A subcontractor employee was using a band saw to cut a 1" metal pipe when a flash fire occurred on the third floor hydrogen fluoride area. Subcontractor employees were removing all piping associated with the Anhydrous Hydrofluoric Acid (AHF) system. These lines were being removed during plant decontamination and demolition (D&D). The subcontractor employee was attempting to cut a 90-degree elbow located at the highest elevation on the 1" line, but the lowest elevation of the overall piping run. Since hydrogen is lighter than air, it is speculated that a minute amount of hydrogen gas had accumulated in the elbow.
Even though Safe Shutdown personnel had previously opened the system and placed it in a safe configuration, residual hydrogen fluoride could have still reacted with the metal piping to create hydrogen gas. The subcontractor employee began to cut the elbow with a portable electric band saw. Once the heat generated by the band saw came into contact with the hydrogen gas, an extremely rapid exothermic chemical reaction with a resulting flash fire occurred. There was no significant impact to on-site or off-site personnel as a result of this occurrence.
The subcontractor employee performing the cutting immediately walked to the temporary safety shower, located less than 15 feet away, and thoroughly rinsed his chemical resistant personal protective equipment (PPE). A second employee readied a fire extinguisher, also less than 15 feet away, but it was never discharged due to the short duration of the event. The first employee left the temporary shower, doffed the outer set of anti-c's, and proceeded to the decontamination area. Upon arrival at the decontamination area, the employee doffed his inner set of anti-c's and showered a second time for twenty minutes as a safety precaution.
The employee exhibited no physical injuries, but was sent to medical for verification. The employee had been wearing cotton anti-contamination clothing and a second set of chemical resistant anti-c's. The flash fire caused the chemical-resistant clothing to melt, similar to a marshmallow, on the right side of the employee's body. The cotton anti-c's located underneath the chemical resistant anti-c's revealed no physical signs of heat damage as a result of the flash fire, most likely due to the employee's quick activation of the safety shower and/or short duration of the event.
The root cause of the event was determined to be a defective or inadequate procedure. The facility did not identify hydrogen off-gas as a potential hazard in the hydrogen fluoride process lines of the plant. If this issue had been properly addressed, it is less likely that this event would have occurred.
Although most of the hydrogen fluoride piping system was open to the atmosphere before the elbow was cut, the lines were not purged. Two plugs of residual material on either side of the elbow remained in place, thus allowing hydrogen to be trapped in the elbow. In the future, piping systems containing hydrogen fluoride or other chemicals should be purged to ensure that they are free from hazards prior to starting D&D activities.