conditions1998In: Proc. Scientific Basis for Nuclear WasteManagement XXII,, 1998, p. 1001-Conference paper (Other academic)
    Abstract [en]

    To evaluate the effect of geochemical conditions in the repository on the solubility of uranium under strongly oxidizing conditions, a mathematical model has been developed to determine the solubility, by utilizing a set of non linear algebraic equations to describe the chemical equilibria in the groundwater environment. The model takes into account the predominant precipitation-dissolution reactions, hydrolysis reactions and complexation reactions that may occur under strongly oxidizing conditions. The model also includes the solubilitylimiting solids induced by the presence of carbonate, phosphate, silicate, calcium, and sodium in the groundwater. The thermodynamic equilibrium constants used in the solubility calculations are essentially taken from the NEA Thermochemical Data Base of Uranium, with some modification and some uranium minerals added, such as soddyite, rutherfordite, uranophane, uranyl orthophosphate, and becquerelite. By applying this model, the sensitivities of uranium solubility to variations in the concentrations of various groundwater component species are systematically investigated. The results show that the total analytical concentrations of carbonate, phosphate, silicate, and calcium in deep groundwater play the most important role in determining the solubility of uranium under strongly oxidizing conditions.

  • 35.
    Liu, Longcheng
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    The effect of hydrogen on oxidative dissolution of spent fuel2002In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 138, p. 69-78Article in journal (Refereed)
    Abstract [en]

    An earlier model for the oxidative dissolution of spent fuel is further developed by including the reductive effect of H-2, which is formed by both the radiolysis of ground-water and the anoxic corrosion of the cast iron insert of the canister. The kinetics of reduction of dissolved uranium species by dissolved hydrogen is derived from a series of previously published experimental studies. The simulation results suggest that the effect of autocatalytic reduction of hexavalent uranium by hydrogen may play an important role in controlling the dissolution of the fuel matrix within a canister. Further experimental studies are required to firmly verify these findings.

  • 36.
    Liu, Longcheng
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Permeability and expansibility of natural bentonite MX-80 in distilled water2011In: Physics and Chemistry of the Earth, ISSN 1474-7065, E-ISSN 1873-5193, Vol. 36, no 17-18, p. 1783-1791Article in journal (Refereed)
    Abstract [en]

    Natural bentonite MX-80 differs from the purified and fully Na-exchanged bentonite in that it contains approximately 20.0% accessory minerals, in addition to the montmorillonite particles. Since the accessory minerals and montmorillonite particles have very different physical and chemical properties, natural bentonite MX-80 is found to expand much more slowly in distilled water, leading actually to a three-component system that has very different hydraulic properties from that of the fully Na-exchanged bentonite. To better understand and simulate the special features of expansion of natural bentonite MX-80 in distilled water, the focus is put primarily on the development of a Kozeny-Carman-like equation for its hydraulic permeability in the same way as it was done for Na-exchanged bentonite. With this permeability model, the dynamic force balance model that was originally developed for colloidal expansion of montmorillonite in a two-component system is applied to the natural MX-80 system. Without making any changes to the model, however, two strategies are used to account for both physical and chemical effects of the accessory minerals. The "lumped" strategy assumes that the accessory minerals are stuck onto the montmorillonite particles in such a way that they behave just like one solid component. The "stepwise" strategy changes the pore water chemistry gradually from initially distilled water to eventually achievement of the equilibrium condition. These strategies are simple but proved to function well. The agreement between the simulations and the experimental results indicates that the two-component dynamic force balance model works well in predicting the general features and the behavior of upward expansion of natural bentonite MX-80 in distilled water in a vertical test tube.

