Sweden's nuclear energy regulator, SSM, has asked plant operators to produce plans in the coming months to shield their reactors from harmful hot weather, its director told Reuters on Monday.

A number of Swedish reactors had to shut down or reduce output as the summer heatwave sent temperatures to record highs in July, with the sea water that is used to cool them becoming much warmer than normal, exceeding safety levels. The last time that SSM, the Swedish radiation safety authority, asked operators to produce plans to modify their reactors was after Japan's Fukushima disaster in 2011. The cost of those changes, which are due by 2020, was in the hundreds of millions of euros.

"We really have to take into consideration what happened this summer. ... We have asked them orally to come with suggestions. Of course there will be a cost. I do not know how much at this stage," said SSM chief Mats Persson.