Deep Geological Disposal Facility Research Articles

Coupled Modelling of Brine Availability in Salt-Based Disposal Facilities - Learning from DECOVALEX 2023

Salt is a potential host rock or caprock for deep geological disposal facilities for radioactive waste and has several favourable properties compared to other candidate rock types. In particular, intact salt has an extremely small porosity and permeability, which limits water availability and transmissivity. This has tended to lead to disposal concepts in salt being considered as ‘mostly dry’ and hence ideally suited to limiting groundwater interactions that could otherwise lead to corrosion of waste packages and mobilisation of radionuclides over isolation timescales (10 5 -10 6 years). However, there is plentiful experimental evidence to suggest that excavations in bedded salt can exhibit non-trivial inflows. These inflows often increase on heating and are thought to arise due to complicated interactions between thermal, hydrogeological and mechanical processes in the salt. Task E in the international collaborative DECOVALEX 2023 programme was established to attempt to better understand how these coupled processes affect brine availability in bedded salt. A summary of the UK team's learning from the task is given in this paper that may be of relevance when constructing models to inform future safety cases for radioactive waste disposal in salt, and other energy geoscience applications where brine interactions are a potentially complicating factor.

Gas migration at the granite–bentonite interface under semirigid boundary conditions in the context of high‐level radioactive waste disposal

AbstractThe corrosion of waste canisters in the deep geological disposal facilities (GDFs) for high‐level radioactive waste (HLRW) can generate gas, which escapes from the engineered barrier system through the interfaces between the bentonite buffer blocks and the host rock and those between the bentonite blocks. In this study, a series of water infiltration and gas breakthrough experiments were conducted on granite and on granite–bentonite specimens with smooth and grooved interfaces. On this basis, this study presents new insights and a quantitative assessment of the impact of the interface between clay and host rock on gas transport. As the results show, the water permeability values from water infiltration tests on granite and granite–bentonite samples (10−19–10−20 m2) are found to be slightly higher than that of bentonite. The gas permeability of the mock‐up samples with smooth interfaces is one order of magnitude larger than that of the mock‐up with grooved interfaces. The gas results of breakthrough pressures for the granite and the granite–bentonite mock‐up samples are significantly lower than that of bentonite. The results highlight the potential existence of preferential gas migration channels between the rock and bentonite buffer that require further considerations in safety assessment.

Influence of the Al Content on the Electrochemical Behavior of Zn-Al Cold-Sprayed Coatings in the Context of the Deep Geological Disposal of Radioactive Waste

For high-level radioactive waste, the French National Radioactive Waste Management Agency is currently developing a 500 m deep geological disposal facility called Cigéo. Carbon steel containers will be used to contain the wastes in the specific conditions of the disposal. The use of a sacrificial coating was studied as an additional protection for the containers against corrosion. A previous work had shown the possibility to use Zn-Al coatings in this specific medium. To optimize the coatings’ performance, the cold-spraying process was considered instead of the previously used wire arc spraying because it can increase the cohesion between the particles in the coating. Moreover, three aluminum contents, i.e., 5, 15 and 25 wt.%, were considered. The characterization of the obtained coatings revealed a strongly heterogeneous composition for the lower Al content (5 wt.%), with local Al contents from 1.3 wt.% Al to 44.5 wt.% Al. The corrosion study was carried out in a specific solution mimicking the pore solution of the surrounding cementitious material designed for disposal at a temperature of 50 °C. First, the polarization curves acquired with coated steel electrodes revealed the pseudo-passive behavior of the 25 wt.% Al coating, while for the other compositions, the coating remained active. Moreover, the higher aluminum content (25 wt.%) induced an important decrease in potential, with a possible risk of hydrogen embrittlement for the protected steel. Secondly, the sacrificial properties were investigated through 6 months of experiments using coated electrodes with cross-like defects and coated electrodes coupled with bare steel electrodes. Whatever the composition of the coating, the protection was maintained, with the 15 wt.% Al coating giving the best performance.

