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Implications of THMC Processes on Long-Term Safety of Geological Disposal...
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Implications of THMC Processes on Long-Term Safety of Geological Disposal of Radioactive Waste

A special issue of Minerals (ISSN 2075-163X). This special issue belongs to the section "Clays and Engineered Mineral Materials".

Deadline for manuscript submissions: 27 September 2024 | Viewed by 4432

Special Issue Editors

Dr. Thanh Son Nguyen

E-Mail Website
Guest Editor
Canadian Nuclear Safety Commission (CNSC), Ottawa, ON K1P 5S9, Canada
Interests: geomechanics; contaminant hydrogeology; coupled THMC processes in geomaterials; deep geological disposal
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Mamadou Fall

E-Mail Website
Guest Editor
Department of Civil Engineering, University of Ottawa, 161 Louis Pasteur, Ottawa, ON K1N 6N5, Canada
Interests: geotechnical engineering; deep geological disposal; THMC processes in geosystems; mining geotechnics

Special Issue Information

Dear Colleagues,

Geological disposal being considered in many countries for the long-term management of radioactive waste consists of emplacing the waste in a repository at depths of hundreds of meters in a suitable rock formation. A deep geological repository (DGR) relies on a multiple, redundant barrier system, with engineered and natural components that act together to contain and isolate the waste for tens of thousands up to a million years. The engineered barrier components are typically the waste container, the bentonite sealing system that surrounds the container in the emplacement room and the host rock formation is the natural component. The primary type of waste considered for deep geological disposal is high-level radioactive waste, such as used fuel from nuclear power plants. High-level waste (HLW) generates heat and would substantially raise the ambient temperature in the DGR and host rock for tens of thousands of years. This heat results in complex coupled processes that perturb the Thermal (T)-Mechanical (M)- Hydraulic (H)- Chemical (C) regime in the multiple barrier system and impact its long-term performance.

Research in coupled THMC processes has been active for the last few decades, resulting in ever-improving capabilities of mathematical models for THMC processes to predict experiments conducted at different research institutions and underground research facilities. Many countries at present have moved past the stage of concept development and fundamental research and have started or are close to implementing geological disposal. Therefore, we believe it is timely to consider the following questions:

  1. How do coupled THMC processes impact the containment and isolation of a DGR? When do they need to be considered and when can they be neglected?
  2. How confident are we that coupled THMC models could be used to evaluate the long-term evolution of a DGR (up to 1 million years) while they are typically developed and validated using short-term experiments?

The editors therefore solicit contributions, either in the form of synthesis discussion or reporting of original research that can shed some light on the above two questions.  Although the focus of this call for papers is on geological of radioactive waste, coupled THMC processes are important in many other underground engineering applications, such as carbon sequestration, petroleum extraction using hot water injection, geothermal energy, mine backfilling, etc. Therefore, contributions from these other fields that shed light on Question 2 above are particularly welcome.

Dr. Thanh Son Nguyen
Prof. Dr. Mamadou Fall
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Minerals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • coupled thermal–hydrological–mechanical–chemical processes
  • geological disposal of radioactive waste
  • long-term safety
  • geomaterials
  • geotechnical
  • barrier

Published Papers (4 papers)

