DECOVALEX Project Research Articles

Comparative verification of hydro-mechanical fracture behavior: Task G of international research project DECOVALEX–2023

Numerical simulations become a necessity when experimental approaches cannot cover the required physical and time scale of interest. One of such area is a simulation of long-term host rock behaviors for nuclear waste disposal and simulation tools involved in the assessment must go through rigorous validation tests. The DECOVALEX project (Development of COupled models and their VAlidation against EXperiments) is dedicated to this purpose by international participants.aawww.decovalex.org. This work is part of the ongoing phase DECOVALEX–2023 (D–2023, Task G) particularly aiming to simulate fracture behaviors under various conditions. Here, we cross-verified a variety of numerical methods including continuous and discontinuous approaches against four benchmark exercises with emphasis on numerical accuracy and parameterization of the various numerical approaches. The systematic inter-comparisons of test cases highlight advantages and disadvantages of the different numerical models. Numerical details on discretization effects (e.g. mesh density and orientation) and domain size were investigated in detail for practical applications. It became evident that meticulous attention to mesh resolution and domain size is imperative for achieving accurate numerical simulations, even for static cracks. Moreover, when comparing numerical methods to closed-form solutions for static cracks, all models successfully reproduced the maximum crack opening but encountered challenges near the crack tips. Finally, the paper discusses how to convert between and therefore compare parameters of various numerical approaches. Our benchmark studies reveal that each model necessitates a distinct number of parameters, even in simple scenarios like static crack aperture benchmarks. It is generally more practical to employ fewer parameters to mitigate model over-parameterization and enhance experimental feasibility.

DECOVALEX-2019: An international collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems

The DECOVALEX Project is an international research collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. DECOVALEX stands for “DEvelopment of COupled Models and VALidation against EXperiments”. The creation of this international initiative, now running for almost 30 years, was motivated by the recognition that prediction of these coupled effects is an essential part of the performance and safety assessment of geologic disposal systems for radioactive waste and spent nuclear fuel, and also for a range of other sub-surface engineering activities. DECOVALEX emphasizes joint analysis and comparative modeling of state-of-the-art field and laboratory experiments, across a range of host rock options and repository designs. Participating research teams are from radioactive waste management organizations, national research institutes, regulatory agencies, universities, and consulting groups, providing a wide range of perspectives and solutions to these complex problems. The most recent phase of the DECOVALEX Project, here referred to as DECOVALEX-2019, started in 2016 and ended in 2019. Modeling teams from 13 international partner organizations participated in the comparative evaluation of seven modeling tasks involving complex field and/or laboratory experiments. This Virtual Special Issue on DECOVALEX-2019 provides an in-depth overview of these collaborative research efforts and how these have advanced the state-of-the-art of understanding and modeling coupled THMC processes.

International collaboration in disposal research: comparative modeling of coupled processes in the DECOVALEX project

Abstract. This presentation gives an overview of an international research collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. The creation of the international DECOVALEX project, now running for more than 25 years, was motivated by the recognition that prediction of these coupled effects is an essential part of the performance and safety assessment of geologic disposal systems for radioactive waste and spent nuclear fuel. DECOVALEX emphasizes joint analysis and comparative modeling of state-of-the-art field and laboratory experiments, across a range of host rock options and repository designs. Participating research teams are from radioactive waste management organizations, national research institutes, regulatory agencies, universities, and consulting groups, providing a wide range of perspectives and solutions to these complex problems. The presentation provides examples of the research contributions made collectively in past DECOVALEX tasks and also touches on the unique modeling challenges tackled in the ongoing project phase, referred to as DECOVALEX-2023. The current phase comprises 17 partner organizations, about 50 modeling teams, and 7 modeling tasks, which cover a broad portfolio from fundamental studies on gas migration to full-scale in situ heater experiments in different host rocks to performance assessment studies. Together, these examples illustrate that the insight and scientific knowledge gained within the DECOVALEX project would not have been possible if one group had studied these complex THMC modeling challenges alone rather than within a truly collaborative setting.

Development of approaches for modelling coupled thermal–hydraulic–mechanical–chemical processes in single granite fracture experiments

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.

