Steam generators the subject of ASN attention

Monday, 26 September 2016

Steam generators at Fessenheim and Bugey are the only current areas of action for the French Nuclear Safety Authority, which has published a list of all the uncovered irregularities in manufacturing paperwork.

Manufacturing at Creusot Forge (Image: Areva)

Following questions over quality control and carbon content of steel in the reactor pressure vessel of the EPR under construction at Flamanville, France, Areva was asked by regulators to review its records for some 400 heavy steel components made at the Creusot Forge plant since 1965.

ASN has now released a list of 87 irregularities concerning components currently in use at French nuclear power plants. The ASN has previously characterised the issues as "inconsistencies, modifications or omissions in the production files, concerning manufacturing parameters and test results." It identified 23 which "in principle have the most significant safety implication", although for 21 of those "ASN concluded that the deviations identified do not compromise the safety of the equipment concerned."

Steam generators are heat exchangers between the water circulating in the reactor's primary coolant 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. Their domed lower head is part of the primary circuit and therefore has an important safety role in ensuring cooling water is always available. There are three steam generators in 900 MWe pressurized water reactors, like Fessenheim 2 and Bugey 4, while the larger reactors feature four.

The remaining issues concern the carbon content of steam generators installed in recent years at Fessenheim 2 and Bugey 4 as part of EDF's preparation for their 30-year periodic safety review and preparation for long-term operation. The parts were manufactured by Japan Casting and Forging Corporation acting as a subcontractor for Areva, EDF said.

In July the ASN suspended the serviceability certificate for one of three steam generators at Fessenheim 2 due to anomalies in the steel of its lower shell. Areva said the internal production record for the part has showed "divergence from the nuclear pressure equipment manufacturing standards". ASN said the forging of the component "had not been conducted in accordance with the technical dossier" submitted to it by Areva.

ASN is considering what action if any to take concerning a steam generator at Bugey 4. The unit is currently shut down for maintenance and ASN said it would declare its opinion before restart.

EDF and Areva are conducting tests which involve measuring the cabron content of steel in various places with the intention of satisfying ASN that the steam generators are indeed fit for purpose.

Researched and written
by World Nuclear News