The U.S. Chemical Safety and Hazard Investigation Board (CSB) has released its final report on the Aug. 11, 2025, explosion at U.S. Steel’s Clairton Coke Works in Clairton, Pennsylvania, finding deficiencies in procedures, facility siting and process safety management contributed to the incident.

The explosion occurred during maintenance on a double-disc gate isolation valve supplying coke oven gas to Battery 13. U.S. Steel employees and contractors from MPW Industrial Services were attempting to exercise the valve, a process intended to confirm that it could fully close and reopen.

According to the CSB, workers had developed an informal practice of applying pressurized water to valve cleanout ports when coke residue prevented valves from closing fully. The practice had been used for at least three years but was not included in the facility's written valve-exercising procedure, and U.S. Steel did not have a formal procedure for washing valves with pressurized water.

During the incident, workers applied pressurized water while the valve's two gates were closed or nearly closed. The water became trapped between the gates, increasing pressure beyond the valve's containment capability. The more than 70-year-old cast-iron valve then failed, releasing toxic and flammable coke oven gas. The gas ignited within minutes, causing the explosion.

Two workers were killed and 11 others injured, including five who suffered serious injuries. The incident also caused an estimated $52.5 million in property damage.

The CSB found that the severity of the incident was compounded by the location of occupied buildings in a transfer area between Coke Batteries 13 and 14. Several buildings, including control rooms and a break room, were located less than 20 feet above the coke oven gas piping. None had been designed or constructed to withstand an explosion, and all were catastrophically damaged.

"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," Drew Sahli, CSB investigator in charge, said in a statement. "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: inadequate procedures and hazard analysis, facility siting and deficiencies in process safety management systems.

The board recommended that U.S. Steel conduct a facility-siting evaluation covering occupied and potentially occupied buildings at Clairton and address identified hazards. It also recommended that the company establish a written procedure for washing valve seats with pressurized water and implement a comprehensive process safety management system for coke oven gas processes.

The CSB also issued recommendations to Nippon Steel North America (NSNA), U.S. Steel's parent company, to establish a corporate process safety governance program and audit facilities for process safety deficiencies. MPW was advised to develop written procedures for cleaning piping systems containing flammable or toxic gases and ensure workers receive appropriate training.

The Clairton investigation began shortly after the explosion, which initially was reported as having killed two workers and injured at least 10 others. The facility, which processes metallurgical coal into coke for steelmaking, operates six coke batteries containing 455 ovens.

The report is the latest in a series of recent CSB investigations highlighting deficiencies in process safety and hazard management. In July, the board's final report on a 2024 fire and toxic chemical release at KIK Consumer Products' Bio-Lab warehouse in Conyers, Georgia, cited inadequate safeguards for reactive chemical storage and renewed recommendations for expanded regulatory oversight of reactive chemical hazards.