[en] The 4. edition of the International Meeting 'Clays in Natural and Engineered Barriers for Radioactive Waste Confinement' took place at the 'Cite Internationale des Congres' of Nantes (France). Approximately 500 participants (from about 20 different countries) attended the meeting. All the abstracts (oral and poster sessions) are included in these proceedings. The purpose of this 4. international conference is to gather specialists in the different disciplines related to clays and clay minerals, with scientists from organisations engaged in radioactive waste disposal, in order to evaluate the progress of the research conducted in that field. Multidisciplinary approaches including geology, mineralogy, geochemistry, rheology, geomechanics of clays are required in order to provide a detailed characterisation 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 disposal facilities. The major objectives for the experimental programs are constituted by the performance evaluation for the natural barrier as well as the impact of repository-induced disturbances upon the confinement properties of clay-rich geological formations. This is being or will be conducted in underground research laboratories, for interpreting the subsequent scientific results, for modelling the long-term behaviour of radioactive waste repositories and for carrying out safety assessment exercises. This conference covers all the aspects of clay characterisation and behaviour relevant to the confinement of radionuclides in clay, considered at various time scales and locations, from the descriptions of basic phenomenological processes to the global understanding of the performance and safety at repository and geological scales. Most of the topics covered by the programme of the conference are in line with the general objectives proposed within the Strategic Research Agenda, elaborated through the Implementing Geological Disposal of Radioactive Waste Technology Platform (IGD-TP). The conference offers a particular opportunity to present the more recent developments and main outputs of research carried out within the framework of national and international cooperative experiments and dedicated European projects. Contributions coming from fields other than radioactive waste disposal; like geological storage of natural gas, sequestration of CO₂, energy storage in underground, chemical waste isolation, etc., also taking advantage of the properties of the clay material, were encouraged. This conference covers all topics concerning the natural argillaceous geological barriers and the clay-based engineered barrier systems, investigated by means of: laboratory experiments on clay samples (new analytical developments), in situ experiments in underground research laboratories, mock-up demonstrations, natural analogues, as well as numerical modelling and global integration approaches (including up-scaling processes and treatment of uncertainties) and monitoring. General strategy for clay based repository concepts Examples of research programmes (national or international) concerning the role of natural and engineered clay barriers for radioactive waste confinement including repository designs, safety assessment, full-scale demonstrations and implementations (e.g. heater tests). Geology and clay characterisation Clay mineralogy, sedimentology, paleo-environment, diagenesis, dating techniques, discontinuities in clay rock, fracturing, self-sealing processes, role of organic matters and microbiological processes, micro and nano characterisation of clay minerals and argillaceous rocks. Geochemistry Pore water geochemistry, clay thermodynamics, chemical retention, geochemical modelling, advanced isotopic geochemistry. Mass transfer Water status and hydraulic properties in low permeability media, pore space geometry, water, solutes and gas transfer processes, colloids mediated transport, large scale movements, long-term diffusion. Alteration processes Oxidation effects, hydration-dehydration processes, response to thermal stress, iron-clay interactions, alkaline perturbation. Geomechanics Thermo-hydro-mechanical behaviour of clay, rheological models, EDZ characterisation and evolution, coupled behaviour and models (HM, THM, THMC). Monitoring Evaluation of the evolution of natural and engineered clays in context to radioactive waste disposal