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The federal government alleged that U.S. Steel could have prevented a catastrophic explosion at Clairton Coke Works near Pittsburgh on Aug. 11, 2025, killing two workers and injuring 11.
The U.S. Chemical Safety and Hazard Investigation Board (CSB) issued a final report a year after the incident, saying the company had no procedure for how to safely conduct maintenance in which workers were attempting to fully close and then reopen a valve supplying toxic and flammable gas to one of the plant’s coke-oven batteries. In a coke oven, coal is burned at very high temperatures to remove impurities like tar and sulfur. Coke is used in the steelmaking process. The Clairton Coke Works was built in 1901.
The accident “should never have happened,” said CSB Chairperson Steve Owens in a press release about the report.
The board—whose conclusions are only advisory and do not require the company to act on them—also accused U.S. Steel of failing to develop a formal protocol for washing valves despite relying on the process during maintenance outages. And the CSB said U.S. Steel failed to construct buildings that could withstand an explosion.
The company did not have adequate process-safety management systems in place that could have prevented or reduced the severity of the incident, which caused an estimated $52.5 million in property damage, CSB said.
The absence of formal procedures for the maintenance operation that led to the explosion also applied to U.S. Steel’s valve-cleaning contractor, MPW, the CSB report said. MPW did not respond to a request for comment.
“The CSB determined the cause of the incident was the over-pressurization of a double-disc gate valve constructed of cast iron, which resulted in the release of flammable coke-oven gas that ignited and exploded,” the agency said in the 85-page report.
The board is charged with investigating, determining and reporting to the public the facts and causes of any accidental chemical release that kills or seriously injures people or causes substantial property damage.
In February, the federal Occupational Safety and Health Administration cited U.S. Steel with nine “serious” violations and one non-serious violation in connection with the 2025 incident, and fined the company $118,214. The company appealed those violations, and the case awaits a hearing in federal court. U.S. Steel said it does not comment on what it called an “active administrative litigation.”
In June 2025, U.S. Steel became a wholly owned subsidiary of Japan’s Nippon Steel Corp. which paid $14.9 billion for the American industrial icon. The new owner agreed to retain the U.S. Steel name, leave its headquarters in Pittsburgh and honor existing trade-union agreements.
In a statement to Inside Climate News, the company said it has established best practices for industrial valve cleaning and updated an internal program that improves the assessment of procedural changes. “Employees have completed comprehensive training on these new program elements and procedural changes,” the statement said.
But it’s unclear whether the statement means U.S. Steel has changed its practices to prevent another such incident, said Matt Mehalik, executive director of the Breathe Project, a nonprofit that advocates for better air quality in the notoriously polluted southwest Pennsylvania region, where U.S. Steel has been a leading cause of air pollution for decades. U.S. Steel was fined at least $15.3 million by the Allegheny County Health Department for violating air-quality standards at its three Pittsburgh-area plants from 2021 to 2025, according to the department’s website.
“There’s been no specific documentation that notes officially a verifiable accountability or enforcement activity related to the followup from the explosion,” Mehalik said.
Any regulatory enforcement would come from OSHA or the Allegheny County Health Department—which enforces air-quality regulations in the region—but U.S. Steel is contesting the OSHA findings, and the health department hasn’t issued any air-quality regulations since before the Clairton incident, so it’s not publicly known whether U.S. Steel has made changes following the explosion, Mehalik said.
On Aug. 10, the day before the first anniversary of the Clairton explosion, Mehalik’s group issued a statement applauding the CSB’s final report but accusing U.S. Steel and Nippon of failing to update processes to prevent a recurrence.
“The findings lead to key questions such as why Nippon continues to retain high-level former U.S. Steel leadership responsible for the situation instead of replacing them with management more attuned to safety,” the Breathe Project said. “Why does Nippon insist on doubling down on these old, outdated and unsafe facilities instead of replacing them with modernized steel-making processes? Why continue to put the community and workers at risk?”
In the new report, the CSB called on U.S. Steel to evaluate the siting of all occupied and potentially occupied buildings at the Clairton plant; to ensure that all siting hazards are mitigated to conform with practices set by the American Petroleum Institute; to write a procedure for washing valve seats with pressurized water; and to do a safety analysis of the valve-washing procedure.
The CSB also accused U.S. Steel of failing for years to establish a formal procedure for performing the maintenance that led to the explosion.
“U.S. Steel had no procedure for how to safely conduct this operation, but for at least three years prior to the incident, U.S. Steel employees had utilized pressurized water to clean valve seats on an ad hoc basis despite the lack of a formal procedure prescribing it,” the report said.
The Clairton incident also had implications for the chemical industry as a whole, the report said. The recommendations called on other companies to write detailed procedures for correctly performing potentially hazardous tasks; to always consider the “potential for over-pressure”; and to design nearby buildings to protect people or equipment from explosions.
“Companies should develop and apply robust safety-management principles to all hazardous processes,” it said.