Clay-Based Materials for Engineered Barriers: A Review
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CLAY-BASED MATERIALS FOR ENGINEERED BARRIERS: A REVIEW
Alain LAJUDIE, Jodl RAYNAL, Jean-Claude PETIT and Pierre TOULHOAT Commissariat AI'Energie Atomique - DCC/DESD/SESD - Centre d'Etudes de Fontenay aux Roses - BP6, 92265 Fontenay aux Roses Cedex France I. INTRODUCTION The potential importance of backfilling and plugging in underground radioactive waste repositories has led differents research institutions around the world (SKB in Sweden, CEA in France, AECL in Canada, etc.) to carry out extensive studies of swelling clay materials for the development of engineered barriers. These materials, which have to be emplaced in underground conditions, should combine a variety of complementary properties from both the hydro-thermo-mechanical and geochemical viewpoints: impermeability, swelling ability inorder to fill all void space, heat transfer and retention capacity for the most noxious radionuclides. For years, the scientific community has acknowledged the fact that smectite clays best exhibit these properties and, thus, most of the research effort has been devoted to this type of materials. The aim of such studies is to try and link the microscopic characteristics of the material (mineralogy, geochemical properties, microstructure, etc.) to its macroscopic behaviour (swelling properties, etc...). II. HISTORICAL PERSPECTIVE
Before reviewing the information acquired on such clay materials, investigated for more than a decade either for reprocessed wastes or for spent fuel, we think interesting to underline the fact that the concept of engineered barriers dates back to the end of the 70's. Indeed, the idea of disposing radioactive wastes in geological formations has a very long history but, until the mid-70's, the concept promoted in several countries was restricted to the waste form (usually glass since reprocessing was a common option at that time), its canister and the host rock (see for instance Petit, [1] for a sociotechnical analysis of this field). In a paper, where the authors
were questioning the capability of the geological formation to confine radionuclides, as well as the ability of the geologist to model it, de Marsily et al. [2] insisted on the geochemical properties of the host rock, and in particular its retardation capability, due to a variety of mechanisms (sorption, ion exchange, etc.). They first introduced the idea that such a function could be reinforced by "an artificial one set up around the wastes. It could be possible, therefore, to reinforce the confinement locally by means of a complementary physical barrier". However no particular suggestion was made about the possible constituents of such an artificial geochemical barrier. Later on, Ringwood [3] suggested that the long-term geological stability of the Synroc constituents (basically titanate minerals) as well as of the Ni3Fe canister he was promoting (a material analogue to some meteorites) could be best guaranteed in particular geochemical conditions. Indeed, the remarkable corrosion resistance in groundwaters of these compounds has only
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