All current regulatory assessment procedures of reactor pressure vessel (RPV) rely essentially on the Charpy impact test, in particular the two parameters characterizing the ductile-to-brittle transition temperature (DBTT) and the upper shelf energy (USE). Such parameters are derived from the surveillance programs aiming at monitoring the vessel materials ageing and embrittlement under irradiation. During the last decade, thanks to the enhanced surveillance strategy developed at SCK•CEN, a large number of crack resistance measurements were collected. Contrary to the upper shelf energy concept which is rather rudimentary, the crack resistance curve including initiation toughness and tearing resistance relies on fracture mechanics. Today, the available database of crack resistance properties includes various RPV materials including forgings and welds irradiated to different fluence levels.
Similarly to the DBTT, a regulatory trend curve established on an empirical basis is used for the vessel integrity assessment. On the other hand, the most important requirement for operation is that the USE should not below an arbitrary threshold energy level of 68J. With the development of fracture mechanics on small size samples, it becomes possible to assess the evolution of the RPV materials properties at upper shelf temperatures with modern concepts based on fracture mechanics. As a result, the embrittlement trend curve at upper shelf should also be adapted accordingly and substitute empirical by physical insight.