The integrity of the tubesheet to shell junction zone of a fixed tubesheet reactor subject to in phase proportional pressure and temperature loading, and under reduced thickness, has been investigated by applying various failure theories and in accordance to design by analysis rules through considering various material hardening parameters. Results of an elastic FE analysis for reduced tubesheet and shell thicknesses at the junction of the shell to tubesheet indicates that the junction zone is subject to large stresses. It is shown that, junction stresses are formed as a result of combined action of channel side shell bending and tubesheet rotation at its rim. Additionally, it is discussed that linearization of a stress path at the vicinity of the junction zone categorize a large portion of calculated stresses as a peak stress and that not all of stresses classified as peak are entirely peak stress. It is further shown that, lower tubesheet thickness can be obtained by identifying the exchanger plastic load. Exchanger plastic load has been calculated through inelastic analysis and in accordance with the twice elastic slope (TES), tangent intersection (TI) and curvature of plastic work (PWC) methods. In contrast to TES and TI methods, which are graphical methods, it is shown that PWC method being a mathematical one produces more accurate plastic load. Failure mode associated with progressive plastic deformation is investigated for various material hardening models. It is shown that exchanger shakes down to cyclic plasticity after showing some initial ratcheting behaviour under multilinear kinematic model. Under this material model, magnitude of plastic strains are small and are confined within surrounding elastic media, this has been proved by existence of elastic core. It is also shown that, ASME elastic- perfectly plastic material model incorrectly predicted ratcheting behaviour.Fatigue analysis for both welded and unwelded region of tubesheet and shell junction at the groove location has been carried out, this has been done to demonstrate that under full load cycles of start-up and shutdown, and under upsets conditions the number of life cycles for this reactor is much larger than the number of operating cycles anticipated to occur during the reactor life. This has been proved by usage of stress method, local strain method and in accordance with experimentally determined equations. Additionally, it is shown that for cases subject to loads of low-cycle in nature, non-cyclic stress-strain material diagram produces higher plastic strains in comparison to the cyclic one and therefore its usage has been advised on similar applications.