A report in Ouest France newspaper said there may be some injuries. M6 radio said five people were slightly injured.
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Officials at the state-backed company's Paris headquarters had no immediate comment and local officials at the plant in western France could not be immediately reached for comment.
The Flamanville 1 and 2 reactors, 1,300 MW each, were build in the 1980s. A new reactor is being built at the site but the explosion did not take place there, a spokeswoman said.
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The nuclear safety authority ASN said it had no information and could not immediately comment.
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