The blast occurred at around 9:19 a.m. during welding work on a storage tank to expand a wastewater disposal facility at its plant in Ulsan, the company said in a statement on Friday.

The Ulsan plant mainly produces polyvinyl chloride, a plastic used in a wide range of products. It is the second-biggest chemical factory operated by Hanwha Chemical, an affiliate of Hanwha Group.

The cause is yet to be determined, but the company officials suspect that the explosion was triggered by a gas leak. They said the project manager did not measure the levels of gas inside the storage unit before allowing laborers to do the welding work.

The police already raided the offices of safety-related teams at Hanwha Chemical’s factory in Ulsan and Hyundai Environment’s office in Busan on Friday and seized job sheets, safety inspection reports and a floor plan for the factory. More raids could follow if necessary, the police said.

It also conducted an autopsy to determine the cause of death of six workers killed in the blast. The result will be released in three to four days, the police said.

The Ulsan branch of the Employment and Labor Ministry is questioning witnesses and reenacting the events that led up to the explosion as part of efforts to find the cause of the accident.

If Hanwha Chemical or Hyundai Environment were found to have neglected their duty to prevent such an explosion from happening, the Labor Ministry will punish the companies for violating occupation safety and health acts, it said. 

By Ock Hyun-ju
(laeticia.ock@heraldcorp.com)