Atomistic simulations of materials for nuclear fusion
Creators: name: Jackson, Matthew Lee
Publisher: name: Imperial College London
Year: 2017

description: Nuclear fusion has held the promise of unlimited clean energy for over fty years. However, owing to the technical challenges of achieving a sustained reaction, this promise remains unrealised. Chief among these challenges is the survivability of reactor materials, which are subject to extreme temperatures and flux of fast neutrons. To aid in understanding damage processes, atomistic simulations have been employed to model the fundamental processes of radiation damage, with some models validated by comparison to inelastic neutron scattering results. Beryllium rich beryllides, in particular the Be12M materials (where M is a transition metal), are under consideration for neutron multiplying applications in fusion reactors, however the basic properties of some of these materials remain poorly characterised. Herein, DFT simulations have been used to clarify the structure of Be12Ti, which was previously in contention. Further, several basic properties of Be12Ti have been predicted, including the thermal expansion, bulk modulus, elastic constants and lattice parameters. The phonon density of states of Be12M (M=Ti/V/Mo/Ta/Nb) and Be13Zr have been predicted, with trends observed based on the mass of the M species. Inelastic neutron scattering has also been performed, and results compared with the simulated phonon density of states. The experimental results were significantly broadened, making analysis difficult. It was found that signal at low energies is attributed to second order reflections, and has better energy resolution than the fi rst order data. When simulated results are artificially broadened, they bear strong qualitative resemblance to experimental results for all materials. Point defects including vacancies, interstitials and antisite defects were investigated in Be12M materials (M=Ti,V,Mo,W) using DFT, with interstitial sites identified for the first time. Beryllium defects are consistently more favourable than transition metal defects. Schottky disorder is the lowest energy intrinsic disorder process in all materials, although beryllium Frenkel is comparable for Be12Ti and Be12V. Small defect clusters were also investigated. Several VBeVBe, VBeVM and MBeBe clusters are stable with respect to the isolated species, although their energies are highly orientation dependent. BeiBei formation is almost always unfavourable, and VMVM is always unfavourable. Non-stochiometry is extremely limited, to the extent that these intermetallics may be considered line compounds. Migration is predicted to be dominated by VBe mediated processes and to be weakly anisotropic. Low energy displacement simulations using empirical potentials were performed for beryllium, tungsten, carbon and tungsten carbide. Displacement was predicted to be strongly dependent on the potential used, as well as the local environment of the displaced species. For beryllium, tungsten and diamond, defect recovery is predicted to be important immediately following the displacement event at energies above the threshold displacement energy. The threshold displacement energy is a strong function of crystallographic direction for all materials. New models have been developed to predict the maximum displacement as a result of a displacement event.