[en] The inner vessel of FBR-1 and 2 separates the hot pool sodium from the cold pool and permits the passage for the stand pipes. Stress analysis of the inner vessel, which is designed based on this requirement has been performed under mechanical loadings for normal operating, Operation Basis Earthquake (OBE) and Safe Shutdown Earthquake (SSE) conditions. Thermal stress analysis has been carried out on the optimized geometry thus obtained. Progressive deformation and creep-fatigue damage have been estimated for normal operating condition by using elastic analysis method as per the RCC-MR code. The critical location is the maximum thermal stress region where the creep damage is greater than one. On considering stress relaxation at this location and anticipating the presence of weld at the critical location, the creep and fatigue damages are found to be 0.7 and 3.89E-08 respectively. Hence the optimized geometry with the given thickness and loading conditions meets all the limits as per RCC-MR. (author)