The　cooling　characteristics　of　the　melt　have　important　influence　on　the　process　of　IVR　in　a　severe　accident.　An　enthalpy　formulation　based　on　fixed　grid　methodology　was　developed　for　the　numerical　solution　of　convection-diffusion　controlled　mushy　zone　phase-change　solidification　of　the　melt.　The　process　of　the　solidification　was　simulated　by　solving　the　N-S　equations　and　the　enthalpy　formulation　equation　using　the　SIMPLE　algorithm.　The　phase-change　model　could　simulate　the　solidification　and　mushy　zone　well,　and　it　was　verified　by　the　use　of　analytical　solutions　and　benchmark　problems.