Effect of Iron Chloride (II) on Bentonites under Hydrothermal Gradients: A Comparative Study between Sodium Bentonite and Calcium Bentonite

This study investigates the performance of two bentonite materials, specifically MX-80 (Na-bentonite) and FEBEX (Ca-Mg-Na-bentonite), employed as engineered barriers in deep geological disposal facilities for the isolation of high-level radioactive waste, contained in metallic canisters. Experiments conducted at the laboratory scale focused on the interaction of these bentonites with FeCl2 powder, used as a soluble iron source, to observe enhanced alteration of the bentonite. The experiments were carried out under a hydrothermal gradient. A dominant Na-Cl-SO4 saline solution was put in contact with the compacted bentonites from the top, while a constant temperature of 100 °C was maintained at the bottom using a heater in contact with the layer of FeCl2. The experimental cells were examined after six months of interaction. Various changes in the physical and chemical properties of the bentonites were observed. An increase in the water content, a reduction in the specific surface area and cation exchange capacity, changes in the distribution of aqueous species, and the formation of secondary minerals were observed. Reaction products formed at the bentonite interface with FeCl2, primarily comprising akaganeite, goethite, and hematite. The smectites showed evident structural modifications, with an enrichment in iron content, and a shift in the exchangeable ion distribution in the case of MX-80 bentonite. This work provides valuable insights into the complex interactions between bentonite barriers and materials that dissolve iron, serving as proxies for deep geological disposal environments and indicating the potential long-term behavior, taking into account higher concentrations of dissolved iron than those expected in a real repository.

Key geotechnical characteristics of clay materials for engineered safety barriers of the deep geological RW disposal facility at the Yeniseiskiy site

Key geotechnical characteristics of clay materials for engineered safety barriers of the deep geological RW disposal facility at the Yeniseiskiy site

Numerical gas flow simulation performed to analyze advective gas flow in a compacted clay material

Burial of nuclear waste in a clay formation can result in the generation of significant amount of hydrogen and other gases due to corrosion of metallic materials under anoxic conditions, radioactive decay of waste, and radiolysis of water. If the rate of gas production exceeds the rate of gas diffusion within the clay porewater, formation and accumulations of a discrete gas phase will occur until the gas pressure becomes large enough to surpass the surrounding material gas-entry pressure; at that critical point occurrence of dilating and advective gas flow is expected [1]. Precompacted bentonite is suggested as a sealing material for the isolation of boreholes, disposal galleries, and deposition holes in a deep geological disposal facility for radioactive waste. Both formation of new porosity and the spread of dilatant pathways have been related to the advective flow of gas in bentonite [2]. Understanding of the processes and mechanisms involved is a key aspect when evaluating the impact of gas flow on repository design and construction of any future facility. A series of gas injection tests on compacted bentonite were carried out at the British Geological Survey within DECOVALEX-2023 (D-23) Task B: MAGIC and LASGIT. The project focuses on the development of new numerical techniques for the quantitative prediction of gas flow in repository systems.
 DECOVALEX-2023 Task B started with Modelling Advection of Gas In Clays (MAGIC). The planned approach of the task includes different stages, with an initial conceptual model development phase. In this work, the D-23 MAGIC has been studied based on the same methodology as previously applied in the work for DECOVALEX-2019 Task A - stage 1A (ENGINEER) using the same material properties and modelling strategy [3, 4]. Based on the actual dimensions of the sample, the 3D finite element model includes the geometry of the sample, the injection rod, the shape of the injection tip, the back pressure and the two filters associated to the location of the sensors. Hexahedral elements have been generated utilizing the CODE_BRIGHT software [5], and the random heterogeneity have been defined considering 3 different materials (M1-M2-M3). Layer-by-layer random permeability distribution was assumed (see Figure 1) applying 2/3–1/6–1/6 weighting for intrinsic permeabilities (ki) equal to 1×10-21, 1×10-20 and 1×10-19 m2, for materials M1, M2 and M3, respectively, as well as incorporating different embedded fracture parameters.
 LASGIT (LArge Scale Gas Injection Tests modelling) is a full-scale demonstration experiment operated by SKB at the Äspö Hard Rock Laboratory Hard Rock Laboratory at a depth of 420 m [6]. The 3D finite element model geometry is generated according to the real dimension of the test. Tests measurements included pressure and rate of gas inflow, gas outflow volume and pore pressure observed at various points of the sample. The gas test 1 of the LASGIT experiment is modeled in this study using a multiphase flow in porous media approach with the aim of facilitating the creation of favorable gas migration routes assuming permeability heterogeneity (Figure 1(b)), the same strategy as the MAGIC random permeability distribution outlined above. In this case, a coupled hydro-gas 3D FEM numerical model has been developed by the UPC/Andra research team to simulate the gas flow tests using the computer software CODE_BRIGHT, including initial permeability heterogeneity throughout the model. The numerical formulation also includes embedded fractures. The proposed model has been able to reproduce satisfactorily the observed behavior of the test measurements.

