Background: The calculation of irradiation-induced dilation of the fuel subassembly duct in LMFBR is important for fast reactor core design.Purpose: To investigate how to calculate the dilation by using finite element method（FEM）.Methods: First

irradiation-induced creep and swelling material models are introduced.Then

a theoretical solution based on a simplified bending plate model is briefly given.Finally

a stress update scheme for the adopted material models is presented and furthermore embedded into ABAQUS user interface UMAT to conduct finite element analysis.Both solutions are compared and discussed.Results: FEM successfully predicts the duct dilation and its solution agrees well with theoretical one in small deformation.Conclusions: The proposed stress update scheme is effective.The accuracy of the theory solution declines when dilation becomes larger.The maximum stress occurs at the duct corner point

and the location has stress relaxation effect.