The Sequoyah nuclear power plant in Tennessee reported to the Nuclear Regulatory Commission that a small explosion had taken place in the electrical switchyard on Sunday morning. The plant declared a site emergency and issued a Notice of Unusual Event.
An investigation revealed damage to an insulator between a Protective Circuit Breaker and a Motor Operated Disconnect. Technicians also found that a transformer was leaking oil. Additionally, when operators attempted to reopen the Motor Operated Disconnects, they found that one of the units would not open on demand.
As a result of the incident, the Unit 1 reactor had a Condensor Circulating Water pump trip. Plant personnel restored the pump following the event.
As a result of the explosion, operators determined that one offsite power source was inoperable. The second source of offsite power continued to function.
A little less than two hours into the event, operators determined that repairs to the faulty circuit had been completed, and Sequoyah terminated the Notice of Unusual Event.
The significance of this event is that Sequoyah has a single switchyard that provides offsite power to the plant’s emergency systems. If the explosion in the switchyard had been a little more severe, the plant could have lost all offsite power. Such an incident occurred at the Surrey nuclear power plant in Virginia last April, when a tornado hit the switchyard and knocked out all offsite power. When offsite power is lost, a nuclear plant is forced to rely on emergency diesel generators to maintain control of cooling systems, which must remain in operation at all times.
The worst loss of offsite power accident was the disaster at Fukushima last March, when an earthquake and tsunami knocked out the plant’s offsite power, leading to the meltdown of three reactors.
(I’d like to welcome several new readers to this list, and apologize for the NukeSpeak in today’s report. If anyone ever has questions about items on this list, please feel free to contact me, and I’ll see if I can answer them. Additionally, I wanted to remind everybody that you can read Event Reports daily at the Nuclear Regulatory Commission’s website at www.nrc.gov.)