Clay formations are investigated worldwide as potential host rock for the deep geological disposal of high-level radioactive waste (HLW). Usually bentonite is preferred as the buffer and backfill material in the disposal system. In the disposal of HLW, heat emission is one of the most important issues as it can generate a series of complex thermo-hydro-mechanical (THM) processes in the surrounding materials and thus change the material properties. In the context of safety assessment, it is important to understand the thermally induced THM interactions and the associated change in material properties. In this work, the thermally induced coupled THM behaviours in the clay host rock and in the bentonite buffer as well as the corresponding coupling effects among the relevant material properties are numerically analysed. A coupled non-isothermal Richards flow mechanical model and a non-isothermal multiphase flow model were developed based on the scientific computer codes OpenGeoSys (OGS). Heat transfer in the porous media is governed by thermal conduction and advective flow of the pore fluids. Within the hydraulic processes, evaporation, vapour diffusion, and the unsaturated flow field are considered. Darcy’s law is used to describe the advective flux of gas and liquid phases. The relative permeability of each phase is considered. The elastic deformation process is modelled by the generalized Hooke’s law complemented with additional strain caused by swelling/shrinkage behaviour and by temperature change. In this study, special attention has been paid to the analysis of the thermally induced changes in material properties. The strong mechanical and hydraulic anisotropic properties of clay rock are described by a transversely isotropic mechanical model and by a transversely isotropic permeability tensor, respectively. The thermal anisotropy is described by adoption of the bedding-orientation-dependent thermal conductivity. The dependency of the thermal conductivity on the degree of water saturation, the dependency of the thermal effects on the water retention behaviour, and the dependency of the effects of the pore pressure variation on the permeability and the anisotropic swelling/shrinkage behaviour have been intensively analysed and the corresponding numerical models to consider those coupling effects have been developed. The developed numerical model has been applied to simulate the laboratory and in situ heating experiments on the bentonite and clay rock at different scales. Firstly the laboratory heating experiment on Callovo-Oxfordian Clay (COX) and the laboratory long-term heating and hydration experiment on MX80 pellets were simulated. Based on the knowledge from the numerical analysis of the laboratory experiments, a 1:2 scale in situ heating experiment of an integrated system of the bentonite engineered barrier system (EBS) in the Opalinus Clay host rock was simulated. All the relevant operation phases were considered in the modelling. Besides, the modelling was extended to 50 years after the heat shut-down with the aim of predicting the long-term behaviours. Additionally, variation calculations were carried out to investigate the effects of the storage capacity of the Opalinus Clay on the thermally induced hydraulic response. In the long-term modelling, the effects of different saturated water permeabilities of buffer material on the resaturation process were analysed. Based on the current researches and model developments, the observed THM behaviours of the bentonite buffer and the clay rock, that is, the measured evolution of temperature, pore pressure, humidity, swelling pressure, and so on in the laboratory and in situ experiments can be reproduced and interpreted well. It is proved that by using both a non-isothermal multiphase flow model and a non-isothermal Richards flow model combined with the corresponding thermal and mechanical models, the major THM behaviours can be captured. It is validated that the developed model is able to simulate the relevant coupled THM behaviours of clayey material under the well-defined laboratory conditions as well as under the complex natural disposal conditions.

