[en] In a sodium cooled nuclear fast reactor the thermal analysis has a particular importance as a consequence of the rapid transients and the high sodium conductivity. The aim of this work is to estimate the value of transient thermal stresses in elastic and plastic fields. In the first part, the loading conditions for different steady and transient operations are examined. A number of tests was planned to simulate in a mock-up the temperature changes in the upper part of the vessel for different thermal transients considered most dangerous: scram to full power, electric supply fault in a pump with reactor scram, pump seizure with reactor scram. A mock-up in scale 1 : 4 simulating the vessel area above the core was built-up. The heat transfer is simulated with a mass transfer; with this simulation it is not considered that the heat conduction in the sodium and the thermal gradients measured are higher than in the reactor. The temperature transients have been obtained with a different salt concentration: the water simulates the hot sodium and the water salted represents the cold sodium. In the second part of the work is considered the thermo-elastic structural response using the FEM analysis. The elastic approach has had two functions: the first is to find a simplified structure in two dimensions that has the same behavior as the 3-D nozzle; the second is to follow the thermal transient to look for the time of maximum stress. For different geometry, in a previous paper, the stress distribution and the influence of some characteristic parameters have been examinated. As a result of this part an axisymmetric geometry has been chosen and also the time of maxim