A near meltdown at an Alabama Nuclear Plant could have been the nuclear disaster that Chernobyl became nine years later. Quick thinking by one operator stopped the reactor's meltdown. However, missteps and convoluted fire protection plans made this fire in history a close call to being one of the worst nuclear disasters on U.S. soil.
On March 22, 1975 at 12pm, the Brown's Ferry plant in Alabama was running at full capacity. At that time, two electricians were working on a maintenance issue in the cable spreading room. They were attempting to seal air leaks by using a foam rubber and a candle to test if the leak had been sealed.
The two electricians would seal the leak, place the candle next to the new seal, and see if the flame moved. The issue, however, was that the foam seal was flammable. One of the electricians placed the candle next to the seal, but the candle flame was too close to the foam and it burst into flames.
Electrician D said, "Because the wall is about 30 inches thick and the opening deep, I could not reach far enough, so Electrical Inspector C asked me for the foam and he stuffed in into the hole. The form is in sheet form, it is a 'plastic' about 2 inches thick, that we use as a backing material."
Electrical Inspector C went on to say, "We found a 2x4 inch opening in a penetration window in a tray with three or four cables going through it. The candle flame was pulled horizontal showing a strong draft. Electrician D tore off two pieces of foam sheet for packing into the hole. I re-checked the hole with the candle. The draft sucked the flame into the hole and ignited the foam which started to smoulder and glow. Electrician D handed me his flashlight with which I tried to knock out the fire. This did not work, and then I tried to smother the fire with rags stuffed in the hole. This also did not work and we removed the rags. Someone passed me a CO2 extinguisher with a horn which blew right through the hole without putting out the fire, which had gotten back into the wall. I then used a chemical extinguisher, and then another, neither of which put out the fire."
A later report showed that the foam used was resilient polyurethane, which caught fire and spread rapidly. When the CO2 was applied, the fire spread to the reactor building side of the leak that was being fixed. The resilient polyurethane foam splattered as it burned and spread. When the second extinguisher was applied, a roaring sound could be heard and a blowtorch effect occurred from the airflow through the hole.
It took fifteen minutes for the fire alarm to alert.
One of the electricians went to a plant guard and told him there was a fire in the turbine fire. The alarm was not immediately sounded because no one was sure of the correct phone number.
According to NRC, the men involved did not follow procedure. In their report, they said, "The Emergency Procedure was not followed by those involved when reporting the fire. The construction workers first attempted to extinguish the fire, whereas the procedure specifies that the fire alarm be sounded first. The guard reporting the fire telephoned the shift engineer's office rather than calling either of the numbers listed in the procedure."
A Near Nuclear Meltdown
The alarm sounded at 12:20pm, but the reactor operators did not shut down the two reactors.
The reactors began to malfunction. The operator of unit 1 noticed all of the pumps in the emergency core cooling system (ECCS) started.
The TVA report said, "[the] control board indicating lights were randomly glowing brightly, dimming, and going out; numerous alarms occurring; and smoke coming from beneath panel 9-3, which is the control panel for the emergency core cooling system (ECCS). The operator shut down equipment that he determined was not needed, only to have them restart again."
This continued for a full 10 minutes before the operators thought to shut off the reactors. He finally shut the reactor down at 12:51pm when the pumps suddenly quit.
At 12:55pm, electricity was lost to the control and power of the emergency core cooling system and some reactor unit 1 equipment (feedwater, core spray, low-pressure ECCS, reactor core isolation cooling system, instrumentation that communicates to control room).
The operator said, "I checked and found that the only water supply to the reactor at this time was the control rod drive pump, so I increased its output to maximum. Panel lights were changing color, going on and off. I noticed the annunciators on all four diesel generator control circuits showed ground alarms. I notified the shift engineer of this condition and said I didn't think they would start. At 1:00pm, the unit 2 operator observed decreasing reactor power, many scram alarms, and the loss of some indicating lights. The operator put the reactor in shutdown mode."
1:20pm - Reactor relief valves control lost.
1:15pm - Nuclear instrumentation lost. Control of only four relief valves were left.
1:45pm - Unit 2 equipment began to fail and the high-pressure ECCS was lost.
1:57pm - PAX phone system failed. No more outgoing calls from the control room could be made.
2:15pm - Control over reactor relief valves restored when the reactor was depressurized using the relief valves.
To prevent a meltdown, the operator made a makeshift condensate booster pump for unit 1. It provided a temporary supply of water that was adequate enough to keep it from melting. The water dropped from 200 inches above the core to only 48 inches, but it was enough. Unit 2 then used a similar pump system.
At 2:43pm 1/4 of the diesel generators failed, leaving the plant with the bare minimum supply of power.
The fire burned for seven hours before firefighters were able to extinguish it. More than 1,600 electrical cables were affected; 628 were related to plant safety. The fire damaged cables that affected power, control systems, instrumentation, and reactor safety systems.
Related: Case Study - Nuclear Power Plant
After the fire, investigations showed shortcomings in fire protection design at nuclear power plants and plant procedures for responding to a fire. Evidence showed that fires in certain locations in the plant could shut down redundant safety systems and components. It would make the reactor difficult to shut down safely.
After the Browns Ferry fire, the NRC revised its fire protection regulations to reasonably ensure a reactor maintains the ability to shut down safely in the event of a fire.
The change minimized potential for fires and explosions; rapidly detected, controlled, and extinguished fires that do occur; and ensures that the operators can shut down the reactor safely.
Nuclear power plants now have multi-layered fire protection. They have fire barriers, fire detection, and fire suppression. Every nuclear power plant must also have a fire protection plan that outlines the program, fire suppression and detection, and the ability to shutdown the reactor if need be. The requirements are listed in 10 CFR 50.48(a).
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