Considerations on the manner of accounting for fast fracture risk in the design of PWR vessels

The French approach to the prevention of fast fracture in PWR vessels is to consider it as a whole and to choose the most convenient way to meet this general goal from an economic and technical point of view. According to this approach, there are no specific limits imposed on such factors as end of life RT NDT or neutron fluence, which are taken as criteria in other countries. The RCCM design and construction code specifications on chemical content and RT NDT for beltline and non-irradiated parts establish a sound basis for safety. However, for the most critical parts, the existence of large margins with respect to fast fracture is demonstrated by analysis for all second, third and fourth category design transients. To this aim, the RCCM code needs to demonstrate specified safety margins, depending on the transient category, for reference defects defined in kind and size, in order to bound realistically any defects which have a chance of occurring in the part during manufacture. This approach, which enables the disclosure of the influence of all significant design factors on fracture risk, ensures the most consistent way to improve design safety.