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