Leaching of α-doped gels

Full text of publication follows: α-doped (Np, Pu, Am) SON68 glasses have been completely altered under hydrothermal conditions at 300 deg. C to form gels. These gels then have been leached at 50 deg. C under static mode, 44 cm-1 and under oxidizing (Eh/HNE ∼ +0.15 V) and reducing (Eh/HNE ∼ -0.25 V, obtained by hydrogenated argon bubbling in the reactor) conditions during one year. The goal of these experiments was to study the actinides release in solution during gels leaching and to determine what mechanism controlled this release at long term. SEM observations of gels showed that there were secondary phases made of needles and sheets with the amorphous gel grains. The XRD patterns indicated that the gels were mainly amorphous with crystallized phases (sodium and calcium alumino-silicates: analcime, albite). The pH values of the leachates decreased versus time from 1 day to 3 months and after they were constant. This decrease of the pH values was due to an increase of dissolved CO2 in the leachates, resulting from samplings for solutions analysis. The pH values obtained during the Am-doped gel leaching were lower of about 0.5 pH unit compared to those observed with Np- and Pu-doped gels. This phenomenon was due to γ radiolysis of the N2- O2-H2O system leading to HNO3 formation. The Am-doped gel dissolution rate, calculated with regard to silicon, was 2 to 3 times higher than those measured for Np- and Pu-doped gels, whatever the oxidation-reduction potential of the leachates. The three alkali metals had not the same behavior during gels leaching and the following relation was found: r(Li) ≅ 2 r(Na), r(Na) ≅ 2 r(Cs) and r(Cs) ≅ r(Si). In fact, alkali metals were preferentially included in crystallized phases, except Cs that was partially trapped in the amorphous silicated gel; this could explain the different behavior between cesium and the other alkali metals. The actinides in solution were found very mainly in colloidal form and the fraction > 0.45 μm represented more than 50% of the released actinides. The soluble neptunium concentrations were about 3 10-7 and 5 10-8 mol.L-1 under oxidizing and reducing conditions and the colloidal fraction increased under reducing medium, in relation with the presence of Np(V) and Np(IV) for respectively oxidizing and reducing media. The plutonium concentrations were about 4 10-9 and 10-9 mol.L-1 under oxidizing and reducing conditions and Pu(IV) prevailed in both media. The plutonium colloidal fraction did not vary according to the Eh value. Americium had the same behavior than plutonium under reducing conditions and its solubility was about 10-9 mol.L-1. Solubility calculations showed that no simple phases (hydroxide, carbonate) controlled the actinides concentrations in solution. (authors)