Reactive transport modelling has wide applications in geosciences. In the field of hydrogeology, it has been utilised to simulate the biogeochemical processes that disperse and degrade contaminants in the aquifer. For geotechnical applications, such as geological CO2 sequestration, the reaction of CO2 with the ambient saline aquifer determines the final success of storage. In a radioactive waste repository, scientists rely on reactive transport models to predict the mobilisation of hazardous radionuclides within space and time. In this work, the multi-component mass transport code OpenGeoSys, was coupled with two geochemical solvers, the Gibbs Energy Minimization Selektor (GEM) and the Biogeochemical Reaction Network Simulator (BRNS). Both coupled codes were verified against analytical solutions and simulation results from other numerical models. Moreover, the coupling interface was developed for parallel simulation. Test runs showed that the speed-up of reaction part had a very good linearity with number of nodes in the mesh. However, for three dimensional problems with complex geochemical reactions, the model performance was dominated by solving transport equations of mobile chemical components. OpenGeoSys-BRNS was applied to a two dimensional groundwater remediation problem. Its calculated concentration profiles fitted very well with analytical solutions and numerical results from TBC. The model revealed that natural attenuation of groundwater contaminants is mainly controlled by the mixing of carbon source and electron donor. OpenGeoSys-GEM was employed to investigate the retardation mechanism of radionuclides in the near field of a nuclear waste repository. Radium profiles in an idealised bentonite column was modelled with varying clay/water ratios. When clay content is limited, Ba-Sr-Ra sulfate solid solutions have a very strong retardation effect on the aqueous radium. Nevertheless, when clay mineral is abundant, cation exchange sites also attract Sr and Ba, thus dominates the transport of Ra.

Anaerobic digestion technologies have been utilized in Vietnam for more than 30 years with thousands of domestic small scale plants, mostly for agricultural and livestock wastes. For municipal solid waste (MSW) the development of biogas plants is far below the current high waste generation rates. The aim of this paper is to present the results of a feasibility assessment of implementing AD to treat the organic fraction of municipal solid waste (OFMSW) in Vietnam. For this purpose, an environmental analysis was performed comparing three treatment scenarios: two hypothetical AD technologies (a wet and a dry fermentation system) and the existing industrial composting facility at Nam Binh Duong Waste Treatment Complex in South Vietnam. This study sought for the technology to recover the most possible resources and energy from the OFMSW, and reduce greenhouse gas (GHG) emissions. The results were then combined with a policy review to support a holistic approach on the feasibility of these technologies in Vietnam. The outcome indicates that by implementing the dry AD system, up to 16.7 GWh of power and 14.4 GWh of heat energy can be generated annually and it can potentially save up to 5,400 Mg of CO2 equivalent per year, presenting the highest resource/energy benefits. The performance of the wet system and composting facility present some advantages particularly if there is a previous segregation of the organic material from the rest of the household wastes. Moreover, current reforms in Vietnam demonstrate the government’s interest in AD technologies, translated into the development of fiscal and financial revenues which incentivize participation from the public and private sector. Finally, these technologies are constantly under development and have the potential to be further improved, which gives hopes that waste treatment systems can be optimized to meet the waste and energy challenges of the future generations. / Phương pháp lên men kị khí đã được áp dụng tại Việt Nam từ hơn 30 năm nay với hàng nghìn các công trình nhỏ chủ yếu xử lý chất thải nông nghiệp và chăn nuôi. Sự phát triển hiện nay của các nhà máy sinh khí biogas còn quá ít cho xử lý lượng phát thải cao rác thải đô thị. Bài báo này trình bày các kết quả việc đánh giá tính khả thi khi áp dụng công nghệ lên men kị khí xử lý phần hữu cơ của chất thải rắn đô thị tại Việt Nam. Với mục đích này, phân tích môi trường được thực hiện để so sánh ba kịch bản xử lý: hai công nghệ lên men kị khí giả định (một cho công nghệ lên men ướt và một cho công nghệ lên men khô) và nhà máy hiện hữu lên men hiếu khí làm phân bón compost tại khu liên hợp xử lý chất thải Nam Bình Dương ở miền Nam Việt Nam. Nghiên cứu này tìm kiếm giải pháp công nghệ để thu hồi nhiều nhất có thể các tài nguyên và năng lượng từ rác thải đô thị và và giảm phát thải khí nhà kính. Các kết quả sau đó được kết hợp với đánh giá chính sách để hỗ trợ cách tiếp cận toàn diện về tính khả thi của các công nghệ này vào Việt Nam. Kết quả cho thấy áp dụng công nghệ lên men kị khí khô có thể tạo ra đến 16,7 GWh điện năng và 14,4 GWh nhiệt năng hàng năm và có khả năng làm giảm đến 8,000 Mg CO2 tương đương mỗi năm, thể hiện lợi ích cao nhất giữa tài nguyên và năng lượng. Hiệu suất của hệ thống lên men kị khí ướt và lên men hiếu khí thể hiện một số lợi thế đặc biệt khi nguyên liệu hữu cơ cho quá trình lên men được tiền phân loại ra khỏi hỗn hợp rác sinh hoạt. Hơn nữa, các đổi mới hiện nay ở Việt Nam thể hiện sự quan tâm của Chính phủ đến các công nghệ lên men kị khí, thể hiện qua sự tăng trưởng tài chính và doanh thu để khuyến khích sự tham gia của khu vực công và tư nhân. Chắc chắn rằng các công nghệ sẽ liên tục được phát triển và có khả năng được cải tiến tốt hơn, mang đến cho chúng ta những hy vọng rằng các hệ thống xử lý chất thải được tối ưu hóa để đáp ứng được các thách thức về chất thải và năng lượng của các thế hệ tương lai.

