Nuclear systems are built from materials satisfying strict specifications which ensure them to sustain operationnal cyclic loadings during very harsh conditions due to irradiation and environment. It has been observed that coolants of GEN IV reactors such as Lead Bismuth Eutectic can have a negative influence on the fatigue life of some candidate materials. Fatigue damage is induced by the nucleation of cracks which finally propagate and cause the rupture of the component. The cracks nucleate at the surface where they immediately interact with the surronding medium. The physical mechanisms accelerating the nucleation and propagation of cracks will be identified by conducting mechanical experiments in three different media such as vacuum, air and LBE. The microstructural modifications at the crack tip will be investigated and correlated with the mechanical results.