The phase chemistries and microstructures of titanate-based ceramics containing simulated high-level nuclear waste with varying sodium contents were compared. Incorporation of relatively low sodium levels (=0.9 wt% NazO) resulted in more complex phase assemblages. The principal hosts for sodium were hibonite and freudenbergite, and, when iron and sodium were present in combination, loveringite was also stabilized. During fabrication, oxygen potential was controlled by Ti-TiOz or TiHz-TiJOS solid-state buffers. These metal and hydride oxygen getters behaved similarly, neither disturbing the phase assemblages nor significantly altering the partitioning of waste elements between radiophases. It is believed that the hydrothermal stability of the sodiumbearing ceramics (containing up to 2.7 wt% NazO) will be comparable to sodium-free material, since less durable sodium-rich phases are encapsulated in a resistant matrix. Extensive formation of glassy phases may cause embrittlement at higher sodium loadings.

Phase stability and effects of sintering atmosphere on the crystalline structure of Np-doped yttria-stabilized zirconia (YSZ) were evaluated in comparison with those same properties and conditions for Ce-doped YSZ. Different sintering atmospheres for Ce-doped YSZ led to differences in phase formation through reduction of the dopant from Ce 4؉ to Ce 3؉ . On the other hand, YSZ specimens containing up to 40 mol% Np formed only a fluorite-structure phase regardless of sintering atmosphere. Yttria-stabilized zirconia thus seems to accommodate Np within a wide range of concentrations and to have excellent phase stability under both oxidizing and reducing atmospheres.

Abstract--The effects of neodymium (Nd) on the transformation of femhydrite to iron oxides was studied. The possible isomorphous substitution of Nd 3+ for Fe 3+ in iron oxides was examined also. Nd was used as an inactive substitute of trivalent radioactive actinide elements. Hydrolysis of ferric nitrate solution containing 0-30 mole % of Nd formed Nd, Fe-rich ferrihydrite as initial precipitates, which were poorly crystalline. Aging of the Nd-containing ferrihydrite in 0.3 M OH-at 40~ and at pH 9.2 at 70~ formed Nd-free goethite and Nd-substituted hematite. The abundance of these crystalline phases was related to Nd in the parent solutions. Phase abundance, unit-cell parameters, and peak width were estimated by use of the Rietveld method.