Reactive transport modelling has wide applications in geosciences. In the field of hydrogeology, it has been utilised to simulate the biogeochemical processes that disperse and degrade contaminants in the aquifer. For geotechnical applications, such as geological CO2 sequestration, the reaction of CO2 with the ambient saline aquifer determines the final success of storage. In a radioactive waste repository, scientists rely on reactive transport models to predict the mobilisation of hazardous radionuclides within space and time. In this work, the multi-component mass transport code OpenGeoSys, was coupled with two geochemical solvers, the Gibbs Energy Minimization Selektor (GEM) and the Biogeochemical Reaction Network Simulator (BRNS). Both coupled codes were verified against analytical solutions and simulation results from other numerical models. Moreover, the coupling interface was developed for parallel simulation. Test runs showed that the speed-up of reaction part had a very good linearity with number of nodes in the mesh. However, for three dimensional problems with complex geochemical reactions, the model performance was dominated by solving transport equations of mobile chemical components. OpenGeoSys-BRNS was applied to a two dimensional groundwater remediation problem. Its calculated concentration profiles fitted very well with analytical solutions and numerical results from TBC. The model revealed that natural attenuation of groundwater contaminants is mainly controlled by the mixing of carbon source and electron donor. OpenGeoSys-GEM was employed to investigate the retardation mechanism of radionuclides in the near field of a nuclear waste repository. Radium profiles in an idealised bentonite column was modelled with varying clay/water ratios. When clay content is limited, Ba-Sr-Ra sulfate solid solutions have a very strong retardation effect on the aqueous radium. Nevertheless, when clay mineral is abundant, cation exchange sites also attract Sr and Ba, thus dominates the transport of Ra.

Nach über 60 Jahren Nutzung ziviler Kernenergie in Deutschland und dem Atomausstieg bis 2022 stellt sich die Frage nach der Endlagerung (hoch-) radioaktiver Abfälle. Für die Endlagersuche in Deutschland trat 2017 das Standortauswahlgesetz (StandAG) in Kraft, das ein partizipatives, wissenschaftsbasiertes und transparentes Verfahren fordert (Deutscher Bundestag, 2020). Um die in diesem Verfahren auftretenden Fragen und Ergebnisse gesellschaftlich und wissenschaftlich fundiert diskutieren zu können, werden von den am Prozess beteiligten Stakeholdern in zunehmendem Maße auch digitale Informationsangebote bereitgestellt. Diese nutzen verschiedene Arten des Informationsdesigns (Umwandlung komplexer Daten in einfachere, nützliche und efektive Informationen, um Bedürfnisse und Ziele Nutzender kontextabhängig zu erfüllen (Herrera, 2013)), u.a. Kartendarstellungen, wie sie auch in anderen partizipativen Verfahren eingesetzt werden, um niederschwellig Information bereitzustellen (Moser et al., 2020). Vertrauen ist besonders wichtig, um den Betroffenen bei der Entscheidung zu helfen, ob sie an öffentlichen Beteiligungsprozessen teilnehmen (Santamaría-Philco & Wimmer, 2018). In früheren Umfragen wurde den verschiedenen Stakeholdern generell ein sehr unterschiedlicher Grad an Vertrauenswürdigkeit zugestanden (Drögemüller, 2017; Hocke, Stolle & Gloede, 2003; TNS Opinion & Social, 2008). Bei kritischer Würdigung der Fähigkeit dieser Stakeholder, die öffentliche Meinung zu beeinflussen, stellt sich die Frage, ob eine Beurteilung des Inhalts anhand des Informationsdesigns oder aufgrund des dem Stakeholder zugestandenen Vertrauens erfolgt. Wissenschaftlichkeit hingegen ist als Kriterium vor allem mit Blick auf das StandAG relevant, welches einen wissenschaftlich fundierten Prozess fordert. Um diese Aspekte zu untersuchen, wurde eine Umfrage durchgeführt. [Aus: Einleitung]

Anaerobic digestion technologies have been utilized in Vietnam for more than 30 years with thousands of domestic small scale plants, mostly for agricultural and livestock wastes. For municipal solid waste (MSW) the development of biogas plants is far below the current high waste generation rates. The aim of this paper is to present the results of a feasibility assessment of implementing AD to treat the organic fraction of municipal solid waste (OFMSW) in Vietnam. For this purpose, an environmental analysis was performed comparing three treatment scenarios: two hypothetical AD technologies (a wet and a dry fermentation system) and the existing industrial composting facility at Nam Binh Duong Waste Treatment Complex in South Vietnam. This study sought for the technology to recover the most possible resources and energy from the OFMSW, and reduce greenhouse gas (GHG) emissions. The results were then combined with a policy review to support a holistic approach on the feasibility of these technologies in Vietnam. The outcome indicates that by implementing the dry AD system, up to 16.7 GWh of power and 14.4 GWh of heat energy can be generated annually and it can potentially save up to 5,400 Mg of CO2 equivalent per year, presenting the highest resource/energy benefits. The performance of the wet system and composting facility present some advantages particularly if there is a previous segregation of the organic material from the rest of the household wastes. Moreover, current reforms in Vietnam demonstrate the government’s interest in AD technologies, translated into the development of fiscal and financial revenues which incentivize participation from the public and private sector. Finally, these technologies are constantly under development and have the potential to be further improved, which gives hopes that waste treatment systems can be optimized to meet the waste and energy challenges of the future generations. / Phương pháp lên men kị khí đã được áp dụng tại Việt Nam từ hơn 30 năm nay với hàng nghìn các công trình nhỏ chủ yếu xử lý chất thải nông nghiệp và chăn nuôi. Sự phát triển hiện nay của các nhà máy sinh khí biogas còn quá ít cho xử lý lượng phát thải cao rác thải đô thị. Bài báo này trình bày các kết quả việc đánh giá tính khả thi khi áp dụng công nghệ lên men kị khí xử lý phần hữu cơ của chất thải rắn đô thị tại Việt Nam. Với mục đích này, phân tích môi trường được thực hiện để so sánh ba kịch bản xử lý: hai công nghệ lên men kị khí giả định (một cho công nghệ lên men ướt và một cho công nghệ lên men khô) và nhà máy hiện hữu lên men hiếu khí làm phân bón compost tại khu liên hợp xử lý chất thải Nam Bình Dương ở miền Nam Việt Nam. Nghiên cứu này tìm kiếm giải pháp công nghệ để thu hồi nhiều nhất có thể các tài nguyên và năng lượng từ rác thải đô thị và và giảm phát thải khí nhà kính. Các kết quả sau đó được kết hợp với đánh giá chính sách để hỗ trợ cách tiếp cận toàn diện về tính khả thi của các công nghệ này vào Việt Nam. Kết quả cho thấy áp dụng công nghệ lên men kị khí khô có thể tạo ra đến 16,7 GWh điện năng và 14,4 GWh nhiệt năng hàng năm và có khả năng làm giảm đến 8,000 Mg CO2 tương đương mỗi năm, thể hiện lợi ích cao nhất giữa tài nguyên và năng lượng. Hiệu suất của hệ thống lên men kị khí ướt và lên men hiếu khí thể hiện một số lợi thế đặc biệt khi nguyên liệu hữu cơ cho quá trình lên men được tiền phân loại ra khỏi hỗn hợp rác sinh hoạt. Hơn nữa, các đổi mới hiện nay ở Việt Nam thể hiện sự quan tâm của Chính phủ đến các công nghệ lên men kị khí, thể hiện qua sự tăng trưởng tài chính và doanh thu để khuyến khích sự tham gia của khu vực công và tư nhân. Chắc chắn rằng các công nghệ sẽ liên tục được phát triển và có khả năng được cải tiến tốt hơn, mang đến cho chúng ta những hy vọng rằng các hệ thống xử lý chất thải được tối ưu hóa để đáp ứng được các thách thức về chất thải và năng lượng của các thế hệ tương lai.

Clay formations are investigated worldwide as potential host rock for the deep geological disposal of high-level radioactive waste (HLW). Usually bentonite is preferred as the buffer and backfill material in the disposal system. In the disposal of HLW, heat emission is one of the most important issues as it can generate a series of complex thermo-hydro-mechanical (THM) processes in the surrounding materials and thus change the material properties. In the context of safety assessment, it is important to understand the thermally induced THM interactions and the associated change in material properties. In this work, the thermally induced coupled THM behaviours in the clay host rock and in the bentonite buffer as well as the corresponding coupling effects among the relevant material properties are numerically analysed. A coupled non-isothermal Richards flow mechanical model and a non-isothermal multiphase flow model were developed based on the scientific computer codes OpenGeoSys (OGS). Heat transfer in the porous media is governed by thermal conduction and advective flow of the pore fluids. Within the hydraulic processes, evaporation, vapour diffusion, and the unsaturated flow field are considered. Darcy’s law is used to describe the advective flux of gas and liquid phases. The relative permeability of each phase is considered. The elastic deformation process is modelled by the generalized Hooke’s law complemented with additional strain caused by swelling/shrinkage behaviour and by temperature change. In this study, special attention has been paid to the analysis of the thermally induced changes in material properties. The strong mechanical and hydraulic anisotropic properties of clay rock are described by a transversely isotropic mechanical model and by a transversely isotropic permeability tensor, respectively. The thermal anisotropy is described by adoption of the bedding-orientation-dependent thermal conductivity. The dependency of the thermal conductivity on the degree of water saturation, the dependency of the thermal effects on the water retention behaviour, and the dependency of the effects of the pore pressure variation on the permeability and the anisotropic swelling/shrinkage behaviour have been intensively analysed and the corresponding numerical models to consider those coupling effects have been developed. The developed numerical model has been applied to simulate the laboratory and in situ heating experiments on the bentonite and clay rock at different scales. Firstly the laboratory heating experiment on Callovo-Oxfordian Clay (COX) and the laboratory long-term heating and hydration experiment on MX80 pellets were simulated. Based on the knowledge from the numerical analysis of the laboratory experiments, a 1:2 scale in situ heating experiment of an integrated system of the bentonite engineered barrier system (EBS) in the Opalinus Clay host rock was simulated. All the relevant operation phases were considered in the modelling. Besides, the modelling was extended to 50 years after the heat shut-down with the aim of predicting the long-term behaviours. Additionally, variation calculations were carried out to investigate the effects of the storage capacity of the Opalinus Clay on the thermally induced hydraulic response. In the long-term modelling, the effects of different saturated water permeabilities of buffer material on the resaturation process were analysed. Based on the current researches and model developments, the observed THM behaviours of the bentonite buffer and the clay rock, that is, the measured evolution of temperature, pore pressure, humidity, swelling pressure, and so on in the laboratory and in situ experiments can be reproduced and interpreted well. It is proved that by using both a non-isothermal multiphase flow model and a non-isothermal Richards flow model combined with the corresponding thermal and mechanical models, the major THM behaviours can be captured. It is validated that the developed model is able to simulate the relevant coupled THM behaviours of clayey material under the well-defined laboratory conditions as well as under the complex natural disposal conditions.

