Electron microscopy and multi-scale modelling of radiation damage recovery in tungsten

• Main Author: Ferroni, Francesco
• Other Authors: Roberts, Steve ; Fitzgerald, Steven ; Tarleton, Edmund
• Published: University of Oxford 2016
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.728740

The thesis is concerned with understanding the recovery mechanisms of radiation damage in tungsten. Tungsten is a key material in next-generation fusion reactors such as ITER, which will be exposed to intense neutron radiation, plasma and high temperatures, and its durability will determine the reactor's competitiveness versus other energy sources. The thesis makes a comprehensive, quantitative study of recovery and its depen- dence on temperature, time, and material purity. It employs both experiments (ion-beam irradiation with ex and in situ analysis via transmission electron mi- croscopy) and multi-scale deterministic and stochastic modelling techniques such as molecular dynamics and dislocation dynamics. Annealing experiments on ultra-pure tungsten reveal the presence of several re- covery stages, and an acceleration of dislocation defect recovery and loop coarsening above ~900°C. They also reveal discrepancies with dislocation climb models in the literature, suggesting the presence of additional recovery pathways leading to coars- ening, most notably conservative self-climb. Presence of helium is found to cause an increase in the equilibrium concentration of defects at a given temperature. Also, very high doses cause morphological changes to the radiation damage structure, from relatively homogeneous defect concentra- tions of dislocations in a narrow size range, to complex dislocations networks with voids. Finally, modelling techniques are able to successfully predict dislocation motion phenomena observed in experiments at high temperature (including quantitatively accurate defect densities), but only after extensive modification of publicly available dislocation dynamics codes to include boundary conditions and stochastic behaviour of crystal defects.