A fireball (about the size of a basketball) developed when a pipefitter used a torch to cut apart a cooling water system pipe connecting a heat exchanger used to cool wall panels in a room housing a nuclear reactor. The water in the cooling system pipes contained untreated tap water supplied from the building main. The operation of the reactor was halted in 1970, fuel elements removed, and the facility closed. The cooling water pipes were left charged with water. In January, 1989, as part of a total decontamination/ decommissioning process, it was necessary to disassemble the cooling water lines. Several months prior to the incident, the cooling water system had been drained at a low lying point. As one of the fitters was cutting a section of the system a fireball suddenly developed and then extinguished. The fitter was on a step ladder and was not singed.
It was noticed that a smell of "sulfur" remained after the flash and upon further investigation that a faint but definite odor of hydrogen sulfide was present in the parts of the pipe which had previously been cut open. Had this odor been detected when cutting other sections of the water pipe, the potential for hazard would have been revealed. It is not unusual in waste systems for anaerobic decomposition to produce flammable gases such as hydrogen sulfide and methane, however, it is unusual for water cooling systems. Apparently there was a sufficient quantity of impurities available in the water to degrade over time and provide the atmosphere necessary to produce a fireball in the presence of a flame.
To avoid the possibility of fire when cutting into a water cooling system, it is advisable to take certain precautions. These include:
Systems which are expected to contain large quantities of undesirable materials, or whose decomposition produces flammable gases should of course be dismantled in such a way that neither the workers nor the environment are subject to harm.