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...