In this paper it is described the work performed on the steel coming from a Spanish BWR Nuclear Power Plant (NPP) vessel currently operating in order to analyse its structural integrity assessment from different procedures. The classical procedures for the characterization of the cuasistatic fracture behaviour in the ductile to brittle transition region [
] are based on conservative correlations obtained from Charpy impact tests. In this work, on the contrary, direct measurements of cuasistatic K
fracture toughness were performed. The results were analysed according to the recent Master Curve methodology [
] in order to obtain the Reference Temperature, T
, describing fairly the response of the material in such a region.