Browns Ferry nuclear power plant.
On March 22 1975, technicians at Browns Ferry nuclear power station were inspecting air leaks in the pressurised cable spreading room. As per procedure, they were using the smoke produced from a candle to judge whether air was escaping or not into the Unit 1 building. The temporary sealant being used to plug the discovered leak was flammable and was set alight by the candle. The fire spread through the leak to the Unit 1 building and damaged control cables to both units. This led to operators loosing control of many critical systems including the emergency core cooling system. The two units were shut down but without adequate control of the cooling system, there was a risk of the top of the fuel becoming exposed and melting from decay heat. The operators managed to use a low pressure condensate pump to establish sufficient forced cooling to keep the core intact.
Had the operators not been able to re-establish forced cooling, we would have had a major disaster on our hands.
In many ways, the ultimate result of the Browns Ferry fire was similar to the Three Mile Island accident. The key risk to core integrity was a lowering of the cooling water level to a point where part of the fuel was exposed. Without the cold sink of the water, the decay heat would build up in the ceramic and melt.
At TMI, this is what happened. At Browns Ferry, it was averted. However, had it not been, then like at TMI, it would have created a huge, expensive mess in the core, which would have hurt the accounts of the Tennessee Valley Authority, but would not have harmed the public.
It shows just how delicate nuclear power stations are.
The key vulnerability in the design of Browns Ferry, which made the event as serious as it was, was the lack of separation between the different wiring trains. It allowed one fire to damage them all including. The most important lesson learnt from this was that redundancy must also be accompanied by isolation. From that point on, redundant wiring trains are kept physically separated to ensure that one incident cannot affect both of them.
Clearly Browns Ferry was had inherent vulnerabilities. The fire demonstrated them so they could be eradicated in future.
Fire suppression and emergency handling procedures were pretty flawed with operators not sure of what to do when the fire alarm sounded.
This clearly was another weakness, though industry at the time was generally pretty bad about safety training, compared to today's standards. Lessons were learnt about better training and particularly in today's more safety oriented culture, personnel are better equipped to respond properly and rapidly to emergency situations.
So why should we not feel unsafe as a result of this?
For a start, it is worth pointing out that this accident happened over thirty years ago and therefore cannot be used to directly impugn the safety standards of today's nuclear technology. The main point is that nuclear reactor design is based on the principle of defence-in-depth. There are multiple layers to protect the outside world from the hazards of radioactive material. Some of these layers were eroded, including some designed to keep the core material intact. However, the final layer, the steel-reinforced concrete containment structure, was not even put to the test in this instance. At TMI, it was and it proved itself. As long as containment ultimately works, any mess that goes on inside is the problem for the utility, not for us.