Atomic scale mobility of the volatile fission products Xe, Kr and I in cubic SiC

Abstract

The migration barriers for the vacancy-assisted migration of fission products in 3C-SiC are reported and analysed in the context of the five frequency model, which enables one to calculate an effective diffusion coefficient from elementary mechanisms. Calculations were carried out using the nudged elastic band method (NEB) with interatomic forces determined from density functional theory (DFT). Justification for treating vacancy-assisted fission product migration as limited to the FCC carbon sublattice is based on the stability of carbon vacancies, unfavourable silicon vacancy formation and the accommodation of fission products on the carbon sublattice. Results show that for most Fermi levels within the band gap the activation energy for I exceeds that of Xe which exceeds that of Kr. Results also indicate that activation energies are higher near the conduction edge, thus, implying that enhanced fission product retention can be achieved through n-type doping of 3C-SiC, which limits the availability of the migration mediating carbon vacancies.

Atomistic and mesoscale simulations to determine effective diffusion coefficient of fission products in SiC

“…The work also reasons that the diffusion paths causing the formation of anti-site defects are energetically unfavorable, and thus their contributions to Ag diffusion can be considered negligible. This multifrequency approach is similar to the previous study by Cooper et al [14] using a simplified five-frequency model. In this work we identified the symmetrically nonequivalent diffusion paths in a more rigorous manner and further examined the hops associated with the following Ag-vacancy pairs: Ag Si -Va Si , Ag C -Va C , Ag Si -Va C , and Ag C -Va Si .…”

“…For Ag C -Va C and Ag Si -Va Si pairs, the migration barriers of the type-I rotation hop are both higher than the type-II rotation hop. This indicates that specific pathways are preferred for Ag-vacancy pairs to move together, and the correlation factor of Ag diffusion will differ from the analytical expression compared to the five-frequency model applied in the study [14].…”

Nonequilibrium ab initio molecular dynamics determination of Ti monovacancy migration rates in B1 TiN

“…II B]. Being often point-defect migration in crystalline solids a simple process involving a single saddle-point in energy along the reaction coordinate [98][99][100][101], Eq. (3) is based on the assumption that the potential energy landscape along the migration path has a sinusoidal-like shape (see supplemental material in Ref.…”