The refinery has ten covered process units each utilizing a threshold amount of a regulated substance. It operated a boiler at the Facility, which combusted natural gas or refinery fuel gas as fuel. On September 28, 2012, natural gas was introduced into the boiler during an attempted start-up. During the attempted startup, the natural gas combusted, which resulted in an unintended flash detonation that killed one worker immediately and led to the death of a second worker on October 18, 2012. Pursuant to section 112(r)(1) of the CAA, an owner/operator of a stationary source producing, processing, handling or storing substances listed pursuant to section 112(r)(3) of the CAA, or any other extremely hazardous substance, has a general duty to: