Explosion in the off-gas system of a nuclear power plant

The event occurred in the stack filter house and was preceded by anomalously high radiation level readings from a monitor in the vent stack. When two plant employees entered the filter house to check the trouble, they were exposed to a radiation level of 6 mR/hr and left.
Another employee, outfitted with proper protective clothing entered the area a few hours later and observed water on the floor and a mis overhead. He refilled the off-gas loop seals with water.

The explosion occurred four hours later. The off-gas loop seals had been blown because of a back pressure generated by a partially blocked filter in the vent stack. The filter was clogged by water that had bypassed an improperly installed demister. Hydrogen escaping from the blown loop seal accumulated in the filter house and was ignited when a relay activated to isolate the loop seals.

The INITIATING CAUSE was the failure of the safeguards (water) seals, due to the build up of pressure due to a clogged filter and cumulation od hydrogen.
The hydrogen released into the vent building, created a flammable mixture with air and ignited from a relay arc.
The ROOT CAUSE could be attributed to the lack of process parameter control and emergency procedure. Hydrogen detection does not seem having been in place as safety measure, and the fact that the water seals had failed does not seem having triggered any mitigating action.

demsiter, filter, off-gas house

unknown