CSB issues update on fatal Clairton Coke Works explosion

The U.S. Chemical Safety and Hazard Investigation Board (CSB) yesterday released an update on the agency’s ongoing investigation into the August 11, 2025, fatal explosion at the U.S. Steel Clairton Coke Works facility in Clairton, Pennsylvania.

Two U.S. Steel employees were killed in the explosion. Five other workers were seriously injured and hospitalized - four U.S. Steel employees and one employee of Veolia Water North America Operating Services. Six additional workers - five U.S. Steel employees and an employee of MPW Industrial Services, Inc. (MPW) - were treated for injuries but not hospitalized.

The CSB announced it sent a team to investigate the accident the day following the incident.

“The focus of the CSB’s investigation is to understand what caused this terrible tragedy and identify actions that can be taken to prevent something like this from happening again,” said Steve Owens, CSB chairperson.

Clairton Coke Works processes raw coal into coke, which is then used as a primary component in steelmaking. The 392-acre facility is located about 15 miles outside of Pittsburgh and is considered the largest coking facility in North America. Nearly 1,300 employees report to the facility daily.

Gas monitors triggered less than one minute before explosion

Preliminary information indicates that the explosion occurred during maintenance activities involving a gas isolation valve in the basement of the Battery 13/14 transfer area at the facility.

U.S. Steel employees, assisted by MPW personnel, were performing a procedure to close and reopen the valve. When they attempted to flush the valve seat with water, gas monitors began alarming. Less than one minute after a verbal and radio evacuation call was issued, an explosion occurred.

In addition to the deaths and injuries, the explosion caused extensive structural damage to the Battery 13/14 transfer area and resulted in prolonged search and rescue operations. The body of one of the fatally injured employees was not recovered until approximately nine hours after the incident.

“Tragedies like this must lead to change” said Sylvia Johnson, CSB board member. “Our investigation will identify not just what went wrong, but what must be done to ensure workers across this country are protected from similar hazards.”

Faulty cast iron valve was over 70 years old

The gas isolation valve involved was a cast iron, 18-inch, double-disc gate valve.  The valve was manufactured more than 70 years ago in 1953 and had been refurbished in 2013. Post-incident examination revealed the valve body had split open along a fully circumferential crack. Additional valves recovered from the scene also showed signs of damage.

The CSB said it is continuing to collect evidence and conduct analysis in several key areas, including:

• The cause and source of the gas release
• Metallurgical analysis of cast iron valve components
• U.S. Steel’s use of cast iron materials in its coke oven gas systems
• Policies and procedures for valve maintenance and exercising
• U.S. Steel’s overall safety management systems

The CSB will issue a final report detailing findings, analyses, and recommendations at the conclusion of its investigation.