Simon Norris

An experimental study of the flow of gas along synthetic faults of varying orientation to the stress field: Implications for performance assessment of radioactive waste disposal

Critical stress theory states that fault transmissivity is strongly dependent upon orientation with respect to the stress tensor. This paper describes an experimental study aimed at verifying critical stress theory using a bespoke angled shear rig designed to examine the relationship between gas flows along a kaolinite-filled synthetic fault as a function of fault dip. A total of 22 gas injection experiments were conducted on faults oriented 0°, 15°, 30°, and 45° to horizontal; both with and without active shear. Gas flow was seen to be complex; repeat gas injection testing showed a consistent gas entry pressure but considerably different, nonrepeatable, gas peak or breakthrough pressure. Gas flow occurred along discrete, dilatant pathways. The physics governing the pressure at which these features formed was repeatable; however, permeability was dependent on the number, distribution, and geometry of the resultant pathways. The nonrepeatable gas response suggests that the number of pathways was dependent on very subtle variations in gouge properties. No fault orientations were seen to exhibit nonflow characteristics, although critical stress theory predicted that two of the investigated fault angles should be effective seals. However, a small variation in gas entry pressure was seen with fault angle as a result of varying normal and shear stress acting on the gouge material. Shear was seen to enhance gas movement by reducing gas entry pressure and increased permeability once gas became mobile. Therefore, in kaolinite gouge-filled faults, shear is not an effective self-sealing mechanism to gas flow.

Bentonite reactivity in alkaline solutions: interim results of the Cyprus Natural Analogue Project (CNAP)

Abstract Bentonite is one of the more safety-critical components of the engineered barrier system in the disposal concepts developed for many types of radioactive waste. It is used due to its favourable properties (including plasticity, swelling capacity, colloid filtration, low hydraulic conductivity, high retardation of key radionuclides) and its stability in relevant geological environments. However, bentonite is unstable under alkaline conditions and this has driven interest in low-alkali cements (leachate pH of 10-11). To build a robust safety case, it is important to have supporting natural analogue data to confirm understanding of the likely long-term performance of bentonite. In Cyprus, the presence of natural bentonite in close proximity to natural alkaline groundwaters permits the zones of potential bentonite/alkaline water reaction to be studied as an analogy of the potential reaction zones in the repository. Here, the results indicate minimal volumetric reaction of bentonite, with production of a palygorskite secondary phase.

A new hydro-mechanical model for bentonite resaturation applied to the SEALEX experiments

Bentonite barriers perform safety critical functions in many radioactive waste disposal concepts, but it is challenging to accurately predict bentonite resaturation behaviour in repository settings. Coupled models of the hydro-mechanical response of bentonite are used to demonstrate understanding of bentonite behaviour in experiments and to predict the response of bentonite in a repository environment. Following trials of a range of numerical approaches, a new model is presented, referred to as the Internal Limit Model, which makes use of key observations on limiting stresses supported in bentonite samples in experimental data. This model is based on the Modified Cam Clay model, and uses the observation that for a given dry density of bentonite, there is a limiting stress that the sample can support, be that stress due to swelling, compaction or suction, to explicitly couple the hydraulic and mechanical models. The model is applied to experimental data from the SEALEX experiments, involving a 70/30 by mass mixture of MX80 bentonite and sand. The model is able to reproduce the experimental data using a single set of parameters for all the experiments considered. This builds confidence that the model will be useful in the future for predictive modelling given appropriate data to characterise the bentonite material being used.

Engineered damage zone sealing during a water injection test at the Tournemire URL

Low-permeability clay formations provide good candidate host rocks for geological disposal of radioactive waste, because there is expected to be limited movement of gas or water through the formation. However, when constructing tunnels, the stress state in the formation around the tunnel will change, which can lead to damage to the formation, changing the bulk hydraulic properties of the formation close to the tunnel. There is the potential for this damaged zone to act as a preferential pathway for fluid flow and radionuclide transport. A water injection experiment is ongoing at the Tournemire underground rock laboratory to investigate the hydraulic properties of the Toarcian argillite in which the laboratory is constructed. Water is injected into the formation at the end of a sealed borehole and moves preferentially in the damaged zone along the borehole walls. The rate of water injection into the rock changed over the first year of the experiment and the causes of this change are investigated in this paper by numerical modelling. The study demonstrates that the change in water injection rate into the argillite can be explained by the evolving hydraulic properties of the damaged zone around the borehole. The findings of the modelling study are discussed in the context of long-term radioactive waste disposal.

A nonlinear elastic approach to modelling the hydro-mechanical behaviour of the SEALEX experiments on compacted MX-80 bentonite

Abstract Hydraulic seals using compacted sand–bentonite blocks are an important part of the closure phase of deep geological disposal facilities for the isolation of many categories of radioactive wastes. An understanding of the hydro-mechanical behaviour of these seals and the ability to model their behaviour is a key contribution to safety cases and licence applications. This work reports the development of a hydro-mechanically coupled model and its application to the simulation of a range of test conditions investigated in the SEALEX experiments conducted by IRSN at Tournemire URL. The work has been conducted as part of the recently completed DECOVALEX-2015 project. Richards’ equation for unsaturated fluid flow is coupled to a nonlinear elastic strain-dependent mechanical model that incorporates a moving finite element mesh, and calibrated against laboratory experiments. Stress and volumetric dependencies of the water retention behaviour are incorporated through the Dueck suction concept extended to take into account permanent changes in water retention behaviour during consolidation. Plastic collapse in laboratory results is modelled with the application of a source term activated by a threshold defined in terms of the net axial stress and net suction. The model is used to simulate both a 1/10 scale mock-up laboratory test and full-scale in situ performance test and is capable of reproducing the major trends in the data with just nine mechanical parameters and an experimentally defined stress threshold.

