Time-Temperature Transformation Phase Diagrams for Developing Advanced Glass Ceramic Nuclear Waste-forms

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The purpose of this project is to understand nucleation and growth of ceramic phases in UK's waste glasses in order to develop monolith glass-ceramic (GC) wasteforms to immobilize critical fission products (FP) that otherwise phase segregate out of the glass matrix. Borosilicate glasses (BSG) have been used for immobilizing high-level waste and while most FP's can be accommodated in the glass matrix, some FPs like Mo, lanthanides, and noble metals have limited solubility in BSG and hence they phase separate/crystallize. Ceramic oxides can incorporate these FPs in their stable crystal structure, and are more resistant to leaching and radiation damage.Thus there is a need to design glass ceramics (GC) that are single-phase borosilicate glass at melting temperatures while undergoing a liquid-phase separation, followed by bulk crystallization into targeted phases upon slow cooling. Thus, even though correct stoichiometry should ensure formation of the desired crystalline phases, varying cooling rates can affect the formation of crystalline phase in the glass matrix. Hence there is a need to develop a fundamental understanding of the phase-transformations leading from melt to GC wasteform as function of cooling rate and composition.Work will involve understanding thermo-chemical factors attracting the crystallization of different ceramic phases inside a CaZn BSG matrix and studying the effects of various cooling rates to mimic those occurring in real large-scale glass wasteform fabrication processes. The microstructure and structural phase of the ceramics will be characterized using diffraction techniques, electron microscopy and chemical analysis tools. In-situ XRD and DSC will also be performed to closely monitor the onset of crystallization. The samples in the final phase of the project will be tested for their leaching (corrosion) and radiation tolerant properties compared against the presently used CaZn BSG.