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Fate of Repository Gases

Final Report Summary - FORGE (Fate of Repository Gases)

Executive Summary:
Various gases will be generated in a repository including hydrogen (from metal corrosion and radyolysis) and methane and carbon dioxide (both from decomposition of organic materials contained in some wastes). Understanding where and how these gases form and how they move through a repository and the surrounding rocks has been the focus of the FORGE project. By using small scale laboratory experiments, large scale field tests (performed at a number of underground research laboratories throughout Europe), data and numerical modelling the results from FORGE are providing information and insight to help guide repository design and predict future radionuclide migration.
The understanding and prediction of the evolution of repository systems over geological time scales requires a detailed knowledge of a series of highly-complex coupled processes. There remains significant uncertainty regarding the mechanisms and processes governing gas generation and migration in natural and engineered barrier systems. It is important to understand the behaviour of gas in a repository system to an adequate level of detail to allow confidence in the assessment of site performance, recognising that a robust treatment of uncertainty is desirable. Of particular importance to the European radioactive waste management programmes are the long-term engineering performance of bentonite buffers, plastic clays, indurated mudrocks and crystalline formations. To reduce uncertainty, FORGE has provided further experimental data to address:
• Corrosion and gas generation rates in repository environments;
• Key issues relating to the migration and fate of repository gases;
• Validation of numerical codes;
• Derivation of new methodologies for up-scaling from laboratory to field to repository scales;
• Optimisation of repository concepts through detailed scenario analysis.
Key messages from FORGE:
• Features, Events and Processes (FEPs) relevant to the consideration of gas in the safety case (the ‘gas issue’) are well-known, although therein there are uncertainties that need to be managed as a standard aspect of developing a safety case.
• Understanding the ‘gas issue’ provides coupled mitigation opportunities that can be considered on repository-specific basis, e.g. inventory optimisation, choice of materials for the Engineered Barrier System (EBS), repository design and repository operation (including repository sealing and closure).
• The relative importance of the ‘gas issue’ in the safety case is a function of the disposal concept under consideration, which is itself a function of the disposal inventory (including the gas source term, the approach to waste treatment and packaging, and how the packaged waste is management prior to emplacement in the repository etc) and the safety functions required to be provided by complementary barriers (e.g. EBS, geology).
• Repository-derived gas needs to be considered at an appropriate level in all repository safety cases. This can be done on the basis of existing knowledge.
• Based on studies undertaken in the EC FORGE project, and on input from complementary studies, we have enhanced our understanding of repository derived gas in relation to a range of concepts for the geological disposal of radioactive waste. Such understanding provides a justification for increased confidence in analyses of the gas issue as undertaken within the safety case.
Project Context and Objectives:
Project context and objectives
The multiple barrier concept is the cornerstone of all proposed schemes for the underground disposal of radioactive wastes. Based on the principle that uncertainties in performance can be minimised by conservatism in design, the concept invokes a series of barriers, both engineered and natural, between the waste and the surface environment. Each successive barrier represents an additional impediment to the movement of radionuclides.
Depending on the disposal concept, engineered barriers may comprise the buffer/backfill medium enclosing the waste containers, the tunnel/borehole liner, and the backfill and high integrity seals placed in the repository access ways or emplacement boreholes. The buffer/backfill medium enclosing the waste will often also provide both a physical and a chemical barrier to radionuclide migration. The functions of the engineered/chemical barriers are:
• To reduce the rate of corrosion of the waste containers and thus extend their life;
• To limit the release of radionuclides from the waste-form to the far-field (geosphere) after container failure;
• To limit the migration of radionuclides along the pathway provided by the access tunnels and shafts of a repository or the boreholes in the case of a deep borehole emplacement.
For L-ILW disposal in vaults, the backfill may be a porous, cementitious grout which is intended to pH-buffer the pore water for an extended time period. Typically the buffer/backfill for HLW might comprise compacted bentonite or other clay-based material, providing a low permeability, alkaline pH-buffered pore water to limit solubility and mobility of certain radionuclides (e.g. actinides), plus good retention/retardation properties including high sorption and a capacity to filter colloids.
The geological barrier is the final impediment to radionuclide migration. Depending on details of the local geology, this may be considered to constitute the host formation itself, extending above, below and laterally away from the repository. Alternatively, the entire sequence of low permeability rocks which may separate the repository from the surface and/or more permeable, water-bearing, strata may be included. The practical realisation of the multiple barrier concept is the primary objective of all stages of a disposal programme, from site appraisal and characterisation through to design and construction. However, the general performance of the repository as a whole (waste, buffer, engineering disturbed zone, host rock), in particular its gas transport properties, are still poorly understood. Issues relating to basic process understanding (e.g. dilational versus displacement flow mechanisms); the long-term integrity of seals, and in particular possible gas flow along interfacial contacts; and up-scaling from laboratory to field conditions have yet to be adequately examined.
