Material testing continues on French steam generators

24 June 2016

Parts of some steam generators at 18 nuclear power units in France may have similar anomalies discovered in the steel of the pressure vessel of the Flamanville EPR, utility EDF has informed the French nuclear regulator.

The upper and bottom heads of the reactor pressure vessel for the EPR under construction as Flamanville 3 were manufactured at Areva's Le Creusot facility in September 2006 and January 2007, respectively. A high carbon content in those parts prompted Areva to review the company's quality management process in 2015.

The French regulator, the Autorité de Sûreté Nucléaire (ASN), requested Areva and EDF conduct an analysis to identify any other components in operating nuclear power reactors that could be affected by a similar anamoly to that of the Flamanville EPR vessel.

SG bottom head - 250 (ASN)
The steam generator vessel bottom (Image: ASN)

The ASN also asked EDF to justify the mechanical resistance of the steam generator bottom heads that had been manufactured by Areva's Creusot Forge facility or by Japan Casting and Forging Corporation.

Steam generators are heat exchangers between the water circulating in the primary circuit - at a temperature of about 350°C and a pressure of 155 bar - and the water in the secondary circuit that supplies steam to the turbines. There are three steam generators in 900 MWe pressurized water reactors, while the larger ones feature four.

The bottom heads are semi-sperical 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", the 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 regulator has identified 18 French reactors - of both 900 MWe and 1450 MWe capacity - that could be affected.

EDF informed the ASN that its initial analysis showed the strength of these components was adequate to keep the reactors in service.

However, the ASN has requested that further investigations are carried out on the parts. EDF is to perform non-destructive tests on the outer srface 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.

The ASN noted that further analyses continues on other forged components of the vessel, the prerssurizer and the steam generators, which may also be affected by the anomaly.

Researched and written
by World Nuclear News