A hydrogen explosion occurred at a 585-MW coal-fired supercritical unit and caused one fatality, injuries to 10 other people, and significant damage to several buildings. The explosion occurred during a routine delivery of hydrogen when a hydrogen relief device failed, which allowed the contents of the hydrogen tank to escape and be ignited by an unknown source. The truck driver was delivering hydrogen gas to fill two 6500 cubic feet storage cylinders at the plant. The hydrogen gas was used to cool the generator at the power plant. At approximately 9:22 a.m., the employer, who was working alone at the time, was in the final stages of filling the cylinders. At this time witnesses heard a loud noise, like high pressure gas venting through a relief valve, and within 15 to 20 seconds, the hydrogen gas exploded, killing the employee and injuring eight co-workers. The investigation revealed that the hydrogen gas over pressurized a rupture disk attached to one of the hydrogen cylinders. As the set pressure of the rupture disk was between 3500 (24 MPa) and 4000 psi (28 MPa), and the maximum pressure that could have be put on the system during filling was 2600 psi (18 MPa), the rupture disk failed well below its designed pressure. The release of hydrogen through the relief vent piping, which terminated outside above the roof, also caused the copper vent tubing to fail and burst apart at a 45 degree elbow below the roof, therefore releasing hydrogen gas under the roof. This allowed the gas to accumulate into a large cloud that subsequently ignited and exploded. The force of the explosion killed the employee and injured nine AEP workers in an adjacent building.