A First-Principles Density Function Theory Study of Tritium Diffusion in Li₂ZrO₃: Application for Producing Tritium

Abstract

The ceramic material Li2ZrO3 has superior thermo-physical and thermo-chemical properties and is highly compatible with other blanket materials used in nuclear reactors. Like LiAlO2, it could be used in the form of an annular pellet in tritium-producing burnable absorber rods (TPBARs) to produce tritium­(3H) upon thermal neutron irradiation of lithium isotope (6Li). The radiation damaged pellet crystal contains vacancies, defects of its constituent elements, and several other trapping sites which hinder the 3H diffusion process and releasing behavior. In this study, we investigate the diffusion mechanisms of 3H and O3H species in Li2ZrO3 ceramic pellet in order to understand the effects on diffusion barriers and diffusion coefficients due to the presence of interstitial and substitutional Li defects, hydroxide (O–3H) vacancy defect, and of the interactions of 3H with O-vacancies in the radiation damaged Li2ZrO3 crystal. We consider several possible diffusion pathways of interstitial and substitutional 3H and its species in a defective supercell and calculate the activation energy barrier profiles. We find that the smallest activation energy barrier is 0.3 eV and corresponding diffusion coefficient at 600 K is 1.93 × 10–9 m2/s for 3H substitutional diffusion. The smallest 3H interstitial diffusion barrier and diffusion coefficient are found to be 0.34 eV and 3.25 × 10–9 m2/s. By examining several channels for diffusion, our results show the most likely diffusion mechanism of 3H migration is occurring along the c-direction by exchange of 3H within and between different Li sites. Our calculated results reveal that the smallest energy barrier for O3H diffusion is 0.75 eV which is when O3H is diffusing to the O–Li vacancy pair and the corresponding diffusion coefficient is 6.31 × 10–13 m2/s. In terms of 3H diffusion, the obtained results indicate that the performance of Li2ZrO3 could be better than γ-LiAlO2, a widely used ceramic material in TPBAR for producing 3H.

First-Principles Study of Tritium Diffusion in the Li3TaO4 Crystal

“…The diffusion coefficient is further investigated to reveal the tritium-diffusion behavior in the Li 3 TaO 4 crystal. According to the previous study of the γ-LiAlO 2 crystal, the diffusion coefficient D can be written as follows , where l is the migration distance, namely the length of the diffusion path; R 0 is the attempt frequency; Z = 6 is the coordination number; E α is the activation energy barrier; K B is the Boltzmann constant; and T is the temperature. According to the theory of Wert and Zener, R 0 can be written as , and the diffusion coefficient can be written as To confirm the accuracy of this equation, the diffusion coefficients in the previous study have been calculated, and the results are consistent with it in the order of magnitude.…”

“…The tritium diffusion coefficients at 600 K are approximately 10 −9 , 10 −12 , and 10 −10 m 2 /s in the above three types of crystals correspondingly. 23,24 In the previous experimental study, the observed order of tritium diffusion coefficients in different lithium ceramic materials was D (Li 8 ZrO 6 ) ≈ D (Li 4 SiO 4 ) > D (Li 2 O) > D (Li 2 ZrO 3 ) > D (Li 2 SiO 3 ) > D (γ-LiAlO 2 ). 39 Thus, from this point of view, we believe that Li 3 TaO 4 could act as a potential tritium breeder, and the diffusion energy barrier of tritium in the Li 3 TaO 4 crystal is not the major factor for preventing tritium release.…”

“…Besides, we compare our results with those of other lithium ceramic materials. In the latest theoretical study, 23,24 the smallest activation energy barrier inside the Li vacancy is 0.3 eV for Li 2 ZrO 3 and 0.63 eV for γ-LiAlO 2 , while our result is 0.39 eV in a type III Li vacancy of Li 3 TaO 4 . The tritium diffusion coefficients at 600 K are approximately 10 −9 , 10 −12 , and 10 −10 m 2 /s in the above three types of crystals correspondingly.…”

“…The diffusion coefficient is further investigated to reveal the tritium-diffusion behavior in the Li 3 TaO 4 crystal. According to the previous study of the γ-LiAlO 2 crystal, the diffusion coefficient D can be written as follows 23,24…”

“…Besides, the tritium-release behavior is affected by various irradiation-induced defects, especially the lithium vacancy, because lithium atoms are constantly knocked out of their positions to generate tritium under irradiation. Several tritium-diffusion behaviors have been studied for Li 4 SiO 4 , Li 2 TiO 3 , Li 2 ZrO 3 , and LiAlO 2 crystals, and the activation energies predicted by related experiments and theoretical works normally range from 0.59 to 1.55 eV. − However, there is no reported theoretical and experimental work on the tritium-diffusion properties of the Li 3 TaO 4 crystal so far.…”