The　pressure　relief　rate　and　loss　rate　of　coolant,　which　affect　largely　safety　of　fission　reactor　systems,　are　determined　by　the　flashing　process　inside　cracked　passages.　Information　about　flashing　phenomenon　is　of　great　significance　for　the　leakage　online　monitoring　system　of　SG　(Steam　Generator),　which　ensures　the　normal　operation　of　SG　and　safety　of　the　reactor　system　during　early　stage　of　tube　rupture　accidents.　In　this　research,　flashing　flow　inside　a　thin　micro-crack　in　heat　transfer　tube　of　SG　has　been　studied　using　ANSYS-FLUENT.　The　classic　cavitation　model　and　evaporation–condensation　model,　in　combination　with　both　the　mixture　two-phase　flow　model　and　Eulerian　two-fluid　model,　are　adopted　to　simulate　flashing　phenomenon.　The　real　geometry　and　operation　conditions　of　AP1000　nuclear　system　are　adopted　to　reflect　the　reality　leakage　process　in　SG.　Two　types　of　micro-cracks　including　axial　crack　and　circumferential　crack,　which　both　could　happen　in　the　reality　operations,　are　considered.　The　CFD　(Computational　Fluid　Dynamic)　results　gained　from　this　research　have　been　compared　with　test　data,　and　good　agreement　is　demonstrated.　The　results　show　that　the　evaporation–condensation　model　under　Eulerian　two-fluid　model　framework　behaves　superior　to　the　classic　cavitation　models　in　simulating　the　flashing　phenomenon　after　evaluating　accuracy　of　leakage　rate　and　rationality　of　thermal　hydraulic　parameters　distribution.　In　addition,　two-phase　choking　flow　phenomenon　is　obtained　by　decreasing　backpressure　of　cracked　tubes.　The　simulation　in　this　research　reveals　the　flashing　process　in　short　and　micro-crack　and　the　results　could　be　good　reference　for　leakage　prediction　of　SG　under　LBB　(Leak　Before　Break)　operations　to　improve　operation　performance　of　SG　and　safety　of　the　whole　nuclear　power　system.　©　2021　Elsevier　B.V.