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The effect of stagnant water zones on retarding radionuclide transport in fractured rocks: An extension to the Channel Network Model
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering. (Division of Nuclear Waste Engineering)ORCID iD: 0000-0002-6049-428X
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering. (Division of Nuclear Waste Engineering)
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering. (Division of Nuclear Waste Engineering)
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering. (Division of Nuclear Waste Engineering)
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. Vol. 540, p. 1122-1135
Keywords [en]
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: urn:nbn:se:kth:diva-213978DOI: 10.1016/j.jhydrol.2016.07.031OAI: oai:DiVA.org:kth-213978DiVA, id: diva2:1139488
Note

QC 20170926

Available from: 2017-09-07 Created: 2017-09-07 Last updated: 2017-09-26Bibliographically 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
Keywords
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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Publisher's full texthttp://www.sciencedirect.com/science/article/pii/S0022169416304619

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