Towards Understanding the Thermodynamic Properties of Magnesium Rich Minerals Using Atomistic Simulations

Abstract

The aim of this thesis is to use atomistic simulation to provide insight in to the structures and thermodynamic properties of magnesium rich mineral phases. The MgO-H2O-CO2 system has been chosen due to its relevance in the area of nuclear waste storage, where corroded Magnox cladding has formed a complex waste, mainly comprising of magnesium rich phases. Chapter 1 outlines previous experimental and computational research conducted, relevant to the identification and classification of Magnox nuclear sludge. A number of magnesium mineral phases have been identified that are likely to be present e.g. Brucite (Mg(OH)2), Magnesite (MgCO3), Hydromagnesite (Mg5(CO3)4(OH)2:4H2O). Chapter 2 describes the computational methodology and validation of the simulation parameters used within this work.The results are presented and discussed in Chapters 3 to 5, where Chapter 3 evaluates various DFT approaches for modelling magnesium phases and examines the effect of including van der Waals (vdW) forces using 3 different implementations.  Results show that the inclusion of vdW is vital to obtaining an accurate representation of the structural and energetic  properties of magnesium rich phases, particularly for the layered phases. Chapter 4 outlines a thermodynamic framework for producing high quality phase diagrams, where optB86b-vdW and optB88-vdW functionals compare well with the experimental phase diagram. All functionals produce similar phases diagrams, where each vdW correction has a constant shift in the pressures from experiment, potentially allowing for a correction factor to be determined. Chapter 5 presents the surface calculations and the interactions of Brucite surfaces with H2O/CO2 and radio-nuclei (90Sr). It can be concluded that {100} surfaces of Brucite displays evidence of being more reactive than {001} surfaces. The stability of strontium defects is heavily dependent on the coordination that the surface ions can achieve. Additionally, 90Sr only shows favourable adsorption on to the {001} surface in the presence of CO2 is also present. Finally, an overview of the conclusions and possible areas for future research can be found in Chapter 6.

Date of Award	28 Feb 2018
Original language	English
Awarding Institution	University of Bath
Supervisor	Steve Parker (Supervisor) & Marco Molinari (Supervisor)