PLAQUEMINE — Federal regulators say portable work lights left inside a large drum are to blame for a series of explosions that rocked the Dow Chemical plant near Plaquemine in the summer of 2023.

The U.S. Chemical Safety and Hazard Investigation Board said Thursday that the lights had been left inside a drum during maintenance work in May 2023. According to its report, metal on the lights deteriorated and created debris that punctured a rupture disc in a tank holding ethylene oxide.

Separate problems with the relief piping had caused it to lose its inert nitrogen atmosphere and let it fill with air. After the air and ethylene oxide mixed, the increased pressure popped a relief valve, and the resulting flame front moved into the vapor space of the drum. The heat caused even more pressure, triggering explosions and fires on July 14, 2023.

Local officials issued a shelter-in-place order for hundreds of residents.

Investigators said they found fragments of a portable work light in the debris and said at least three lights had been left behind. As the lights broke down in the ethylene oxide, some parts flowed into other equipment, causing pump vibrations that initially took the unit down. An operator restarted the unit two minutes later.

"This catastrophic incident should never have happened," board chairman Steve Owens said. "The workers did not remove all the work lights from inside the drum and Dow did not have an effective procedure in place to ensure that they did so."

The board also found that Dow did not have adequate procedures to confirm the drum was clean before it was sealed and restarted. Investigators also found that nitrogen had slowly leaked out of the pressure relief piping over time and the piping had filled with air, allowing the ethylene oxide to ignite.

Ethylene oxide is a flammable chemical and a known human carcinogen.