  The production of energy through nuclear fusion poses serious challenges
related to the stability and performance of materials in extreme conditions. In
particular, the constant bombardment of the walls of the reactor with high
doses of He ions is known to lead to deleterous changes in their
microstructures. These changes follow from the aggregation of He into bubbles
that can grow and blister, potentially leading to the contamination of the
plasma, or to the degradation of their mechanical properties. We
computationally study the behavior of small clusters of He atoms in W in
conditions relevant to fusion energy production. Using a wide range of
techniques, we investigate the thermodynamics of the clusters and their
kinetics in terms of diffusivity, growth, and breakup, as well as mutation into
nano-bubbles. Our study provides the essential ingredients to model the early
stages of He exposure leading up to the nucleation of He bubbles.