Numerical modeling of heating and hydration experiments on bentonite pellets

In a common, designed disposal system for high-level radioactive waste, bentonite is used worldwide as a buffer material due to its favorable thermodynamic properties. In-situ experiments at different scales are being conducted in several underground laboratories to investigate the long-term coupled thermo-hydro-mechanical (THM) and chemical behavior of bentonite. Simultaneously, numerous laboratory experiments under well-defined conditions were performed to determine the material properties of bentonite. Since 2012, a laboratory heating and hydration test on MX-80 bentonite has been performed by CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas). The obtained data during the test were analyzed by different research teams within the international DECOVALEX project in order to understand the coupled THM behavior of bentonite and furthermore to determine the thermal-mechanical properties of bentonite. This paper presents a numerical model for fully-coupled THM processes in bentonite based on the finite element method program OpenGeoSys (Kolditz et al., 2012a). In this model, the description of heat conduction is based on Fourier's law, and advection is also considered in the heat transfer. As an important coupling factor from hydraulic process to thermal process, the dependency of thermal conductivity on the water saturation is intensively analyzed. A multiphase flow model considering water evaporation and vapor diffusion is used to describe the hydraulic process. Special attention was also paid on the analysis of the water retention behavior. An elastic constitutive model based on generalized Hook's law was applied to describe the material's mechanical behavior complemented with consideration of the thermally induced strain and the swelling deformation. Good agreement between the calculated and measured data has been achieved concerning temperature, relative humidity, total stress, and water intake. The main coupling processes and bentonite behavior in the experiment could be captured and well analyzed in this model. However, further investigations are also needed, especially for the long-term THM process with respect to water intake.

The Effect of Stress on Flow and Transport in Fractured Rock Masses Using an Extended Multiple Interacting Continua Method with Crack Tensor Theory

We present an extended multiple interacting continua (Ex-MINC) model of fractured rock masses that uses Oda’s crack tensor theory to upscale the hydraulic and mechanical properties. The Ex-MINC concept includes separate connected continua representing active fractures, inactive fractures, and matrix to represent the fracture-matrix system. The crack tensor theory was used to calculate the stress-dependent permeability tensor and compliance tensor for individual grid blocks. By doing this, we transformed a discrete fracture network model into a grid-based continuum model. The Ex-MINC model was verified against an existing analytical solution, and the entire Ex-MINC/crack tensor model approach was applied to a benchmark test (BMT) related to coupled stress, fluid flow, and transport through a 20-×20-m model domain of heavily fractured media. This BMT was part of the international DECOVALEX project for the development of coupled models and their validation, thus providing us with the opportunity to compare our results with the results of independent models. We conducted the coupled hydraulic and mechanical modeling with TOUGH-FLAC, a simulator based on the TOUGH2 multiphase flow code and the FLAC3D geomechanical code. The results of our simulations were generally consistent with the results of the other independent modeling approaches and showed how inactive fractures impeded solute transport through the fractured system by providing an additional fracture surface area as an avenue for increasing fracture matrix diffusion.

Modelling of hydro-mechanical coupling and transport in densely fractured rock mass

A numerical model is presented to study hydro-mechanical coupling process based on the rigid body spring method and the discrete fracture network model. In order to realistically simulate the constitutive behaviours of fractures, a non-linear constitutive model is employed, in which non-linear normal stress deformation relationship, tangential slipping failure and dilation effects are considered. To investigate coupling effects on transport, a particle tracking procedure is also incorporated to account for solute movement. Stress effects on flow field and transport are studied on a typical fracture network provided by the DECOVALEX project. Results show that it is the non-linear normal deformation that mainly influences flow and transport when the confining stress is relatively low and the stress difference is not large. When the stress increases and stress difference becomes larger, dilation effect gradually becomes the governing mechanism. These results are compared with other researcher’s work and reasonable agreements are obtained.

Non-isothermal flow in low permeable porous media: a comparison of Richards’ and two-phase flow approaches

The present work compares the performance of two alternative flow models for the simulation of thermal-hydraulic coupled processes in low permeable porous media: non-isothermal Richards’ and two-phase flow concepts. Both models take vaporization processes into account: however, the Richards’ model neglects dynamic pressure variations and bulk flow of the gaseous phase. For the comparison of the two approaches first published, data from a laboratory experiment are studied involving thermally driven moisture flow in a partially saturated bentonite sample. Then a benchmark test of longer-term thermal-hydraulic behavior in the engineered barrier system of a geological nuclear waste repository is analyzed (DECOVALEX project). It was found that both models can be used to reproduce the vaporization process if the intrinsic permeability is relative high. However, when a thermal-hydraulic coupled problem has the same low intrinsic permeability, only the two-phase flow approach provides reasonable results.

