Development and application of topological methods to characterise radiation damage effects in borosilicate and iron phosphate glasses

The immobilization of high level nuclear waste is a very important aspect of the nuclear industry. In general, it is necessary to stabilise high level nuclear waste into a form that will retain its integrity for extended periods of time. The resulting wasteforms must be able to retain their durability and integrity for the timescale for which the incorporated radioactive elements emit radiation. This time scale extends to thousands of years and it is not possible to establish the long-term reliability of the new wasteforms only by experimental methods. Computational simulations of the wasteforms have the ability to provide detailed information regarding the structural changes in the wasteform due to the creation of an radiation damage at short timescales that can be used along with experimental approaches to predict the long term behaviour of the wasteforms. Traditional methods used to analyse radiation damage effects in computer models of glass wasteforms are based on the Wigner-Seitz method which ignores the properties of specific bonds and the number of broken bonds associated with the displacement of a particle from its initial position. Thus, it is necessary to develop novel computational methods to characterise the radiation damage effects with increased accuracy. Work presented in this thesis, outlines the development of new topological based approaches to the characterisation of radiation damage effects in computer models of recoil damaged borosilicate and iron phosphate glasses. This method utilises a modified set of the well known Steinhardt order parameters and introduces a new set of distance-dependent order parameters, referred to as Hermite order parameters. The methods were developed using zircon crystal as a test structure, to establish the accuracy of the new approach, and then applied to the irradiated borosilicate and iron phosphate glass models to determine the behaviour of the glasses under irradiation. Additional structural analysis of the simulated structures was performed using primitive ring statistics. The results of the analysis show that one of the topological methods proposed in this work succeeds in providing new insights regarding the effects of radiation damage in terms of bond defects. The simulated structures show significant tolerance to irradiation. For the borosilicate glass models, the Steinhardt and Hermite order parameters based methods suggest that the silica network is almost completely recovered, in contrast with the predictions of the Wigner-Seitz method, according to which a significant number of silicon particles are permanently damaged. Additionally it is found that the majority of the damage is due to broken B-O bonds. For the iron phosphate glasses the topological analysis suggest that only a small percentage of P-O bonds are affected by the creation of the damage cascade. Using the Steinhardt order parameters method it is also revealed that a radiation damage event affects the geometry of the \ce{SiO4} and \ce{PO4} tetrahedra in the borosilicae and iron phosphate glass models respectively, by creating variations in the values of O-Si-O, O-P-O angles and Si-O, P-O bond lengths. Primitive ring statistics analysis in the borosilicate glass models reveal that the creation of a radiation damage cascade favours the formation of higher order primitive rings. However, the distribution of the ring sizes in the recovered structures is very close to the one of the undamaged glass models, suggesting a strong recovery of the network of the glasses. An attempt to perform a primitive ring statistics analysis in the iron phosphate glasses failed to provide any results, as no primitive rings were detected in line with existing models for the structure of phosphate glasses.