The authority and local police officials said there had been no radiation leak. Some five hours after the explosion, the authority announced that the episode was over. The site, about 20 miles from Avignon, has no nuclear reactors, the authority said. A spokesman for the French power utility E.D.F., which owns the site, said, “It is an industrial accident, not a nuclear one.”
Olivier Isnard, an emergency manager at France’s Institute for Radioprotection and Nuclear Safety, said that the explosion took place in the foundry of the waste processing plant, which was melting down about four tons of used, mildly radioactive metal objects. The cause of the explosion was not yet known, he said, but he emphasized that the level of radiation — about 67,000 becquerels — contained in the molten metal was minor.
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“This is very, very low — nothing close to the radioactivity you would find inside a nuclear power plant,” he said.
Even so, firefighters set up a security perimeter around the installation.
The spokesman for E.D.F. said that the foundry oven was used to destroy two types of low-level waste — “metallic waste, like tools and pumps,” and “burnable waste, like gloves or technicians’ overalls.”
He said that the fire caused by the explosion had been “controlled.”
The French Interior Ministry said that the workers were not contaminated. The Nuclear Safety Authority said one of the four injured people was in serious condition.
The facility where the explosion took place is known as the Centraco — short for Centre Nucléaire de Traitement et de Conditionnement — and is owned by Socodei, a subsidiary of E.D.F. It is near the Marcoule nuclear research center, one of France’s oldest. The Marcoule plant uses recycled nuclear waste to produce MOX fuel for reactors.
Mr. Isnard said that initial tests at the Centraco site showed no change to environmental radiation levels, and that the foundry building’s conditioning and ventilation systems continued to function normally. The safety dispatched a crisis team and a group of specialized firefighters to take air and soil samples for further analysis, he said.
Nathalie Kosciusko-Morizet, a government minister responsible for energy and environmental issues, was expected to arrive at the accident site later Monday. Her aides declined to make any immediate comment.
Cécile Duflot, a leader of the French Green Party, asked the government “for the greatest transparency, in real time, about the situation and the environmental and health consequences.”
France gets 77 percent of its electricity from nuclear power, a much higher ratio than in other large economies.
J. David Goodman contributed reporting from New York.
This article, “Blast at French Nuclear Site Is Said to Kill One Person,” originally appeared at The New York Times.