* Regulator says Flamanville reactor fit for service
* EDF says ready to replace flawed reactor cover
* Japan's JSW, not Areva's foundry, would forge new cover
* Greenpeace calls on minister to halt reactor construction (Adds ASN, EDF comments)
By Geert De Clercq
PARIS, June 28 (Reuters) - EDF's Flamanville 3 nuclear reactor being built in northwest France is fit for service despite weak spots in its steel, but the utility will have to replace the reactor cover by 2024 at the latest, nuclear regulator ASN said.
The ASN's provisional ruling - which will be followed by a final ruling in October after consultation of the public - is in line with recommendations in a report by its technical arm IRSN, which says Flamanville can start up safely but will need constant extra monitoring over its lifetime and will need to replace its reactor cover after a few years of operation.
"The ASN considers that there are sufficient safety margins in order to start up the reactor," ASN Director Pierre-Franck Chevet told reporters on Wednesday.
Following the discovery of carbon concentrations in the Areva-made base and cover of the Flamanville reactor late 2014, ASN had ordered EDF and reactor maker Areva to do extensive testing of the reactor's steel and has reviewed the results of these tests in the first half of this year.
The stakes were high for EDF and Areva because it would have cost billions of euros and taken years to repair the vessel if ASN had ruled that the flawed steel would prevent a start-up of the reactor.
The ASN's green light for Flamanville, slated for start-up in late 2018, is also a European Commission precondition for approving EDF's planned takeover of Areva's reactor business.
The conclusions of the ASN's report, reported by Reuters on Friday, state that the base of the reactor can operate safely over its expected 60-year lifetime given extra monitoring.
But its cover, which has numerous perforations for control rods and instrumentation, could not be tested adequately and unless EDF can devise new tests that will show the reliability of the cover's steel, it will have to replace it by 2024 latest, ASN ruled, going further than the report's advice.
In a call with reporters, the head of EDF's Flamanville 3 project Laurent Thieffry said the utility would comply with the ASN's ruling and would give itself two years to try and come up with new tests.
It also confirmed, as reported by Reuters on Monday, that it had already ordered a new cover which would be forged by Japan Steel Works, not by Areva foundry Creusot Forge, which manufactured the flawed current cover and base.
Thieffry said replacing the cover would take up to nine months and would cost about 100 million euros.
ASN's Chevet said the problems with the Flamanville cover could also impact other European Pressurised Reactors (EPR), notably two EPRs under construction in Taishan, China, whose base and covers have also been manufactured by Creusot Forge.
Chevet said representatives of Chinese utility CGN and its regulator had been present during the discussion of the report by a group of experts on Monday and Tuesday.
EDF's Thieffry said the Chinese would benefit from the French investigation into the reactor's steel, but that it would be up to CGN to decide what course of action to take.
A fourth EPR under construction in Finland is not affected as its reactor vessel components have all been forged in Japan.
EDF plans to build two of the same Areva-designed European Pressurized Reactor (EPR) models in Hinkley Point, Britain.
Greenpeace called on energy and environment minister Nicolas Hulot to halt the Flamanville construction.
"The ASN is no longer able to decide on safety in all independence and cannot resist industry pressure, simply because the survival of the entire French nuclear industry is at stake with the approval of this reactor vessel," Greenpeace said.
Additional reporting by Benjamin Mallet; Editing by David Goodman and Susan Thomas