  • 37.
    Liu, Longcheng
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Shahkarami, Pirouz
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Meng, Shuo
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Solute transport along a single fracture in a porous rock: a simple analytical solution and its extension for modeling velocity dispersion2017In: Hydrogeology Journal, ISSN 1431-2174, E-ISSN 1435-0157Article in journal (Refereed)
    Abstract [en]

    A simple and robust solution is developed for the problem of solute transport along a single fracture in a porous rock. The solution is referred to as the solution to the single-flow-path model and takes the form of a convolution of two functions. The first function is the probability density function of residence-time distribution of a conservative solute in the fracture-only system as if the rock matrix is impermeable. The second function is the response of the fracture-matrix system to the input source when Fickian-type dispersion is completely neglected; thus, the effects of Fickian-type dispersion and matrix diffusion have been decoupled. It is also found that the solution can be understood in a way in line with the concept of velocity dispersion in fractured rocks. The solution is therefore extended into more general cases to also account for velocity variation between the channels. This leads to a development of the multi-channel model followed by detailed statistical descriptions of channel properties and sensitivity analysis of the model upon changes in the model key parameters. The simulation results obtained by the multi-channel model in this study fairly well agree with what is often observed in field experiments—i.e. the unchanged Peclet number with distance, which cannot be predicted by the classical advection-dispersion equation. In light of the findings from the aforementioned analysis, it is suggested that forced-gradient experiments can result in considerably different estimates of dispersivity compared to what can be found in natural-gradient systems for typical channel widths.

  • 38.
    Löfgren, Martin
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    A Conceivable Technique of Measuring Sorption Coefficients in Intact Rock Using an Electrical Potential Gradient as the Driving Force for Migration2004In: Scientific Basis for Nuclear Waste Management Symposium XXVII / [ed] Oversby, VM; Werme, LO, 2004, p. 683-688Conference paper (Refereed)
    Abstract [en]

    Sorption coefficients are traditionally obtained in batch experiments if the sorbent is strongly or intermediately sorbing. In a batch experiment the rock is crushed and this could increase the surface area as well as induce new and fresh surfaces. Therefore there is some concern whether sorption coefficients obtained in batch experiments represents those of intact rock. Performing sorption experiment by diffusion in intact rock with intermediately and strongly sorbing species seems impossible in practice due to extremely long experimental times. In this paper the possibility of increasing the migration rate in the rock by two of three orders, thus enabling KD measurements of intermediately sorbing species in intact rock, is discussed. The increase in migration rate has already been achieved successfully in so called through electromigration experiments using non-sorbing species. Here a potential gradient acts as the main driving force. In our experiments the migration rate was increased 320 times by using a potential drop of only 9 volts.

  • 39.
    Löfgren, Martin
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Formation factor distributions from the Oskarshamn site investigation.Manuscript (preprint) (Other academic)
  • 40.
    Löfgren, Martin
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Formation factor logging by electrical methods - Comparison of formation factor logs obtained in situ and in the laboratory2003In: J CONTAM HYDROL, 2003, Vol. 61, no 1-4, p. 107-115Conference paper (Refereed)
    Abstract [en]

    In this paper, a new in situ method for obtaining the formation factor, which is essential for the matrix diffusion, is described and tested in intrusive igneous rock. The method is based on electrical resistivity measurements in rock where the pore water and rock resistivities are essential parameters. The method is based on electromigration instead of diffusion as in traditional diffusion experiments. In previous works, quantitative formation factors of rock have been obtained by electrical methods in the laboratory. Here, a similar approach is used in situ. An in situ logging campaign was performed by SKB during 2000 in the 1700-m-deep borehole KLX02 in Laxemar, Sweden. The rock resistivity was measured with the slimhole Dual Laterolog from Antares. The groundwater resistivity was measured with the Difference Flow Meter from Posiva. A formation factor log was obtained with the maximum vertical resolution of 10 cm. In order to validate the log, 100 rock samples were taken from the bore core, and a formation factor log was obtained by using electrical methods in the laboratory. Both direct current (DC) and alternating current (AC) were used. The measurements on the core confirmed that the in situ log was quantitative, but with a possible systematic error. The in situ formation factors were on average about 1/3 to 1/5 of the laboratory formation factors, depending on depth.

