There was no risk of a radioactive leak after the blast, caused by a fire near a furnace in the Centraco radioactive waste storage site, said officials.

The owner of the southern French plant, national electricity provider EDF, said it had been "an industrial accident, not a nuclear accident". The cause of the blast was not yet known, said the company.

The explosion hit the area at 11:45 local time on September 12. A security cordon was set up as a precaution.

The International Atomic Energy Agency (IAEA) said it was in touch with the French authorities to learn more about the nature of the explosion.

France's Environment Minister Nathalie Kosciuscko-Morizet visited the site the same day, to help carry out a precise evaluation of the possible radiological impact of this accident.

Marcoule was opened in 1955 and is one of France's oldest nuclear sites, though it has been extensively modernised.

All the country's 58 nuclear reactors have been put through stress tests in recent months, following the disaster at Japan's Fukushima nuclear plant which was hit by an earthquake and tsunami.

France is the world's most nuclear-dependent country, relying on nuclear power to meet 75 percent of its energy needs.