Corrosion evaluation for engineered barriers in crystalline host rock – challenges for container development in the German site selection process

Abstract. Crystalline rock is being considered as a potential host rock for a deep geological disposal facility for high-level radioactive waste in the ongoing site selection process in Germany. As crystalline rock is commonly highly fractured, the designation of a containment-providing rock zone is potentially not possible. In this case, according to German law (§ 23 (1) and (4) StandAG), the safe containment of the nuclear waste for 1 million years must be guaranteed by engineered and geo-engineered barriers. Therefore, it is of great importance to evaluate and understand processes that might lead to corrosion of the canister from the outside or from the inside. The long-term integrity of the waste canister depends on various parameters such as the material, microstructure, and coating of the canister; the properties of the geo-engineered barriers; the local pore water composition; the radiolysis on the inside of the container; and the tectonic strain. In a recent project (Long-Term Integrity of Canisters in Crystalline Host Rock – LaKris), the challenges of determining the durability of waste canisters in crystalline host rock have been addressed by comparing and evaluating different international concepts for canister design and discussing their applicability to the requirements in Germany. Further focus lies on the evaluation of predominant corrosion mechanisms while also considering the geochemical environment of the crystalline host rock in Germany that the canisters might be subjected to. This contribution will highlight the activities within the project with regard to evaluating the corrosion properties of different potential canister materials such as copper, cast iron, and stainless steel. The aim is to deepen the understanding of individual processes and their combined effects as well as their classification into favourable or unfavourable processes and conditions for the derivation or justification of requirements. It also includes a review of the existing requirements on canister integrity according to the regulatory framework as well as a discussion about important hydrogeochemical parameters, which could influence the corrosion of the canister, including the limited availability and uncertainty of such data.

Scalable block preconditioners for saturated thermo-hydro-mechanics problems

We are interested in the modelling of saturated thermo-hydro-mechanical (THM) problems that describe the behaviour of a soil in which a weakly compressible fluid evolves. It is used for the evaluation of the THM impact of high-level activity radioactive waste exothermicity within a deep geological disposal facility. We shall present the definition of a block preconditioner with nested Krylov solvers for the fully coupled THM equations. Numerical results reflect the good performance of the proposed preconditioners that show to be weakly scalable until more than 2000 cores and more than 1 billion degrees of freedom. Thanks to their performance and robustness, a real waste storage problem on a scale, to our knowledge, unprecedented in the field, can be addressed.

Geochemical interactions at the steel-bentonite interface caused by a hydrothermal gradient

Bentonites are used in deep geological disposal facilities as an engineered barrier to isolate high level radioactive waste, contained in metallic canisters. The present study, performed at laboratory scale, evaluated the behaviour of MX-80 (Na-bentonite) and FEBEX (Ca-Mg-Na-bentonite) in contact with carbon steel, subjected to a hydrothermal gradient. A dominant Na-Cl-SO4 saline solution was injected towards the compacted bentonite from the top, while a heater, located at the bottom in contact with the steel disc, maintained a constant temperature of 100 °C. The cells were studied after one and six months of interaction. Changes in the physical (water content and specific surface area) and chemical (cation exchange capacity and element distribution) properties of the bentonite were observed, as well as the formation of a corrosion layer on the steel, at the interface with bentonite, mainly composed of magnetite, maghemite and hematite. The bentonites were mainly altered at the mm scale, being enriched in iron content, and changing their ion distribution to Ca-dominant smectite (in MX-80 bentonite).

