Computational Study of Deuterium Diffusion in Boron PFM

In a thermonuclear fusion reactor, constant plasma bombardment can contaminate the plasma-facing material (PFM) of the reactor and subsequent diffusion of the contaminating particles may impact the PFM longevity or effectiveness. For certain choices of PFM, such as boron, the diffusion of contaminating particles has not been adequately studied experimentally or computationally. To bridge this gap, we performed molecular dynamics (MD) simulations of deuterium diffusion in crystalline alpha-rhombohedral boron based on an existing ReaxFF force-field for the involved species. Furthermore, we employed bond-boost hyperdynamics to accelerate the simulations, thereby sampling a substantially large number of instances of diffusion and extending the simulation timescale accordingly. It is observed that while the potential barrier separating two adjacent deuterium binding sites in boron is not excessively large, the geometry of the transition path connecting these sites creates constrictions that significantly reduce the rate of diffusion. Additionally, it is observed that diffusion only occurs along layers defined by the icosahedral structure of the boron crystal lattice. Along with a quantitative analysis of the diffusion rate, these observations help to assess the suitability of boron as a PFM.