Within a repository corrosion of ferrous materials under anoxic conditions will lead to the formation of hydrogen. Radioactive decay of the waste and the radiolysis of water particularly in HLW will produce additional gas. If present, biodegradable wastes will produce carbon dioxide and methane through microbial action and other (minor) species may also be generated. If the gas production rate exceeds the rate of diffusion of gas molecules in the pores of the engineered barrier or host rock, the solubility limit of the gas will be exceeded, leading to the formation of a discrete gas phase. Gas would continue to accumulate until its pressure becomes sufficiently large for it to enter the engineered barrier or host rock. Understanding gas generation and migration is thus one of the key issues in the assessment of repository performance and was the focus of the FORGE project.
The generation of gases in the repository environment is of concern for a number of reasons:
• Pressurisation of waste containers;
• Perturbation of the groundwater flux;
• Effect on repository backfill and seals;
• Effect on engineered disturbed zone (EDZ) and self-sealing properties;
• Effect on host-rock mass transport properties;
• Effect on heat dissipation;
• Release of active gases;
• Displacement of contaminated groundwater.
Uncertainties of particular importance to FORGE are:
• The definition of long-term corrosion rates of ferrous metals under repository conditions (WP 2);
• A better understanding of the processes and mechanisms governing gas migration in clay-based engineered barriers and host rocks (WP 3, 4 and 5);
• The effect of elevated gas pressures on the movement of groundwater and aqueous borne contaminants (WP 4);
• The role of gas on the evolution of the near field and the EDZ (WP 4);
• The possible coupling of effects to compromise repository performance (WP 1).
To address these fundamental issues, the FORGE project was structured in such a way as to provide new insights into the processes and mechanisms governing gas generation (WP2) and migration (WP3-5) through the acquisition of new experimental data, aimed at repository performance assessment (WP1). FORGE will also help to address the paucity of high-quality data currently available for future activities such as benchmarking and validation of numerical codes for the quantitative prediction of gas flow, the development of HM models for the prediction of EDZ and near-field processes and to assist in the assessment of the long-term evolution of the potential geological barriers.
To fully understand the impact of gas generation and transport on current engineered barrier concepts, its effect on self-sealing within the engineered damaged zone (EDZ) and its long-term effect on the hydrogeological characteristics of the far-field, requires an integrated, multi-disciplinary project to address these key research areas. The FORGE proposal is a pan-European project, with links to international radioactive waste management organisations, regulators and academia specifically designed to tackle the key research issues associated with the generation and movement of repository gases in radioactive waste disposal facilities. The work packages are:
• WP 1 Treatment of gas in performance assessment;
• WP 2 Gas generation;
• WP 3 Engineered barrier systems;
• WP 4 Disturbed host rock formations;
• WP 5 Undisturbed host rock formations.
Project Results:
Project objectives
The multiple barrier concept is the cornerstone of all proposed schemes for the underground disposal of radioactive wastes. Based on the principle that uncertainties in performance can be minimised by conservatism in design, the concept invokes a series of barriers, both engineered and natural, between the waste and the surface environment. Each successive barrier represents an additional impediment to the movement of radionuclides.
Depending on the disposal concept, engineered barriers may comprise the buffer/backfill medium enclosing the waste containers, the tunnel/borehole liner, and the backfill and high integrity seals placed in the repository access ways or emplacement boreholes. The buffer/backfill medium enclosing the waste will often also provide both a physical and a chemical barrier to radionuclide migration. The functions of the engineered/chemical barriers are:
• To reduce the rate of corrosion of the waste containers and thus extend their life;
• To limit the release of radionuclides from the waste-form to the far-field (geosphere) after container failure;
• To limit the migration of radionuclides along the pathway provided by the access tunnels and shafts of a repository or the boreholes in the case of a deep borehole emplacement.
For L-ILW disposal in vaults, the backfill may be a porous, cementitious grout which is intended to pH-buffer the pore water for an extended time period. Typically the buffer/backfill for HLW might comprise compacted bentonite or other clay-based material, providing a low permeability, alkaline pH-buffered pore water to limit solubility and mobility of certain radionuclides (e.g. actinides), plus good retention/retardation properties including high sorption and a capacity to filter colloids.
The geological barrier is the final impediment to radionuclide migration. Depending on details of the local geology, this may be considered to constitute the host formation itself, extending above, below and laterally away from the repository. Alternatively, the entire sequence of low permeability rocks which may separate the repository from the surface and/or more permeable, water-bearing, strata may be included. The practical realisation of the multiple barrier concept is the primary objective of all stages of a disposal programme, from site appraisal and characterisation through to design and construction. However, the general performance of the repository as a whole (waste, buffer, engineering disturbed zone, host rock), in particular its gas transport properties, are still poorly understood. Issues relating to basic process understanding (e.g. dilational versus displacement flow mechanisms); the long-term integrity of seals, and in particular possible gas flow along interfacial contacts; and up-scaling from laboratory to field conditions have yet to be adequately examined.
Within a repository corrosion of ferrous materials under anoxic conditions will lead to the formation of hydrogen. Radioactive decay of the waste and the radiolysis of water particularly in HLW will produce additional gas. If present, biodegradable wastes will produce carbon dioxide and methane through microbial action and other (minor) species may also be generated. If the gas production rate exceeds the rate of diffusion of gas molecules in the pores of the engineered barrier or host rock, the solubility limit of the gas will be exceeded, leading to the formation of a discrete gas phase. Gas would continue to accumulate until its pressure becomes sufficiently large for it to enter the engineered barrier or host rock. Understanding gas generation and migration is thus one of the key issues in the assessment of repository performance and was the focus of the FORGE project.