Shear strength and shear stiffness analysis of compacted Wyoming-type bentonite

Wyoming-type bentonite is one of the materials to be used as part of the Engineered Barrier System (EBS) in deep geological disposal facilities for the safe disposal of spent nuclear fuel.In the KBS-3 type repository, the canisters containing the spent fuel assemblies are surrounded by bentonite buffer. Therefore, it is necessary to understand the bentonite behaviour in shearing conditions for the long-term performance assessment of the canisters. Bentonite shearing could arise, for example, due to potential post-glacial earthquakes.The deviatoric stress at failure and the shear modulus of a compacted Wyoming-type bentonite were measured using different shearing tests. Tests were carried out on unsaturated samples at three laboratories. The shearing tests on unsaturated samples were unconfined and triaxial compression tests, hollow cylinder tests and simple shear tests. Unconfined and triaxial compression tests were also carried out on saturated samples. The failure was quite ductile when tests were carried out with confining pressure.The samples were compacted at different dry densities and degrees of saturation and tested at different confining pressures and strain levels to study their influence on the shear properties. Shear strength and shear modulus were studied based on models that consider the double porosity structure of the bentonites. Previously published triaxial tests and resonant column test results were also included in this study.In combination with these models, several alternatives for the definition of the constitutive stress were checked for the case of bentonite. A novel, approach, involving the results of mercury intrusion porosimetry in estimating the constitutive stress, was introduced.The shear strength calculated by the models showed a good agreement with the measured values. In some cases, unrealistic large tensile stresses were predicted. Furthermore, the models can assess the tendency of the shear modulus as a function of the shear strain, degree of saturation and dry density.

Sensitivity study of risk to future generations from geological disposal of radioactive waste in high strength rock in the UAE

The United Arab Emirates (UAE) Government's Policy is to institute a safe and permanent geological disposal facility (GDF) for the final disposal of higher activity radioactive wastes (HAW). The UAE has yet to identify or decide on a disposal site, and geological data specific to the characterisation of a GDF host rock either lacks sufficient details or is not available in the public domain. However, initial screenings of geological environments in the UAE give an indication of potential suitable host rock formations. This paper presents the results of a study into the factors that influence the risk to future generations from the disposal of Spent Nuclear Fuel (SNF) in a deep underground geological disposal facility. For the purpose of this study the GDF was assumed to be in a High Strength Rock (HSR) geological environment in the Khor Fakkan (KF) Block. GoldSim's Contaminant Transport code was used to model the potential dispersion of radioactivity from a GDF. which enabled the calculation of risk to future generations. The GDF radiological source term was based on the UAE's forecasted SNF inventory. The model created from GoldSim emphasizes the transport of radionuclides through the GDF's engineered barrier systems (EBS), through the surrounding geosphere and into the biosphere. Once in the biosphere the radiation dose to the reference receptor is determined via ingestion from well drinking water, or inhalation through atmospheric dispersion. The paper explores the factors that influence the assessment of risk to people both now and in the future. The analyses presented in this paper are not meant to be used in site selection decision making or in the assessment of the suitability of the location to host a GDF for nuclear waste. The selection of a disposal site in the UAE would depend on a more detailed analysis of site-specific conditions.

Chromium Doped UO2-Based Ceramics: Synthesis and Characterization of Model Materials for Modern Nuclear Fuels.

Cr-doped UO2 as a modern nuclear fuel type has been demonstrated to increase the in-reactor fuel performance compared to conventional nuclear fuels. Little is known about the long-term stability of spent Cr-doped UO2 nuclear fuels in a deep geological disposal facility. The investigation of suitable model materials in a step wise bottom-up approach can provide insights into the corrosion behavior of spent Cr-doped nuclear fuels. Here, we present new wet chemical approaches providing the basis for such model systems, namely co-precipitation and wet coating. Both were successfully tested and optimized, based on detailed analyses of all synthesis steps and parameters: Cr-doping method, thermal treatment, reduction of U3O8 to UO2, green body production, and pellet sintering. Both methods enable the production of suitable model systems with a similar microstructure and density as a reference sample from AREVA. In comparison with results from the classical powder route, similar trends upon grain size and lattice parameter were determined. The results of this investigation highlight the significance of subtly different synthesis routes on the properties of Cr-doped UO2 ceramics. They enable a reproducible tailor-made well-defined microstructure, a homogeneous doping, for example, with lanthanides or alpha sources, the introduction of metallic particles, and a dust-free preparation.

Hydro-mechanical response of crushed argillite and bentonite mixtures as sealing material

Mixture of excavated Callovo-Oxfordian (COx) argillite and MX80 bentonite has been proposed as potential sealing material for the French deep geological disposal facility of radioactive waste. The mixtures hydro-mechanical behaviour was studied in order to assess their response on design requirements. At the as-compacted state, soil fabric analysed by the MIP technique, showed single-modal pore distribution for pure COx and bimodal pore distribution for samples containing MX80. All tested mixtures developed homogeneous behaviour during compaction with no effect of MX80 presence. Swelling potential was evaluated by hydration tests under 4 MPa on highly compacted material (2.0 Mg/m 3 ). Swelling increase and saturated permeability reduction was obtained on bentonite content increment. Combining results of the present experimental work and remarks of previous researchers, the characteristic saturation potential slope was determined and exploited to estimate the swelling under 1, 4 and 7 MPa for various COx/MX80 mixtures compacted at different pressures. The experimental verification of the adopted method suggested an approach of identifying possible combinations in terms of compaction and bentonite content that can fulfill design requirements. • Mixtures compressibility at initial water content is governed by COx argillite. • Bentonite presence results in bimodal pore size distribution. • Saturation potential slope estimates swelling under varied stresses for OC samples.

