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Neretnieks, Ivars
Publications (8 of 8) Show all publications
Winberg-Wang, H., Neretnieks, I. & Voutilainen, M. (2019). A Note on the Use of Uranine Tracer to Visualize Radionuclide Migration Experiments: Some Observations and Problems. Paper presented at American-Nuclear-Society (ANS) Student Conference, APR 05-07, 2018, Univ Florida, Gainesville, FL. Nuclear Technology, 205(7), 964-977
Open this publication in new window or tab >>A Note on the Use of Uranine Tracer to Visualize Radionuclide Migration Experiments: Some Observations and Problems
2019 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 205, no 7, p. 964-977Article in journal (Refereed) Published
Abstract [en]

Uranine is a dye commonly used in tracer experiments; it is chosen for its high visibility even at low concentrations. Uranine solutions are slightly denser than water at the same temperature. However, in laboratory experiments uranine solutions have been known to occasionally show unpredictable flow behaviors. This paper investigates the possible effect of light-induced density change to explain some of these behaviors. Uranine has a wide light absorption spectrum for visible light, which can heat the dye solution and lower its density to below that of the surrounding water, which induces buoyancy-driven flow. Simulations are made in both one dimension and two dimensions to determine the extent of the effect. The results are then compared to different experiments with unanticipated flow patterns.

Place, publisher, year, edition, pages
Taylor & Francis, 2019
Keywords
Uranine, fluorescein tracer experiments, flow patterns in fractures, light-induced density-driven flow, buoyancy flow in fractures
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-255342 (URN)10.1080/00295450.2019.1573620 (DOI)000472556500009 ()2-s2.0-85063529203 (Scopus ID)
Conference
American-Nuclear-Society (ANS) Student Conference, APR 05-07, 2018, Univ Florida, Gainesville, FL
Note

QC 20190807

Available from: 2019-08-07 Created: 2019-08-07 Last updated: 2019-12-06Bibliographically approved
Shahkarami, P., Neretnieks, I., Moreno, L. & Liu, L. (2019). Channel network concept: an integrated approach to visualize solute transport in fractured rocks. Hydrogeology Journal, 27(1), 101-119
Open this publication in new window or tab >>Channel network concept: an integrated approach to visualize solute transport in fractured rocks
2019 (English)In: Hydrogeology Journal, ISSN 1431-2174, E-ISSN 1435-0157, Vol. 27, no 1, p. 101-119Article in journal (Refereed) Published
Abstract [en]

The advection-dispersion equation, ADE, has commonly been used to describe solute transport in fractured rock. However, there is one key question that must be addressed before the mathematical form of the so-called Fickian dispersion that underlies the ADE takes on physical meaning in fractures. What is the required travel distance, or travel time, before the Fickian condition is met and the ADE becomes physically reasonable? A simple theory is presented to address this question in tapered channels. It is shown that spreading of solute under forced-gradient flow conditions is mostly dominated by advective mechanisms. Nevertheless, the ADE might be valid under natural flow conditions. Furthermore, several concerns are raised in this paper with regard to using the concept of a field-scale matrix diffusion coefficient in fractured rocks. The concerns are mainly directed toward uncertainties and potential bias involved in finding the continuum model parameters. It is illustrated that good curve fitting does not ensure the physical reasonability of the model parameters. It is suggested that it is feasible and adequate to describe flow and transport in fractured rocks as taking place in three-dimensional networks of channels, as embodied in the channel network concept. It is argued that this conceptualization provides a convenient framework to capture the impacts of spatial heterogeneities in fractured rocks and can accommodate the physical mechanisms underlying the behavior of solute transport in fractures. All these issues are discussed in relation to analyzing and predicting actual tracer tests in fractured crystalline rocks.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
Solute transport, Channel network concept, Non-Fickian behavior, Advection-dispersion equation, Matrix diffusion
National Category
Geophysical Engineering
Identifiers
urn:nbn:se:kth:diva-245940 (URN)10.1007/s10040-018-1855-6 (DOI)000458520300007 ()2-s2.0-85053217977 (Scopus ID)
Note

QC 20190308

Available from: 2019-03-08 Created: 2019-03-08 Last updated: 2019-03-18Bibliographically approved
Neretnieks, I. & Winberg-Wang, H. (2019). Density-Driven Mass Transfer in Repositories for Nuclear Waste. Nuclear Technology, 205(6), 819-829
Open this publication in new window or tab >>Density-Driven Mass Transfer in Repositories for Nuclear Waste
2019 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 205, no 6, p. 819-829Article in journal (Refereed) Published
Abstract [en]

