Pipe corrosion, maintenance, and labor unrest have nearly half of French nuclear reactors offline. 

The result is France's Worst Energy Crisis Since the 1970s.

Twenty-six of France's 56 nuclear reactors are offline for maintenance or because of corrosion on piping that cools the reactor cores. Fixing the corrosion is taking longer than expected at several reactors, delaying their restart by as much as six weeks, according to regulatory filings and a French nuclear executive familiar with the matter.

Labor unrest is another obstacle. Strikes at 18 reactors owned by EDF SA, France's state-controlled power giant, have delayed their restart by several weeks, threatening the government's plans to have all of them back online by the end of the winter. EDF and union leaders said they reached an agreement Friday on salary increases, ending the strikes.

EDF, the world's largest owner of nuclear plants, is one of Western Europe's most important power companies. Its fleet of reactors normally exports large quantities of low-cost nuclear power to neighboring countries, helping stabilize prices across the region.

The situation changed drastically this year, when France swung from being one of Europe's largest exporters of electricity to a net importer because of the outages at its reactors. The rash of outages has officials worried that France and the broader region might run short of electricity in the winter, when power demand in Europe peaks.