The site of the accident (left) at Marcoule is very close to where a recent accident happened at Tricastin nuclear power plant, roughly equidistant between Marseilles and Lyons and very close to Nîmes and Avignon.
It is a MOX (extremely dangerous Plutonium + Uranium fuel, recycled from nuclear weapons) producer plant, it also produces tritium (heavy hydrogen).
One worker was killed and four others injured, one of them with serious burns.
Allegedly the accident did not release any radiation outside the plant but that's what they always say by default, so wait and see...
The official info says that the explosion happened in an oven used to melt metallic waste of little or no radioactivity.
The cause of the blast is not yet known.
An anonymous official said to AFP that there is risk of a radioactive leak (which would naturally go to the Rhône river and the Mediterranean Sea).
While some media claim that Marcoule has no nuclear reactors on site, Wikipedia informs that it has four in fact: G1, G2, G3 and Phénix.
What is clear is that we cannot leave with this Damocles' sword on our heads: the advantages are almost nil and the dangers immense. Nuclear energy stop now!
Update (Sep 15):
According to Ex-SKF and Le Figaro, the exploded furnace had broken down a week earlier and the day of the accident was the first day to work again. It is an oven to burn or melt low radioactivity material like workers' gloves.
At the moment of the accident, metals were not melting properly at some 1500 degrees Celsius and a worker improperly used a crowbar to try to break the items. However it is not clear if this was the cause of the explosion.
The area around the oven remains at high temperature and nobody has been able to approach it, so the investigation remains in wait.