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Abstract The paper summarises the studies performed in Spain as regards the selection and characterization of clays suitable for sealing and backfilling of radioactive waste repositories. This research began in the 1980s under the auspices of ENRESA，the Spanish agency for nuclear waste management，and started by a survey of apt clay deposits and suppliers. The characterization of the clays and the criteria followed for their further selection were those already accepted by the international community：mineralogical purity，retention properties，plasticity，low permeability，high swelling pressure and thermal conductivity. These initial studies resulted in the selection of two bentonite deposits，whose detailed characterizations were carried out by several laboratories. These included the determination of thermo-hydro-mechanical properties，and of the impact of pre-heating，temperature and addition of quartz on these properties. The high expandability and low permeability of these materials led to the modification of the available experimental techniques and to the design of new equipment. The Cortijo de Archidona deposit(Almería) was finally selected and the bentonite taken there has been the object of various research projects that have ended in this bentonite being one of the best characterised from the mineralogical，thermal，hydraulic，mechanical，geochemical and alterability points of view. Besides，the behaviour of this bentonite under the conditions of a repository has been studied at laboratory and natural scale.
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Received: 30 August 2005
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