The　interfacial　interaction　of　uranium　mononitride　(UN)　with　water　from　the　environment　unavoidably　leads　to　corrosion　of　nuclear　fuels,　which　affects　a　lot　of　processes　in　the　nuclear　fuel　cycle.　In　this　work,　the　microscopic　adsorption　behaviors　of　water　on　the　UN(001)　surface　as　well　as　water　dissociation　and　accompanying　H-2　formation　mechanisms　have　been　investigated　on　the　basis　of　DFT+U　calculations　and　ab　initio　atomistic　thermodynamics.　For　adsorption　of　one　H2O　monomer,　the　predicted　adsorption　energies　are　-0.88,　-2.07,　and　-2.07　eV　for　the　most　stable　molecular,　partially　dissociative,　and　completely　dissociative　adsorption,　respectively.　According　to　our　calculations,　a　water　molecule　dissociates　into　OH　and　H　species　via　three　pathways　with　small　energy　barriers　of　0.78,　0.72,　and　0.85　eV,　respectively.　With　the　aid　of　the　neighboring　H　atom,　H-2　formation　through　the　reaction　of　H*　+　OH*　can　easily　occur　via　two　pathways　with　energy　barriers　of　0.61　and　0.36　eV,　respectively.　The　molecular　adsorption　of　water　shows　a　slight　coverage　dependence　on　the　surface　while　this　dependence　becomes　obvious　for　partially　dissociative　adsorption　as　the　water　coverage　increases　from　1/4　to　1　ML.　In　addition,　based　on　the　＂ab　initio　atomistic　thermodynamic＇＇　simulations,　increasing　H2O　partial　pressure　will　enhance　the　stability　of　the　adsorbed　system　and　water　coverage,　while　increasing　temperature　will　decrease　the　H2O　coverage.　We　found　that　the　UN(001)　surface　reacts　easily　with　H2O　at　room　temperature,　leading　to　dissolution　and　corrosion　of　the　UN　fuel　materials.