The　Module　In-vessel　Degradation　severe　accident　Analysis　Code　(M1DAC)　is　developed　by　Xi＇an　Jiaotong　University　to　meet　the　domestic　demand　of　software　autonomous　scheduling.　The　whole　processes　of　in-vessel　severe　accident　can　be　analyzed　by　the　code　with　credible　results.　The　code　is　composed　of　five　main　modules:　the　core　early　behavior　module,　the　core　degradation　module,　the　debris　bed　module,　the　RPV　melt　behavior　module　and　the　thermal　hydraulic　module.　These　modules　are　combined　with　each　other　and　can　simulate　almost　all　the　severe　accident　phenomena　in　the　vesseL　The　core　early　behavior　module　contains　models　for　phenomena　as　the　core　heat　up　after　the　core　exposure,　the　cladding　oxidation　reaction,　the　cladding　ballooning,　the　fuel-cladding　interaction　and　other　related　phenomena.　Core　degradation　module　can　calculate　the　core　melt　and　relocation　process　in　the　core.　A　new　candling　model　is　developed　to　determine　the　behavior　of　melting　material　outside　the　fuel　rods,　inc　hiding　the　melt　flow　and　freeze.　The　debris　bed　module　can　be　used　to　analyze　the　process　of　the　formation　and　cooling　of　the　debris　both　in　the　core　and　lower　plenum.　RPV　melt　behavior　module　treats　the　heat　transfer　of　the　corium　pool　to　the　lower　head　and　determines　the　property　of　the　in-vessel　retention　of　the　corium.　The　thermal　hydraulic　module　is　well　developed　to　interact　with　all　the　other　modules　and　combine　them　to　a　uniform　MIDAC　code.　A　preliminary　validation　of　the　code　with　TMI-2　accident　has　already　been　done　and　the　simulation　results　show　good　agreement　with　the　real　plant　data.　Copyright　©　(2015)　by　American　Nuclear　Society　All　rights　reserved.