Brazilian clays as potential buffer materials for radioactive waste final storage

Abstract

Clayey materials have been adopted in most nuclear waste producing countries, as a key constituent in engineered barrier for final disposal facilities at all levels of radioactive wastes (LILW-SL, LILW-LL and HLW). The following study presents a thorough characterization upon five Brazilian clay rich deposits, mostly smectite bearing clays, aiming to evaluate their expected performance as clay buffer under the conditions associated to a Low and Intermediate Level Waste Repository (RBMN); being the former a matter of national strategic interest. Samples coming from the Brazilian states of Bahia, Maranhão, Pará and Paraíba were treated and analyzed by means of X-Ray diffraction as a main technique, and complemented by FTIR, LALLS, XRF and SEM-EDS, in order to stablish mineralogical composition, particle size distribution and chemical composition. Moreover, several standard clay treatments over the <1 μm size fraction were carried out to reveal information regarding layer charge, major interlayer cations, unit formula and other crystal features of smectite species present in a mineralogical assembly, enabling the construction of a molecular model over which would be realistic to simulate the diffusion of radionuclides. Results obtained on 133Cs adsorption experiments indicate that mineralogical composition would probably be the single most influential factor controlling transport capacity of positively charged radionuclides in the current setup, and can be expressed in terms of smectite contents, favoring montmorillonite rich materials containing majorly Na+ as compensating cation in interlayer position. Thus, the obtained data will be useful in the testing of optimal compaction conditions to obtain the most suitable buffer material for the repository design. However, this trend is yet to be contrasted against hydraulic conductivity measurements and swelling pressure to see how they match.