Spatially dependent kinetics of helium in tungsten under fusion conditions

Tungsten is the prime candidate material for divertor applications in future nuclear reactors (e.g. ITER and DEMO). In the present work, a spatially dependent cluster dynamics model is developed to investigate and understand the microstructure evolution of tungsten under low energy helium implantation and neutron irradiation varying over bulk length scales of millimetres and irradiation time scales of hours. The diffusion of helium, helium clusters and their trapping at neutron induced defects is simulated along the tungsten monoblock depth. The temperature gradient resulting from a steady state heat load of 10 MWm-2 along the monoblock depth is considered and its influence on the evolution of defects is discussed. The trapping of helium at vacancies and the associated formation of helium-vacancy clusters is found to be pronounced in the sub-surface layers. A significant influence of helium detrapping from grain boundaries and dislocations, along with its resolution from clusters, on the helium diffusion length scales is observed. Additionally, the effect of helium cluster mobility is investigated and overall lower retention in the monoblock bulk is observed through significant release of helium at the surface.