Mechanistic Study of Tritium Retention and Permeation in Hydroxyl-Functionalized Nuclear Graphite for Thorium Molten Salt Reactors.

In thorium molten salt reactors (TMSRs), nuclear graphite is subjected to direct contact with high-temperature molten salts and intense neutron irradiation, leading to oxidation and structural evolution that critically influence tritium retention and release. Motivated by oxidation processes in TMSR environments, this work employs first-principles calculations based on hydroxyl (-OH)-functionalized graphite edge models to systematically investigate the effects of edge functionalization on hydrogen/tritium adsorption, migration, and release. The results indicate that, compared with pristine graphite edges, all edge configurations exhibit markedly enhanced hydrogen adsorption, while hydroxyl functionalization induces pronounced spatial heterogeneity in adsorption strength. Strongly bound sites act as effective hydrogen trapping centers, whereas adjacent weak-binding sites facilitate surface migration. Hydrogen diffusion along graphite edges generally proceeds with moderate energy barriers (∼1.0-2.0 eV), and the overall reaction kinetics are governed by the final desorption step. Regarding hydrogen desorption, the H2 desorption barriers at AC edges are markedly lower than those for H2O, whereas at ZZ edges, H2 is the predominant desorption product. Reconstructed edges generally display higher migration and desorption barriers, indicating stronger hydrogen confinement. Electronic structure analysis shows pronounced charge accumulation around C-H and O-H bonds upon adsorption, and the introduction of hydroxyl groups induces the formation of intermediates such as CH-(OH), significantly altering hydrogen migration pathways and desorption kinetics. This study reveals the synergistic effects of hydroxyl functionalization and edge reconstruction on hydrogen/tritium behavior at graphite edges, providing qualitative atomic-scale insights from simplified hydroxyl-functionalized edge models into tritium retention and release, with the understanding that molten salt interactions, temperature effects, and irradiation evolution are not explicitly included.