* France relies on nuclear for three-quarters of its power
* Hit hard by closure of ageing reactors for safety tests
* Power cuts could be imposed during winter cold snaps
* Similar design of reactors creates risk of generic flaws (Adds detail on French nuclear dependence)
By Bate Felix and Geert De Clercq
PARIS, Nov 8 (Reuters) - France could impose power cuts this winter due to an electricity shortage, an unprecedented step in the wealthy nation which would expose the vulnerabilities of its dependence on nuclear power.
The warning was issued on Tuesday by grid operator RTE, which said power supply had been hit by the closure of around a third of the country’s ageing nuclear reactors for safety checks. The country’s regulator has ordered a review of the strength of crucial steel components after the discovery of manufacturing irregularities.
France relies on nuclear for three-quarters of its power, more than any other country. RTE said the amount of nuclear power available was at a record low for this time of year, around 10,000 megawatts lower than a year ago - equivalent to more than twice the consumption of Paris and Marseille combined.
“During some periods of the day in winter, and during some days, we may need to use exceptional measures to guarantee the balance of electricity demand and supply on the network,” RTE President Francois Brottes told reporters at a news conference.
RTE would start by boosting power imports and could also pay some industrial customers to switch off their machinery or curb usage, but Brottes said the gird operator might also have to impose short, rolling power blackouts in parts of the country.
Power supplies are likely to be most stretched in the first three weeks of December, RTE said. With about a third of French homes heated by electricity, the country is highly sensitive to cold snaps.
The discovery last year of weak spots in the steel of the EPR reactor state-backed utility EDF is building in Flamanville in northwest France led nuclear regulator ASN to take a closer look at manufacturing procedures of state-owned reactor builder Areva.
In May, the ASN said the anomalies found in Flamanville had also been discovered in reactors being operated by EDF and ordered safety tests on 18 out of EDF’s 58 reactors.
Unlike other nuclear countries such as the United States and China, which have used different reactor models and suppliers, all French reactors are pressurised water reactors made by the same manufacturer, a forerunner of Areva.
This standardisation allowed France to build reactors relatively quickly and cheaply, but also created the risk that a generic design flaw or manufacturing problem would affect many reactors and incapacitate a large part of the fleet. Green activists have warned of this possible scenario for years.
Traditionally, France is a net exporter of power, but RTE said it could become a net importer during cold snaps this winter, bringing in up to 7,000-9,000 megawatts from abroad.
France is well linked to neighbouring countries via interconnector cables with a capacity of nearly 10 percent of France’s generating capacity.
But Britain has warned that power supply this winter will be tight, Belgium has had problems with the availability of its own nuclear reactors, Italy is chronically short of power and links with Spain have relatively low capacity.
That leaves just Germany and Switzerland as reliable backups, although Germany’s large reliance on intermittent renewable energy makes it less suitable as a provider of baseload power.
“The outlook is pessimistic, notably for the first three weeks of December,” said a Paris-based power trader, adding that power outages could easily happen.
The reactor closures are weighing on the power sales of EDF, which has cut its nuclear production target three times this year. They are also forcing the utility to buy expensive power on the market, further weighing on its profitability.
Ratings agency Moody’s said on Tuesday that EDF was unlikely to benefit from rising power prices offsetting the expected shortfall in volumes.
Last week, EDF issued its second profit warning of the year, lowering its 2016 core earnings forecast to 16-16.3 billion euros from the original 16.3-16.8 billion euros.
The company finally secured approval from the British government in September to go ahead with its 18 billion pound ($22 billion) project to build two nuclear reactors at Hinkley Point in England.
EDF has faced internal dissent over the project, with many critics saying the company’s balance sheet is already too stretched. EDF needs to borrow money just to pay its dividend, and will have to spend about 50 billion euros ($55.1 billion) on upgrading its ageing nuclear fleet and several billion more for its planned takeover of the reactor division of Areva ($1 = 0.8059 pounds; $1 = 0.9074 euros)
Additional reporting by Geert De Clercq and Sybille de La Hamaide; Editing by Andrew Callus and Pravin Char