Nuclear-grade　graphite　IG-110,　an　isotropic　fine-grained　solid　material,　is　widely　studied　for　its　applications　in　high-temperature　gas-cooled　reactors　(HTGRs).　Gas　diffusion　is　a　crucial　parameter　in　understanding　mass　transport　phenomena　in　nuclear　graphite　during　the　dehumidification　and　operational　processes　of　HTGRs.　Despite　the　importance　of　gas　diffusion　modeling,　limited　numerical　frameworks　have　been　developed　to　predict　diffusion　coefficients　within　the　microstructure　of　nuclear-grade　graphite.　In　this　study,　geometric　models　of　nuclear　graphite　were　obtained　using　X-ray　computed　tomography,　and　the　dimensionless　diffusivity　of　nitrogen　was　calculated　using　the　lattice　Boltzmann　method　(LBM)　and　electrical　conduction　simulations.　The　computational　model　was　validated　against　experimental　data,　showing　a　close　alignment　between　the　numerical　approach　and　the　experimental　results.　Additionally,　the　experiment　found　that　gas　diffusion　within　nuclear　graphite　logically　decreases　with　increasing　gas　pressure　and　remains　unaffected　by　confining　pressure.　These　theoretical　findings　are　useful　for　understanding　water　transport　in　nuclear　graphite　during　the　dehumidification　process.