  • 41.
    Löfgren, Martin
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    The pore connectivity of intrusive igneous rock: an important factor in radionuclide retention?2004Article in journal (Other academic)
  • 42.
    Löfgren, Martin
    et al.
    Kemakta Konsult AB.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Through-electromigration: A new method of investigating pore connectivity and obtaining formation factors2006In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 87, no 3-4, p. 237-252Article in journal (Refereed)
    Abstract [en]

    The retardation of radionuclides and other contaminants in fractured crystalline rock is strongly associated with the diffusive properties of the rock matrix. At present, the scientific community is divided concerning the question of long-range pore connectivity in intrusive igneous rock. This paper presents a fast new method, called the through-electromigration method, of obtaining formation factors and investigating pore connectivity. The method involves the migration of an ionic tracer through a rock sample with an electrical potential gradient as the main driving force. The method is analogous to the through-diffusion method but the experimental time is reduced by orders of magnitude. This enables investigations of pore connectivity, as measurements can be made on longer samples. In a preliminary investigation, the new method is compared to the traditional through-diffusion method as well as to rock resistivity methods. The diffusive properties of nine granitic rock samples from Laxemar in Sweden, ranging from 15 to 121 turn in length, have been investigated and the results are compared.

  • 43.
    Mahmoudzadeh, Batoul
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Liu, Longcheng
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Evolution of fracture aperture mediated by dissolution in a coupled flow channel–rock matrix–stagnant zone systemManuscript (preprint) (Other academic)
    Abstract [en]

    Fracture aperture is an important entity controlling fluid flow in natural fractures in rocks. The aperture of fractures in crystalline rocks may decrease or increase by different mechanical and chemical mechanisms. A model to describe the evolution of fracture aperture mediated by dissolution and precipitation is presented in this study. It includes advective flow in the fracture that can carry in or away dissolved minerals. The model also accounts for the fact that dissolved minerals in the fracture plane, in both flow channel and stagnant water zone, can diffuse into the adjacent porous rock matrix. The analytical solution obtained in the Laplace space is then used to study evolution of the fracture aperture under combined influence of stress and flow, in a pseudo-steady-state procedure. The simulation results give insights into the most important processes and mechanisms that dominate the fracture closure or opening under different circumstances. It is found that the times involved for any changes in fracture aperture are very much larger than the times needed for concentrations of dissolved minerals to reach steady state in the rock matrix, the stagnant water zone and the flow channel. This suggests that the steady state model can be used to assess the evolution of concentration of dissolved minerals in the rock fracture. Moreover, it is shown that diffusion into the rock matrix, which acts as a strong sink or source for dissolved minerals, clearly dominates the rate of concentration change and consequently the rate of evolution of the fracture aperture.

  • 44.
    Mahmoudzadeh, Batoul
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Liu, Longcheng
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Rock fracture closing moderated by pressure solution2015In: Future Communication Technology and Engineering - Proceedings of the 2014 International Conference on Future Communication Technology and Engineering, FCTE 2014, CRC Press/Balkema , 2015, p. 269-275Conference paper (Refereed)
    Abstract [en]

    Fracture apertures may decrease or increase by different mechanical and chemical mechanisms when the fractures are subject to stress. A model is presented to describe fracture closure/opening that accommodates pressure dissolution at contacting asperities as well as free-face dissolution/precipitation at free faces of the fracture and of the rock matrix. The derived analytical model accounts for the fact that dissolved minerals carried by flowing water along the fracture can not only diffuse into and out of the adjacent rock matrix but also at first diffuse into the stagnant water zone existing in part of the fracture plane and then from there into and out of the rock matrix adjacent to it. The analytical solution is used to study fracture closure/opening rate in a pseudo steady state, PSS, procedure. This simple model allows us to gain some insights into which processes and mechanisms have the larger impact on the fracture aperture under different circumstances.

