OSHA Hazard Information Bulletins
Anaerobic Decomposition in Cooling Water Systems

March 5, 1990




  • Director
  • Office of Field Programs


  • Director
  • Directorate of Technical Support


  • Hazard Information Bulletin Anaerobic Decomposition in Cooling Water Systems

The Directorate of Technical Support issues Hazard information Bulletins (HIB) as needed to provide relevant information regarding unrecognized or misunderstood safety and health hazards, inadequacies of materials, devices, techniques, and engineering controls. HIBs are initiated based on information provided by the field staff, studies, reports and concerns expressed by safety and health professionals, employers, and the public. Information is compiled based on a comprehensive evaluation of available facts, literature, and in coordination with appropriate parties. HIBs do not necessarily reflect an OSHA policy.

Recently information from the Department of Energy indicated that a health and safety risk could exist while cutting and salvaging abandoned cooling water systems.

In 1989, a fireball developed when a pipefitter used a cutting torch on a cooling water system pipe. The piping connected a heat exchanger used to cool wall panels in a room housing a nuclear reactor. The system had been abandoned for 19 years. The water in the cooling system pipes contained untreated tap water supplied from the building main. The pipes for this system had been left charged with water. The water in the pipes was drained several months before the cutting and salvaging operation. While cutting these pipes, a fireball suddenly developed and lasted for a short time. Fortunately, the pipefitter was not hurt.

Upon investigation, a hydrogen sulfide odor was present in parts of the pipe which had been previously cut open. Apparently explosive gases were generated within the piping system. The gases were ignited by the cutting torch.

Anaerobic decomposition is the probable causative agent for the production of the explosive gases. Anaerobic decomposition can occur under certain conditions, when microorganisms or bacteria (capable of existing without oxygen) and organic materials are brought together. Flammable gases such as hydrogen sulfide and methane may be produced. This reaction is not unusual in waste processing facilities; it is, however, unusual to find hazardous quantities of such gases in water cooling systems. Precautions to be taken when cutting into water cooling system include:

  1. Before cutting open any water line flush the system with fresh water.
  2. Drain the system, allowing for proper disposal of contents, after assuring that the area is appropriately ventilated.
  3. Open valves
  4. Ventilate the area before cutting to permit the safe dissipation of hazardous vapors and gases.

Please distribute this bulletin to all area offices, state plan states, and consultation project officers.