Evaluating the importance of different coupled thermal, hydraulic, mechanical and chemical process simulations during fluid flow experiments in fractured novaculite and fractured granite

Abstract Fluid migration in the subsurface has the potential to induce changes in fluid pressure distribution, temperature distribution, mechanical stresses and the chemistry of both the fluid and the natural geological material it is flowing through. In many situations, the change in all of these processes gives a coupled response, in that one process feeds back to another. When trying to understand fluid flow through naturally and artificially fractured systems, it is important to be able to identify the relative importance of the processes occurring and the degree of interactions between them. Modelling of such highly nonlinear coupled flow is complex. Current and predicted computational ability is not able to simulate discretely all the known and physically described processes operating. One approach to coping with this complexity is to identify the relative importance and impact of relevant processes, dependent on the application of interest. Addressing such complexity can be particularly important when the characteristics of natural and disturbed geological materials are being evaluated in the context of disposal of radioactive waste or other geo-engineering systems where an understanding of the long-term evolution is required. Based on a series of coupled (THMC—Thermal, Hydraulic, Mechanical and Chemical) experimental investigations on the flow of fluid through fractured novaculite and granite crystalline rock samples, several couplings are examined where there is both a significant kinetic chemical control as well as mechanical and temperature control on the fluid flow behaviour. These interactions can be shown both in the literature and experimentally to have a significant effect on the rate of fluid flow through fractures. A new discrete numerical approach and a new homogenous approach are used to model the experimental results of coupled flow through fractures. The results of these modelling approaches are benchmarked both against one another and against the experimental results, and then the processes included in the approaches are ranked in order of impact.

An experimental model of episodic gas release through fracture of fluid confined within a pressurized elastic reservoir

We present new experiments that identify a mechanism for episodic release of gas from a pressurized, deformable reservoir confined by a clay seal, as a result of the transition from bulk deformation to channel growth through the clay. Air is injected into the center of a thin cylindrical cell initially filled with a mixture of bentonite clay and water. For sufficiently dry mixtures, the pressure initially increases with little volume change. On reaching the yield stress of the clay-water mixture, the lid of the cell then deforms elastically and an air-filled void forms in the center of the cell as the clay is driven radially outward. With continued supply of air, the pressure continues to increase until reaching the fracture strength of the clay. A fracture-like channel then forms and migrates to the outer edge of the cell, enabling the air to escape. The pressure then falls, and the clay flows back toward the center of the cell and seals the channel so the cycle can repeat. The phenomena may be relevant at mud volcanoes.

Radioactive waste confinement: clays in natural and engineered barriers – introduction

There is general agreement internationally (Nuclear Energy Agency, OECD 2008) that geological disposal provides the safest long-term management solution for higher-activity radioactive waste. Many countries (e.g. Canada, Finland, France, Switzerland, Sweden, UK and USA) have chosen to dispose of all or part of their radioactive waste in facilities constructed at an appropriate depth in stable geological formations. The development of a repository (sometimes also referred to as a geological disposal facility) on a specific site requires a systematic and integrated approach, taking into account the characteristics of (i) the waste to be emplaced, (ii) the enclosing engineered barriers and (iii) the host rock and the geological setting of the host rock. Three main rock types are usually considered for geological disposal: crystalline rocks, salt and clays. Each type includes bedrock formations with a relatively broad spectrum of geological properties. The engineered barriers contain different types of materials, such as metals, concrete and natural materials, such as clay. This Special Publication highlights the importance of clays and clayey material in the development of almost all national geological disposal systems (for further information on the uses of clay proposed by a range of national waste management programmes, see, for example, ANDRA 2016; Bundesamt für Strahlenschutz 2016; COVRA 2016; Nagra 2016; NWMO 2016; ONDRAF/NIRAS 2016; Ontario Power Generation 2016; Posiva 2016a; PURAM (RHK Kft.) 2016; SKB 2016; UK Government 2016a, b; United States Department of Environment 2016). Clays exhibit many interesting properties, which are exploited in the development of most geological disposal systems. Clays are used both as host rock and as material for engineered barriers. Whatever their use, clays present various characteristics that make them high-quality barriers to the migration of radionuclides and chemical contaminants towards the surface environment. As host rocks, clays are, in addition, hydrogeologically, geochemically and mechanically stable over geological time-scales, i.e. millions of years. The disposal system as a whole

Clays in Natural and Engineered Barriers for Radioactive Waste Confinement: an introduction

The concept of engineered geological disposal has been developed for the safe long-term management of long-lived radioactive waste. This involves emplacement of radioactive waste in deep geological repositories that contain and isolate the waste and, consequently, protect humans and the environment. The ‘multiple barrier concept’ is the cornerstone of all proposed schemes for the geological disposal of radioactive waste. Based on the principle that uncertainties in performance can be minimized by conservatism in design, the concept invokes a series of complementary barriers, both engineered and natural (geological), between the waste and the surface environment. Each successive barrier represents an additional impediment to the movement of radionuclides. Regarding engineered barriers, for lowand intermediate-level waste the waste may be incorporated in a relatively stable and inert matrix such as cement, bitumen, lead-alloy or polymer resin (the choice varying depending on the waste management organization); glass may be used in the case of certain high-level reprocessing wastes. Owing to the very low leach-rate of glass in groundwater, vitrification is widely accepted to be one of the best methods of immobilizing the aqueous products from the reprocessing of spent fuel. Many waste containers will provide some form of physical barrier to groundwater. However, because of the relatively small volumes of waste involved, spent fuel, vitrified waste and other highly active wastes will be totally encapsulated in corrosion-resistant metal canisters that are designed to prevent groundwater entry for extended time periods in excess of 100 000 years. 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 rate of hydraulic transport; † 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.