French utility EDF has informed the nuclear safety authority Autorité de Sûreté Nucléaire (ASN) that some primary bottom heads of steam generators installed at 18 of EDF's 58 reactors in France could have a significant carbon concentration similar to the anomaly found on the reactor pressure vessel at the company's Flamanville-3 EPR under construction in northern France. Areva, EDF and ASN reported in April 2015 that chemical tests Areva performed in 2012 on steel similar to that used in the reactor vessel top and bottom heads at Flamanville-3 showed a high carbon content in the vessel heads, which can reduce the ability for the component to withstand crack propagation.
The discovery of that anomaly led ASN and EDF to investigate whether it is also present on other components in operation at existing reactors, ASN said. The bottom heads at the 18 reactors were manufactured either by Areva's Le Creusot forge facility or by Japan Casting and Forging Corporation in Japan, ASN said ongoing analyses on reactor vessels, pressurisers and steam generators could show that other forged components may also be affected by the anomaly.
The bottom heads are semi-spherical forged parts that form the lower part of the steam generators, helping to contain water in the primary circuit. "The quality of design, manufacture and monitoring during operations is an important issue", ASN said. Steam generator bottom heads can be produced using different technologies, it noted. Those likely to be affected by a high carbon content are those forged from a solid steel ingot. The 18 French reactors which may be affected include those of both 900MWe and 1450MWe capacity.
EDF informed ASN that its initial analysis showed the strength of these components was adequate to keep the reactors in service. However, ASN has requested further investigations and EDF is to perform non-destructive tests on the outer surface of the vessel bottoms to identify areas having a high carbon content. It will also carry out ultrasonic tests to look for signs of defects in the components. ASN noted that further analyses continue on other forged components of the vessel, the pressurizer and the steam generators, which may also be affected.