A MULTISCALE MODELING FRAMEWORK TO PREDICT TRITIUM TRANSPORT IN  IRRADIATED γ-LITHIUM ALUMINATE

Mark Lanza

Gamma-phase lithium aluminate (γ-LiAlO₂) is an essential ceramic breeder material for applications such as Tritium-Producing Burnable Absorber Rods (TPBARs), where its performance is governed by tritium transport. Current engineering models rely on empirical correlations, which lack the predictive, mechanistic foundation necessary to account for variations in microstructure, irradiation conditions, and defects. Limited understanding of tritium diffusion pathways, trapping mechanisms, and chemistry in irradiated γ-LiAlO₂ hinders the accurate prediction of diffusivity as a function of radiation damage, composition, impurities, and microstructural diversity. This research addresses these challenges by establishing and validating a physics-based, multiscale modeling framework for tritium transport in irradiated γ-LiAlO₂.The developed framework bridges atomistic-level phenomena and engineering-scale transport behavior to address key modeling gaps. Density Functional Theory (DFT)-computed data provides parameters for hopping energetics and trapping mechanisms, while graph-theoretical Kinetic Monte Carlo (KMC) simulations are used to compute intrinsic diffusivity in defect-free γ-LiAlO₂. To capture the effects of radiation-induced defects, the framework incorporates a semi-analytical model based on the Grand Canonical Ensemble (GCE) and Fermi-Dirac statistics to predict effective diffusivity considering multi-occupancy trapping at lithium vacancies (VLi). The findings reveal distinct behaviors in tritium transport. KMC simulations indicate that tritium in pristine γ-LiAlO₂ exhibits rapid diffusion via interstitial hopping, with a low activation energy of approximately 0.02 eV. However, parameterization of the GCE model with DFT data reveals the complex binding behavior of lithium vacancies (VLi). When benchmarked against the TMIST-3A dataset, the diffusion-only model significantly overestimates tritium release. In contrast, the trapping-inclusive framework demonstrates that lithium vacancies act as near-perfect sinks for tritium, mechanistically explaining the experimentally observed levels of tritium retention. This analysis establishes that irradiation-induced defect trapping, and not intrinsic diffusivity, is the dominant rate-limiting mechanism governing tritium transport in γ-LiAlO₂. The multiscale framework developed in this work establishes a physics-based foundation for understanding tritium transport in γ-LiAlO₂ as a defect-driven, kinetically-limited process. By transitioning tritium transport modeling from empirical approaches to mechanistically-grounded science, this framework enables a deeper understanding of tritium breeding materials and provides a critical basis for improving the design and optimization of ceramic breeder systems in nuclear applications.

A MULTISCALE MODELING FRAMEWORK TO PREDICT TRITIUM TRANSPORT IN IRRADIATED γ-LITHIUM ALUMINATE

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