* Activists say EDF and Areva knew about reactor vessel problems
* Flamanville key for EDF takeover of Areva reactor unit
* Much-delayed nuclear regulator's ruling due by Sept. 21
By Geert De Clercq
PARIS, April 19 (Reuters) - A group of activists has filed a legal challenge with the French prime minister's office against the extension of EDF's licence for construction of the Flamanville nuclear reactor in northern France.
The move by Greenpeace and other anti-nuclear groups is in response to safety concerns over the Flamanville reactor and is a precursor to elevation of their challenge to the State Council, the country's highest administrative court.
The lobby groups said in a statement that the licence, issued in 2007 and renewed this year, should not have been granted because EDF and reactor supplier Areva were aware of technical shortcomings at Areva's Creusot Forge nuclear foundry since 2005.
In 2014 Areva discovered that the lid of the Flamanville reactor vessel manufactured by Creusot Forge showed abnormally high carbon concentrations, which weaken its steel.
Nuclear regulator ASN is investigating whether the irregularities threathen the safety of the reactor and whether EDF, which also plans to build two Areva-designed EPR reactors at Hinkley Point in Britain, can proceed with Flamanville's start-up in 2018 as planned.
"For more than 10 years, EDF and Areva allowed the manufacturing of faulty components for nuclear plants, including for the EPR (European Pressurised Reactor) in Flamanville, and the ASN has allowed this," the group's statement said.
One member of the group, l’Observatoire du Nucléaire, said in a separate statement that the ASN had committed a major error in December 2013 by allowing EDF to install the reactor containment vessel in the reactor building despite being aware of the problems.
It said that today the vessel could only be extracted by destroying part of the reactor.
"This is an unacceptable option for EDF, which is putting maximum pressure on the ASN to force it to validate the use of this faulty vessel," l’Observatoire du Nucléaire said.
EDF and the ASN were not immediately available for comment on Wednesday.
Xavier Ursat, EDF's head of new nuclear operations, said last month that he was confident the ASN would declare Flamanville safe.
ASN chief Pierre-Franck Chevet has said that a ruling on Flamanville will be solely based on safety requirements.
The regulator has repeatedly delayed its ruling on Flamanville since the end of 2015 and last month said a decision would be made by Sept. 21 at the latest.
EDF's planned takeover of Areva's reactor unit Areva NP -- part of a government-led rescue of virtually bankrupt Areva -- is conditional on the ASN's green light for Flamanville. (Editing by David Goodman)