  • 45.
    Mahmoudzadeh, Batoul
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Liu, Longcheng
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Solute transport in a single fracture involving an arbitrary length decay chain with rock matrix comprising different geological layers2014In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 164, p. 59-71Article in journal (Refereed)
    Abstract [en]

    A model is developed to describe solute transport and retention in fractured rocks. It accounts for advection along the fracture, molecular diffusion from the fracture to the rock matrix composed of several geological layers, adsorption on the fracture surface, adsorption in the rock matrix layers and radioactive decay-chains. The analytical solution, obtained for the Laplace-transformed concentration at the outlet of the flowing channel, can conveniently be transformed back to the time domain by the use of the de Hoog algorithm. This allows one to readily include it into a fracture network model or a channel network model to predict nuclide transport through channels in heterogeneous fractured media consisting of an arbitrary number of rock units with piecewise constant properties. More importantly, the simulations made in this study recommend that it is necessary to account for decay-chains and also rock matrix comprising at least two different geological layers, if justified, in safety and performance assessment of the repositories for spent nuclear fuel.

  • 46.
    Mahmoudzadeh, Batoul
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Liu, Longcheng
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Solute transport in fractured rocks with stagnant water zone and rock matrix composed of different geological layers-Model development and simulations2013In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 49, no 3, p. 1709-1727Article in journal (Refereed)
    Abstract [en]

    A model is developed to describe solute transport and retention in fractured rocks. It accounts for the fact that solutes can not only diffuse directly from the flowing channel into the adjacent rock matrix composed of different geological layers but also at first diffuse into the stagnant water zone occupied in part of the fracture and then from there into the rock matrix adjacent to it. In spite of the complexities of the system, it is shown that the analytical solution to the Laplace-transformed concentration at the outlet of the flowing channel is a product of two exponential functions, and it can be easily extended to describe solute transport through channels in heterogeneous fractured media consisting of an arbitrary number of rock units with piecewise constant geological properties. More importantly, by numerical inversion of the Laplace-transformed solution, the simulations made in this study help to gain insights into the relative significance and the different contributions of the rock matrix and the stagnant water zone in retarding solute transport in fractured rocks. It is found that, in addition to the intact wall rock adjacent to the flowing channel, the stagnant water zone and the rock matrix adjacent to it may also lead to a considerable retardation of solute in cases with a narrow channel.

  • 47.
    Mahmoudzadeh, Batoul
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Liu, Longcheng
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Moreno, Luis
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Solute transport through fractured rock: Radial diffusion into the rock matrix with several geological layers for an arbitrary length decay chain2016In: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 536, p. 133-146Article in journal (Refereed)
    Abstract [sv]

    The paper presents a model development to derive a semi-analytical solution to describe reactive solute transport through a single channel in a fracture with cylindrical geometry. The model accounts for advection through the channel, radial diffusion into the adjacent heterogeneous rock matrix comprising different geological layers, adsorption on both the channel surface, and the geological layers of the rock matrix and radioactive decay chain. Not only an arbitrary-length decay chain, but also as many number of the rock matrix layers with different properties as observed in the field can be handled. The solution, which is analytical in the Laplace domain, is transformed back to the time domain numerically e.g. by use of de Hoog algorithm. The solution is verified against experimental data and analytical solutions of limiting cases of solute transport through porous media. More importantly, the relative importance and contribution of different processes on solute transport retardation in fractured rocks are investigated by simulating several cases of varying complexity. The simulation results are compared with those obtained from rectangular model with linear matrix diffusion. It is found that the impact of channel geometry on breakthrough curves increases markedly as the transport distance along the flow channel and away into the rock matrix increase. The effect of geometry is more pronounced for transport of a decay chain when the rock matrix consists of a porous altered layer.