DECOVALEX Project: from 1992 to 2007

The DECOVALEX project is a unique international research collaboration, initiated in 1992, for advancing the understanding and mathematical modelling of coupled thermo-hydro-mechanical (THM) and thermo-hydro-mechanical-chemical (THMC) processes in geological systems—subjects of importance for performance assessment of radioactive waste repositories in geological formations. From 1992 up to 2007, the project has made important progress and played a key role in the development of numerical modelling of coupled processes in fractured rocks and buffer/backfill materials. The project has been conducted by research teams supported by a large number of radioactive-waste-management organizations and regulatory authorities, including those of Canada, China, Finland, France, Japan, Germany, Spain, Sweden, UK, and the USA. Through this project, in-depth knowledge has been gained of coupled THM and THMC processes associated with nuclear waste repositories, as well as numerical simulation models for their quantitative analysis. The knowledge accumulated from this project, in the form of a large number of research reports and international journal and conference papers in the open literature, has been applied effectively in the implementation and review of national radioactive-waste-management programmes in the participating countries. This paper presents an overview of the project.

Results from an International Simulation Study on Coupled Thermal, Hydrological, and Mechanical Processes near Geological Nuclear Waste Repositories

As part of the ongoing international DECOVALEX project, four research teams used five different models to simulate coupled thermal, hydrological, and mechanical (THM) processes near waste emplacement drifts of geological nuclear waste repositories. The simulations were conducted for two generic repository types, one with open and the other with back-filled repository drifts, under higher and lower postclosure temperatures, respectively. In the completed first model inception phase of the project, a good agreement was achieved between the research teams in calculating THM responses for both repository types, although some disagreement in hydrological responses is currently being resolved. In particular, good agreement in the basic thermal-mechanical responses was achieved for both repository types, even though some teams used relatively simplified thermal-elastic heat-conduction models that neglected complex near-field thermal-hydrological processes. The good agreement between the complex and simplified process models indicates that the basic thermal-mechanical responses can be predicted with a relatively high confidence level.

The DECOVALEX III project: A summary of activities and lessons learned

Initiated in 1992, the DECOVALEX project is an international collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical (THM) processes in geologic systems. The project has made important scientific achievements through three stages and is progressing in its fourth stage. It has played a key role in the development of mathematical modeling and in situ testing of coupled THM processes in fractured rock and buffer/backfill materials, a subject of importance for performance assessment of radioactive waste geologic repositories. This paper summarizes studies under the most recent stage of the project, DECOVALEX III (2000-2003). These studies include those of two major field experiments: (a) the FEBEX experiment at Grimsel, Switzerland, investigating coupled THM processes in a crystalline rock-bentonite system, and (b) the Drift Scale Test (DST) experiment at Yucca Mountain, Nevada, investigating coupled THM processes in unsaturated tuff. These are two of the largest multiyear heater tests undertaken to date for the study of coupled THM processes in geological systems. In addition, three so-called benchmark tests are also studied to evaluate the impact of coupled THM processes under different scenarios and geometries. Within the DECOVALEX project, multiple research teams participated in each of the studies, using different approaches and computer codes. Comparisons of results have provided insight into coupled THM processes, which in turn has stimulated further development of our modeling capabilities. Lessons learned from these studies are discussed. The scientific advances and enhanced insight gained through this kind of international cooperation illustrate the effectiveness of the DECOVALEX project.

Prediction of the hydro-mechanical response during shaft sinking for the proposed Nirex Rock Characterisation Facility near Sellafield, Cumbria, United Kingdom

Abstract The coupled hydro-mechanical responses in the fractured volcaniclastic rocks of the Borrowdale Volcanic Group, Sellafield, Cumbria, to a long-term pump test and the proposed shaft sinking phase of Nirex's Rock Characterisation Facility was studied by research teams under the auspices of the second phase of DECOVALEX project. This paper discusses the background and objectives of the study and provides a brief summary of the results and major scientific and logistical lessons learnt.

DECOVALEX—An international co-operative research project on mathematical models of coupled THM processes for safety analysis of radioactive waste repositories

An international co-operative research project has been established under the name DECOVALEX for theoretical and experimental studies of coupled thermal, hydrological and mechanical processes in hard rocks. The capability of modelling coupled phenomena is of particular importance to the safety assessment of geologic disposal of radioactive waste materials. Different mathematical models and computer codes have been developed by different national research teams and these are used to study the so-called bench mark test and test case problems developed within this project. Bench mark tests are defined as initial-boundary value problems of a generic nature, and test cases are experimental investigations of part or full aspects of coupled thermo-hydro-mechanical processes in hard rocks. Presented in this introductory paper are the motivation, organization and approaches of the project, definitions of bench mark tests and test case problems and descriptions of mathematical models used in Phase I of the DECOVALEX project. A brief introduction to the Phase II and Phase III of the project is also presented.