The generation of gases in the repository environment is of concern for a number of reasons:
• Pressurisation of waste containers;
• Perturbation of the groundwater flux;
• Effect on repository backfill and seals;
• Effect on engineered disturbed zone (EDZ) and self-sealing properties;
• Effect on host-rock mass transport properties;
• Effect on heat dissipation;
• Release of active gases;
• Displacement of contaminated groundwater.
Uncertainties of particular importance to FORGE are:
• The definition of long-term corrosion rates of ferrous metals under repository conditions (WP 2);
• A better understanding of the processes and mechanisms governing gas migration in clay-based engineered barriers and host rocks (WP 3, 4 and 5);
• The effect of elevated gas pressures on the movement of groundwater and aqueous borne contaminants (WP 4);
• The role of gas on the evolution of the near field and the EDZ (WP 4);
• The possible coupling of effects to compromise repository performance (WP 1).
To address these fundamental issues, the FORGE project was structured in such a way as to provide new insights into the processes and mechanisms governing gas generation (WP2) and migration (WP3-5) through the acquisition of new experimental data, aimed at repository performance assessment (WP1). FORGE will also help to address the paucity of high-quality data currently available for future activities such as benchmarking and validation of numerical codes for the quantitative prediction of gas flow, the development of HM models for the prediction of EDZ and near-field processes and to assist in the assessment of the long-term evolution of the potential geological barriers.
To fully understand the impact of gas generation and transport on current engineered barrier concepts, its effect on self-sealing within the engineered damaged zone (EDZ) and its long-term effect on the hydrogeological characteristics of the far-field, requires an integrated, multi-disciplinary project to address these key research areas. The FORGE proposal is a pan-European project, with links to international radioactive waste management organisations, regulators and academia specifically designed to tackle the key research issues associated with the generation and movement of repository gases in radioactive waste disposal facilities. The work packages are:
• WP 1 Treatment of gas in performance assessment;
• WP 2 Gas generation;
• WP 3 Engineered barrier systems;
• WP 4 Disturbed host rock formations;
• WP 5 Undisturbed host rock formations.
Achievements
WP-1 - Treatment of Gas in Performance Assessment
Objectives
Describe the current treatment of gas issues in long-term safety assessments for intermediate-level waste, high-level waste and spent nuclear fuel.
Identify the merits and shortcomings of the current treatment of gas issues in long-term safety assessments.
Analyse the limitation of previous studies and the different types of uncertainties related to gas transport in the host rock and engineered barriers.
Discuss the needs for additional studies of gas migration issues and how they can support future assessments. Identify relevance of work being progressed by FORGE WPs 2-5, and the expected benefit to be gained from progressing the work.
Integrate the information produced by the other WPs throughout the duration of the project, considering the implications for the current state of the art.
Examine the extent to which other WPs’ findings provide satisfactory answers to the needs identified at the outset of the project.
Define “scenarios” related to gas issues. Discuss the uncertainties related to the scenarios. Identify the need of models and data for the treatment.
Make recommendations for, and propose an updated treatment of, gas issues in relation to long-term safety assessments, based on the integrated findings from the project. This will consider, for example, issues to be dealt in a safety case and future needs, and a set of recommendations related to the design of the repository in relation to gas issues.
To undertake a suite of calculations to progress understanding in repository-scale gas migration, linking to the output of WPs 2-5 as appropriate. To include benchmark studies on repository-scale numerical simulations of gas migration. To develop methodologies for dealing with heterogeneities; development of high performance calculation methods to handle large mesh sizes and complex geometries; investigating upscaling methodologies to simulate gas migration at repository scale; integrating ‘small scale’ information into km-scale model.
Significant Results
Benchmark studies (based on ANDRA concept):
• Simulations of gas movement at repository scale show gas flow is very sensitive to local variations in gas transport properties - gas takes pathway with lowest resistance;
• Disturbed host rock around excavations (galleries or disposal cells) and the access ways to repository will therefore most probably act as preferential pathways for gas migration.
• During migration, free gas is always in contact with water (present in the partially desaturated pores) - dissolution can take place.
• Only a very small part of the total generated gas volume (if ever) may reach access ways as free gas.
• Overall, most of the gas is migrating by diffusion in dissolved form towards surrounding geology.
With respect to gas migration, it is possible to model a whole repository taking into account both large and very small scale features.
To achieve such a numerical simulation some simplifications have to be considered (e.g. no complex mechanical coupling), and upscaling techniques have to be addressed
Overall, these simulations give good estimations of gas pressures variations but less accurate estimations of gas fluxes.
Some attempts were made to introduce a simple ‘proof of principle’ mechanical coupling (in order to roughly take into account ‘pathway dilation’ processes), however, there is currently insufficient information to properly parameterize such a model.
Two phase flow / mechanical coupling models:
Localization of gas pathways for low permeability porous media such as indurated rock is difficult to handle with classical two phase flow models (based on generalized Darcy law for each phase, permeability depending only on water saturation).
