PARTITIONING OF LA BETWEEN SOLID AND SOLUTION DURING THE AGING OF SI-AL-FE-LA-CA GELS UNDER SIMULATED NEAR-FIELD CONDITIONS OF NUCLEAR WASTE-DISPOSAL

Lanthanum-bearing gels of various stoichiometries (Si-Fe +/- Al +/- Ca ) and structures (hydrated Na-bearing and anhydrous Na-free materials) , were aged in oxidizing aqueous solutions at 90 degrees 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 equal to 2, whereas crystallization failed for Si/(Fe + Al) 2.7. Clays are able to trap L a, probably as an interlayer cation. In some experiments, a significan t amount of La was also associated with newly formed silicated amorpho us phases (APs), the precipitation of which from the solution was enha nced by a decrease of temperature. Extrapolation of these results to n uclear waste disposal conditions suggests that the radionuclide-rich h ydrated amorphous layer (AL) which forms during the leaching of nuclea r 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. Toward s the far-field conditions, heavy elements are likely to be found in n ewly formed clays and silicated amorphous phases.