CSB Issues Final Report on Fatal Explosion at Pennsylvania Steel Plant

A final report released by the U.S. Chemical Safety and Hazard Investigation Board (CSB) explains the causes of the Aug. 11, 2025, explosion at the U.S. Steel Clairton Coke Works facility in Clairton, Pennsylvania. The incident killed two workers, injured 11 others, and caused more than $52 million in property damage.

The explosion occurred while workers for MPW, a contractor for U.S. Steel, were “exercising” a valve in a pipe containing coke oven gas, a highly flammable mixture produced during the production of iron and steel. Exercising valves involves closing and opening them to ensure they maintain a full range of motion. According to CSB, coke oven residue prevented the valve from fully closing. With the assistance of a U.S. Steel supervisor, the contractors attempted to flush the valve with pressurized water, a practice that was common at the plant but was not documented. CSB Chairperson Steve Owens described the use of water to flush valves at the Clairton site as “an ad hoc informal procedure.”

During the flushing procedure, the water accumulating inside the valve caused it to fail, releasing coke gas into the surrounding area, where it found an ignition source and exploded. The valve that failed was more than 70 years old and was made of cast iron, a brittle material that is not recommended for equipment in hazardous applications, CSB states.

At the time of the explosion, the two workers who were killed, and two others who were seriously injured, were in or near control rooms and a break room located directly above the piping for the coke oven. “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,” said CSB Investigator Drew Sahli. “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’s report includes recommendations for U.S. Steel to conduct a siting evaluation for buildings at the plant, develop a written procedure for washing valve seats, and implement a comprehensive process safety management system at the Clairton facility. CSB also recommends that Nippon Steel North America, which acquired U.S. Steel in June 2025, implement process safety and conduct regular audits at all its facilities. MPW should develop written policies and procedures for cleaning pipes that contain flammable or toxic gas and ensure their workers are properly trained, CSB said.

For more information, refer to CSB’s news release and final report (PDF).