The Durability of Single, Dual, and Multiphase Titanate Ceramic Waste Forms for Nuclear Waste Immobilization

A significant amount of the energy used in the United States comes from nuclear power, which produces a large amount of waste materials. Recycling nuclear waste is possible, but requires a way to permanently fix the unusable radionuclides remaining from the recycling process in a stable, leach resistant structure. Multiphase titanate ceramic waste forms are one promising option under consideration. However, there is insufficient work on the long term corrosion of the individual phases, as well as the multiphase systems of these ceramics. These multiphase titanate ceramic waste forms have three targeted phases: hollandite, pyrochlore, and zirconolite. Hollandite is a promising candidate for the incorporation of Cs, while pyrochlore is readily formed with lanthanides, such as Nd, the most prevalent lanthanide in the waste stream. The third targeted phase, zirconolite, is for the incorporation of zirconium and the actinides. This work looks into the formation of single phase systems of lanthanide titanates, formation of dual phase systems of Ga doped Ba hollandites and Nd titanate, durability of single phase hollandites and multiphase model systems using Vapor Hydration Testing (ASTM C 1663-09), dissolution of dual phase systems of Ga doped Ba hollandites and Nd titanate using Product Consistency Testing (ASTM C 1285-02), as well investigating how grain size affects amount of alterative phases formed using Vapor Hydration Testing. The dual phase systems of hollandites and Nd titanate show significant amounts of secondary phases forming, heavily influenced by the composition of hollandite used in the systems. The most significant phase present was BaNd2Ti5O14. This phase proves to be problematic due to the degradation to the hollandite structure. Using Vapor Hydration Testing to investigate single and multiphase systems presented many some possible alteration phases that could occur in the long term aging of these ceramics. Most notably, Cs rich phases were found in nearly every system, meaning that different hollandites produced similar phases. Using Product Consistency Testing to investigate dissolution in dual phase systems of hollandites and Nd titanate led to the conclusion that as the volume fraction of hollandite increases, so does the stability of the hollandite in the systems