Phase-Pure and Multiphase Ceramic Waste Forms - Microstructure Evolution and Cesium Immobilization

Abstract

Efforts of this thesis are directed towards developing ceramic waste forms as a potential replacement for the conventional glass waste forms for the safe immobilization and disposal of nuclear wastes from the legacy weapons programs as well as commercial power production. The body of this work consists of two equal parts with first focused on multiphase waste form containing hollandite as major phase and the later, on single-phase hollandites for Cs incorporation.

Part I: Multiphase waste forms: Hollandite-rich multiphase waste form compositions processed by melt-solidification and spark plasma sintering (SPS) were characterized, compared, and validated for nuclear waste incorporation. Phase identification by X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) confirm hollandite as the major phase present in these samples along with perovskite, pyrochlore and zirconolite. Distribution of select elements observed by wavelength dispersive spectroscopy (WDS) maps indicate that Cs forms a secondary phase during SPS processing, which is considered undesirable. On the other hand Cs partitioned into hollandite phase in melt-processed samples. Further analysis of hollandite structure in melt-processed composition, by selected area electron diffraction (SAED), reveals ordered arrangement of tunnel ions (Ba/Cs) and vacancies, suggesting efficient Cs incorporation into the lattice.

Following the microstructural analysis, the crystallization behavior of the multiphase composition during melt-processing was studied. The phase assemblage and evolution of hollandite, zirconolite, pyrochlore, and perovskite type structures during melt processing were studied using thermal analysis, in-situ XRD, and scanning electron microscopy (SEM). Samples prepared by melting followed by annealing and quenching were analyzed to determine and measure the progression of the phase assemblage. Samples were melted at 1500°C and heat-treated at crystallization temperatures of 1285°C and 1325°C corresponding to exothermic events identified from differential scanning calorimetry (SC) measurements. Results indicate that the selected multiphase composition partially melts at 1500°C with hollandite coexisting as crystalline phase. Perovskite and zirconolite phases crystallized from the residual melt at temperatures below 1350°C. Depending on their respective thermal histories, different quenched samples were found to have different phase assemblages including phases such as perovskite, zirconolite and TiO2.

Part II: Single phase waste forms Hollandites with compositions Ba1.15-xCs2xCr2.3Ti5.7O16 have been identified as promising lattices to host Cs. Series of compositions with 0 ≤ x ≤ 1.15 were prepared by sol-gel synthesis, characterized, and analyzed for Cs retention properties. Phase-pure hollandites adopting monoclinic symmetry (12/m) were observed to form in the compositional range 0 ≤ x ≤ 0.4. Structural models for the compositions: x = 0, 0.15, and 0.25, were developed from Rietveld analysis of powder XRD and neutron diffraction data. Refined anisotropic displacement parameters (βij) for Ba/Cs along the tunnel direction. in addition, weak super lattice reflections have also been observed in XRD patterns. Our data suggests the presence of supercell structures with ordered tunnel cations for the phase-pure hollandites studied.

Finally, the performance of phase-pure hollandites have been evaluated qualitatively by chemical durability testing and ion-irradiation experiments. Elemental analysis of the leachants after 7-day leach tests show that Cs and Cr were extracted from the lattice together. No direct correlation between structural parameters or Cs content was observed. The simulated light-ion (He2+) and heavy-ion (Kr3+) irradiation experiments reveal that all the hollandite compositions studied undergo amorphization during α-decay events, and the extent of it increases with the Cs content.

In summary, the present work validates melt-processing as an effective method to prepare multiphase waste forms with the desired phase assemblage. Ba1.15-xCs2xCr2.3Ti5.7O16 hollandite has been identified as an effective ceramic host for Cs immobilization and appropriate structural models for hollandites with different Cs levels have been developed. The structural information may be used to study or simulate the lattice-environment interaction.