EdF's plan to switch to 18-month operating cycles at its oldest reactor series has been put at risk because signals from a new eddy current probe designed to detect fine circumferential cracks in steam generator tubes couldn't be interpreted with sufficient certainty during the probe's first use at Bugey 4.
The probe identified a dense bundle of short longitudinal cracks as circumferential cracking. However, EdF was not able to convince regulators that there were no circumferential cracks.
French nuclear safety authority DSIN said that EdF must either put in place a reliable detection system or it must keep the reactors on 12-month cycles.
Mathieu Schuler, director of the NSSS Inspectorate, said the probe couldn't guarantee detection thresholds in terms of angle and section for circumferential crack indications required. DSIN wants circumferential cracking detected earlier if plants are to operate half as long again between inspections.