Explosion in the waste decay tank of a nuclear power plant

An in-tank explosion occurred in a gaseous waste decay tank while the nuclear plant was in cold shutdown. The gaseous hydrogen present in the tank formed a flammable mixture and ignited due to air contamination of the inert nitrogen system used to control the hydrogen-oxygen concentrations in the tank.
This resulted in a release of about 8.8 curies of noble gases and minor tank damage.

Generally, the gas in pressurized water reactor waste gas systems is hydrogen rich and the oxygen concentration must be controlled to prevent flammable gas mixtures. Flammable concentration of gas mixtures can be prevented by limiting either the hydrogen or the oxygen concentration to less than 3 percent.
The source of air was identified as instrument air leaking through check valves at the cross connections between instrument air and nitrogen lines. Under normal operating conditions, the pressure in the instrument air system is higher than that of the nitrogen system. These cross connections had been installed in response to a safety improvement request. The nitrogen system provided a backup gas supply to the air-operated steam supply valve for the steam-driven auxiliary feed water pump. This backup was installed to provide a "safety grade" auxiliary feed water system that satisfies the single-failure criteria. Other cross connections, which apparently did not leak air into the nitrogen system, had been previously installed to provide a redundant gas supply to the air-operated pressurizer relief valves and the associated block valves.

The INITIATING CAUSE of the in-tank explosion was the accidental entrance of air in a hydrogen reach atmosphere.

The source of air was identified as instrument air leaking through check valves at the cross connections between instrument air and nitrogen lines. Under normal operating conditions, the pressure in the instrument air system is higher than that of the nitrogen system. These cross connections had been installed in response to a safety improvement request. The nitrogen system provided a backup gas supply to the air-operated steam supply valve for the steam-driven auxiliary feed water pump. This backup was installed to provide a "safety grade" auxiliary feed water system that satisfies the single-failure criteria. Other cross connections, which apparently did not leak air into the nitrogen system, had been previously installed to provide a redundant gas supply to the air-operated pressuriser relief valves and the associated block valves.
The ROOT CAUSE could be identified in a limitation of a previous risk assessment, which brought to the safety improvement which enabled this incident. The installation of N2-air cross-connections was addressing the risk of single-failure pressure-activated valves, but the possibility of an entrance of air into the nitrogen system had been overseen. The risk assessment should have better considered the overall system hazards due to the modifications.

gaseous waste decay tank, check valve

In waste gas systems of a pressurised water reactor, the gasis hydrogen-rich and the oxygen concentration must be controlled to prevent flammable gas mixtures.This can be prevented by limiting either the hydrogen or the oxygen concentration to less than 3 percent. In this plant, oxygen presence was limited to less than 3% by injecting nitrogen gas.

According to a source, the explosion bent the bolts of an inspection hatch on the tank, allowing radioactive gasses in the tank to escape into a radioactive waste room. From there, the radioactive material was released into the atmosphere (about 8.8 curies of noble gases ).

The NRC conclude its report by saying that that licensees needed to devote more attention to the potential effects of nitrogen-air system cross connections when systems are modified to use nitrogen as a backup to air systems. If cross connections exist, the potential for the formation of flammable gas mixtures should be evaluated.
More in general, a sampling program to avoid formation of flammable gas mixtures in tanks should be considered.