The reactor pressure vessel (RPV) of nuclear power plant is a heavy component built by welding thick parts like rings, nozzles and domes and which is submitted to neutron irradiation and thermal ageing. During the fabrication process, low alloy RPV steels are quenched to obtain a bainitic microstructure. Because of the thickness of the wall of the vessel, the cooling rate during the quench varies from few tens of degrees per minutes in the centre to few degrees per second at the vicinity of the surface of the shell [1]. In addition, macro and micro segregation inherited of the solidification of the ingot modify locally the chemical composition of the steel [2]. Welding of the different parts of the vessel and the cladding deposit in the inner face introduce different heat affected zones (HAZ) with coarse and refined grains. Consequently, the microstructure and the initial mechanical properties are not uniform through the wall thickness and along the axis of the vessel. As an example, the transition temperature can vary by several tens of degrees through the thickness [3] and [4].