Partitioning of La between solid and solution during the ageing of Sisingle bondAlsingle bondFesingle bondLasingle bondCa gels under simulated near-field conditions of nuclear waste disposal

Lanthanum-bearing gels of various stoichiometries (Sisingle bondFe ± Al ± Ca) and structures (hydrated Na-bearing and anhydrous Na-free materials), were aged in oxidizing aqueous solutions at 90°C and under a strong thermal gradient. The presence of La in the starting material did not inhibit the crystallization of ferrous clays (nontronite) for compositions with ratios Si/(Fe + Al) less-than-or-equals, slant 2, whereas crystallization failed forSi(Fe + Al) > 2.7. Clays are able to trap La, probably as an interlayer cation. In some experiments, a significant amount of La was also associated with newly formed silicated amorphous phases (APs), the precipitation of which from the solution was enhanced by a decrease of temperature. Extrapolation of these results to nuclear waste disposal conditions suggests that the radionuclide-rich hydrated amorphous layer (AL) which forms during the leaching of nuclear waste glasses is likely not to crystallize into clays. The hydrated amorphous layer should thus be a stable secondary barrier against the dispersion of radionuclides from leached nuclear waste glasses. Towards the far-field conditions, heavy elements are likely to be found in newly formed clays and silicated amorphous phases.