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Research

16 pages, 7857 KiB  
Article
Coupled Processes at Micro- and Macroscopic Levels for Long-Term Performance Assessment Studies of Nuclear Waste Repositories
by Hua Shao, Eike Radeisen, Jürgen Hesser, Wenqing Wang and Olaf Kolditz
Minerals 2024, 14(5), 453; https://0-doi-org.brum.beds.ac.uk/10.3390/min14050453 - 25 Apr 2024
Viewed by 382
Abstract
Performance assessment of nuclear waste repositories requires state-of-the-art knowledge of radionuclide transport properties. Additionally, the short-term development under thermal pulses and the long-term development of the near field—due to influences such as gas generation—must be evaluated. Key thermal-hydro-mechanical-chemical processes are strongly coupled on [...] Read more.
Performance assessment of nuclear waste repositories requires state-of-the-art knowledge of radionuclide transport properties. Additionally, the short-term development under thermal pulses and the long-term development of the near field—due to influences such as gas generation—must be evaluated. Key thermal-hydro-mechanical-chemical processes are strongly coupled on different spatial and temporal scales. To understand these coupling mechanisms, numerous material models and numerical codes have been developed. However, the existing constitutive approaches—which have been adapted to describe small-scale laboratory experiments and validated against real-scale field observations—are often unable to capture long-term material behavior with sufficient precision. To build the confidence, a more comprehensive understanding of the system at micro- and macroscopic scales is required. Most observed macroscopic processes result from microscopic changes in the crystal structure and/or crystalline aggregates, as well as changes in material properties under the influence of various factors. To characterize these physical fields in crystals, microscopic investigations, such as visualization, or geophysical methods are introduced to verify the understanding at the microscale. Two cases are demonstrated for the presented concept using microscale information: one deals with the mechanically and thermally driven migration of fluid inclusions in rock salt, the other with dilatancy-controlled gas transport in water-saturated clay material. Full article
(This article belongs to the Special Issue Implications of THMC Processes on Long-Term Safety of Geological Disposal of Radioactive Waste)
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27 pages, 9168 KiB  
Article
A Multiphysics Model for the Near-Field Evolution of a Geological Repository of Radioactive Waste
by Uy Vo, Mamadou Fall, Julio Ángel Infante Sedano and Thanh Son Nguyen
Minerals 2023, 13(12), 1535; https://0-doi-org.brum.beds.ac.uk/10.3390/min13121535 - 10 Dec 2023
Cited by 1 | Viewed by 1039
Abstract
The safety and robustness of Deep Geological Repositories (DGRs) are of paramount importance for the long-term management of spent nuclear fuel from electricity generation. The introduction of a multi-barrier system, which includes the host rock formation and an engineered barrier system (including the [...] Read more.
The safety and robustness of Deep Geological Repositories (DGRs) are of paramount importance for the long-term management of spent nuclear fuel from electricity generation. The introduction of a multi-barrier system, which includes the host rock formation and an engineered barrier system (including the bentonite buffer), has been a widely used approach to ensure the safety of DGRs. The assessment of the long-term safety of DGRs involves the mathematical modeling of the coupled thermal–hydraulic–mechanical–chemical (THMC) processes that occur in the near-field of the DGRs and their impact on the behaviour and engineering properties of the bentonite buffer. This paper presents a review of the THMC-coupled processes that arise in the bentonite buffer as well as a mathematical model governing such coupled processes. The model is verified against existing analytical solutions and validated against measured data of a thermal diffusion experiment in a sand bentonite column. Also, scoping analyses were performed to assess the influence of coupled THM processes on solute transport in clayrocks. The results of the numerical model closely matched those of the analytical solutions and experimental data demonstrating the capability of the provided mathematical model as well as the numerical approach in enhancing our comprehension of DGR behaviour. This enhanced comprehension will be valuable for safety prediction and assessment in the context of DGRs. The work presented in this paper is part of the Canadian Nuclear Safety Commission’s (CNSC) regulatory research to gain independent knowledge on the safety of the geological disposal of radioactive waste. Full article
(This article belongs to the Special Issue Implications of THMC Processes on Long-Term Safety of Geological Disposal of Radioactive Waste)
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23 pages, 7225 KiB  
Article
Prediction of Long-Term Geochemical Change in Bentonite Based on the Interpretative THMC Model of the FEBEX In Situ Test
by Liange Zheng and Ana María Fernández
Minerals 2023, 13(12), 1522; https://0-doi-org.brum.beds.ac.uk/10.3390/min13121522 - 05 Dec 2023
Viewed by 1165
Abstract
Since nuclear energy is crucial in the decarbonization of the energy supply, one hurdle to remove is the handling of high-level radioactive waste (HLW). Disposal of HLW in a deep geological repository has long been deemed a viable permanent option. In the design [...] Read more.
Since nuclear energy is crucial in the decarbonization of the energy supply, one hurdle to remove is the handling of high-level radioactive waste (HLW). Disposal of HLW in a deep geological repository has long been deemed a viable permanent option. In the design of a deep geological repository, compacted bentonite is the most commonly proposed buffer material. Predicting the long-term chemical evolution in bentonite, which is important for the safety assessment of a repository, has been challenging because of the complex coupled processes. Models for large-scale tests and predictions based on such models have been some of the best practices for such purposes. An 18-year-long in situ test with two dismantling events provided a unique set of chemical data that allowed for studying chemical changes in bentonite. In this paper, we first developed coupled thermal, hydrological, mechanical, and chemical (THMC) models to interpret the geochemical data collected in the in situ test and then extended the THMC model to 200 years to make long-term prediction of the geochemical evolution of bentonite. The interpretive coupled THMC model shows that the geochemical profiles were strongly affected by THM processes such as evaporation/condensation, porosity change caused by swelling, permeability change, and the shape of concentration profiles for major cations were largely controlled by transport processes, but concentration levels were regulated by chemical reactions, and the profiles of some species such as pH, bicarbonate, and sulfate were dominated by these reactions. The long-term THMC model showed that heating prolongs the time that bentonite becomes fully saturated in the area close to the heater/canister; however, once the bentonite becomes fully saturated, high concentrations of ions in bentonite near the heater, which was observed in the field test, will disappear; illitization continues for 50 years but will not proceed further. Full article
(This article belongs to the Special Issue Implications of THMC Processes on Long-Term Safety of Geological Disposal of Radioactive Waste)
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20 pages, 6140 KiB  
Article
Properties of Bentonite-Based Sealing Materials during Hydration
by Mahsa Shafaei Bajestani, Othman Nasir and Won Taek Oh
Minerals 2023, 13(11), 1412; https://0-doi-org.brum.beds.ac.uk/10.3390/min13111412 - 04 Nov 2023
Viewed by 1047
Abstract
A typical deep geological repository (DGR) design consists of a multi-barrier system, including the natural host rock and the engineered barrier system. Understanding the swelling behavior of bentonite-based sealing materials (BBSM), as a candidate material for the engineered barrier system, is crucial for [...] Read more.
A typical deep geological repository (DGR) design consists of a multi-barrier system, including the natural host rock and the engineered barrier system. Understanding the swelling behavior of bentonite-based sealing materials (BBSM), as a candidate material for the engineered barrier system, is crucial for DGR’s long-term safety. In this study, a hydromechanical (HM) column-type test was designed to model the hydration of BBSM from the underground water and determine the resulting swelling pressure in vertical and radial directions. Five hydration tests were carried out on identical compacted samples of 70% bentonite and 30% sand (70-30 bentonite-sand) mixtures with a dry density of 1.65 g/cm3 for varied durations of hydration, between 1 day and 120 days. The experiments were performed parallel to the compaction direction. Following each HM column-type test, the advancement of the wetting front was determined for each test. After 120 days, 56,339 mm3 of water infiltrated the sample and the wetting front reached over 50% of the sample height. The evolution of axial swelling pressure revealed an initial increase in swelling pressure with time in all tests, followed by a reduction in the rate at later times. After early stages of swelling, radial sensors showed an increase in swelling pressure. After 120 days, the radial pressure sensor closest to the hydration front showed 52% more radial pressure than the axial swelling pressure. Full article
(This article belongs to the Special Issue Implications of THMC Processes on Long-Term Safety of Geological Disposal of Radioactive Waste)
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