Multi-scale microstructure evolution of tungsten under neutron and plasma loads

Abstract

Tungsten (W) owing to its excellent high temperature properties, is the candidate material for plasma facing components in fusion reactors such as ITER and DEMO. However, the lifetime of the tungsten based plasma facing component and thereby the lifetime of the reactor, is dictated by the extreme particle (neutron and ions) and heat loads, and is not very well understood. The fast neutrons result in the generation of point and clustered lattice defects, which further interact with the plasma based helium ions, leading to nucleation and growth of helium bubbles. Additionally, these interactions in combination with high temperatures influence the microstructural evolutionby grain growth and recrystallization process, ultimately affecting the mechanical and thermal properties. Thus, an in-depth understanding on the role of helium ions in conjunction with heat and neutron loads is crucial for predicting the microstructure evolution under fusion conditions accurately.In the present work, a multi-scale model describing the simultaneous effect of the defect generation by neutron irradiation and helium implantation from the plasma, considering irradiation time scales of hours and component length scales is developed. At atomic length scales, the generation of defects such as the vacancies, self-interstitial atoms, their clustering and the trapping of helium at defects and their clusters are modelledusing a kinetic rate theory approach. Additionally, these microstructurallevel interactions are linked to mesoscopic length scales by considering the diffusion of mobile defects along the tungsten monoblock depth (ITER specifications). The spatially varying defect concentrations from the model are also used to obtain a measure of the spatially varying lattice stored energy, thereby allowing to link the effect of helium with mechanisms such as recrystallization and grain growth. The influence of the helium resolution from existing bubbles and microstructural sinks on the helium diffusionlength scales is investigated. Furthermore, the effect of helium cluster mobility on the overall helium retention in tungsten is found to be less-pronounced