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

The disposal system as a whole The disposal system consists of the waste, the engineered barriers and the host rock (Figs 1 & 2). Geological disposal provides protection of people and the environment without human intervention being necessary once the facility is closed (source: ONDRAF/NIRAS 2016). Note that the depth at which a disposal facility could be constructed is dependent on, for example, the national strategy, the geology and the disposal concept. In the case of the UK, the facility will be located at a depth between 200 and 1000 m below the surface (UK Government 2016a).
Each of these elements fulfils, separately or in complementary fashion, multiple safety functions (e.g. isolation, containment). The 'defence-in-depth' principle requires multiple levels of protection, which are designed to enhance safety through their diversity and redundancy. It is the whole system that must be taken into account in a safety case, and not each system component separately.

Clays as host rocks
France (ANDRA 2016) and Switzerland (Nagra 2016) have chosen to dispose of their high-level and long-lived intermediate-level radioactive waste in indurated clays -in the Callovo-Oxfordian and Opalinus Clay formations, respectively. In Switzerland, Opalinus Clay is also the proposed host rock for low-and intermediate-level waste. In Belgium, the technical solution recommended for the longterm management of high-level and long-lived low-and intermediate-level waste is geological disposal in poorly indurated clay (Boom Clay or Ypresian clays) (ONDRAF/NIRAS 2016). In the Netherlands, clays are considered as a potential host rock for the disposal of all types of radioactive waste (COVRA 2016). Other countries also consider the use of clays as host rock (e.g. NWMO 2016; UK Government 2016b).
The research and development studies performed internationally over several decades have highlighted the favourable properties of clays in relation to geological disposal (for further information see the work of the respective national waste management organisations, e.g. . These favourable properties can be listed as follows: † very little water movement -thanks to their low permeability, there is practically no water movement in clays. Radionuclide and chemical contaminant transport via this medium is thus strongly delayed; † diffusive transport -given the limited water movement, transport in clays is essentially diffusive, which means species migrate primarily under the influence of their concentration gradient, and migration under the influence of porewater movement is minimal; Artist's impression of one potential layout of a geological disposal facility for UK higher activity wastes (# UK Crown copyright 2014). The precise layout and design will depend on the inventory for disposal and the specific geological characteristics at the site in question. † retention capacity -clays have a strong retention capacity for many radionuclides and chemical contaminants. Their migration through clays is thus considerably delayed; † buffer effect -clays display a significant buffer effect with regard to chemical perturbations. The thickness of the clay that is chemically perturbed by the disposal facility is, therefore, very limited; † self-sealing capacity -clays show a high capacity for self-sealing. Any fractures and fissures that occur, in particular those created by excavation activities, close quite rapidly; † stability -the selected clay host rocks and, therefore, their favourable properties have remained unchanged over millions of years. The migration of natural chemical species through these clay host rocks has remained diffusive during at least the last million years; † vertical homogeneity -radionuclide and chemical contaminant transport properties are very homogeneous almost throughout the entire thickness of the selected clay host rocks; † lateral continuity -clays are present within simple geological structures, with a significant lateral continuity, which facilitates their largescale characterization.
Clays may also contribute to the safety of disposal systems whose host rock is not clay, by being present in their geological environment.

Clays as engineered barriers
The favourable properties of clay (low permeability, self-sealing, stability) also make it a material of choice for engineered barriers. Clay is mainly planned for use as buffer, backfill or sealing material. † Buffer material -the empty space between the disposal package and the host rock is filled with clay. For instance, bentonite, a swelling clay, is used as buffer material filling the voidage between the disposal packages and the host rock in the disposal facility designs selected in, for example, Canada (NWMO 2016), Finland (Posiva 2016a), Sweden (SKB 2016) and Switzerland (Nagra 2016) (Fig. 3). † Backfill material -clay (for instance in the form of blocks or pellets) is used to fill excavated spaces (placement rooms, access ways), sometimes in combination with other materials. † Sealing material -clay, sometimes in combination with other materials, is used to isolate parts of the disposal facility. Seals are works of limited dimensions with specific purpose placed at key locations of the disposal facility. For instance, in France (ANDRA 2016), the seals aim to limit water flow within the underground facilities.
The geological disposal facilities in operation, under development or being planned in many countries thus contain, or are proposed to contain, clay, whatever the selected host rock. Table 1 lists some of these facilities, sorted according to the progress of the national geological disposal programme. The use of clay as buffer, backfill or in sealing is indicated.