In geologic repositories for nuclear waste located in crystalline rocks, the waste is surrounded by a bentonite buffer that in practice is not permeable to water flow. The nuclides must escape by molecular diffusion to enter the seeping water in the fractures of the rock. At high water-seepage rates, the nuclides can be carried away rapidly. The seepage rate of the water can be driven by the regional hydraulic gradient as well as by buoyancy-driven flow. The latter is induced by thermal circulation of the water by the heat produced by radionuclide decay. The circulation may also be induced by salt exchange between buffer and water in the fractures. The main aim of this paper is to explore how salt exchange between the backfill and mobile water in fractures, by buoyancy effects, can increase the escape rate of radionuclides from a repository. A simple analytical model has been developed to describe the mass transfer rate induced by buoyancy. Numerical simulations support the simple solution. A comparison is made with the regional gradient-driven flow model. It is shown that buoyancy-driven flow can noticeably increase the release rate.

Place, publisher, year, edition, pages
Taylor & Francis, 2019
Keywords
Radionuclide release rate, repositories, buoyancy-driven flow, channeling, equivalent flow rate
National Category
Physical Sciences Geophysical Engineering
Identifiers
urn:nbn:se:kth:diva-254128 (URN)10.1080/00295450.2018.1537460 (DOI)000470315400006 ()2-s2.0-85057240527 (Scopus ID)
Note

QC 2220190620

Available from: 2019-06-20 Created: 2019-06-20 Last updated: 2019-12-06Bibliographically approved
Bond, A. E., Chittenden, N., Fedors, R., Lang, P., McDermott, C., Neretnieks, I., . . . Yasuhara, H. (2018). Coupled THMC modelling of single fractures in novaculite and granite. In: 2nd International Discrete Fracture Network Engineering Conference, DFNE 2018: . Paper presented at 2nd International Discrete Fracture Network Engineering Conference, DFNE 2018, Seattle, United States, 20 June 2018 through 22 June 2018. American Rock Mechanics Association (ARMA)
Open this publication in new window or tab >>Coupled THMC modelling of single fractures in novaculite and granite
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2018 (English)In: 2nd International Discrete Fracture Network Engineering Conference, DFNE 2018, American Rock Mechanics Association (ARMA), 2018Conference paper, Published 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.

Place, publisher, year, edition, pages
American Rock Mechanics Association (ARMA), 2018
National Category
Mineral and Mine Engineering
Identifiers
urn:nbn:se:kth:diva-241479 (URN)2-s2.0-85059422972 (Scopus ID)
Conference
2nd International Discrete Fracture Network Engineering Conference, DFNE 2018, Seattle, United States, 20 June 2018 through 22 June 2018
Note

QC 20190123

Available from: 2019-01-23 Created: 2019-01-23 Last updated: 2019-01-23Bibliographically approved
Meng, S., Liu, L., Mahmoudzadeh, B., Neretnieks, I. & Moreno, L. (2018). Solute transport along a single fracture with a finite extent of matrix: A new simple solution and temporal moment analysis. Journal of Hydrology, 562, 290-304
Open this publication in new window or tab >>Solute transport along a single fracture with a finite extent of matrix: A new simple solution and temporal moment analysis
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2018 (English)In: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 562, p. 290-304Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Dispersion, Fractured rocks, Matrix diffusion, Péclet number, Solute transport model, Temporal moment analysis
National Category
Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:kth:diva-228725 (URN)10.1016/j.jhydrol.2018.05.016 (DOI)000438003000022 ()2-s2.0-85047099016 (Scopus ID)
Funder
Swedish Nuclear Fuel and Waste Management Company, SKB
Note

QC 20180529

Available from: 2018-05-29 Created: 2018-05-29 Last updated: 2018-07-27Bibliographically approved
Liu, L., Neretnieks, I., Shahkarami, P., Meng, S. & Moreno, L. (2017). Solute transport along a single fracture in a porous rock: a simple analytical solution and its extension for modeling velocity dispersion. Hydrogeology Journal
Open this publication in new window or tab >>Solute transport along a single fracture in a porous rock: a simple analytical solution and its extension for modeling velocity dispersion
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2017 (English)In: Hydrogeology Journal, ISSN 1431-2174, E-ISSN 1435-0157Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Springer Berlin/Heidelberg, 2017
Keywords
Fractured rocks - Velocity dispersion - Mathematical model - Matrix diffusion - Taylor dispersion
National Category
Other Chemical Engineering Chemical Process Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-213979 (URN)10.1007/s10040-017-1627-8 (DOI)000423051600020 ()2-s2.0-85026908664 (Scopus ID)
Note

