We　have　investigated　the　low-index　(111),　(110),　and　(100)　surfaces　of　NpO2　as　well　as　the　adsorption　and　dissociation　behaviors　of　water　on　these　surfaces　by　using　DFT　+　U　method.　The　calculated　surface　energies　for　the　(111),　(110),　and　(100)　surfaces　are　0.81,　1.14,　and　1.67J　m(-2),　respectively.　Based　on　our　calculations,　the　presence　of　surface　oxygen　vacancy　enhances　the　dissociation　adsorption　of　water　molecule.　There　is　no　significant　coverage　dependence　for　molecular　adsorption　of　water　on　the　NpO2　(111),　(110),　and　(100)　surfaces.　The　average　adsorption　energy　of　water　in　dissociative　states　increases　slightly　from　1/4　to　2/4　ML　and　rises　significantly　with　the　coverage　increase　to　1　ML　on　these　surfaces.　In　addition,　we　have　analyzed　the　water　adsorption　on　the　NpO2　surfaces　for　different　temperatures　and　H2O　partial　pressures　and　have　plotted　the　pressure-temperature　phase　diagrams　by　using　the　＂ab　initio　atomistic　thermodynamics＂　approach.　(C)　2015　Elsevier　B.V.　All　rights　reserved.