Multidisciplinary research and this volume
Organizations responsible for implementing geological repositories, such as ANDRA, Bundesamt für Strahlenschutz, COVRA, Nagra, NWMO, ONDRAF/NIRAS, Ontario Power Generation, Posiva, PURAM (RHK Kft.), SKB, the UK Government and the United States Department of Environment, are undertaking significant programmes of research -nationally and internationally -to advance understanding of clays and their contribution to ensuring safe long-term management of long-lived radioactive waste. Multidisciplinary approaches, including geology, mineralogy, geochemistry, rheology, the physics and chemistry of clay minerals and assemblages, are required in order to provide a detailed characterization of the geological host formations considered for the disposal of radioactive waste and to assess the behaviour of engineered and natural barriers when submitted to various types of perturbations induced by such facilities. Existing and proposed experimental programmes in underground research laboratories are designed to evaluate the performance of the natural barrier and the impact of repository-induced disturbances upon the confinement properties of clay-rich geological formations. Interpreting the subsequent scientific results, modelling the long-term behaviour of radioactive waste repositories and carrying out safety assessment exercises all further contribute to the knowledge base. This Geological Society, London, Special Publication contains 25 papers of scientific studies presented at the 6th conference on 'Clays in natural and engineered barriers for radioactive waste confinement', hosted by ONDRAF/NIRAS (the Belgian National Radioactive Waste Management Agency) and held in Brussels, Belgium, in 2015. The papers cover a range of outputs from the Brussels meeting and provide insight for the reader of the range of clay-related work currently being undertaken internationally in relation to natural and engineered barriers for radioactive waste confinement.
Since 2002, national waste management organizations have developed this conference into the most important event for all kinds of scientists from all over the world dealing with the disposal of radioactive waste. These conferences, each attracting a large number of papers and attendees from around the world, have contributed significantly to the outstanding scientific level of research relevant to argillaceous media in the context of the disposal of radioactive waste.
The 2015 meeting covered aspects of clay characterization and behaviour considered at various temporal and spatial scales relevant to the confinement of radionuclides in clay, from basic phenomenological process description to the global understanding of the performance and safety at repository and geological scales. Special emphasis was put on the modelling of processes occurring at the mineralogical level within the clay barriers.
The 25 papers published in this Special Publication have been classified according to different areas within the field of disposal of radioactive waste research. The assignment of a study to one of the topic areas was not always easy because many studies provide information of, and for, different aspects of disposal research. With that caveat, the papers in this volume consider research into argillaceous media under the following topic area headings: large-scale geological characterization; general strategy for clay-based disposal systems; geomechanics; mass transfer; bentonite evolution; and gas transfer. Vandersteen et al. (2016) discuss data analysis and modelling in relation to regional aquifer hydrogeochemistry in the confined aquifer system below the Boom Clay (NE Belgium).

Large-scale geological characterization
In their paper, Gravesen et al. (2016) report on an assessment of Palaeogene and Neogene clay deposits in Denmark as possible host rocks for disposal of low-and intermediate-level radioactive waste.

General strategy for clay-based disposal systems
Approaches to evaluate and underpin the technical feasibility of the Belgian disposal concept are discussed by Doudou et al. (2016a).
Outputs from the European Commission DOPAS Project (Posiva 2016b), which considered the full-scale demonstration of plugs and seals, including constituent clay materials and as will be used in a repository, are discussed in the next three papers. Doudou et al. (2016b)

Mass transfer
It is important to understand how groundwater and other chemical species, as might derive from waste disposed in a repository, could potentially move through natural and engineered clay barriers. As well as understanding mass transfer at the very small scale, it is also important to consider how a very detailed conceptual model for the movement of groundwater and other chemical species, for example, can be upscaled for inclusion in a larger-scale model of an evolving repository. Kolomá & Č ervinka (2016) report on a study of 85 Sr transport through a column filled with crushed granite in the presence of bentonite colloids, while Aertsens et al. (2016) propose an improved model for through-diffusion experiments, with application to strontium and tritiated water (HTO) diffusion in Boom Clay (as relevant to the Belgian national programme) and compacted illite. Meng & Pfingsten (2016) discuss multispecies random walk simulations in radial symmetry -the model concept, benchmark and application to HTO, 22 Na and 36 Cl diffusion in clay; Bildstein et al. (2016) report on a study to gain insight into corrosion processes from numerical simulations of an integrated iron-claystone experiment. Toprak et al. (2016) present coupled thermohydro-mechanical modelling of engineered barriers for the final disposal of spent nuclear fuel isolation and, moving to a larger scale, Schaedle et al. (2016) combine high-resolution two-phase with simplified single-phase simulations in order to optimize the performance of performance assessment/ safety assessment simulations for a deep geological repository for radioactive waste.

Bentonite evolution
Bentonite clay will be emplaced in a repository as part of the engineered barrier system. Over the time-scale of interest typically considered in the evaluation of the performance of a repository (up to 1 million years post-closure), it is possible that the bentonite will evolve. It is important to understand the extent of possible bentonite evolution, and whether or not any related change to properties of the bentonite affect its ability to perform as part of a system of natural and engineered barriers. Kröhn (2016) considers conceptual models and mathematical descriptions relating to bentonite re-saturation. Dolder et al. (2016) discuss the alteration of MX-80 bentonite backfill material by high-pH cementitious fluids under lithostatic conditions, presenting an experimental approach using core infiltration techniques. Potential chemical erosion of bentonite is considered by Reijonen & Marcos (2016), who draw on evidence from nature to consider if information can be derived for use in the development of understanding of bentonite evolution in a repository.

Gas transfer
As well as a consideration of how groundwater could move through a clay (see the eight papers in the mass transfer section), it is important to understand how gas -as derived from, for example, the corrosion of waste containers, radiolysis of groundwater -could interact with clay in the engineered barrier system and host rock. Bénet et al. (2016) model water and gas flow through an excavation damaged zone in the Callovo-Oxfordian argillites in the framework of a single porosity model, while Smutek et al. (2016) report on the gas permeability, breakthrough behaviour and re-sealing ability of Czech Ca -Mg bentonite. Finally, Jacops et al. (2016) report on a study to measure diffusion coefficients of dissolved helium and argon in three potential clay host formations: Boom Clay (of relevance to the Belgian national programme), Callovo-Oxfordian Clay (of relevance to the French national programme) and Opalinus Clay (of relevance to the Swiss national programme).
The collection of different topics presented in this Special Publication demonstrates the diversity of geological repository research, which will continue as national programmes progress towards their respective licence applications for repository implementation. This continued research will allow uncertainties relating to, for example, the behaviour of an evolving repository system to be reduced; will increase safety margins; and will help with disposal system optimization. As such, geological repository research is a challenging and engaging task for current scientists and hopefully will provide an interesting and long-term career path that is attractive to young scientists.
The 7th conference on 'Clays in natural and engineered barriers for radioactive waste confinement' will be held in Davos, Switzerland, in September 2017, hosted by the Swiss National Radioactive Waste Management Agency, Nagra.