In large steel ingots, the material condition known as carbon macrosegregation is defined as variations in carbon content that exist in the ingots, ranging in length from centimeters to meters. This is a common phenomenon that occurs during the solidification of large steel ingots. Existence of carbon macrosegregation is undesirable in forged components, as localized carbon levels may exist that exceed material specification limits and due to the potential to reduce fracture toughness and ductility in these regions. Forging vendors follow specific procedures to mitigate the existence of carbon macrosegregation in the components they manufacture, which involves removing the portions of the ingot that experience elevated levels of carbon.
A current industry issue being investigated is the existence of excessive carbon macrosegregation in large forging materials within operating nuclear plant components. This issue was discovered after demonstrations of compliance with new fracture toughness requirements were performed by AREVA as required by the French Nuclear Safety Authority (ASN). Destructive testing on Reactor Pressure Vessel (RPV) heads manufactured by AREVA subsidiary, Creusot Forge, was performed which identified elevated carbon bands around the center section of the forged heads. Ongoing investigations by the industry, including the U.S. Nuclear Regulatory Commission (NRC), to identify the existence of carbon macrosegregation in large forged primary system components in U.S. reactors have not yet identified any conditions adverse to safety.
Structural Integrity, in conjunction with industry groups, has been involved during these ongoing investigations, providing subject matter expertise in material science and probabilistic fracture mechanics to assess any potential risks or vulnerabilities that may exist in the U.S. nuclear fleet. One such investigation performed is an evaluation on the significance that the presence of carbon macrosegregation has on the fracture toughness of RPV beltline materials that are bounding for the U.S. fleet. Probabilistic fracture mechanics analyses were performed to determine the limiting through-wall cracking frequency values for RPV materials with reduced fracture toughness due to carbon macrosegregation, following the methodology and acceptance criteria used in the Alternate PTS Rule, 10 CFR 50.61a “Alternate Fracture Toughness Requirements for the Protection Against Thermal Shock Events.” Again, as of the publishing of this article, these ongoing investigations have not identified any conditions adverse to safety with respect to carbon macrosegregation in U.S. nuclear fleet components with large forgings.