  • 48.
    Meng, Shuo
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Liu, Longcheng
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Mahmoudzadeh, Batoul
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Moreno, Luis
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Solute transport along a single fracture with a finite extent of matrix: A new simple solution and temporal moment analysis2018In: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 562, p. 290-304Article in journal (Refereed)
    Abstract [en]

    A new simple and robust solution, based on uniform steady-state flow velocity, is developed for the problem of solute transport in a fracture-matrix system with a finite penetration depth of a radioactive contaminant into the rock matrix. The solution is an extension of Liu et al. (2017) to finite penetration depth and an alternative solution strategy to the problem solved by Sudicky et al. (1982). The solution takes the form of a convolution of two functions. The first function describes the probability density function of the residence time distribution of a conservative solute resulting merely from advection and Fickian dispersion. The second function is actually the impulse response of the fracture-matrix system in the case of a plug flow without any hydrodynamic dispersion. As a result, the effects of Fickian dispersion and matrix diffusion on solute transport are decoupled, and thus the resulting breakthrough curve can be analyzed in terms of those two functions. In addition to this, the derived Péclet numbers of those two functions, based on temporal moments, are also found to be associated with the derived Péclet number of the resulting breakthrough curve. By comparing the Péclet numbers of those two functions, the contribution of Fickian dispersion and matrix diffusion to solute spreading is determined in a straightforward way. This can aid to find out the dominating mechanism on solute transport, and therefore the performance of breakthrough curve.

  • 49.
    Moreno, Luis
    et al.
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Crawford, James
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Modelling of solute transport under flow conditions varying in time, using the channel network model2004In: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXVII / [ed] Oversby, VM; Werme, LO, 2004, Vol. 807, p. 797-802Conference paper (Refereed)
    Abstract [en]

    Transport of radionuclides from a repository to the biosphere is calculated in the presence of varying boundary conditions. This may occur in Sweden, for example, during post-glacial rebound. To calculate the transport of radionuclides, the flow field has to be calculated at each time step. An alternative approach is to determine the flow field at certain time intervals; i.e., modelling the dynamic flow system as a sequence of punctuated steady states. This can be done in a rather simple way by using the Channel Network model (CNM), in which radionuclide transport is calculated by particle tracking. Intuitively one would expect such a simple procedure to be permissible and lead to small errors if the time intervals are so short that only small changes in flow rates and flow directions take place. In this paper, the technique is applied to a study case. This describes a repository located at a sub sea level location outside the coastline; where, owing to the land lift that is taking place in Scandinavia the coastline is advancing towards the repository. An important issue to be determined is the minimum number of time intervals that need to be used in order to obtain a reliable solution to radionuclide transport. It is found that the number of time intervals needed is strongly dependent on the variation in the boundary conditions. For the changing conditions used in the example tested, a number on the order of 5-9 time intervals allows a sufficiently good representation of the transport.

  • 50.
    Moreno, Luis
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Crawford, James
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Modelling of transport in fractures with complex matrix properties2006In: Scientific Basis for Nuclear Waste Management XXIX, WARRENDALE, PA: MATERIALS RESEARCH SOCIETY , 2006, Vol. 932, p. 267-274Conference paper (Refereed)
    Abstract [en]

    In the ongoing Swedish site investigations it has been found that the rock matrix adjacent to many open fractures has been altered. The alteration can extend from mm to several cm. The altered rock can have different sorption and diffusion properties compared to the undisturbed rock and this may influence the retardation of the nuclides. The paper presents how the Channel Network model has been adapted to handle diffusion into a matrix composed of several layers with different properties in addition to the infinite undisturbed matrix. For one channel, the solution for the Residence Time Distribution, RTD, may be found in the Laplace-plane. For the transport in the Channel Network, a particle tracking technique is used to determine the paths followed by solute particles. The RTD for this path is obtained using convolution, which in the Laplace-plane means multiplication of the transfer functions for each channel. The inversion to the time-plane is made by numerical inversion of the Laplace transforms for each path. The method has been tested with data from the TRUE (Tracer Retention Understanding Experiments) project, Task 6F, fluid flow and solute transport in two features in 100 in scale where a complex matrix was modelled. The model was used to predict the transport of the tracers (I-129), Cs-137, and Am-241) over some 20 m. The paper also addresses how the RTD is influenced by the different retardation mechanisms under Site Characterisation (SC) as well as Performance Assessment (PA) conditions.

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