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The equilibrium concentrations of plutonium, americium and neptunium in salt brines in contact with cementitious waste forms in the presence of complexing agents have been measured.In Q-brine, a saturated solution rich in magnesium chloride (pH near 6.5) the actinide concentration is increased by complex formation but limited by an adsorption equilibrium with the corroded cement product.In saturated sodium chloride solution (pH 12 after equilibration with cement) the concentration of actinides is constrained by the solubility of their respective hydroxides. This is attributed to the combined action of hydrolysis and displacement of actinides by calcium ions.Thermodynamical computations using the EQ3/6 code have been performed to predict the stability range of actinide complexes. The results obtained for the americium/EDTA complex agree well with experimental data.

Migration characteristics of HTO and 237 Np were analyzed through experiments performed with a fractured granite core in the Chemlab2 probe at the Äspö hard rock laboratory in Sweden. These experimental data covered the elution of HTO and 237 Np as well as information of retained actinides onto the fracture surfaces. The flow field was simulated with a two-dimensional variable aperture channel model. The mass transport process was described with a randomwalk method. Three types of basic migration processes were considered : advection, sorption and matrix diffusion. By a combination of these processes, specific effects of each process on transport were evaluated. The retardation of the neptunium were analysed quantitatively by the model. The results were compared to the experimental findings in terms of the elution curves and of distribution patterns on the fracture surface. The elution curves of HTO were used to determine the hydraulic properties in the fractured core such as the extent of a dispersion effect according to the flow rates. After termination of the migration test, the rock core was opened, and the retained radioactivities onto the fracture surfaces were measured to investigate an extend of 237 Np retardation and the retardation mechanism. A retardation factor was obtained by comparing the simulated results to the distribution of sorbed tracers along the flow path. The validity of sorption data from batch tests was also discussed for estimating migration of sorbing tracers.