For small scale experiment, using two phase flow models without any coupling with mechanical effects or any evolution of rock properties due to gas pressure and/or deformations leads to low accuracy results, especially on laboratory-scale experiments in which localized pathways can be monitored.
For large scale experiments when gas is injected in undisturbed host rock, such models seem to give a good approximation of the experimental results. This could be due to less accurate measurements in situ, but also to homogenization effect at metre scale.
• Two phase flow models seem to give a good representation of gas migration on large scale experiments in undisturbed indurated rock.
• At small scale, “classical” two phase flow model is not well adapted to reproduce all the details of the test.
For a repository in Opalinus Clay, the studies in FORGE strengthen the overall understanding of how the repository will evolve and also the understanding of effects of gas on the host rock.
As gas pressures increase in the near field, it is clear from field experiments that gas entry will first occur along the EDZ at a pressure well below that of the far-field in situ stress.
Two-phase flow models are able to describe the pressure evolution and gas flow.
In situ and laboratory tests confirm that the movement of gas can occur in undisturbed Opalinus Clay by two-phase flow and that experimental results can be successfully modelled with extended two-phase flow models that incorporate mechanical coupling.
The results also show that gas can move through the rock without significant change of rock integrity.
Furthermore, little flow occurs as a result of gas invasion and the post-gas invasion hydraulic properties are not changed, thus the rock can be considered undamaged by the gas transport.
WP2 Gas generation
Objectives
This work package will examine the rate of hydrogen production in order to provide information in support of repository performance assessment. The key processes to be better understood are the impact of radiation on near field materials and the role of chemical processes, in particular the corrosion of metals. Experiments will measure hydrogen production rates from the corrosion of steel in contact with bentonite under different test conditions, improving our understanding of these issues. Understanding the effectiveness of processes which may mitigate the volume of hydrogen close to the point of origin, are an important input for assessing the influence of the global evolution of hydrogen generation within the repository. Short and longer term irradiation experiments will be undertaken in parallel to understand the influence of the boundary conditions of the tests and the temporal evolution of parameters. Feedback from the long term experiment will be available by month 36 of the project.
Significant Results
Experiments on carbon steel showed:
• Corrosion rate in compacted bentonite in neutral pH disposal environments is greatly accelerated (up to tens of μm/a) at least in the first month compared to the rate in bentonite porewater (the long-term corrosion rate has been determined in several prior studies to be a few μm/a).
• Initial corrosion rate is significantly higher at elevated temperature (70°C) than at lower temperatures. Rate decreases rapidly, no significant temperature dependence after approximately one month. Should not be a significant issue for post-closure assessment.
• Gamma radiation at dose rates in the range of 50-100 Gy/h enhances both hydrogen production and the corrosion rate – need to extrapolate to ‘general’ repository conditions of lower dose rates or the presence of clay.
In the case of cementitious environments, further study may be required for some conditions, e.g. changes to corrosion rates and associated gas generation rates influenced by:
• pH changes and loss of carbon steel passivity;
• Effect of organic degradation products.
Microbial issues where further (in situ) studies may be warranted:
• Microbial corrosion of steel and copper: not studied in FORGE, but considered in prior laboratory studies.
• Utilisation of hydrogen as an electron donor by microbes (e.g. sulphate-reducing bacteria) - this process is normally conservatively ignored in assessing gas pressure build-up, but can reduce gas pressure.
Gas generation processes generally well-understood for different types of metals, including how generation rates are globally affected by changes in experimental conditions / conditions in repository.
Effects of higher short-term gas production rates need to be checked for the specific EBS design and hydraulic boundary conditions.
WP3 - Engineered Barrier and Seals
Objectives
Gas generation from either the waste form or the engineered barriers is an unavoidable but generally undesired effect in most European repository concepts for radioactive waste. Gas generation and migration can potentially alter the hydraulic and mechanical properties of the repository (possibly the thermal and chemical properties as well). The purpose of this work package is to investigate gas migration processes and the consequences of gas migration in the EBS of the repositories. The WP will deliver results that can be used for:
Direct qualitative and quantitative confirmation of the consequences of gas migration to be used in long-term safety assessments;
Scientific knowledge about the gas migration processes to be used in the development of conceptual models;
Quantitative data to be used in the development and testing of numerical models for the simulation and prediction of gas migration and its consequences.
The work in this work package will be divided into 4 areas:
Bentonite URL Experiments: Field-scale experiments with bentonite buffers and seals that can be used directly as a confirmation in safety assessment and also give important information on the effects of up-scaling and realistic boundary conditions.
Bentonite Laboratory Experiment: The test will complement the URL in the sense that it is possible to investigate the importance of different parameters and processes (materials, boundary conditions, etc) and provide detailed and high quality data to both conceptual and numerical models.
Interface Laboratory Experiment: Test that will be specifically designed to study the importance of interfaces between different materials or construction parts for the gas migration processes. These tests will supply data to modelling, but also aid in the interpretation of other laboratory and field scale tests.
Concrete Laboratory Experiments: Tests that will be used to study the gas migration process in concrete structures and barriers in the repository. The aim is to study the effects of degree of saturation, gas pressures and alteration of the cement as well as effects of gases on the cementitious materials themselves and provide detailed and high quality data to both conceptual and numerical models.
