Simulation calculation of the influence of interstitial atoms on the desorption behavior of tritium in nuclear graphite

Abstract

In order to explore the effect of interstitial atoms on tritium desorption behavior of nuclear graphite, the adsorption and desorption behaviors of hydrogen atoms on interstitial atoms were simulated by first-principles. The results indicated that the interstitial atoms can gradually adsorb 3 hydrogen atoms until the sp3 hybrid structure is formed. The CH3 structure with dangling bonds tends to desorb as ionic CH3, which will combine with free H or other CH3 to form CH4 or C2H6. The CH (or CH2) structure tends to polymerize with other CH (or CH2) structures to form C2H2 (or C2H4), which reaction energy barrier is slightly lower than that of the CH3 structure; Although the migration energy of the interstitial atoms with H adsorbed on the graphite flakes is not much different from that of the interstitial atoms without H, the energy barrier of their recombination with single vacancies is significantly greater than that of the interstitial atoms without H. Therefore, it can predict that the large number of interstitial atoms in decommissioned nuclear graphite will become important sites for chemisorption of tritium. In addition, the reaction energy barrier of the interstitial atoms possessed large adsorption energy with tritium gas desorption is significantly higher than that with carbon containing tritium products (including C2T2, C2T4, CT4 or C2T6). This work will provide technical guidance and support for the heat treatment of the adsorbed tritium in decommissioned nuclear graphite.