Nuclear plants of the next generation , so called GEN IV reactors, are today a major contender for the safe delivery of clean and environmental respectful energy. Some of these reactors are planned to use a liquid metal eutectic of lead and bismuth (LBE) as a coolant. The eutectic melts at 125°C and then behaves similar to water , is circulated around the reactor core to extract the heat. The materials used for the structure of the reactor have to satisfy strict specifications which ensure them to sustain operational cyclic loadings during harsh conditions due to irradiation and environment. The prospective studies already accomplished have shown that coolants of this type can have a negative influence on the mechanical properties of some of the candidate materials. They are corrosive and can induce a degradation of the fatigue performance and fracture resistance. It was found that under certain conditions, the number of cycles for the initiation of a crack was reduced and that the propagation rate of the crack was increased. Different physical mechanisms have been proposed in the literature to explain the results but none of them was really able to properly describe the observed effects. Comparison tests have also shown that the environment is a key factor for the materials's response. In an attempt to identify the physical mechanisms controlling the nucleation and propagation of cracks, experiments in three different media such as vacuum, air and LBE will be conducted in the selected material. The microstructural modifications at the crack tip in the different environments will be observed using techniques such as SEM, TEM and FIB which should allow the candidate to give a detailed description of the microstructure at the surface of the crack and close below it. These observations will be correlated with the mechanical results and used to propose an explanation of the mechanisms going on in the presence of LBE.