(Montel) France’s ASN nuclear safety authority and EDF failed to reach a “consensus” after the utility asked the watchdog for permission to delay repairs on cracked safety pipes, said its deputy head Julien Collet.
“For them, it's a question of industrial organisation. They would prefer to inspect and repair only what is strictly necessary in the short term,” he said.
“We have no problem with that. On the other hand, there must be a robust justification that this does not pose a problem in terms of safety.”
To keep reactors in service , EDF must consider crack propagation – any widening, lengthening or increase in the number of cracks over time, the deputy head said.
However, “for the moment, there is no consensus on the propagation speed”.
Impact on output
In its initial request, EDF assumed a propagation speed of 0.5 mm/year based on simulations to justify keeping reactors in service for longer. But this rate is half the 1mm/year speed assumed by ASN, the watchdog said in a letter to EDF dated 30 March.
However, this hypothesis did not explain “the extension of the very deep crack” observed at the beginning of the year on a repaired weld at the Penly 1 reactor, ASN said. This crack was 23 mm deep for a pipe thickness of 27 mm.
Delaying repairs would allow EDF to wait for planned outages, which take place every 18 months or so and last 60-80 days, to do the work, a nuclear division manager involved in the repairs told Montel.
This would give the utility enough time to replace the damaged welds without having to significantly extend shutdowns and cut production, he said.
EDF declined to comment.
Last September, the technical safety body IRSN said in a report that the rate of propagation of stress corrosion cracks depended heavily on the oxygen content of the water circulating in the pipes.
Referred to experts
If the oxygen content was perfectly controlled, a propagation rate of 0.5 mm/year seemed “realistic” for a crack a few millimeters deep in a weld that had not been repaired during construction. But in other cases, higher speeds had to be considered.
ASN had referred EDF’s demand for delays to a group of independent experts, Collet said. The group met on 25 and 26 May, but so far ASN and EDF have declined to comment on the outcome.
Last month, EDF cut the duration of outages at six 1.3 GW reactors affected by corrosion by up to 28 days, with four of the units scheduled to start earlier in the winter.