It is common for the most reactor pressure vessel (RPV) of light-water reactors used today, that the RPV made of ferrite-perlitic steel is cladded with an austenitic layer, which well resists corrosion influence. The cladding is manufactured by welding, the technology (mode of application, number of layers) is different by each manufacturer. This is the layer which comes into contact with the working media first, and is of course in cohesive contact with the RPV material. The aim of this study is to demonstrate the new approach, used for assessing the safety of the reactors today. This approach plays a decisive role in the lifespan calculation of already operating nuclear power stations, including the extension of their service period.