There is not a separate modelling task within WP 3. Instead the modelling is integrated into the different tasks in three ways:
Each team is responsible for the interpretation and conceptual modelling of their own experimental work and that will be reported together with the experimental results.
There is a group for numerical modelling of the experiments integrated in the different tasks. This group consists of CIMNE-UPC, SKB, ANDRA, IRSN and NAGRA. The purpose of the numerical modelling group is to be available to model selected experimental results from the entire WP 3 and will also cooperate with the modelling groups from the other WPs.
Interpretation of the results from experiments/simulations to assess the long term performance of the engineered barriers with respect to gas issues and will serve as input to safety/performance assessment.
WP 3 will have direct links with all the other work packages. Some results can be directly applied in PA (WP 1). The gas generation rates discussed in WP 2 will serve as boundary condition for the processes Gas migration in the EBS gives the inner boundary condition to the transport processes in the EDZ and the geosphere (WP 4 & WP 5). Many processes may be common for bentonite buffer and a clay host rock (WP 4).
Significant Results
Bentonite-based barriers
Two-phase flow is the dominating transport mechanism in unsaturated or partially saturated bentonite (also for saturated sand-bentonite mixtures if the sand content is sufficiently high).
Classical two-phase flow models cannot correctly represent gas migration in a compacted saturated bentonite. • High gas pressure may significantly delay the saturation of the bentonite.
If the gas pressure reaches a higher value than the pressure in the bentonite a mechanical interaction will occur, leading to either:
• Consolidation of the bentonite; and/or
• Formation of dilatant pathways (allowing gas mobility).
Dilatant pathways exhibit spatio-temporal evolution – localised outflows during gas breakthrough and no measurable desaturation in any test samples.
A detailed stress analysis is required to capture the transition from consolidation to dilatant pathway formation; effect is clearly geometry dependent, but other factors may be involved:
• When the gas pressure reaches the sample pressure (e.g. as seen in LASGIT);
• At an overpressure at about 20-30%;
• At pressures 2-3 times higher than the sample pressure.
Self-sealing of bentonite always occurs after a gas migration event.
Concrete barriers
For other materials studied in FORGE, the deformation of the solid phase is less important and therefore two-phase flow can be considered as the main mechanism for gas flow even near water saturation. Improved database and process understanding has been gained for:
• Gas permeability in concrete under different conditions;
• How carbonation, from CO2 gas, will affect the permeability of concrete.
Gas migration in interfaces
Gas will generally move along the interface between the clay and another material in a saturated system (because gas entry pressure in the interface is generally lower than in the surrounding materials).
Interfacial flow depends on surface roughness, geomechanical properties, wettability, etc of the materials.
In a saturated system bentonite/bentonite interfaces will seal (or heal – as demonstrated by the development of cohesion) and will not be preferential pathways for gas. Gas pressure induced re-opening of healed interfaces is not observed.
Shear displacement in the contact zone due to pressurisation of a plug will not result in mechanically-induced pathways because the saturated bentonite behaves plastically.
WP4 - Disturbed host rock (DHR) formations
Objectives
Construction of any underground opening results in a re-distribution of the local stress field. At depth it is possible to remove mass from the system (e.g. tunnelling), but it is not possible to remove the stress. Therefore the rock surrounding the opening has to accommodate the load that was originally borne by the removed rock, leading to a localised stress concentration. In most geological setting, rocks at depth are at a point of limiting equilibrium, i.e. they are at a stress state just short of failure. Therefore, any stress re-distribution is likely to result in failure of the host rock. Failure is usually observed in the form of a complex fracture network, which is heterogeneous in distribution around a circular tunnel opening because of the heterogeneous stress distribution. The orientation of stress with respect to the fracture network is known to be important. The complex heterogeneous stress trajectory and heterogeneous fracture network results in a broad range of stresses and stress directions acting on the open fracture network. During the open stage of the repository, stress will slowly alter as shear movements occur along the fractures, as well as other time-dependent phenomena. As the repository is back filled, the stress field is further altered as the backfill settles and changes volume due to resaturation. Therefore, a complex and wide ranging stress regime and stress history will result. As such, there is a need to understand the roles of the stress tensor, the stress path and associated mechanical deformation in determining permeability changes affecting the sealing efficiency of the host rock. The work package is split into three main areas, laboratory, field and numerical simulation. The objectives of this integrated work package are:
perform laboratory scale experiments to: (a) provide data to test, develop and validate theoretical frameworks and predictive tools to analyse the effects of the stress tensor, the stress path and associated mechanical deformation in determining permeability changes affecting the water and gas sealing efficiency of the host-rock following repository closure, (b) examine the role of the stress tensor orientation with fracture orientation and examine the conditions under which fractures become conductive, (c) examine possible radionuclide movement in an artificially damaged plastic clay formation (Boom Clay) supported by X-ray tomographic techniques (a technique successfully applied in the SELFRAC Project), (d) examine the permeability evolution of the seal plug/host rock interface supported by X-ray tomographic imaging of test cores
perform field-scale experiments to: (a) provide a comprehensive insight into the hydro-mechanical behaviour of a fractured EDZ in an indurated mudrock formation (Opalinus Clay) in transporting gas along the backfilled tunnels and seals; (b) examine EDZ-sealing, radionuclide (Radionuclide) migration and gas movement in a plastic clay formation (Boom Clay) simulating the expected sequence of phenomena in a medium-level waste (MLW) repository that could lead to gas-driven radionuclide transport; (c) investigate the hydro-mechanical behaviour of the EDZ in a disturbed crystalline rock formation (granite) and its role in the movement of repository gases; (d) examine issues of up-scaling from laboratory to field-scale experiments
Undertake detailed numerical modelling of laboratory and field scale experiments with particular emphasis placed on the assessment and application of constitutive models to describe hydraulic and gas flow properties in a clay-based EDZ. This will be facilitated through the development of strong interactive links between modeling and experimental teams.
