During　a　core　disruptive　accident　in　a　sodium-cooled　fast　reactor,　the　molten　fuel　or　steel　is　solidified　into　debris　particles　that　form　a　debris　bed　in　the　lower　plenum.　When　the　debris　bed　starts　boiling,　the　flow　of　the　coolant　and　vapor　together　relocates　and　flattens　the　debris　particles.　This　process　is　called　debris　relocation.　The　thickness　of　the　debris　bed　and　its　porosity　have　significant　influence　on　the　cooling　ability　of　the　fuel　debris　in　the　lower　plenum.　Therefore,　it　is　necessary　to　evaluate　the　transient　changes　in　the　shape　and　thickness　of　the　debris　bed　in　the　relocation　behavior　for　accident　analysis.　To　simulate　the　relocation　behavior,　a　debris-relocation　model　based　on　the　COMMEN　code　was　developed　in　this　study.　The　debris-relocation　model　is　based　on　the　shear　strength　mechanism,　which　is　widely　applied　in　soil　mechanics.　Shear　strength　is　a　function　of　the　density,　position,　and　shape　characteristics　of　particles.　The　debris　bed　is　relocated　only　when　the　shear　stress　of　the　global　debris　particles　is　greater　than　the　shear　strength.　By　incorporating　the　relocation　model　into　the　COMMEN　code,　the　relocation　transient　processes　of　different　particle　beds　in　a　nitrogen-injection　experiment　were　simulated,　and　the　results　were　compared　with　experimental　results.　The　analysis　indicates　that　the　proposed　debris-relocation　model　can　effectively　predict　the　occurrence　of　debris-bed　relocation　and　reasonably　simulate　its　transient　process,　although　future　improvement　is　necessary.　©　2020　Elsevier　B.V.