One person has been killed and four injured, one seriously, in a blast at the Marcoule nuclear site in France.
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 (09:45 GMT). A security cordon was set up as a precaution.
But interior ministry spokesman Pierre-Henry Brandet later said there had been no leak of radiation, neither inside nor outside the plant.
None of the injured workers was contaminated by radiation, said officials. The worker who died was killed by the blast and not by exposure to nuclear material.
The Centraco treatment centre belongs to a subsidiary of EDF. It produces MOX fuel, which recycles plutonium from nuclear weapons. There are no nuclear reactors on site.
The French nuclear programme does not have a stellar record of transparency. In environmental circles, particular opprobrium is reserved for officials who in 1986 claimed the Chernobyl accident would have no impact on France – a statement lampooned as indicating officials believed radioactive fallout observed national boundaries.
What this incident implies for the future of the French nuclear programme is not entirely clear. If it remains a relatively minor matter, it will probably be passed off as the type of thing that regrettably happens in all types of industrial facility.
However, Marcoule is on the list of candidate sites to host one of the European Pressurised Water Reactors (EPRs) that according to government policy are to provide the next generation of French citizens with nuclear electricity.
The EDF spokesman said blast happened in a furnace used to burn waste, including fuels, tools and clothing which had been used in nuclear energy production but had only very low levels of radiation.
“The fire caused by the explosion was under control,” he said. Another official later said the incident was over.
The International Atomic Energy Agency (IAEA) said it was in touch with the French authorities to learn more about the nature of the explosion.
Speaking on the sidelines of a scheduled meeting of the IAEA’s board, Director General Yukiya Amano said the organisation’s incident centre had been “immediately activated”.
France’s Environment Minister Nathalie Kosciuscko-Morizet visited the site on Monday, to “help carry out a precise evaluation of the possible radiological impact of this accident”.
“For the time being, no exterior impact has been detected,” the AFP news agency quoted a ministry spokesman as saying.
“There are several detectors on the outside and none of them detected anything, the building is sound.”
Marcoule was opened in 1955 and is one of France’s oldest nuclear sites, though it has been extensively modernised.
It is located in the Gard department in Languedoc-Roussillon region, near France’s Mediterranean coast.
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.
EDF’s share prices fell by more than 6% as news of the blast emerged.
France is the world’s most nuclear-dependent country, relying on nuclear power to meet 75% of its energy needs, so safety in the industry is a highly sensitive issue, says the BBC’s Christian Fraser in Paris.
In June, France announced it was investing 1bn euros (£860m) in nuclear power, including a significant boost for safety research.
French nuclear giant Areva is developing the next generation of nuclear reactors and has been involved in a huge publicity campaign since the Fukushima disaster to reassure the public of the safety of nuclear energy.
Other countries in Europe, including Germany, Italy and Switzerland, have said they will reduce or phase out their use of nuclear power over the next few years.