Abstract: The COMMEN code was developed by China Institute of Atomic Energy for the analysis of core severe accident in sodium-cooled fast reactor, which consists of thermal hydraulics module, fuel pin and structure module and neutronics module. The detailed pin model in COMMEN code was described in this paper. In order to get detailed description of radial temperature distribution in the fuel pellet, the node inside the fuel pellet was divided. Besides, UTOP (unprotected top of power) accident of China Experimental Fast Reactor (CEFR) was calculated and analyzed in terms of reactivity feedback with COMMEN code which contains the detailed pin model, and the calculation results were compared between SAS4A code and COMMEN code to test and verify the detailed pin model. The results show that the results of fuel temperature using the detailed pin model are in good agreement with the results using SAS4A code, and the developed COMMEN code is suitable for UTOP accident analysis.