Investigators suspect a chemical reaction involving hydrogen caused the blast at metal and chemical company Mitsubishi Materials Corporation’s Yokkaichi plant, about 220 miles west of Tokyo.
The blast appeared to have occurred when plant workers were rinsing heat exchange equipment at the silicone plant during maintenance, according to a Mie prefectural police official, who spoke on condition of anonymity due to department rules.
Fire department officials said that 17 people were taken to hospitals.
The dead and injured were all believed to be workers at the factory.
In October, the Japanese government admitted it could cope with one of the world’s worst nuclear disasters of all time alone: Fukushima.
Reports emerged of leaks, radiation exposure and other issues, from rats gnawing cables to workers mistakenly removing pipes and exposing six colleagues to radioactive water.
The International Research Institute for Nuclear Decommissioning (IRID), set up about a month ago, was launched with a press conference in Japan but almost no international fanfare.