The U.S. Chemical Safety and Hazard Investigation Board (CSB) released its final report on the fatal August 11, 2025, explosion at the U.S. Steel Clairton Coke Works facility in Clairton, Pennsylvania. The incident fatally injured two people, injured 11 others, five of them seriously, and caused an estimated $52.5 million in property damage.

The U.S. Steel Clairton Coke Works facility was built in 1901 and is the largest coke manufacturing facility in the Western Hemisphere. U.S. Steel currently operates six coke batteries at the Clairton facility, with a total of 455 ovens.
The explosion occurred during a maintenance operation in which U.S. Steel employees and contractors from MPW Industrial Services (MPW) were attempting to fully close and reopen a double disc gate isolation valve in piping that supplied coke oven gas to Battery 13.

CSB found that U.S. Steel maintained an operating procedure for exercising Battery 13's cast iron double gate isolation valve and other valves at the facility. Exercising a valve involved closing it and then reopening it to ensure that it could operate through its full range.

U.S. Steel employees told the CSB that they occasionally had difficulty fully closing a valve because coke oven residue accumulated in the valve seat over time. CSB also found that U.S. Steel had a practice of injecting steam or high-pressure water into a valve's cleanout port when a valve would not close fully in an attempt to remove residue.

However, the company's exercising procedure did not mention the use of water, and U.S. Steel had no formal procedure for washing valves with water. Despite the lack of a formal procedure, employees had used pressurized water to clean valves on an ad hoc basis for at least three years before the incident.
On the day of the incident, a U.S. Steel supervisor decided to exercise the Battery 13 isolation valve to confirm that it operated correctly. The supervisor arranged for MPW contractors to use a pump truck to apply pressurized water to the valve.

While applying pressurized water to the valve's seats, the workers closed or nearly fully closed the valve's double gates, creating an enclosed space between the gates. The pressurized water filled the space inside the valve and caused pressure to increase beyond the valve's ability to contain it. The valve then failed catastrophically, releasing toxic flammable coke oven gas into the surrounding area. Within minutes, the released gas ignited and exploded.
The failed valve was made from cast iron and was more than 70 years old. It was originally manufactured in 1953 and refurbished in 2013. CSB noted that cast iron is widely recognized as a brittle material and that numerous safety publications either prohibit or warn against using cast iron equipment in certain hazardous applications, including for flammable substances such as coke oven gas.

The explosion occurred in the transfer area between Batteries 13 and 14, where multiple buildings were routinely occupied by employees. The buildings were located less than 20 feet directly above the coke oven gas piping that was the source of the release. None of the buildings were designed or built to withstand an explosion, and all were catastrophically damaged.

At the time of the explosion, the two fatally injured workers were each in or near separate control rooms directly above the coke oven gas piping. Two of the five seriously injured workers were inside a break room, also directly above the piping.

One of the fatally injured employees was propelled by the force of the explosion and found underneath rubble by emergency responders on the ground level adjacent to the coke batteries. The other fatally injured worker was buried in debris and found by a search and rescue team roughly nine hours after the explosion.

Both workers in the break room were seriously injured by debris from the room's walls and ceiling. One suffered burns, broken ribs, broken vertebrae and a broken tibia. The other suffered explosion shrapnel to his face, burns and broken bones in his spine, ankle, lower legs, knees and hands. One worker freed himself from the rubble, crawled out of the debris and found help. The other was trapped under debris and unable to free himself; emergency responders located him alive roughly four hours after the explosion.
Nine other people outside the buildings were also injured, three of them seriously.

CSB identified three key safety issues that contributed to the severity of the incident: procedures and hazard analysis, facility siting, and process safety management systems. Following its investigation, CSB made recommendations to U.S. Steel, Nippon Steel North America (NSNA) and MPW.

CSB recommended that U.S. Steel conduct a siting evaluation for all occupied and potentially occupied buildings at the Clairton Coke Works and mitigate identified facility siting hazards. It also recommended that U.S. Steel develop a written procedure for washing valve seats with pressurized water and develop a comprehensive process safety management system for all coke oven gas processes at the Clairton facility.

CSB recommended that NSNA develop a corporate process safety governance program to implement process safety management at its facilities, conduct regular audits throughout the company's organization and promptly correct process safety deficiencies.

For MPW, CSB recommended developing written policies and procedures for cleaning piping systems containing flammable or toxic gas and ensuring that all workers involved in those operations are trained on the policies and procedures.

Ad hoc informal procedures lead to tragedy

In a recent quote, CSB Chairperson Steve Owens, said, “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. It should never have happened.”

CSB Investigator in Charge Drew Sahli added, “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. Had these buildings been located in a different area of the facility, away from coke batteries, this incident could have been far less severe.”

CSB Board Member Sylvia Johnson added, “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. 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.”