Density functional theory simulations have been carried out to investigate the diffusion of interstitial tritium (T, 3H) in three Al-rich (&gt;60% at. Al) iron aluminide phases found in the Fe-Al coating of Tritium Producing Burnable Absorber Rod (TPBAR). The phases identified by STEM imaging were hexagonal FeNiAl5, monoclinic Fe4Al13, and orthorhombic Fe2Al5. An atomistic analysis of the structures of FeNiAl5 and Fe4Al13 indicates that these phases can be viewed as the stacking of two layers. The structural particularity of Fe2Al5, is that channels are present along the c-axis. These channels have a variable Al vacancy content, giving rise to an approximate composition of Fe2Al5. Therefore, three stoichiometries for Fe2Alx phase, namely Fe2Al4, Fe2Al5, and Fe2Al6, have been studied to evaluate the impact of Al vacancy concentration on tritium diffusion behavior. Simulations using the nudged elastic band method indicated that tritium has preferential diffusion pathways in FeNiAl5 and Fe4Al13. In FeNiAl5, tritium preferentially diffuses along the a-axis in between layers, while in Fe4Al13, tritium preferentially diffuses across layers, along the b-axis. In the case of Fe2Alx phases, it was found that the tritium diffusion pathway is affected by the Al vacancy concentrations in the channels. Based on the calculated energy barriers, the diffusion coefficients for tritium in Fe-Al phases have been determined.

Tritium diffusion in Fe-Al aluminide coating bulk phases

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