QC 20170918

Available from: 2017-09-07 Created: 2017-09-07 Last updated: 2018-02-02Bibliographically approved
Shahkarami, P., Liu, L., Moreno, L. & Neretnieks, I. (2016). The effect of stagnant water zones on retarding radionuclide transport in fractured rocks: An extension to the Channel Network Model. Journal of Hydrology, 540, 1122-1135
Open this publication in new window or tab >>The effect of stagnant water zones on retarding radionuclide transport in fractured rocks: An extension to the Channel Network Model
2016 (English)In: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 540, p. 1122-1135Article in journal (Refereed) Published
Abstract [en]

An essential task of performance assessment of radioactive waste repositories is to predict radionuclide release into the environment. For such a quantitative assessment, the Channel Network Model and the corresponding computer program, CHAN3D, have been used to simulate radionuclide transport in crystalline bedrocks. Recent studies suggest, however, that the model may tend to underestimate the rock retarding capability, because it ignores the presence of stagnant water zones, STWZs, situated in the fracture plane. Once considered, the STWZ can provide additional surface area over which radionuclides diffuse into the rock matrix and thereby contribute to their retardation.

The main objective of this paper is to extend the Channel Network Model and its computer implementation to account for diffusion into STWZs and their adjacent rock matrices.

In the first part of the paper, the overall impact of STWZs in retarding radionuclide transport is investigated through a deterministic calculation of far-field releases at Forsmark, Sweden. Over the time-scale of the repository safety assessments, radionuclide breakthrough curves are calculated for increasing STWZ width. It is shown that the presence of STWZs enhances the retardation of most long-lived radionuclides except for 36Cl and 129I.

The rest of the paper is devoted to the probabilistic calculation of radionuclide transport in fractured rocks. The model that is developed for transport through a single channel is embedded into the Channel Network Model and new computer codes are provided for the CHAN3D. The program is used to (I) simulate the tracer test experiment performed at Äspö HRL, STT-1 and (II) investigate the short- and long-term effect of diffusion into STWZs. The required data for the model are obtained from detailed hydraulic tests in boreholes intersecting the rock mass where the tracer tests were made.

The simulation results fairly well predict the release of the sorbing tracer 137Cs. It is found that over the short time-scale of the tracer experiment, the effect of diffusion into STWZs is not as pronounced as that of matrix diffusion directly from the flow channel, and the latter remains the main retarding mechanism. Predictions for longer time-scale, tens of years and more, show that the effect of STWZs becomes strong and tends to increase with transport time. It is shown that over the long times of interest for safety assessment of radioactive waste repositories, STWZs can substantially contribute to radionuclide retardation, though for the short time-scales the impact is not very strong and is not expected to affect the results of short-term field experiments.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Channel Network Model, Stagnant water zones, Matrix diffusion, Radionuclide transport, CHAN3D program, Fractured rocks
National Category
Chemical Process Engineering Other Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-213978 (URN)10.1016/j.jhydrol.2016.07.031 (DOI)000382269500088 ()2-s2.0-84979030705 (Scopus ID)
Note

QC 20170926

Available from: 2017-09-07 Created: 2017-09-07 Last updated: 2019-09-04Bibliographically approved
Winberg-Wang, H. & Neretnieks, I.Visualisation of mass transfer between source and seeping water in a variable aperture fracture-Impact of tracer density.
Open this publication in new window or tab >>Visualisation of mass transfer between source and seeping water in a variable aperture fracture-Impact of tracer density
(English)Manuscript (preprint) (Other academic)
Abstract [en]

An experiment with a vertical slot with horizontally seeping water with a dye diffusing from below was performed to help validate and visualise the Q-equivalent model, which describes the mass transfer rate from a source into flowing water such that in a repository for nuclear waste. The Q-equivalent model is used for quantifying mass transport in geological repositories. However, the tracer propagated much slower and to a lesser extent than predicted by the model. It was found that the tracer gave rise to a small density gradient, which induced buoyancy-driven flow, overwhelming that driven by the horizontal hydraulic gradient. This dramatically changed the mass transfer from the dye source into the water in the slot. For the release of contaminants, this can have detrimental as well as beneficial effects, depending on positive or negative buoyancy is induced. These observations led to an analysis of when and how density differences in a repository can influence the release and further fate of escaping radionuclides in waste repositories. This and other experiments also showed that laboratory experiments aimed to visualise flow and mass transfer processes in fractures could be very sensitive to the heating of the dye tracers by the lighting in the laboratory. 

Keywords
tracer experiments, repositories, safety assessment
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-264913 (URN)
Funder
Swedish Nuclear Fuel and Waste Management Company, SKB, C7246
Note

QC 20191212

Available from: 2019-12-04 Created: 2019-12-04 Last updated: 2019-12-12Bibliographically approved
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