(489d) Density Functional Theory Study of the Tritium Diffusion, Desorption, and Trapping on the ?-LiAlO2 (100) Surface

γ-LiAlO₂ enriched with the ⁶Li isotope is one of the important components of tritium-producing burnable absorber rods (TPBARs). This oxide is located between the zircaloy-4 liner and nickel-plated zircaloy-4 tritium getter. When irradiated in a pressurized water reactor (PWR), the ⁶Li pellets absorb neutrons, simulating the nuclear characteristics of a burnable absorber rod, and produce tritium ( ). The specie chemically reacts with the metal getter, leading to the formation of a metal hydride. Currently, the understanding of ³H transport through the ceramic pellets and the barrier/cladding system is hampered by the lack of fundamental data particularly related to hydrogen isotope solubility and diffusivity.¹ Recently, by employing first-principles density functional theory (DFT) calculations, we have investigated the energetics of the charged defects (including vacancies, interstitials, antisite defects, and ³H substitutional defects) of the γ-LiAlO₂ used in TPBARs and identified the trends in stability properties of various point defects under TPBAR operating temperature and O₂ partial pressure conditions, and the tritium diffusion pathways in γ-LiAlO₂ in the presence of interstitial and substitution AL Li defects, hydroxide (O-³H) vacancy defect, Al (or Zr) defect, and of the interactions of ³H with O-vacancies.^2-9

Once the is produced in the bulk of γ-LiAlO₂, it diffuses to its surface. It is possible that atoms bind with surface atoms or become trapped at vacancy sites. With increasing the number of diffused from bulk to surface, upon recombination and desorption from surface sites, they will escape from the γ-LiAlO₂ surface to form different species (possibly being T, OT, T₂, or T₂O, which need to be identified) and are captured by the getter. Hence, exploring the diffusion from bulk to surface, corroborated with surface adsorption, diffusion and recombination and followed by desorption from surface as various types of species becomes a very important task as an integral part of the solubility and diffusivity in the pellets. Based on DFT, in this study, we explored the diffusion, desorption, and trapping on the most stable γ-LiAlO₂ (100) surface. Firstly, we investigated the diffusion to the nearest neighbor (NN), 2^nd NN and 3^rd NN Li vacancy (V_Li) and the TO diffusion to a bound O and Li vacancy (V_LiO). The results showed that the energy barriers of T diffusion to the NN, 2^nd NN, and 3^rd NN V_Li (1.40, 2.05 and 2.93 eV) on (100) surface are larger than those of T diffusion in the bulk (0.63, 1.51, and 0.86 eV).² The energy barriers of TO diffusion on the surface to the NN, 2^nd NN, and 3^rd NN (1.95, 2.25 and 2.96 eV) V_Li are comparable with those of T diffusion on the same surface, but are much smaller than those of TO diffusion in the bulk (2.17, 4.23, and 3.44 eV).² Secondly, we considered these T/TO diffusion on the defective (100) surface under irradiation from MD simulation. By analogy with 2^nd NN and 3^rd NN V_Li on the corresponding perfect surface, the energy barriers for T and TO diffusion cross the hole to 2^nd NN and 3^rd NN V_Li are 4.74, 4.85 and 1.78, 2.28 eV respectively. Therefore, the order of energy barrier for substitution T diffusion in bulk and surfaces is: bulk < surface < defective surface, which is reversed for TO diffusion, because the space is an advantage for TO but is a disadvantage for T diffusion. Furthermore, we explored the diffusion from bulk to the (100) surface with/without V_Li. The obtained energy barrier for diffusion from bulk to (100) surface with/without V_Li is 1.37/1.59 eV, and for T to desorb from the surface is 4.36/3.10 eV with/without V_Li respectively. Thus, it is easy for to diffuse from bulk to (100) surface but is difficult for T to desorb from the surface. Finally, we considered the trapping in the surface V_Li. Our results indicated that one V_Li can trap two , and two V_Li can trap a maximum of three . More T could be adsorbed on the nearby Li/Al top site, and even combined into T₂ or T₂O molecule. The T₂ molecule can desorb from surface spontaneously while the T₂O desorption needs energy of 0.69 eV. Therefore, the T₂ and T₂O molecules are main products from (100) surface of γ-LiAlO₂. Since the T₂O is formed by two T with a surface O which only has one coordination associated with two V_Li, increasing V_Li can bring out more such O to produce more T₂O.

Reference:

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