Simulations of Glass and Ceramic Systems for Nuclear Waste Applications
Glass and ceramic materials have been used for the immobilisation of nuclear wastes. A greater understanding of these materials and the way in which they interact will lead to better wasteform design and performance. For this reason, atomic scale simulations were performed on glass, ceramic and glass-ceramic systems relevant to nuclear waste applications.
Here molecular dynamics techniques are used to form models of vitreous silica, alkali silicate, alkali borosilicate and alkali aluminoborosilicate glasses using a melt-quench procedure. The interactions occurring at interfaces between these glass compositions and (100) and (110) surfaces of ceramics exhibiting the rock-salt crystal structure are considered. Three different methods for interface formation are considered and their relative merits explored.
In particular, the changes in the modifier and network structure of glasses, encountered at glass-ceramic interfaces were investigated. Considerable modifier enhancement at interfaces was observed. In addition, considerable differences in the modifier distribution were found between (100) and (110) interfaces. Interfacial energies were calculated for each interface; of the interfaces considered it was found that (110) interfaces consistently gave lower energies than comparable (100) interfaces. A borosilicate waste glass composition was compared to a similar glass which had small additions of aluminium and magnesium. In general, interfacial energies for this aluminoborosilicate glass composition were found to be lower than those corresponding to the borosilicate glass.
Partial ordering of the glass network adjacent to the glass-ceramic interfaces was observed. Borate and silicate polyhedra in the glass close to glass-ceramic interfaces were found to adopt preferred orientations based on the structure of the ceramic surface. At interfaces with the (100) surfaces of CaO, SrO and BaO, glass anions were found to sit over interstitial sites. By comparison, at interfaces with the (100) surface of MgO, glass anions were associated with cations in the ceramic surface.
The presence of the ceramic interface was found to cause layering of the glass network. Regularly spaced boron/silicon and oxygen rich layers were found parallel to the plane of all glass ceramic interfaces except that formed between SiO 2 and the (100) surface of MgO. The lack of layering in this system was attributed to the absence of network modifiers in this system.
Rare earth pyrochlore materials have been proposed as host materials for nuclear waste applications. A systematic series of calculations on A 3+2 B 4+2 O 7 pyrochlores were performed. In particular the results of static energy minimisation and molecular dynamics simulations were used to explore transformations occurring between pyrochlore, defect fluorite and amorphous states. Predictions were made for the volume changes experienced as a consequence of these transformations. It was found that volume changes were lowest for compositions close to the pyrochlore-defect fluorite phase boundary. Finally, pyrochlore to defect fluorite transformation temperatures were predicted for rare-earth zirconate pyrochlores.