To characterize the mechanical properties of the residual carbon segregation of the Flamanville EPR RPV heads, a testing program have been organized by Framatome and performed in 3 laboratories.
Three sacrificial pieces have been used for this program, allowing the investigation of different carbon contents, coupled with different quenching conditions (thanks to a specimen sampling at different positions in the heads thickness).
As expected, the increase of the carbon content leads to an increase of the tensile properties, a shift toward the higher temperature of the brittle-to-ductile transition and a lowering of the toughness in the ductile regime. In the worst case, compare to the RTNDT measured in the frame of acceptance tests of the heads, the temperature to be considered to envelop all the experimental results with the toughness curve of the RCC-M code is 20°C higher. The extensive test program allows a quantification of these impacts for the integrity assessments of the RPV.
It is also demonstrated that the segregated material remains homogeneous, with a classical failure mode by cleavage. Master Curve approach is also applicable and used to characterize the carbon content impact on the toughness properties.