Cluster dynamics simulations of tritium and helium diffusion in lithium ceramics

Abstract

Tritium (T) and He diffusion in LiAlO2 and LiAl5O8 phases influences the performance of tritium producing burnable absorber rods (TPBARs) by affecting the gas release, swelling and thermal conductivity of Li-bearing ceramic pellets. Frenkel pair defects and clusters created by irradiation can attract T and He interstitials and form clusters of the type HeixLi,HeixAl,HeixO,TixLi,TixAlandTixO,1≤x≤4 in a Li, Al or O vacancy site (notation denotes x He or T atoms in a 1 Li, 1 Al or 1 O vacant site). The concentration and mobility of each of these clusters collectively contribute to the diffusion of the He and T gases in LiAlO2 and LiAl5O8. In this work, free energy cluster dynamics simulations implemented in the Centipede code, are used to obtain the concentration and diffusivities of these clusters which are then used to calculate the total diffusivity of T and He gases in LiAlO2 and LiAl5O8. The results show that diffusivity of T is at least one order of magnitude higher in LiAlO2 as compared to that in LiAl5O8 whereas He diffusion is 2–13 orders of magnitude higher in LiAlO2 as compared to that in LiAl5O8. There is a higher concentration of highly diffusive species (T interstitials and Ti03Li for the case of tritium and Hei01Li, Hei02Li and Hei03Li for the case of He) in LiAlO2 than in LiAl5O8 which increase the total diffusion of T and He in LiAlO2.