Methane (CH4) source of Greenhouse Gases should be considered; CH4 is formed by composting under anaerobic conditions. Using microbial Methane oxidation is a solution with low cost and effective. In this study, 27 bio-filters and 18 laboratory-scale bioreactors were used to investigate the potential for CH4 removal in biogas. The CH4, Dinitrogen monoxide (N2O) and Carbon dioxide (CO2) concentrations at the inlet and outlet of the air purifier were measured by gas chromatography. The results showed that the CH4 concentration decreased in experiments while the CO2 and N2O content increased in all experiments. An experiment was conducted with 1 kg of biofilter material with the input of 800 ppm CH4 contained in a 5-liter flask for 49 hours containing. The results also showed that the CH4 concentration decreased by 71% after 20 hours and N2O was formed in the reactor. / Mê-tan (CH4) là nguồn khí gây nên hiệu ứng nhà kính cần được quan tâm, khí CH4 được sinh ra trong quá trình ủ vi sinh trong điều kiện kị khí. Một giải pháp với chi phí thấp là sử dụng vi sinh vật oxy hóa khí CH4 cố định trên giá thể là vật liệu sử dụng trong thiết bị lọc sinh học. Trong nghiên cứu này, 27 thiết bị lọc sinh học trên thực tế và 19 bình lọc tại phòng thí nghiệm đã được sử dụng nhằm mục đích khảo sát khả năng loại bỏ CH4 có trong khí sinh học. Nồng độ khí CH4, N2O và CO2 ở đầu vào và đầu ra bể lọc khí được đo đạc bằng phương pháp sắc ký khí. Kết quả cho thấy nồng độ khí CH4 giảm sau khi qua hệ thống lọc sinh học ở một số bình, trong khi nồng độ khí CO2 và N2O lại tăng lên ở tất cả các bình. Khi khảo sát khả năng oxi hóa CH4 ở nồng độ 800 ppm của 1kg vật liệu thiết bị lọc sinh học chứa trong bình phản ứng thể tích 5L với thời gian 49 giờ. Kết quả cho thấy nồng nồng độ CH4 giảm 71% sau 20 giờ. Tuy nhiên, N2O đã được ghi nhận có hình thành trong bình phản ứng đó.

Bei der Suche nach einem geologischen Tiefenlager für radioaktive Abfälle müssen alle Risiken für die Integrität des Endlagers und die Sicherheit der umgebenden Biosphäre berücksichtigt werden. Nach der Einlagerung radioaktiver Abfälle kommt es durch verschiedene chemische und physikalische Prozesse innerhalb des Endlagers zur Gasbildung. Die kontinuierliche Gasbildung kann den Gasdruck im verschlossenen Endlager erheblich erhöhen. Das Multibarrierensystem des Endlagers, das häufig aus tonhaltigen Materialien besteht, verhindert, dass sich das Gas verflüchtigt. Die Auswirkungen eines hohen Gasdrucks und die damit verbundenen mechanischen Verformungen können sich negativ auf das Barrieresystem des Endlagers auswirken. Ein genaues Verständnis und die Reproduzierbarkeit der gekoppelten Prozesse ist daher unerlässlich.\\ Experimentelle Ergebnisse deuten darauf hin, dass die Gasmigration bei hohen Gasdrücken über lokal begrenzte, preferentielle Strömungs-pfade erfolgt. In dieser Arbeit wurden verschiedene numerische Ansätze entwickelt, um diese Strömungspfade in numerischen Modellen effektiv zu reproduzieren. Ein entwickelter Ansatz nutzt mikroskalige Eigenschaften, die aus Messungen der Porengrößendichte gewonnen werden. Diese werden hochskaliert und in die Eigenschaften des porösen Mediums integriert. Darüber hinaus ist ein bimodales Porenmodell entwickelt worden, das speziell das dehnungsabhängige Wasserrückhaltevermögen beschreibt und einen verringerten Gaseintrittsdruck und erhöhten Anteil der Makroporosität simulieren kann. Die Simulationsmethoden beinhalten zusätzlich die Verwendung von dehnungsabhängigen Permeabilitätsmodellen für die implizite Darstellung der Strömungspfade. Diese Modellierungsansätze werden dann zur Simulation verschiedener Experimente angewandt, wodurch ein detaillierter Vergleich zwischen berechneten Parametern und experimentellen Ergebnissen ermöglicht wird.