The numerical data generated in WP 4 will be used in the development and validation of process models aimed at repository performance assessment. As such, WP 4 has direct links with WP’s 1, 2, 3 and 5.
Significant Results
Our understanding of fundamental processes governing gas flow in the EDZ has significantly improved as a result of FORGE.
Relevant mechanisms affecting gas transport properties include stress and pore pressure conditions, stress history, orientation of EDZ fractures to the stress field, strains and hydro-chemical porewater-rock interactions (e.g. swelling, precipitation, filtration, erosion).
Gas flow is initially focussed within the repository excavation damaged zone (EDZ), the network of discrete EDZ fractures acting as a preferential pathway for gas migration. Flow in EDZ highly localised along the largest EDZ fractures, exhibiting a complex inter-dependence between fracture transmissivity and the distribution of radial stress around tunnel.
Tests show temporal evolution of flow behaviour.
The evolution of the EDZ as a gas transport path is controlled by a variety of features, events and processes, such as the connectedness of EDZ fracture network, the resaturation of the repository near field, pore pressure recovery, build-up of swelling pressures in the clay-bearing EBS, rock creep in response to the local stress field and by the nature of the actual gas source term (gas generation rates, gas species).
Linking to WP3, models have been developed that consider pathway dilatancy using a number of different approaches in order to better represent the data and to improve simulations.
Generic modelling studies emphasised the relevance of the spatial variability of rock properties in gas transport simulations. The inclusion of subscale variability of certain rock properties (strength, permeability) enabled models to simulate gas flow localisation.
WP5 - Undisturbed host rock formations
Objectives
To establish the conditions under which the different gas migration processes are dominant;
To identify how those processes can be modelled and to determine the values of the main parameters;
To measure the parameters that may have an impact on the long-term safety as a consequence of enhanced radionuclide transport through the host rock.
Gas generation and migration may have an impact on the hydraulic and the mechanical properties of the host rock. Consequently, these processes could affect the safety function of the host rock to retard and spread in time the release of radionuclides.
Earlier studies have shown that the ratio between the gas generation rate and the diffusive gas flux through the undisturbed host rock determines the development of a separate gas phase as well as the rate of increase of gas pressure. The two-phase flow properties of the host rock will determine the gas pressure at which gas flow will start as well as the quantity of water that will be displaced. The latter is particularly important in case of MLW disposal where gas generation and radionuclide release in the near-field can occur at the same time. There is now a general consensus that in the case of plastic clay-rich clays and in particular bentonite, classic concepts of porous medium two-phase flow are inappropriate and continuum approaches to modelling gas flow may be questionable depending on the scale of the processes and resolution of the numerical model. The mechanisms controlling gas entry, flow and pathway sealing in general clay-rich media are not yet fully understood. The “memory” of dilatant pathways within a clay could impair barrier performance.
WP 5 will have direct links with all the other work packages. Results directly relevant to PA will be introduced into WP 1. Gas migration in the undisturbed host rock sets the outer boundary condition to the transport processes in the EDZ and the engineered barriers (WP 3 & WP 4). Many processes may be common for bentonite buffer (WP 3) and a clay host rock. The experimental results will be used for models applied (WP 1).
Significant Results
Major experimental challenges for undisturbed clay host rock have been identified and considered in improved test protocols.
Difficult to create a gas flow into an intact clay host rock as the gas entry pressure and water retention is very high.
Both laboratory and in situ experiments show that very little water is displaced by gas phase flow through undisturbed clay.
Evidence for hydro-mechanical coupling and pathway dilatancy found as gas transport mechanism in very carefully-performed laboratory experiments in which all mechanical and hydraulic boundary conditions are well controlled and sample is near water saturation (or saturated).
As its gas entry pressure is generally lower, free gas will preferentially flow through the EDZ rather than in intact rock in a clay host rock.
Implicit and explicit formulations have been tested to introduce hydro-mechanical coupling in gas transport simulations (also WP4-relevant):
• Implicit formulations are based on an extension of classical two phase flow codes to cope with fluid flow and gas transport processes in deformable media. Successful applications are reported for gas permeability tests, associated with low and moderate volumetric strains in response to the gas pressure build-up;
• Explicit formulations (fully coupled HM modelling) were needed in other cases to reproduce the main features of the experimental results.
