(305c) Stability and Interactions of Point Defects in Lithium Metal Oxides for the Tritium-Producing Burnable Absorber Rod Applications

Point Defect
Energetics in Lithium Metal Oxides for Tritium-Producing Burnable Absorber Rod
Applications

Yueh-Lin
Lee¹, Jamie Holber¹, Hari P.
Paudel¹, Dan C. Sorescu¹, and Yuhua
Duan¹

¹National
Energy Technology Laboratory, 626 Cochrans Mill Road,
P.O. Box 10940, Pittsburgh, Pennsylvania 15236-0940, USA

Ceramics,
such as γ-LiAlO₂, Li₂TiO₃, and Li₂ZrO₃,
are candidate systems for their suitability as blanket materials in
tritium-producing burnable absorber rods (TPBARs). After adsorbing neutrons, ⁶Li
is readily converted to tritium through
reaction [1]. The oxide defects
and structural disorder may enhance the diffusion of Li and tritium species
within TPBAR materials. Previous experimental results have indicated that
interactions between tritium and irradiation-induced defects could play a key
role in the observed tritium solubility and diffusivity and overall can modify
the tritium release from TPBAR materials [2,3].
Furthermore, previous theoretical studies in bulk Li₂O and Li₂TiO₃
[4-6] have suggested that both Li and O vacancies can act as trapping sites for
tritium species.

In
this work, the phase stability and defects energetics of Li_xM_yO_z
(M=Al, Ti, and Zr) materials are investigated using density functional theory
(DFT) calculations and ab initio thermodynamic
analysis. Theoretical investigations were performed for several types of
charged defects including vacancies, interstitials, antisite defects, and ³H
substitutional defects to identify the trends of the point defects stability
under TPBAR operating temperature and oxygen partial pressure conditions [7,8]. The interplay between defects stability, defects
interactions, the change of the environmental chemical potentials, and the
intrinsic Fermi energy level under the TPBAR operating conditions are
discussed. As shown in the Figure 1 below, our results suggest that
interactions between the tritium and the point defects are dependent on the
electron chemical potential across the band gap, i.e. the Fermi energy
level. By considering the intrinsic Fermi level position for a range of
external conditions, i.e. P(O₂)=10^-5atm at T=1000K to P(O₂)=10^-25atm at T=1000K, the results obtained suggest that
γ-LiAlO₂ phase exhibits stronger interactions between the ³H
interstitial and the V_Li, V_O,
and O_int defects than Li₂TiO₃
and Li₂ZrO₃ materials, indicating a higher tendency to
trap ³H at these point defects [7] and correspondingly a lower
efficiency for tritium recovery in TPBARs.

Among
the three TPBARmaterials investigated, i.e., Li₂TiO₃,
Li₂ZrO₃, and γ-LiAlO₂, our results
indicate that the point defect formation energies are higher
in γ-LiAlO₂ than in the other
two materials. The increased defect stability in γ-LiAlO₂ correlates also with the experimental
tritium/hydrogen diffusivities which are the slowest in this material. At the
same time, the higher point defect formation energies observed in γ-LiAlO₂
indicate a greater stability of this material with the possibility to better withstand the long-term irradiation at high
temperatures in a reactor. Our theoretical results provide thermodynamic
guidance for the factors governing the tritium transport properties [9], point
defect energetics and the phase stability in Li_xM_yO_z
(M=Al, Ti, and Zr) systems under TPBAR operating chemical potential conditions
[10].

References:

[1] Senor,
D.J., Recommendations for Science and Technology in Support of the Tritium
Sustainment Program, PNNL-27216. 2017, PNNL

[2] Moriyama,
H.; Tanaka, S.; Noda, K., Irradiation effects in ceramic breeder materials.
Journal of Nuclear Materials, 1998. 258(Part 1):
p. 587-594.

[3]
Kobayashi, M., et al., Dependency of irradiation damage density on tritium
migration behaviors in Li2TiO3. Journal of Nuclear
Materials, 2014. 447(1): p. 1-8.

[4]
Shah, R.; Devita, A.; Payne, M.C., Ab-Initio Study of Tritium Defects in Lithium-Oxide.
Journal of Physics-Condensed Matter, 1995. 7(35):
p. 6981-6992.

[5]
Murphy, S.T., Tritium Solubility in Li₂TiO₃ from
First-Principles Simulations. The Journal of Physical
Chemistry C, 2014. 118(51): p. 29525-29532.

[6]
Tanigawa, H.; Tanaka, S., Ab-initio
study on interaction of hydrogen isotopes with charged defects in lithium
oxide. Journal of Nuclear Materials, 2002. 307-311:
p. 1446-1450.

[7] Lee, Y.-L; Holber, J.; Paudel, H. P..; Sorescu, D. C.;
Sensor, D.; Duan, Y., “Density Functional Theory
Study of the Point Defect Properties for the γ-LiAlO₂, Li₂ZrO₃,
and Li₂TiO₃ Materials”, (2018) to be submitted.

[8] Broberg, D., et al., PyCDT: A
Python toolkit for modeling point defects in semiconductors and insulators. Computer Physics Communications, 2018. 226, p.
165-179

[9] Paudel, H. P., Lee, Y. -L., Senor, D. L., Duan, Y., “Tritium Diffusion Pathways in γ-LiAlO₂
Pellets Used in TPBAR: a first-principles density functional theory
investigation”, Journal of Physical Chemistry C (2018) revision submitted.

[10] Paudel, H. P., Lee, Y.-L., Holber,
J., Sorescu, D. C., Duan,
Y., “Fundamental Studies of Tritium Solubility and Diffusivity in LiAlO₂
and Lithium Zirconates Pellets Used in TPBAR”,
Tritium Science Program FY17 Report, DOE/NETL-PUB-21464, Nov. 2017