[en] Washing vessels are provided to wash the spent fuel subassembly for removing the sticking sodium after the removal from core. Similarly preheating vessels are provided to preheat fresh subassembly prior to transferring it to core for power generation. The washing vessels and preheating vessels are preheated to around 200 °C and expected to wash about 1500 and preheat about 2000 subassemblies respectively during the entire life of the vessel. The heating and cooling (thermal cyclic loading) of vessel may produce fatigue damage due to cyclic thermal gradients. The both vessel are analyzed using ABAQUS FE software for estimation of maximum permissible thermal gradients in various directions to avoid the crack initiation due to fatigue damage in vessel. The weld location is considered for analysis as it is more prone to failure for thermal cycling loading. For washing vessels, the maximum permissible stress range is found to be 310 MPa for keeping the fatigue damage as unity. Hence, the maximum permissible temperature difference between diametrical ends (557 mm) of the vessel is 498 °C, without axial temperature gradients. Similarly, the permissible axial temperature difference at a distance of 557 mm is 593 °C, without any circumferential temperature gradient. In the case of both directional gradients, the maximum allowable temperature difference is 389 °C, measured between both ends of diameter and axial distance of 557 mm. Similarly, for preheating vessel, the values have been found out. These values can be used as guideline while washing operation and preheating is performed. (author)