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The apparent diffusivities (Da) for cesium and tritium in compacted sodium bentonite saturated with distilled water were measured at ambient temperature. The dry density of the bentonite had a significant effect on Da for cesium, which decreased with increasing the dry density in the range of 0.2 to 2.0 Mg/m 3 . Α batch-Äi/ for cesium measured using a high solution/bentonite ratio is not applicable to predict Da(Cs) with a model based on linear sorption, because the ionic strength of pore water in the compacted bentonite is probably higher than the value of the solution for the batch sorption test in the existing experimental condition.

Dry density of bentonite affected the diffusivity for the radionuclide and the apparent diffusion coefficient decreased with increasing dry density of bentonite.Small effect of dry density of bentonite on retardation factor which was derived from the apparent diffusion coefficient was indicated for 137 Distribution coefficient derived from the apparent diffusion coefficient showed a tendency of decreasing with increasing dry density of bentonite except for 237 Np and 241 Am whose coefficients were unchanged and were higher than values from Sorption Data Base of OECD/NEA. The decrease of the distribution coefficient may be caused by the change of the ion concentration in the pore water. The higher distribution coefficients of 237 Np and 241 Am may be caused by sorption, ion exclusion and molecular filtration.

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The solubility of Np(IV) in carbonate media was studied for determination of thermodynamic data. The concentration of dissolved Np(IV) was measured with ionic strengths of 2.0 and 2.9 M (M ≡ mol dm −3 ), hydrogen-ion concentration exponent (pH c = − log[H + ]) of 12.0, 12.9 and 13.0, and with the total carbonate concentration (C T ) from 5 × 10 −2 M to 7× 10 −1 M by both oversaturation and undersaturation methods. It was found that the solubility of Np(IV) increased with increasing C T . Due to the dependencies of the solubility on C T and pH c , the dominant aqueous species of Np(IV) in highly basic solutions was expected to be NpCO 3 (OH) 4 4− . Together with a carbonatohydroxo complex of Np(CO 3 ) 2 (OH) 2 2− estimated in our previous study, the apparent equilibrium constants of the carbonatohydroxo complexes were obtained at the respective ionic strength while the equilibrium constants at the zero ionic strength were extrapolated. The obtained equilibrium constants supported the literature values strongly, and contributed to improve the reliability of the thermodynamic data.