\\ Die mit dem Open-Source-Code OpenGeoSys durchgeführten Simulationen zeigen, dass bestimmte Merkmale der Gasmigration durch gesättigten Bentonit oder Tonstein erfolgreich reproduziert werden können. Heterogene Verteilungen der Materialeigenschaften können genutzt werden, um bevorzugte Strömungspfade und plötzliche Gasdurchbrüche zu simulieren. Eine Kombination aus dehnungsabhängigen Modifikationen der intrinsischen Permeabilität und der Wasserrückhalte-kurve ermöglicht eine verbesserte Modellierung impliziter präferentieller Pfade. Das sich daraus ergebende tiefere Verständnis der zugrundeliegenden Prozesse der Gasmigration kann das Sicherheitsanalyse für ein geologisches Tiefenlager für radioaktive Stoffe verbessern.:I Background 1 INTRODUCTION 1.1 Motivation 2 1.2 Objectives 4 1.3 Structure 5 II Theory and Numerics 2 GEOTECHNICAL CONTEXT AND FLUID TRANSPORT 2.1 Deep Geological Repository 9 2.2 Gas production in a DGR 11 2.2.1 Anaerobic metal corrosion 12 2.2.2 Water radiolysis 12 2.2.3 Biodegradation 13 2.3 Fluid transport mechanisms 13 2.3.1 Unsaturated initial conditions 13 2.3.2 Saturated initial conditions 14 2.3.2.1 Advective or diffusive transport of dissolved gas 14 2.3.2.2 Visco-capillary two-phase flow 16 2.3.2.3 Dilatancy-controlled gas flow 18 2.3.2.4 Fracture-controlled gas flow 19 3 NUMERICS AND METHODOLOGY 3.1 Balance equations 21 3.2 Mechanical models 23 3.3 Enhanced permeability functions 24 3.4 Spatial heterogeneity 27 3.5 Bimodal water retention in bentonite 29 4 APPLICATION AND MODEL DESCRIPTION 36 4.1 Application within the DECOVALEX Project 36 4.1.1 Task A – COx claystone 37 4.1.2 Task B – MX-80 Bentonite 38 4.2 Additional gas flow tests 40 III Simulations 5 RESULTS OF GAS FLOW TESTS WITH MX-80 BENTONITE 5.1 Heterogeneous Gas Entry Pressure from Measurements (Paper I) 45 5.2 Bimodal Heterogeneous Gas Entry Pressure (Paper II) 46 5.3 Strain-dependent Bimodal Water Retention Model (Paper III) 47 5.4 Strain-dependent Pore-Model with Heterogeneity 48 6 RESULTS OF GAS FLOW TESTS WITH COX CLAYSTONE (PAPER IV) 51 7 DISCUSSION 53 IV Summary 8 CONCLUSIONS AND OUTLOOK 57 A APPENDIX A 72 A.1 List of Publications 72 B APPENDIX B B.1 Paper I 75 B.2 Paper II 87 B.3 Paper III 109 B.4 Paper IV 121 / In the search for a deep geological repository for radioactive waste, all risks to the integrity of the repository and the safety of the surrounding biosphere must be considered. Following the emplacement of radioactive waste, various chemical and physical processes within the repository induce gas generation. Continuous gas generation can significantly increase the gas pressure in the sealed repository. The repository's multi-barrier system, which often consists of clayey materials such as clayrock and bentonite, prevents the gas from dissipating. The resulting high gas pressure and the associated mechanical deformations can have a negative impact on the multi-barrier system of the repository, e.g. by creating fluid pathways. A sufficiently precise understanding and reproducibility of the processes associated with gas migration through clayey materials is therefore essential for the safety analysis of a repository for radioactive waste.