A French nuclear reactor has gone into an unscheduled shutdown, state energy giant EDF said on Friday, placing fresh pressure on a national power grid already strained by freezing temperatures.

The company said the shutdown, on the number two reactor at the Cattenom plant in the northeast of the country, was caused by a broken alternator in a non-nuclear part of the reactor and posed no threat to public safety.

But the fault will take several days to repair and the reactor will remain offline at a time of high demand. EDF said it thought household consumption had peaked on Wednesday and would fall as the weather warms up.

Another French reactor, at the Tricastin plant in the Rhone valley, stopped on Thursday, but was due to go back into service on Friday. France has 58 reactors in all, and 75 percent of its power is nuclear-generated.