In　this　paper,　improvements　to　the　multigroup　cross-section　perturbation　model　have　been　proposed　and　applied　in　the　self-developed　UNICORN　code,　which　is　capable　of　performing　the　total　sensitivity　and　total　uncertainty　analysis　for　the　neutron-physics　calculations　by　applying　the　direct　numerical　perturbation　method　and　the　statistical　sampling　method　respectively.　The　narrow　resonance　(NR)　approximation　was　applied　in　the　multigroup　cross-section　perturbation　model,　implemented　in　UNICORN.　As　improvements　to　the　NR　approximation　to　refine　the　multigroup　cross-section　perturbation　model,　an　ultrafine-group　cross-section　perturbation　model　has　been　established,　in　which　the　actual　perturbations　are　applied　to　the　ultrafine-group　cross-section　library　and　the　reconstructions　of　the　resonance　cross　sections　are　performed　by　solving　the　neutron　slowing-down　equation.　The　total　sensitivity　and　total　uncertainty　analysis　were　then　applied　to　the　LWR　pin-cells,　using　both　the　multigroup　and　the　ultrafine-group　cross-section　perturbation　models.　The　numerical　results　show　that　the　NR　approximation　overestimates　the　relative　sensitivity　coefficients　and　the　corresponding　uncertainty　results　for　the　LWR　pin-cells,　and　the　effects　of　the　NR　approximation　are　significant　for　sigma((n,gamma))　and　sigma((n,elas))　of　U-238.　Therefore,　the　effects　of　the　NR　approximation　applied　in　the　total　sensitivity　and　total　uncertainty　analysis　for　the　neutron-physics　calculations　of　LWR　should　be　taken　into　account.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.