PARIS, Feb 7 (Reuters) - Corrosion of fuel rods at 25 of EDF's 58 French nuclear reactors could lead the French nuclear watchdog to tighten maintenance rules for the utility, investigative website Mediapart said on Friday.
The website, which published what it says is an EDF document obtained from an anonymous sender, said French nuclear watchdog ASN would produce a report on the subject in the "coming days".
ASN was not immediately available for comment.
A spokesman for EDF said ASN had raised the question of fuel rod corrosion but he said there was no safety problem and the issue would have no impact on the reactors' operations.
"This is a technical issue, which is the subject of discussions between EDF and the ASN," he added.
Mediapart said EDF fuel rods -- zirconium alloy tubes that hold the enriched uranium pellets that power the nuclear reaction - showed signs of corrosion.
It said that 13 of EDF's high-power 1,300 megawatt reactors and 12 of its 900 megawatt reactors - among its oldest - were affected by fuel rod corrosion. The rods hang directly in the water of the reactor's primary cooling circuits, where they give off their heat.
"Because of the high number of plants concerned, dealing with this problem will inevitably have an impact on the fleet's functioning and therefore on the quantity of nuclear electricity produced," Mediapart said.
The corrosion could lead ASN to force EDF to use nuclear fuel for shorter periods of time and replace it more often with new material, reducing the availability of the nuclear fleet.
Stricter regulation following Japan's 2011 Fukushima nuclear disaster has lead to increased maintenance work at EDF's ageing fleet of 58 nuclear reactors, reducing the availability of its plants and the group's profit margins. (Reporting by Michel Rose and Benjamin Mallet; editing by Geert De Clercq and Keiron Henderson)