EDF's poor nuclear forecasts put off power traders

* Since mid-April nuclear power forecast to rise sharply

* Actual output systematically below projections

* Traders say cutting exposure to France's power market

* France's regulator says keeping close eye

By Michel Rose and Muriel Boselli

PARIS, July 4 (Reuters) - A persistent gap between the French electricity grid's forecasts of available nuclear power and the amount that eventually feeds into the network is discouraging participation in a small power market with little price movement, traders said on Wednesday.

State-owned utility EDF, which operates France's 58 nuclear reactors, provides power grid RTE with capacity data, which it then uses to produce short-term, medium-term and long-term availability forecasts.

French power generators in 2010 started publishing projected output data per unit of production as well as unplanned outages and restarts to help traders accurately gauge the supply/demand balance, which could encourage trading in the market.

France's energy regulator, the European Commission and traders had put increasing pressure on EDF, the former power monopoly, to boost data transparency of its reactors, which provide 75 percent of the country's power mix.

"It's now been two months that unavailability is higher than what is forecast," one London-based trader said.

"It's one thing to see mistakes from time to time, but every day - this is not acceptable," the trader said, adding the margin of error could at times reach 23 percent.

"This leads to a loss of confidence in the French market, with prices people cannot compute, and this is worrying," a Geneva-based trader said.

Traders said the constant gap between forecasts and actual generation could give EDF's own traders an unfair advantage, leading some to mull the possibility of lodging a complaint to the European Federation of Energy Trading (EFET).

An EFET spokeswoman declined to comment.

EDF said the utility was relaying accurate information in a speedy manner and declined to comment on traders' suspicions.

Delays in restarts are possible because of the high level of nuclear outages, an EDF spokeswoman said.

"We are at a turning point because reactors are ageing," she said. "We have many reactors undergoing once-a-decade maintenance; we have a lot of work going on."

EDF traditionally carries out maintenance in the spring and the autumn to help ensure reactors are available to meet peak electric demand in the summer and winter.

A spokeswoman at France's energy regulator (CRE) said the watchdog was keeping a close eye on the issue but could not say whether this would lead to an investigation.

CONSISTENTLY FALLING SHORT

Since mid-April, weekly forecasts of nuclear availability in the short and medium term have predicted sharp increases for the subsequent week, but capacity coming online has consistently failed to pick up.

The gap coincides with a record high level of nuclear outages at EDF's 19 plants with off-line capacity at 20,600 MW, or 32.6 percent, according to Reuters data.

As an example, forecasts for the week of June 25 published on RTE's website 10 days in advance predicted nuclear capacity of 45,055 MW. Actual capacity that week was 14 percent less at 38,680 MW on average.

The discrepancy was even greater for the week of May 28, when actual capacity reached a 2012 low of 35,727 MW. The first medium-term forecast for that week issued on March 2 was for 48,598 MW.

And the day before May 28, the forecast was still for a capacity of 39,358 MW, more than 9 percent higher than the actual online capacity.

Even if the record low in nuclear availability is unlikely to provoke price spikes this summer, while hydropower reserves have been high and temperatures have been relatively cool for the season, there could be consequences for the liquidity of the French market.

The French electricity market is around a fourth the size of Germany's with just over 5 million megawatt hours traded in June on the EPEX Spot bourse.

"We despair. If they want to kill the market they are going the right way about it," another London-based trader said. (Editing by Jane Baird)