CSB releases final report into fatal U.S. Steel Clairton Coke Works explosion

The CSB today released its final report on the fatal Aug. 11, 2025, explosion at U.S. Steel’s Clairton Coke Works in Clairton, Pennsylvania.

The incident occurred when toxic flammable coke oven gas was released and exploded - killing two people, injuring 11 others, five of them seriously, and causing an estimated $52.5 million in property damage.

“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.”

— Steve Owens, CSB Chairperson

Flammable coke oven gas piped back into ovens for fuel

At U.S. Steel’s Clairton Coke Works, the coke batteries have three levels: a top level where workers pour coal into the top of ovens, a “bench” level (along the lower blue dashed line) where the control rooms and break room were located, and the “basement” underneath which contained the coke oven gas supply piping. Source: CSB

The U.S. Steel Clairton Coke Works facility was built in 1901 and is the largest coke manufacturing facility in the Western Hemisphere. The Clairton facility processes coke inside “coke ovens,” which are connected by common walls and operated as one unit, called a “coke battery.” U.S. Steel Clairton currently operates six coke batteries, with a total of 455 ovens.

The process of producing coke involves heating raw metallurgical coal to approximately 2,000 degrees Fahrenheit for a minimum of 18 hours. The nearly pure carbon that remains in the oven is called “coke,” which is often used as fuel in blast furnaces for iron and steel production.

During the production process, gases, known collectively as “coke oven gas,” are released and driven off into an off-gas piping system. Coke oven gas is highly flammable, toxic, and colorless with a sulfurous odor.

After tar, ammonia, light oil, and elemental sulfur is removed, the remaining coke oven gas is a mixture of hydrogen, methane, nitrogen, and carbon monoxide, which is used, in part, to burn as fuel to heat the coke ovens.

U.S. Steel lacked formal valve cleaning procedure

The CSB illustrated the likely sequence of events that caused the valve to fail in its final report. Workers were attempting to “exercise” the Battery 13 gas isolation valve to ensure it moved through its entire range before isolating the battery for other planned repairs. Source: CSB

The fatal 2025 explosion occurred during a maintenance operation during which U.S. Steel employees and contractors from MPW Industrial Services (MPW) attempted to fully close and reopen a double disc gate isolation valve in the piping that supplied coke oven gas to Battery 13.

The CSB found U.S. Steel maintained an operating procedure for “exercising” Battery 13’s cast iron double gate isolation valve, as well as other valves at the facility. Exercising a valve included closing it and then reopening it, to ensure that the valve could successfully operate through its full range.

U.S. Steel employees told the CSB that they occasionally had difficulty fully closing a valve when exercising it due to the coke oven residue that accumulated in the valve seat over time.

The CSB also found U.S. Steel had a practice of injecting steam or high-pressure water into a valve’s cleanout port when the 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 the company had no formal procedure for washing valves with water.

Nevertheless, despite the lack of a formal procedure for using water, U.S. Steel employees had used pressurized water to clean valves on an ad hoc basis for at least three years prior to the incident.

“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.”

— Sylvia Johnson, CSB Board Member

Failed cast iron valve was 70 years old

When workers closed the valve gates, pressurized water split open the valve housing, breaking the valve seal and releasing flammable gas which then escaped through the crack. The 70-year-old valve was made from brittle cast iron. Source: CSB

On the day of the incident, a U.S. Steel supervisor decided to exercise the Battery 13 isolation valve, to confirm that the valve operated correctly. The supervisor arranged for contractors from MPW, a company that provides industrial cleaning services, 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, which created an enclosed space between the two gates. The pressurized water filled this space inside the valve and caused the pressure inside the valve 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 over 70 years old. The valve was originally manufactured in 1953 and had been refurbished in 2013.

Cast iron is widely recognized as a brittle material, and numerous safety publications either prohibit or warn against using cast iron equipment in certain hazardous applications, including for flammable substances like coke oven gas.

Workers injured & killed in rooms directly above piping

The explosion occurred in an area located between Batteries 13 and 14 called the “transfer area,” where there were multiple buildings 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 those buildings were designed or constructed to withstand an explosion, and all of them 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, and two of the five seriously injured workers were inside a break room, also directly above the coke oven gas piping.

In Dec. 2025, the CSB issued two interim safety recommendations to U.S. Steel regarding facility siting. While the company has since chose to administratively require employees to take breaks in other locations, the CSB’s final report outlines potential safety hazards from relocating the control rooms and personnel to a building 100 feet from the battery transfer area. Source: CSB

One of the fatally injured employees was propelled by the force of the explosion and was 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 was found by a search and rescue team roughly nine hours after the explosion.

Both of the workers in the break room at the time of the explosion were seriously injured by debris from the room’s walls and ceiling. One of them 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 of these two workers freed himself from the rubble, crawled out of the debris, and found help. The other was trapped under debris and was unable to free himself. Emergency responders located him, alive, roughly four hours after the explosion.

“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.”

— Drew Sahli, CSB Investigator in Charge

Nine other people outside of those buildings were also injured, three of them seriously.

CSB recommended developing PSM procedures, facility citing evaluation

The CSB identified three key safety issues that led to the severity of the incident:

1. Procedures and hazard analysis
2. Facility siting
3. Process safety management (PSM) systems.

As a result of its investigation, the CSB made a number of recommendations to U.S. Steel, Nippon Steel North America (NSNA), and MPW.

The CSB recommended U.S. Steel conduct a siting evaluation for all occupied and potentially-occupied buildings at the Clairton Coke Works and mitigate all facility siting hazards that are identified.

The CSB also recommended U.S. Steel develop a written procedure for washing valve seats with pressurized water to minimize the hazards of the operation, and develop a comprehensive PSM system for all coke oven gas processes at the Clairton facility.

Additionally, the CSB recommended that NSNA develop a corporate process safety governance program that will implement PSM at its facilities, as well as conduct regular audits of facilities throughout the company’s organization and promptly correct process safety deficiencies.

The CSB recommended that MPW develop written policies and procedures for cleaning piping systems containing flammable or toxic gas and ensure that all workers involved in such operations are trained on the policies and procedures.