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Radionuclide migration through fractured rock for arbitrary-length decay chain: Analytical solution and global sensitivity analysis
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.ORCID iD: 0000-0002-6049-428X
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.ORCID iD: 0000-0001-6801-9208
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.ORCID iD: 0000-0001-8241-2225
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
2015 (English)In: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 520, 448-460 p.Article in journal (Refereed) Published
Abstract [en]

This study presents an analytical approach to simulate nuclide migration through a channel in a fracture accounting for an arbitrary-length decay chain. The nuclides are retarded as they diffuse in the porous rock matrix and stagnant zones in the fracture. The Laplace transform and similarity transform techniques are applied to solve the model. The analytical solution to the nuclide concentrations at the fracture outlet is governed by nine parameters representing different mechanisms acting on nuclide transport through a fracture, including diffusion into the rock matrices, diffusion into the stagnant water zone, chain decay and hydrodynamic dispersion. Furthermore, to assess how sensitive the results are to parameter uncertainties, the Sobol method is applied in variance-based global sensitivity analyses of the model output. The Sobol indices show how uncertainty in the model output is apportioned to the uncertainty in the model input. This method takes into account both direct effects and interaction effects between input parameters. The simulation results suggest that in the case of pulse injections, ignoring the effect of a stagnant water zone can lead to significant errors in the time-of-first arrival and the peak value of the nuclides. Likewise, neglecting the parent and modeling its daughter as a single stable species can result in a significant overestimation of the peak value of the daughter nuclide. It is also found that as the dispersion increases, the early arrival time and the peak time of the daughter decrease while the peak value increases. More importantly, the global sensitivity analysis reveals that for time periods greater than a few thousand years, the uncertainty of the model output is more sensitive to the values of the individual parameters than to the interaction between them. Moreover, if one tries to evaluate the true values of the input parameters at the same cost and effort, the determination of priorities should follow a certain sequence.

Place, publisher, year, edition, pages
2015. Vol. 520, 448-460 p.
Keyword [en]
Fractured rock, Transport model, Stagnant water, Chain decay, Analytical solution, Global sensitivity analysis
National Category
Oceanography, Hydrology, Water Resources Geology
Identifiers
URN: urn:nbn:se:kth:diva-160756DOI: 10.1016/j.jhydrol.2014.10.060ISI: 000348255900037Scopus ID: 2-s2.0-84916242371OAI: oai:DiVA.org:kth-160756DiVA: diva2:791690
Note

QC 20150302

Available from: 2015-03-02 Created: 2015-02-27 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Solute Transport in Fractured Rocks: The Effect of Stagnant Water Zones and Velocity Dispersion
Open this publication in new window or tab >>Solute Transport in Fractured Rocks: The Effect of Stagnant Water Zones and Velocity Dispersion
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The focus of this thesis is on the development of new models to improve our understanding of radionuclide transport in the repository “far-field” in fractured rocks. The proposed models contribute to the channel network concept and describe the recently developed models with stagnant water zones (STWZs) and channels with variable aperture allowing to consider their possible impacts on the overall transport of radionuclides in fractured rocks. New conceptual models are also proposed to better understand hydrodynamic dispersion in fractured rocks by taking into account velocity distribution within tapered channels, i.e., Fickian-type dispersion, and between different flow paths, i.e., velocity dispersion, as embodied in the proposed multi-channel model.

The results of both deterministic and probabilistic analyses reveal that over the long times of interest for safety assessment of high-level radioactive waste repositories, STWZs can substantially enhance the retardation of both short- and long-lived nuclides, with the exception of the non-sorbing species, i.e., 36Cl and 129I. Nevertheless, over the short time-scales the impact of STWZs is not very strong and is not expected to affect the results of short-term field experiments. It is also shown that the proposed multi-channel model can explain the apparent scale dependency of the dispersion coefficient that is often observed in tracer experiments. It is further discussed that the interpreted results of short-range tracer experiments cannot necessarily give information on what would take place over longer distances because the spreading mechanisms are expected to be entirely different. Usefulness of the continuum model to interpret tracer experiments is, thereafter, discussed and arguments are presented to support the premise that it is more physically meaningful to describe flow and transport as taking place in a three-dimensional network of channels.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 37
Keyword
Channel network concept; radionuclide transport; stagnant water zones; velocity dispersion; modeling and simulation
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-214040 (URN)978-91-7729-523-5 (ISBN)
Public defence
2017-10-06, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170911

Available from: 2017-09-11 Created: 2017-09-10 Last updated: 2017-09-18Bibliographically approved

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Shahkarami, PirouzMoreno, Luis

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