Studies on the immobilization of technetium under near field geochemical conditions

The immobilization of technetium dissolved in salt brines, which are in contact with simulated spent fuel and HAW glass packages, has been studied under aerobic conditions at room temperature. The retention effect of spent fuel packages has been investigated by adding UO2, hull (zircaloy-4) and container material (iron) in appropriate ratios to NaCl- and MgCl2-rich salt brines containing an initial TcO-4 concentration of 10-5 mol/L. The time dependence of the Tc concentration, pH and Eh has been measured simultaneously. Analoguous investigations have been carried out with borosilicate HAW glass. According to this scenario, container material (stainless steel) has been added to the salt solutions. In the system simulating the disposal of spent fuel, the Tc concentration decreases to less than 10-8 mol/L within 50 days, while in the system simulating the disposal of vitrified HAW, the immobilization of Tc is less efficient. In the latter case, even after 500 days there are still about 50-80% of the initial Tc dissolved. In both systems the addition of sulfide has a positive influence on the immobilization of Tc. The results are discussed in terms of reduction of Tc(VII) to sparingly soluble Tc(IV) hydrous oxide, sorption of technetium and precipitation of Tc2S7. (orig.)