Оценка требований к геологической среде при выборе участка недр для захоронения радиоактивных отходов в глубокие геологические формации

The paper analyzes the structure and the contents of main regulations on the subsoil siting of deep geological radioactive waste disposal facility which was done based on the geological research performed to assess the “Yeniseiskiy” site in the Krasnoyarsk Territory. It also discusses some specific inconsistencies found in different documents, as well as ambiguities and inaccuracies in their content. The paper provides some recommendations on their upgrading.

Influence of thermal treatment on the disposability of spent ion exchange resins in a deep geological repository: a French case

The filtration of contaminated aqueous waste streams from nuclear applications produces spent ion exchange resins (IER) which can be classified either as low level Waste (LLW) or as intermediate level Waste (ILW). For the purpose of the work conducted in the framework of the European THERAMIN project, studies considered spent IER waste form to be routed to the French deep geological disposal facility in the Callovo-Oxfordian formation (Cigéo). This form of waste is known to release hydrogen by radiolysis degradation, reactive species and complexing compounds. Today, for disposability in Cigéo, direct cementation of IER is the main immobilization process. This work aims to evaluate the application of the incineration-vitrification with plasma process, SHIVA, on a mixture of zeolites, diatoms, and spent IER in regards to the reference immobilization process by cementation. Production of an alumina-borosilicate type glass using SHIVA incineration-vitrification process was considered in order to examine the impact of this process for managing the specificities of this IER waste form with the additional advantage of minimizing the disposal volume. The potential benefits that thermal treatment can provide in the context of Cigéo for such spent IER waste form are evaluated based on the physical characterizations and leaching experiments carried out by CEA on the resulting vitrified product. The influence of thermal treatment on several characteristics in line with a generic list of Waste Acceptance Criteria (WAC) defined within the THERAMIN project have been qualitatively evaluated considering their potential influences on operational and long-term safety.

Radiation Damage Effects in Chlorite Investigated Using Microfocus Synchrotron Techniques

A detailed understanding of the mechanisms and effects of radiation damage in phyllosilicate minerals is a necessary component of the evaluation of the safety case for a deep geological disposal facility (GDF) for radioactive waste. Structural and chemical changes induced by α-particle damage will affect the performance of these minerals as reactive barrier materials (both in the near and far-field) over time scales relevant to GDF integrity. In this study, two examples of chlorite group minerals have been irradiated at α-particle doses comparable to those predicted to be experienced by the clay buffer material surrounding high-level radioactive waste canisters. Crystallographic aberrations induced by the focused 4He2+ ion beam are revealed via high-resolution, microfocus X-ray diffraction mapping. Interlayer collapse by up to 0.5 A is prevalent across both macrocrystalline and microcrystalline samples, with the macrocrystalline specimen displaying a breakdown of the phyllosilicate structure into loosely ...

MECHANICAL CHARACTERISTICS AND WATER RETENTION PROPERTIES OF UNSATURATED “BENTONITE-BASED ENGINEERED BARRIER MATERIALS” BASED ON CONTROLLED-SUCTION TESTS

放射性廃棄物の地層処分においては，処分場閉鎖後に地下水の浸入に伴い人工バリアシステムは不飽和状態から飽和状態に向かって遷移するが，廃棄体からの発熱やガス発生により飽和度が一時的に低下する現象も起こる．しかし，ベントナイト系人工バリア材の不飽和力学特性や保水特性に関しては十分なデータが取得されておらず，上記のような過渡的な挙動を予測するための適切な数値モデルを構築できないのが現状である．本研究では，ベントナイト系人工バリア材を対象にマトリックサクションを制御した各種要素試験を行い，吸水・脱水による膨潤・収縮変形特性，不飽和圧密変形特性，不飽和せん断強度特性および保水特性をマトリックサクションとの関係で整理，考察するとともに，それぞれの特性に対する既往の構成モデルの適用性について議論した．

