The Nuclear Safety Authority said no radioactive leaks have been detected in the blast at 12:37 p.m. (1037 GMT, 6:37 a.m. EDT) at an oven in the Centraco nuclear site. The accident was under control within the hour, the agency said in a statement.
Centraco is located on the grounds of another nuclear site, Marcoule, in the Languedoc-Roussillon region near the Mediterranean Sea.
"According to initial information, the explosion happened in an oven used to melt radioactive metallic waste of little and very little radioactivity," the statement said. "There have been no leaks outside of the site."
Those injured were not contaminated with radiation, and the outside of the building that houses the oven showed no sign of damage or contamination either, the agency said in a separate statement.
Officials from France's EDF power company, whose subsidiary operates Centraco, stressed that there was no nuclear reactor on the site and that no waste treated at the site of the explosion came from a reactor. Spokeswoman Carole Trivi said a fire broke out after the explosion, but it has since been brought under control.
The cause of the blast was not immediately known, and an investigation has been opened, Trivi said.
The material at Centraco comes from nuclear sites and therefore is mildly radioactive, spokeswoman Carole Trivi said. She said the site treats mostly waste from EDF's own power plants, as well as a small amount of material from hospitals or medical research labs.
Nothing comes from weapons manufacture, she said.
Staff at the plant reacted to the accident according to planned procedures, the Nuclear Safety Authority said in the statement. The local Midi Libre newspaper reported that no quarantine or evacuation measures had been immediately undertaken.
A separate statement from France's Nuclear and Alternative Energy Commissions added that the site of the explosion remains sealed and its ventilators were operating.
Centraco is located on the 300-hectare Marcoule site, which also houses a research center and four industrial sites, including one that makes Mox, a fuel made from plutonium and uranium.
The head of the Vienna-based International Atomic Energy Agency, Yukiya Amano, said his organization's "incident and emergency center was immediately activated and has sent requests for detailed information."
France is the world's most nuclear-dependent country in the world, with the lion's share of its electricity coming from the 58 nuclear reactors that dot the country. France is also a major exporter of nuclear power, treats nuclear waste from around the world, and state-owned nuclear giant Areva is one of the country's most prominent companies.
The kind of soul-searching about using nuclear power that swept the world following Japan's March 11 tsunami and the disaster at the Fukushima nuclear plant have been largely absent in France, which has stuck firmly to its pro-nuclear policy.
In June, President Nicolas Sarkozy pledged that France will stick to a plan to invest euro1 billion ($1.37 billion) in future nuclear reactors.
By contrast, neighboring Germany took eight of its older reactors off the grid in the wake of the Japanese disaster and lawmakers have voted to shut the country's nine remaining nuclear plants by 2022.
Still, French environmentalists have long called for the end to the country's nuclear program, and the Europe Ecology-Greens party urged transparency in responding to Monday's accident.