Due　to　the　complex　products　and　irradiation-induced　defects,　it　is　hard　to　understand　and　even　predict　the　thermal　conductivity　variation　of　materials　under　fast　neutron　irradiation,　such　as　the　abrupt　degradation　of　thermal　conductivity　of　boron　carbide　(B4C)　at　the　very　beginning　of　the　irradiation　process.　In　this　work,　the　contributions　of　various　irradiation-induced　defects　in　B4C　primarily　consisting　of　the　substitutional　defects,　Frenkel　defect　pairs,　and　helium　bubbles　were　re-evaluated　separately　and　quantitatively　in　terms　of　the　phonon　scattering　theory.　A　theoretical　model　with　an　overall　consideration　of　the　contributions　of　all　these　irradiation-induced　defects　was　proposed　without　any　adjustable　parameters,　and　validated　to　predict　the　thermal　conductivity　variation　under　irradiation　based　on　the　experimental　data　of　the　unirradiated,　irradiated,　and　annealed　B4C　samples.　The　predicted　thermal　conductivities　by　this　model　show　a　good　agreement　with　the　experimental　data　after　irradiation.　The　calculation　results　and　theoretical　analysis　in　light　of　the　experimental　data　demonstrate　that　the　substitutional　defects　of　boron　atoms　by　lithium　atoms,　and　the　Frenkel　defect　pairs　due　to　the　collisions　with　the　fast　neutrons,　rather　than　the　helium　bubbles　with　strain　fields　surrounding　them,　play　determining　roles　in　the　abrupt　degradation　of　thermal　conductivity　with　burnup.