Physico-chemical characterization of clays at Kalpakkam nuclear plant site towards assessment of radioactive waste disposal

Highlights: → Colloidal clay particles in groundwater have the potential to transport radionuclides in the ground environment. → Transport of radionuclides through groundwater depends on the nature of the clay colloids and its zeta potential. → The low CEC value of the kaolinite clay colloids ensures reduced potential for transport of radionuclides. → The low cation exchange capacity of the clay mineral in the study area emphasizes the requirement of suitable backfill materials around disposal facilities. - Abstract: In order to evaluate the safety of a radioactive waste repository in a geological formation, it is necessary to study the prevailing sub surface geology surrounding the disposal facility. The Kalpakkam subsurface was found to comprise of charnockite/hypersthene granite which is overlain by clay, sandy clay and sand. Clays can act as a barrier for groundwater flow and attenuate radionuclide migration. Accordingly, in the present study, clay samples were collected at chosen depths in the subsurface within the study area at Kalpakkam and colloidal clay fractions were separated. The clay colloids after separation were characterized for their size, shape, morphology, elemental composition and zeta potential. Photon correlation spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction technique and zetasizer were used to characterize the colloidal particles. The detailed characterization and analysis has revealed that the major clay mineral present in the subsurface at Kalpakkam is kaolinite. Minor quantities of smectite and illite were also identified in some clay samples. It was observed that the clays present in the study area are having low cation exchange capacity for radionuclides and emphasizes the requirement of proper backfill materials around disposal facilities for retardation of radionuclide migration. The occurrence of clay minerals at depths within the aquifer thickness warrants analysis of groundwater samples for suspended colloidal clay content. These suspended clay particles in groundwater have potential to transport of radionuclides in the ground environment along with groundwater flow depending on the nature of clay colloids and its zeta potential. Above investigations with respect to clay characterization would help the designer in incorporating the adequate safety measures in construction of near-surface disposal facility.