First-Principles Mechanistic Insights into Steam-Enhanced Tritium Desorption from Nuclear Graphite.

Carrier gas plays a crucial role in the thermal treatment method for removing tritium from irradiated nuclear graphite. Experiments studies have demonstrated that the addition of steam can enhances tritium removal from graphite, while the mechanism of the promotion remained unclear. In this study, based on density functional theory (DFT), we investigated the mechanisms of tritium removal from graphite using HO as the carrier gas, focusing on two primary processes: direct isotope exchange and adsorption-desorption methods. The activation energy for tritium removal from graphite via direct isotope exchange ranges from 3.18 to 5.10 eV, which is associated with the strength of the C-T bond in graphite. The adsorption-desorption mechanism involves the adsorption of HO in the form of hydroxyl group and hydrogen, followed by desorption of hydroxyl group together with tritium preadsorbed in graphite matrix. The activation energy for this process is dependent on the adsorption activation energy of HO, ranging from 2.01 to 3.52 eV. Following the adsorption-desorption method, tritium in nuclear graphite is primarily desorbed as tritiated water (HTO), and the activation energy was found to be significantly lower than that of direct isotope exchange. Therefore, the tritium adsorbed in nuclear graphite would be desorbed as HTO after combining with the hydroxyl group in steam. These findings study provide valuable theoretical guidance for the removal of tritium from irradiated nuclear graphite through thermal treatment methods.