CLAIRTON, Pa. — Federal investigators released a final report on Tuesday regarding the fatal explosion at the U.S. Steel Clairton Coke Works facility.
The release and explosion of toxic, flammable coke oven gas killed two workers, injured 11 others and caused an estimated $52.5 million in property damage on Aug. 11, 2025.
According to the U.S. Chemical Safety and Hazard Investigation Board, the explosion was caused by unapproved maintenance practices, poorly placed personnel buildings and deficient safety management systems.
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Steve Owens, chairperson of the CSB, criticized the facility’s operational practices following the findings.
“This deadly incident was the result of an ad hoc informal procedure, poor facility siting and an ineffective process safety management system at the Clairton facility,” Owens said. “It should never have happened.”
Built in 1901, the Clairton facility is the largest coke manufacturing plant in the Western Hemisphere. The site currently operates six coke batteries containing a total of 455 ovens.
Coke production involves heating raw metallurgical coal to approximately 2,000 degrees Fahrenheit for a minimum of 18 hours. The process leaves behind nearly pure carbon called coke, which is burned as fuel in blast furnaces for steel manufacturing. The process releases highly flammable and toxic coke oven gas into an off-gas piping system. Once tar, ammonia, light oil and elemental sulfur are removed, the remaining gas mixture of hydrogen, methane, nitrogen and carbon monoxide is reused as fuel to heat the ovens.
The CSB said the fatal explosion occurred during a maintenance operation involving U.S. Steel employees and contractors from MPW Industrial Services. Workers were attempting to fully close and reopen a double disc gate isolation valve supplying coke oven gas to Battery 13. Facility staff routinely performed this “exercising” procedure to ensure valves could move through their full range. However, coke oven residue frequently accumulated in the valve seats, making full closure difficult.
To clear the residue, U.S. Steel employees had used pressurized water on an ad hoc basis for at least three years prior to the explosion. The company had no formal written procedure for washing valves with water, according to the CSB. On the day of the incident, a supervisor arranged for MPW Industrial Services contractors to connect a pump truck and apply pressurized water to the valve seats. Closing the double gates created an enclosed space inside the valve. The trapped pressurized water exceeded the valve’s pressure capacity, causing a catastrophic structural failure that released gas into the facility.
The failed cast iron valve was manufactured in 1953, making it more than 70 years old and had been refurbished in 2013. Safety publications prohibit or warn against using brittle cast iron equipment in hazardous applications involving flammable gas.
The release occurred in a transfer area between Batteries 13 and 14, directly underneath routinely occupied work buildings located less than 20 feet above the piping. None of the structures were built to withstand an explosion and all suffered catastrophic destruction.
Drew Sahli, CSB investigator in charge, highlighted the hazards of placing personnel structures near hazardous process lines.
“When buildings are occupied by personnel, they must be adequately designed or located to protect the personnel or equipment from fires, explosions, or toxic releases,” Sahli said. “Had these buildings been located in a different area of the facility, away from coke batteries, this incident could have been far less severe.”
The two workers killed in the blast were inside or near separate control rooms directly above the gas piping. Emergency responders located one victim on the ground level after the blast, while search and rescue teams found the second victim underneath rubble roughly 9 hours later. Two workers inside a nearby break room suffered severe injuries, including facial shrapnel, burns and multiple fractures. One worker crawled out of the debris to seek help, while emergency crews rescued the second worker from the rubble 4 hours after the explosion. Nine other people outside the buildings were also injured, three of them seriously.
CSB Board Member Sylvia Johnson emphasized the need for formal operating guidelines and oversight.
“This incident was the result of workers routinely performing a task incorrectly over a period of years until it ultimately led to a catastrophic explosion,” Johnson said. “Companies must outline procedures for any task that could potentially cause harm to workers and make sure that workers are fully trained on those procedures.”
Following the investigation, the CSB issued safety recommendations to U.S. Steel, Nippon Steel North America and MPW Industrial Services. The board recommended that U.S. Steel conduct facility siting evaluations for occupied buildings, establish written procedures for pressurized water valve cleaning and implement a comprehensive safety management system for coke oven gas processes. The CSB recommended that Nippon Steel North America create a corporate safety governance program with regular facility audits and that MPW Industrial Services establish written cleaning procedures for flammable gas piping. The CSB is an independent federal agency that conducts root-cause investigations and issues safety recommendations rather than fines or citations.
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