The safety cases of the French 900-MWe and 1300-MWe reactor pressure vessels (RPV) have been reviewed respectively in 2010 and 2015 by the French Nuclear Safety Authority (ASN) and the Institute for Radiological Protection and Nuclear Safety (IRSN) in order to decide of the continued operation for the next ten years after their third decennial outage.
The demonstration of the RPV serviceability, based on a deterministic approach, includes an assessment of the risk of crack initiation under normal and accidental conditions. The fracture criterion defined by the French utility, takes into account a size effect on fracture toughness in the ductile-to-brittle transition of ferritic steel by the application of a correction factor to the fracture toughness estimated on the French code RCC-M fracture toughness reference curve. This correction related to the crack length is built on the observation that the failure probability follows a Weibull distribution and on the assumption that the RCC-M KIC curve can be associated to a reference crack length of 100 mm.
A first crack length correction has been judged inappropriate by ASN and IRSN in 2010. A new temperature-dependent correction has then been proposed in 2014. Nevertheless, numerous elements presented in this paper have led ASN and IRSN to the conclusion that this new correction should not be applied to the RCC-M KIC curve. In particular the RCC-M KIC curve already integrates intrinsically a size effect on fracture toughness.