The mechanisms of transport and retaidation processes, chemistry and migration behaviour of radionuclides of fission products
and actinides in engineered barriers, especially bentonites, have been summarised. A “critical group of radionuclides” is
proposed for thorough investigation of their retardation properties in natural sorbents. The evaluation of accessible data
of retardation and transport parameters relevant for the conditions of underground deep repository to be projected is carried
out from point of view their possible inclusion into a data-bank. The effect of solubilities and dissolution reactions, diffusion
and sorption/desorption, complexation and variations in the aqueous phase composition, pH-value and oxidation-reduction properties
and other phenomena on Kd values is discussed. Attention is devoted to thermodynamic modelling and its validation for evaluation
of various phenomena in the buffer/backfill-ground water systems.