| The long-term stability of nuclear waste repositories depends to a great extent on the chemical behavior of a few long-lived isotopes, technetium and actinides, and on the effects of radiation damage on actinide host phases. This thesis addressed the problem by investigating the dissolution reactions of several prototype ceramic and glass waste forms containing technetium and uranium.; The experimental approach was to synthesize single phase ceramics and to make use of a reference borosilicate glass. The ceramic phases were Ca(Zr,U)Ti(,2)O(,7)(zirconolite), (Ca,U)(Ti,Fe)O(,3) (perovskite) and Mg(,2)(Ti,Tc)O(,4)(spinel). Radiation-damaged materials were obtained by the thermal neutron fission of the ('235)U dopant. Materials were reacted with aqueous solutions at temperatures between 25(DEGREES) and 90(DEGREES)C, and pH values ranging from 0 to 11, under oxidizing and reducing conditions.; Zirconolite proved to be an extremely stable waste form, and dissolution kinetics could be measured at pH = 0. At other pH values, the rate of dissolution of metamict zirconolite was no faster than that of the crystalline material within limits of error.; Perovskite is a considerably less stable radionuclide host. Dissolution rates could be measured at all pH values. The dissolution kinetics were linear except that saturation effects were seen due to precipitation of TiO(,2) and also CaCO(,3). Perovskite was very sensitive to radiation damage with the metamict material dissolving at a rate 1000 times that of the crystalline material at pH = 1.32.; The attempt to immobilize technetium in glass and spinel was successful. In both cases it is necessary that the oxygen fugacity of the environment be adjusted so that the Tc('4+) is the stable state.; The dissolution behavior of technetium from spinel is complex. There is an initial rapid increase followed by a plateau, after which the dissolution becomes linear with time. These results have been interpreted in terms of a build-up of a porous titanium oxide layer on the surface.; Dissolution of technetium from the borosilicate glass in acid regimes followed the parabolic kinetics characteristic of diffusion barriers. When the oxygen activity was reduced, the technetium release fell below the limits of detection (1 ppb). |
