To complete the manufacturing process qualification of the closure head and bottom head domes of the Flamanville EPR RPV in 16MND5 ferritic steel (similar to A508 cl3), AREVA NP performed in 2014 material testing on a sacrificial dome so-called UA Sup. These tests revealed that the Charpy impact energy at 0°C — in the positive segregation zone at ¼ thickness — was lower than 60 J, the minimum value required by the French regulation. Thereafter, this result was explained by the high carbon content close to 0.3 % — against 0.18 % in the ladle — due to the forging process. The positive segregation was insufficiently eliminated to avoid high carbon content in certain location.
As the level of Charpy impact energy is an indicator of the level of fracture toughness, a thorough material characterization of the segregation zone, as well as a brittle fracture analysis have been proposed and carried out by AREVA NP to demonstrate that the Flamanville EPR RPV domes are tough enough for a safe operation.
The principles of the demonstration approach were reviewed by ASN and IRSN in 2015 and the final safety case was submitted to the ASN and IRSN review in December 2016. The latter led to the conclusion that, even if the segregation zone fracture toughness is lowered in comparison to that of the acceptance test zone, this does not put into question the Flamanville EPR RPV fitness for service as long as adequate in-service inspection of the domes is periodically performed.
This paper aims at presenting the key elements of the IRSN assessment from part of which this conclusion was drawn, namely the material testing program, the description of the loadings in normal and accidental conditions, and the brittle fracture analysis.