One person was killed and four injured in a blast at a nuclear site in the south of France on Monday as the government sought to play down fears of a radioactive leak.

There was "no cause for concern", Environment Minister Nathalie Kosciuscko-Morizet said at the scene of the explosion at Codolet in the Rhone Valley, near the southern city of Nimes.

"None of the detectors on site, both inside and outside, have detective any radioactive leak," she said after meeting family members of the victims, adding the cause of the blast was not yet known.

France's state nuclear regulator had earlier warned there was a risk of a leak.

National electricity provider EDF confirmed one person died in the explosion in an oven at the site.

One of the injured was in a serious condition, France's Nuclear Safety Authority (ASN) said, officially declaring the incident "over".

"This accident has no radiological risk or need for population protection," the ASN said, adding it had suspended a crisis cell set up to deal with the incident.

The blast hit the Centraco nuclear waste treatment centre belonging to EDF subsidiary Socodei, said a spokesman for the Atomic Energy Commissariat (CEA).

"Initial reports suggest there was an explosion in an oven used to melt metallic low- and very low-level radioactive waste," the ASN said.

An EDF spokesman added: "This is an industrial accident, not a nuclear accident."

"In this kind of oven, there are two sorts of waste: metallic waste such as valves, pumps and tools and combustible waste such as technicians' work outfits or gloves," the spokesman said.

An expert at the Institute for Radioprotection and Nuclear Safety in Paris, Olivier Isnard, said radioactivity levels in the oven were around 17 becquerels per kilo -- "very, very low" -- at the time of the blast.

"We don't expect there to be an impact on the environment," Isnard said, adding that samples would be taken from grass, soil and dust on cars to confirm this.

The interior ministry said no one was evacuated from near the site nor were any workers confined following the blast.

Those injured "have not been contaminated" and the fatality was caused by the explosion, the ministry said.

The site is about 20 kilometres (12 miles) north of the historic city of Avignon which throngs with tourists at this time of the year.

EDF's share price dropped over six percent on the news of the blast.

France said in June it would invest one billion euros ($1.4 billion at the time) in nuclear power development while boosting research into security.

France produces most of its energy from nuclear power. Some countries, notably EU neighbour Germany, have rejected atomic power after the Fukushima reactor disaster in Japan.

Italy, meanwhile, said the blast highlighted its own vulnerability to atomic accidents on the territory of its neighbours.

"Unfortunately, as we have always said, the (nuclear) stations near our borders, 19 of them beyond the Alps, will not spare us in case of dysfunction," said Secretary of State for Economic Development Stefano Saglia.

Environment Minister Stefania Prestigiacomo said Rome was "monitoring" the situation at the site.

The Marcoule site, considered the heart of France's nuclear reprocessing industry, employs about 5,000 people and holds four civil nuclear installations alongside government defence activities.

Environmental lobby group Greenpeace demanded total and immediate transparency from the authorities.

"It's essential for local populations to be informed in real time about the situation and possible radioactive discharge," said campaigner Yannick Rousselet.

Kosciuscko-Morizet said three probes were underway: by the ASN, the military police, and the labour inspectorate.

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