Measuring the water retention curves of an indurated clay (e.g. Opalinus clay) reveals new issues to be addressed in a future research programme:
• No significant difference was noted between applying matrix or total suction, suggesting that osmotic suction (i.e. porewater chemistry) has only a minor impact on gas transport. Dedicated investigations need to assess impact of porewater chemistry on the water retention behaviour of indurated clays;
• Increasing mechanical stress seems to further increase the capillary strength value which in any case is already high (6 to 34 MPa). Dedicated studies of stress dependence of the two phase flow parameters needed.
Key achievements of FORGE
FORGE has enhanced:
• Information and understanding
• Methods, models and computer codes
• Qualitative safety arguments
• Quantitative assessments
• Development and maintenance of expertise, collaboration
Gas-related uncertainties and areas for further targeted work remain (e.g. EC CAST* proposal)
Uncertainties are not ‘show-stoppers’ in terms of adequate and appropriate consideration of gas in the safety case NOW.
“Managing uncertainty” approach is a required component of any safety case
• Deploy approach for gas issues
Overall Key Messages from FORGE
Considering the output from the EC FORGE project as detailed in this report and in related references, the overall key message in relation to the updated treatment of gas generation and migration in the safety case are noted below. Progressing beyond FORGE, these key messages and the outcomes from the EC FORGE project now need consideration in safety case studies undertaken by national implementing organisations, to ensure the derived understanding is embedded and to ensure repository-derived gas is managed such that the safety case is compliant with national regulatory requirements.
• Features, Events and Processes (FEPs) relevant to the consideration of gas in the safety case (the ‘gas issue’) are well-known, although therein there are uncertainties that need to be managed as a standard aspect of developing a safety case.
• Understanding the ‘gas issue’ provides coupled mitigation opportunities that can be considered on repository-specific basis, e.g. inventory optimisation, choice of materials for the Engineered Barrier System (EBS), repository design and repository operation (including repository sealing and closure).
• The relative importance of the ‘gas issue’ in the safety case is a function of the disposal concept under consideration, which is itself a function of the disposal inventory (including the gas source term, the approach to waste treatment and packaging, and how the packaged waste is managed prior to emplacement in the repository etc) and the safety functions required to be provided by complementary barriers (e.g. EBS, geology).
• Repository-derived gas needs to be considered at an appropriate level in all repository safety cases. This can be done on the basis of existing knowledge.
• Based on studies undertaken in the EC FORGE project, and on input from complementary studies, we have enhanced our understanding of repository derived gas in relation to a range of concepts for the geological disposal of radioactive waste. Such understanding provides a justification for increased confidence in analyses of the gas issue as undertaken within the safety case.

Potential Impact:
FORGE has had a key role in enhancing and developing European expertise in gas migration ensuring that the partners are global leaders in this fast developing and important area of science. This pan-European approach has provided enhanced understanding of gas migration issues in relation to radioactive waste disposal which build confidence in the applied methods for the long-term prediction of gas related effects in the performance assessment of a deep geological disposal facility for radioactive waste.
It has developed new insights into the processes and mechanisms governing gas generation and migration through the acquisition of new experimental data, aimed at repository performance assessment. The new high-quality data is available for future prediction of repository performance and to assist in the assessment of the long-term evolution of the potential geological and engineered barriers.
While FORGE was designed to reduce some of the uncertainties associated with understanding gas migration in a radioactive waste repository context, many of the results from the project are also applicable to the petroleum and carbon dioxide storage sectors.
As a world-leading project, FORGE has established the European Community as a world centre of excellence on repository gas migration issues. This will facilitate knowledge transfer to both newer member states and to future potential members of the European Union. Through associates in Canada and Japan FORGE has enhanced the understanding of the ‘gas issue’ worldwide.
The training courses and workshops, the former attended by people from a number of newer EU member states and Korea as well as Italy, has allowed wide dissemination of the outcomes of the FORGE project.
FORGE has had a comprehensive schedule of reporting (over 80 technical reports) all of which are available as pdf versions from the FORGE web site. Some of these reports are also being published by the lead partners in their technical reports series.
Many of the reports form the basis of peer reviewedpapers that have been published or currently progressing through publication processes. The co-ordinator is currently editing a Special Publication of the Geological Society on the outcomes of FORGE which will have approximately 15 peer reviewed contributions from FORGE partners and associates. Publication is expected in late 2014 or early 2015.
Four ‘popular’ publications have been written on FORGE. These were a two page article in Planet Earth (Autumn 2011), the NERC quarterly magazine covering NERC research aimed at the interested general public, a three page article in International Innovation (July 2013), a one page article in Pan European Networks – Government (August 2013) and a three page article in EU Research (2014 Vol 1).
The co-ordinator was an invited keynote speaker at the 4th Clays in Natural & Engineered Barriers for Radioactive Waste Confinement meeting held in Nantes in March 2010 and gave a presentation on FORGE. This was published in an ANDRA report that formed part of the published proceedings of the meeting. FORGE was well represented at 4th Clays in Natural & Engineered Barriers for Radioactive Waste Confinement meeting and particularly at the 5th Clays in Natural & Engineered Barriers for Radioactive Waste Confinement meeting held in Montpellier in October 2012 with posters and presentations on FORGE work. It is expected that the project will also be well represented at the 6th Clays in Natural & Engineered Barriers for Radioactive Waste Confinement meeting to be held in Brussels in April 2015. Output from FORGE has been presented at many international meetings including EGU (2012, 2013 and 2014), Migration 2013 and Goldschmidt (2013 and 2014).
