The Mie Prefecture Police Department reported that the explosion occurred during a maintenance and cleaning operation at the chemical plant. According to Mitsubishi Materials, a heat exchanger exploded outside one of its facilities while it was being cleaned by approximately 20 workers and supervisors. The company speculated that the explosion could have been caused when a chemical agent inside the heat exchanger was exposed to air.
The fire department reported that three of the five who died were Mitsubishi Materials employees and at least one person was taken to Mie University Hospital.
Yokkaichi is about 200 miles from Tokyo.
[The Japan Times]