EdF told French regulators in May that it had misjudged the urgency of adjusting set points for steam generator secondary water level on its 1300MWe PWR reactors as promised in connection with a switch to a new fuel management mode in 1998.
EdF found that the set points should have been changed immediately, instead of waiting until the units' 20-year outages.
Safety experts said the potential consequences of the anomaly were still being reviewed, but the event has been rated at INES level 1, because it reflects a defect in quality assurance.
The need to change the set point was discovered through studies underpinning EdF's safety case for taking the large PWRs from 12-month to 18-month operating cycles with higher enriched fuel, done in 1997-98. Safety experts said that it had nothing to do with fuel enrichment or cycle length, but was associated with better steam generator flow modelling methods used in recent safety studies.
Jean-Claude Legendre, vice president for technical support of EdF Nuclear Generation, said that all the set points should be modified within four months. Under the previous schedule, the modifications would have been done between 2005 and 2010.