The co-ordinator was also invited to present the outcomes of FORGE at Euradwaste 2013, held in Vilnius, Lithuania during October 2013.
An international symposium and workshop was held in Luxembourg during February 2013 to disseminate the results of FORGE. The meeting was attended by about 120 people from within and outside FORGE and included representatives of implementers and regulators as well as scientists from Europe, North America and Japan. The meeting included presentations on why gas is an important issue for safety cases for radioactive waste disposal and the key outcomes from the FORGE project as well as presentations and posters on all aspects of the project. There were discussions on the impacts of gas on hydraulic properties, mechanical properties, solute migration, chemical properties and containment in relation to a repository. The participant’s opinion on whether any gas related phenomena required further generic consideration.
During the FORGE project four annual general meetings and a kick-off meeting have been held at which the emerging results of the project have been presented. These meetings were attended by between 80 and 120 people comprising project members, associates and other interested parties, the meeting being open to anyone wishing to attend. The meetings were held at:
• Aspo, Sweden (kick-off) – February 2009
• Bure, France – December 2009
• Prague, Czech Republic – December 2010
• Solothurn, Switzerland – December 2011
• Ghent, Belgium – December 2012
As well as providing a forum to present results and interact with members working in different work packages these meetings provided the opportunity for members of both the experimental teams and modelling teams to understand the scope and limitations of each other’s data and methods.
During the course of each meeting there was an optional opportunity to visit an underground research laboratory (Aspo, Bure, Josef Stolla, Mont Terri and Mol respectively) to gain firsthand experience of the URL and examine in-situ experiments including some forming part of the FORGE research programme. This provided particularly useful insight into experimental procedures and limitations for those modelling the experimental results.
During FORGE a series of themed workshops have been held to encourage discussion and development in topics important within the project. The main workshops have covered:
• Modelling gas migration (Paris, September 2009 and Brugg (Switzerland), January 2011);
• Bentonite (Keyworth, September 2011);
• Testing of low permeability materials (Keyworth, February 2013);
• The treatment of gas in repository design and performance assessment (London, January 2012).
While these workshops were largely attended by participants in the FORGE project they were open to participants outside FORGE.
The Bentonite/Testing of low permeability materials workshop format is continuing after completion of FORGE with the next workshop planned for Lille in October 2014 with plans for a bi-annual meeting thereafter.
In addition to these each work package held at least annual meetings, open to all working in the project, to develop and integrate the experimental and modelling work in each of the topic areas.
A five day training course on ‘Gas Migration in Geological Disposal Conditions’ was held at INR, Pitesti, Romania in September 2012. The course included presentations on many aspects and outcomes of FORGE including testing of low permeability materials, modelling gas/fluid flow and application of low permeability materials in radioactive waste disposal. One day was devoted to hands-on experimental testing of bentonite based materials. The course was well attended with representation from Korea, Italy, Romania, UK, Spain, Belgium and Bulgaria with about 25 attendees including professionals from WMOs, regulators and universities and a number of undergraduate and postgraduate students studying in the topic area.
FORGE has established a website at www.FORGEproject.org or www.bgs.ac.uk/forge from which all reports arising from FORGE, presentations given at meeting and training courses etc are available from. All reports are freely down.
A ‘Mobility Fund’ was included in FORGE to support the travel and subsistence costs of more junior scientists so that they can exchange between contributing institutions to allow them to gain expertise and experience outside their own institution and to attend FORGE meetings where other sources of funding was not available. The fund financed 3 people, including two PhD students, to attend the FORGE symposium in Luxembourg. Several FORGE students had the opportunity to spend time at other institutions in part funded by the mobility fund.
Because of its nature FORGE has not resulted in any patents or specific foreground IPR. It has, however, developed significant insights into the generation and migration of gas in a repository context, both ‘hard’ rock and clay rock hosted. FORGE deliberately did not set out to develop new computer codes but rather to use existing codes for modelling activities.
Several large scale experiments established or continued within FORGE are ongoing. These include the Lasgit experiment at Aspo and gas migration experiments at Josef Stolla, Bure and Mont Terri. These experiments will continue to provide data and information on gas migration issues.
FORGE has provided new and enhanced existing expertise in areas relating to gas generation and migration that is continuing to work in the field. By involving WMOs, regulators and research institutes/universities in the project FORGE has fostered links between these sectors and has greatly increased understanding of, for example, the limitations of experiments and modelling, between the different groups.
As a result of its experimental and modelling activities FORGE has provided new data and insights into the behaviour of gas in a repository context, for example, evidence that in some circumstances that gas flow through clays and bentonite is via dilatant pathways. Many of the observations from FORGE have improved understanding of gas transport and inform how it may be considered in the development of safety cases in future.



List of Websites:
www.FORGEproject.org

or

www.bgs.ac.uk/forge

Dr. R.P.Shaw
British Geological Survey
Environmental Science Centre
Keyworth
Nottingham
NG12 5GG

Tel: +44 (0) 115 9363545
e-mail: rps@bgs.ac.uk