* ASN gives generic green light for EDF reactors

* Extra tests needed, EDF says will be done quickly

* French forward power prices tumble, EDF post gains (Adds details, quotes, background and shares)

PARIS, Dec 5 (Reuters) - French winter electricity supply worries could ease after nuclear safety regulator ASN gave approval in principle on Monday for EDF to restart some nuclear reactors involved in a safety investigation.

ASN said it agreed with EDF that some nuclear reactors whose steam generators have high carbon concentrations could be restarted, although the utility would have to carry out extra tests and the restart of each individual reactor would be subject to ASN approval.

ASN is checking 12 of the state-controlled utility’s reactors after raising concerns that certain steam generator channel heads could contain a zone with a high carbon concentration, which could weaken the resilience of the steel and its ability to resist the spreading of cracks.

The checks have delayed the restart of several reactors, forcing EDF to prolong their outage periods and raising fears that France, which depends on its 58 nuclear reactors for 75 percent of its electricity needs, could face a tight supply this winter.

French and European power prices have hit new highs over the past weeks due to the tight supply fears.

ASN’s comments sent the French power price for January delivery down 19.3 percent to 65 euros ($70) a megawatt hour (MWh) by 1642 GMT on Monday. February and March prices also fell and first quarter was off 17 percent.

A London-based power trader said ASN’s statement opened the door for an early restart of some reactors and also suggested it was ruling out a complete shutdown or costly replacement of the steam generators.

“Today, we are able to give rather positive news,” ASN head Pierre-Franck Chevet told Reuters after the news conference.

“The restart of the EDF reactors could happen quickly, it is in the hands of EDF,” Chevet said, adding that ASN estimates that EDF could get its documents ready for each plant in about two weeks per reactor.

Seven of the reactors are expected to resume production on Dec. 31.

A spokeswoman for EDF said the company will work quickly to carry out the extra tests on the reactors involved and provide the additional information to ASN. It is ready to restart the reactors once it receives the green light from the watchdog, the spokeswoman said.

EDF’s shares rose sharply after ASN’s announcement and had gained 2.28 percent by 1703 GMT, outperforming the European utilities index, which was down nearly 1 percent.

“There is evidently some relief in the market to what ASN has just said. But there is still fragility in the supply situation in France,” a European power trader said.

In a separate statement, French nuclear safety institute IRSN, which provides expertise to ASN on the investigation, said that tests had shown no risk of rupture of steam generators on 900-megawatt reactors except for Bugey 4, Fessenheim 1 and Tricastin 4.

“Our investigations have shown that there are no risks of a brutal rupture,” IRSN’s deputy director Thierry Charles, told Reuters. ($1 = 0.9326 euros) (Additional reporting by Nina Chestney in London; Editing by Adrian Croft and Susan Fenton)