as reported by Kyodo News. Interestingly, if you go to Kyodo News site the article is not there. It was carried by several local/regional papers but in print only, as far as I know.
Rough translation from the original Japanese article, as appeared in Kumamoto Nichinichi Shinbun (9/15/2011):
French nuclear facility: Explosion took place right after the restart of the furnace operation
Employee says "The melting furnace broke down last week"
(Kyodo News-Paris) Regarding the explosion that took place on September 12 at the low-level radioactive waste treatment center (CENTRACO) in Marcoule in southern France, it was revealed that the melting furnace that exploded had broken down last week, and it was restarted on the day of the accident. French papers including Le Figaro reported it on September 14 as information from the sources in the French prosecutor's office who interviewed the employees at the center.
According to the employees testimony, the melting furnace, which is used to melt low level radioactive waste like workers' gloves, protective clothes and metal pumps, was stopped last week after "numerous problems" had been detected. The operation resumed in the morning of September 12, but metals weren't melted well. When an employee tried to break the lump using a stick-shaped object, the molten metal spewed from the lump, causing the explosion.
French prosecutors are set to start investigation on death and injury by negligence in a few days.
From the beginning of the accident, the French media had pointed out the possibilities of heated metal coming in contact with water, or of some chemical reaction taking place near the furnace as the cause of the explosion.
However, the prosecutors say, "To promote further melting using an equipment is a well-known procedure, even if it not practiced widely", indicating they haven't ruled out other possibilities.
According to the major French TV network TF1, the area around the furnace remains in high temperature and no one can approach anywhere near it, and the prosecutors and the French nuclear control bureau haven't made much progress in their investigation.
Le Figaro's article on September 13 had these details:
Furnace was set at 1,500 degrees Celsius but the metal didn't completely melt;
The employee tried to break the molten mass with a crowbar ("barre à mine").
But just as TEPCO has the manual for the outside world and the "real" manual(s) for the workers to follow, breaking the molten metal blob with a crowbar probably is part of the "real" manual at the French plant.