Abstract
Graphite is used as a moderator in advanced gas cooled nuclear reactors (AGRs) across the country. The graphite is damaged over time due to a bombardment of neutrons and this has a wide range of efects on the physical properties of the graphite. This thesis focuses of the dimensional change of the nuclear graphite. The first part of this thesis is based on two dimensional dislocation dynamics (2D-DD) and describes a program which has been written to model the movement of line defects in an anisotropic elastic continuum. Classical elasticity theory is applied to a single crystal containing dislocations to calculate the dimensional change of the crystal and the energy stored in the continuum by the dislocations. Several modes are described, beginning with the standard model of dimensional change which concerns point defect aggregation into prismatic loop dislocations. Extending these theories the program has been developed further to model the dimensional change as a result of gliding basal dislocations. This program was created as a proof of concept model to help show that significant energy can be stored in the lattice by dislocations as well as by the well established point defect energies. The second part of this thesis uses ab initio calculations to measure geometries and energies of line defects in bilayer graphene and graphite which can be used to quantify the models used in the two dimensional dislocation dynamics. Density functional theory (DFT) with a local density approximation (LDA) has been utilised as implemented by the AIMPRO package. These calculations use state of the art filtration methods to allow optimisations on many atom structures which were previously unattainable. The first DFT calculation of a basal dislocation dipole in bilayer graphene has been carried out, a structure which has previously only been optimised using classical molecular dynamics. The results of these ab initio calculations can be used to quantify the 2D-DD results.