Simulation calculation of the migration and combination of interstitial atoms in nuclear graphite

BackgroundThe interstitial atoms in nuclear graphite of the reactor will lead to the change of the properties and structure of the nuclear graphite. However, the migration and configuration of interstitial atoms in nuclear graphite are not well understood.PurposeThis study aims to clarify the migration and combination mechanism of interstitial atoms in nuclear graphite by First-principles calculation.MethodIn this research, the Vienna Ab-initio Simulation Package was used to calculate the properties of the system. First-principles calculation was used to study the interaction between interstitial atoms and the migration and combination behavior of interstitial atoms. The diffusion energy barrier was estimated by the climbing image nudge elastic band (CI-NEB) method.ResultsThere is a gravitational force between the interstitial atoms on the same graphite sheet, but it can be negligible while the distance between two interstitial atoms is greater than 0.5 nm. The interstitial atoms tend to migrate and combine along the armchair direction. The combination of two interstitial atoms reduces the structural energy by about 6.0 eV. The energy barrier of the entire migration and combination is about 0.4 eV during the migration process.ConclusionThe most stable cluster structure of interstitial atoms in graphite is the chain-like structure, and the secondary is the spiro structure of the closely adjacent interstitial atoms. When the number of interstitial atoms is more than 6, the structure of interstitial atoms exists as a cluster containing six-membered carbon rings and stacked with the bottom surface vertically into an AB configuration. If more interstitial atoms are combined, a new graphite sheet will eventually be formed.