Washington, D.C. August 10, 2026 — Today, 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 occurred when toxic flammable coke oven gas was released and exploded -- fatally injuring two people, injuring 11 others, five of them seriously, and causing an estimated $52.5 million in property damage.
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.”
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.
The August 11, 2025 fatal 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 the piping that supplied coke oven gas to one of U.S. Steel’s coke oven batteries, called Battery 13.
The CSB found that 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 that 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, that 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 utilized pressurized water to clean valves on an ad hoc basis for at least three years prior to the incident.
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.
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. 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.
Nine other people outside of those buildings were also injured, three of them seriously.
CSB Investigator in Charge Drew Sahli said, “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.”
The CSB identified three key safety issues that led to the severity of the incident: (1) procedures and hazard analysis, (2) facility siting, and (3) process safety management 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 that 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 that 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 process safety management 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 process safety management 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.
CSB Board Member Sylvia Johnson said, “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.”
The CSB is an independent, nonregulatory federal agency charged with investigating incidents and hazards that result, or may result, in the catastrophic release of extremely hazardous substances. The agency’s core mission activities include conducting incident investigations to identify root cause of releases; formulating preventive or mitigative recommendations based on investigation findings and advocating for their implementation; issuing reports containing the findings, conclusions, and recommendations arising from incident investigations; and conducting studies on chemical hazards.
The agency's board members are appointed by the President subject to Senate confirmation. The Board does not issue citations or fines but makes safety recommendations to companies, industry organizations, labor groups, and regulatory agencies such as OSHA and EPA.
Please visit our website, www.csb.gov. For more information, contact Director of External Affairs Hillary Cohen at [email protected].