* Pipes at 20 reactors may not be earthquake-proof - EDF
EDF says work ongoing to reinforce pipes
* News adds to worries over French winter power supply
* French forward power prices at new highs (Adds EDF, analyst comments, shares)
By Bate Felix
PARIS, Oct 11 (Reuters) - French utility EDF is repairing pumping station pipes at 20 nuclear reactors after discovering they might not be strong enough to withstand earthquakes, posing a risk to cooling systems, it said on Wednesday.
In a note to French nuclear regulator ASN, EDF classified the problem as a 'level 2' incident out of 7 on the international nuclear and radiological event scale (INES).
A company spokeswoman said the repairs would not lead to a halt in production at the reactors involved and would not affect nuclear power output.
ASN could not immediately be reached for comment.
"In-depth investigations have revealed thinning of the metal in certain sections of piping, thus making it impossible to guarantee their seismic resistance in the event of a design-base-type earthquake," EDF said in its note.
"This could potentially cause flooding in the pump houses of twenty reactor units, thereby resulting in the functional loss of both reactor cooling water systems," it added.
EDF said pipes at nine reactors - Belleville 1-2, Cattenom 3-4, Dampierre 1-2, Golfech 1-2, and Saint-Laurent-des-Eaux B1 - had been reinforced.
Work is ongoing at five reactors which are currently offline - Chinon B3, Cruas 1, Dampierre 3, Nogent 1, Saint-Laurent-des-Eaux B2 - and will be completed before they restart.
EDF said that for six other reactors now online - Cattenom 1-2, Chinon B4, Cruas 4, Dampierre 4, Nogent 2 - reinforcements had been completed on one of each reactor's two cooling water systems, ensuring the plants could operate safely even in the event of an earthquake.
The utility added that at nine other reactors - Cruas 2-3, Paluel 3-4, Saint-Alban 1-2 and Tricastin 1-3-4 - investigations had revealed that a risk of pump-house flooding or loss of the reactor cooling water system could not be ruled out, and reinforcement works were underway.
The problem at these reactors, however, does not carry a high risk and is classified as level zero, EDF said.
CONCERNS OVER FRENCH POWER SUPPLY
EDF did not say how long the works would last, adding fresh uncertainty over French nuclear power supply in the weeks ahead.
Analysts at Jefferies, which has a "buy" rating on EDF shares, said in a note that pipe reinforcement was a simple, regular procedure likely to only take a few days, and was part of regular seismic maintenance at reactors.
"Overall, we see no material impact on (earnings) guidance," it said.
There are growing concerns that France, which depends on atomic power for over 75 percent of its electricity needs, could face tight power supplies in the coming weeks due to prolonged outages at several reactors.
After several delays in the restart of some reactors following maintenance and a review ordered by the nuclear watchdog, 21 out of France's 58 nuclear reactors are offline, putting available nuclear capacity at 63.80 percent of the total.
The supply worries have driven French and German forward power prices to new heights. French power contracts for 2018 delivery hit a record high of 43.65 euros ($51.71) a megawatt hour (MWh) on Wednesday, up 2.71 percent.
EDF's shares were up 2.21 percent at 10.8 euros, their highest since Sept. 29.
The company, which has lowered its 2017 nuclear generation target to 385-392 terawatt hours (TWh) from 390-400 TWh, said on Monday its cumulative nuclear output so far this year was down 1.3 percent compared with same period last year.
($1 = 0.8442 euros) (Reporting by Bate Felix; Editing by Sudip Kar-Gupta and Mark Potter)