\\ Experimental evidence suggests that, under high gas pressure, gas migration predominantly occurs through localized, preferential pathways characterised by spontaneous breakthroughs. In this work, various numerical approaches have been developed to effectively reproduce these pathways in numerical models. One developed approach utilises microscale properties obtained from measurements of pore size density. These are upscaled and integrated into the properties of the porous medium. Additionally, a bimodal pore model is developed specifically to capture strain-dependent water retention, allowing the simulation of gas entry pressure reduction and macroporosity increase within preferential pathways. The simulation methods include the use of strain-dependent permeability models for the implicit representation of these pathways. These modelling approaches are then applied to replicate different experiments, allowing a detailed comparison between calculated parameters and experimental results.\\ The simulations, performed with the open-source code OpenGeoSys, show that certain features of gas migration through saturated bentonite or claystone can be successfully reproduced. Heterogeneous distributions of material properties can be used to simulate preferential pathways and sudden gas breakthroughs. A combination of permeability models as well as strain-dependent modifications of the intrinsic permeability and the water retention curve provide opportunities for improved modelling of implicit preferential pathways. The resulting deeper understanding of the underlying processes of gas migration can strengthen the safety case for a deep geological repository for radioactive materials.:I Background 1 INTRODUCTION 1.1 Motivation 2 1.2 Objectives 4 1.3 Structure 5 II Theory and Numerics 2 GEOTECHNICAL CONTEXT AND FLUID TRANSPORT 2.1 Deep Geological Repository 9 2.2 Gas production in a DGR 11 2.2.1 Anaerobic metal corrosion 12 2.2.2 Water radiolysis 12 2.2.3 Biodegradation 13 2.3 Fluid transport mechanisms 13 2.3.1 Unsaturated initial conditions 13 2.3.2 Saturated initial conditions 14 2.3.2.1 Advective or diffusive transport of dissolved gas 14 2.3.2.2 Visco-capillary two-phase flow 16 2.3.2.3 Dilatancy-controlled gas flow 18 2.3.2.4 Fracture-controlled gas flow 19 3 NUMERICS AND METHODOLOGY 3.1 Balance equations 21 3.2 Mechanical models 23 3.3 Enhanced permeability functions 24 3.4 Spatial heterogeneity 27 3.5 Bimodal water retention in bentonite 29 4 APPLICATION AND MODEL DESCRIPTION 36 4.1 Application within the DECOVALEX Project 36 4.1.1 Task A – COx claystone 37 4.1.2 Task B – MX-80 Bentonite 38 4.2 Additional gas flow tests 40 III Simulations 5 RESULTS OF GAS FLOW TESTS WITH MX-80 BENTONITE 5.1 Heterogeneous Gas Entry Pressure from Measurements (Paper I) 45 5.2 Bimodal Heterogeneous Gas Entry Pressure (Paper II) 46 5.3 Strain-dependent Bimodal Water Retention Model (Paper III) 47 5.4 Strain-dependent Pore-Model with Heterogeneity 48 6 RESULTS OF GAS FLOW TESTS WITH COX CLAYSTONE (PAPER IV) 51 7 DISCUSSION 53 IV Summary 8 CONCLUSIONS AND OUTLOOK 57 A APPENDIX A 72 A.1 List of Publications 72 B APPENDIX B B.1 Paper I 75 B.2 Paper II 87 B.3 Paper III 109 B.4 Paper IV 121

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