Small-strain shear stiffness of compacted bentonites for engineered barrier system

The shear modulus (G) of two different bentonites was measured by means of a resonant column apparatus. The samples were compacted at different dry densities and degrees of saturation and tested with different confinement pressures and strains levels for studying the influence of these parameters on the shear modulus. The results show similar tendencies in both bentonites: the shear modulus increases as the dry density increases and exhibits maximum shear modulus when degree of saturation is around 80%. An empirical equation, taking into account the microstructure of the clays, is used to evaluate the shear modulus at small strains as a function of dry density and degree of saturation. Although the values of the shear modulus measured are similar in both bentonites for a given stress and degree of saturation, there is difference in the elastic strain limit of the soil. Bentonite clay is going to be part of the Engineered Barrier System (EBS) in deep geological disposal facilities for the long-term confinement of spent nuclear fuel. In order to fully understand their long-term performance, their behaviour in shearing conditions should be assessed.

Радиологическое обоснование контроля содержания радионуклидов в контексте обеспечения долговременной безопасности пунктов захоронения

Purpose: this paper concentrates on the development and validation of an approach enabling to identify radiologically relevant radionuclides in radioactive waste at different stages of their management, as well as to implement their direct instrumental control.
 Material and methods: The research was carried out by comparing relevant radiological effects produced by radionuclides contained in activation RW from VVER-440 unit (reactor vessel, vessel internals and fuel rod cladding) on human at different stages of RW management. AKDAM-2.0 complex was used to evaluate the changes in the radionuclide inventory of materials during their irradiation and cooling.
 Results: The study enabled to calculate relevant contribution of different radionuclides to the overall activity and dose rate from external and internal human exposure at different stages of RW management. This paper proposes a novel approach to identify radiologically relevant radionuclides based on a comparative evaluation of their contribution to the dose rate their activity, half-lives and migration properties. Relevant lists of radionuclides were developed for the studied materials. These radionuclides should be considered in the safety assessments of RW management practices and the post-closure safety demonstration for deep geological disposal facilities (DGDF). The study suggests that direct instrumental control is feasible for a number of radionuclides along with the use of an integrated approach enabling to evaluate the content of other radionuclides.
 Conclusions: Further validation proved the viability of the proposed approach in the development of lists of radiologically relevant radionuclides. It has been found that the generated list is consistent with international practice used to demonstrate DGDF safety. It should be noted that only a small number of radionuclides should be considered radiologically relevant notwithstanding a variety of radionuclides generated due to material activation. In some cases, certain radionuclides are required to be added to the lists specified in the appendices to Radiation Safety Standards (RSS). However, the direct instrumental control covering the entire list of radiologically relevant radionuclides (for each RW management stage) is considered not feasible and hard to implement. The study shows that an integrated approach combining direct instrumental control for certain radionuclides, periodic destructive tests and inventory calculations is a preferred option. At the stage of periodic radiation monitoring in the disposal facility, such instrumental control should be focused on the evaluation of disposal system properties affecting its safety (hydrogeochemical, hydrogeological characteristics and etc.).

Gas network development in a precompacted bentonite experiment: Evidence of generation and evolution

In a deep geological disposal facility for radioactive waste, precompacted bentonite is proposed as a sealing material for the isolation of boreholes, disposal galleries and deposition holes. The advective movement of repository gas in bentonite has been linked to the development of new porosity and propagation of dilatant pathways. For the first time we present a detailed analysis of stress field data during the generation and evolution of a gas network. A new experimental dataset, from a highly instrumented test, clearly shows the strong coupling between stress, gas pressure and flow in bentonite. Multiple discrete propagation events are observed, demonstrating spatial variability and time-dependency as permeability within the clay develops. Analysis of the stress data before, during and after gas entry indicates a heterogeneous stress field initially develops, resulting from the development of these pathways. The flow network is dynamic and continues to spatially evolve after gas entry, such that permeability under these conditions must be time-dependent in nature. Perturbation of the stress field is significant before all major gas outflow events, presumably resulting from the requirement to propagate an effective gas network before outflow is possible. In contrast, no major flow perturbations are detected which did not correlate with fluctuations in the stress field. The controls on the distribution and geometry of the resulting flow network are unclear, as well as its long-term evolution and stability. These will be beneficial in the assessment of gas pressure evolution as part of safety case development.