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Browns Ferry Nuclear Power Plant  (Source: svsnowgoose.com)
A valve on its residual heat removal system was stuck shut, which prompted in-depth inspections by the NRC

Nuclear power has received a lot of criticism lately due to the 9.0-magnitude earthquake that rocked Japan on March 11. U.S. Senators urged the Nuclear Regulatory Commission (NRC) to repeat the inspection of nuclear power in the United States after it was already deemed safe, and French President Nicolas Sarkozy called for a global nuclear review after visiting Japan.

Despite all of these negative reviews, there are also many advocates who see the benefits of cheap, clean and reliable nuclear power as an alternative energy source. U.S. President Barack Obama even embraced nuclear energy in last year's State of the Union address

No matter which side you're on, many can agree that safety comes first, and now, federal regulators are concerned about the safety of an Alabama nuclear plant after its emergency cooling system failed. 

The Browns Ferry nuclear power plant is located near Athens, Alabama and is run by the Tennessee Valley Authority. A valve on its residual heat removal system was stuck shut, which prompted in-depth inspections by the NRC.  

According to reports, there have been previous problems with the valve. Originally, the valve failed sometime after March 2009, but wasn't identified until October 2010 when the plant was being refueled. The valve was fixed at that time, and was labeled as a manufacturer's defect. Despite the plant's effort to fix the valve and inspect all others like it, the NRC criticized the plant for not finding the valve issue sooner through routine inspections.

"The valve was repaired prior to returning the unit to service and Browns Ferry continued to operate safely," said Victor McCree, the NRC's Region ll administrator. "However, significant problems involving key safety systems warrant more extensive NRC inspection and oversight." 

Had there been an emergency, the NRC worries that the faulty valve could have prevented the emergency cooling systems from working correctly. For this reason, the NRC will continue to review the safety culture, organization and performance of the plant. 

"The results of this inspection will aid the NRC in deciding whether additional regulatory actions are necessary to assure public health and safety," said McCree.  

For the time being, the NRC has issued a red finding against the Brown Ferry nuclear plant, which is the most severe ranking given to a plant for inspection. Only five red findings have been issued in the U.S. in the past decade. It is unknown whether the Tennessee Valley Authority will appeal the finding from the NRC at this time.



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RE: oh please...
By Solandri on 5/12/2011 9:34:12 PM , Rating: 2
MIT NSE put together a nice chart of the residual heat generated.
http://mitnse.com/2011/03/16/what-is-decay-heat/

It's about 7% right after SCRAM. It drops to:
2.25% in 4 minutes
1% after about 5 hours
0.61% in 1 day
0.35% in 1 month
0.21% in 1 year

Do note though that an AP1000 reactor is supposed to generate about 1000 MW of electricity. That translates into about 3000 MW of thermal energy generated when operating, so 0.21% after a year translates into 6.3 MW.

Water has a heat capacity of 4.187 kJ/kgK and a heat of vaporization of 2270 kJ/kg. So if you're dumping cooling water at 20 degrees C and venting it as 100 C steam, you'd need 6300 kJ/s / (4.187 kJ/kgK * 80 K + 2270 kJ/kg) = 2.4 kg per second of water being vaporized. In a day, that's 209 tons of water being vaporized. So it's vital that there be some sort of secondary cooling system to recondense the steam back into water.

The passive cooling capacity of an AP1000 reactor basically amounts to a huge tank of water which can cool it without human intervention for a while (I don't recall how long). It's not an end-all solution, it's a stopgap meant to tide things over until you can fix/implement other cooling systems. I believe pebble bed reactors are the only design which have truly passive safety (and that's only maintained as long as you can keep the main thermal chamber sealed from outside oxygen - if oxygen gets it, the graphite coating starts burning and you have a really big problem).

On the flip side, you have to ask what level of risk is acceptable? Nothing in life is risk-free. Currently, you're more likely to die from a random lightning strike than from a nuclear accident. While all this hand-wringing over passive safety and possible accidents is well-intentioned, it's counterproductive if fear leads you to choose a less effective and more dangerous power generation technology (anything other than nuclear) just because their dangers are not as well publicized.


RE: oh please...
By randomly on 5/13/2011 9:36:54 AM , Rating: 2
The AP1000 can go 3 days after a SCRAM on the water in the passive cooling tank. Unlike the Fukushima reactors there is no need to inject water under high pressure into the reactor vessel itself so you do not need a high pressure pump system.

Natural convection loops transfer the decay heat from the core to the containment dome and the water tank supplies water that is sprayed on the outside of the dome to cool it. The water tank is at ambient pressure and you can just refill it with a hose if you need to. Just about any water will do since it never enters the reactor containment or core.

Everything is gravity fed so no electrical power is required at all to maintain cooling for as long as needed.

Nuclear reactor designs have come a long way in the half a century since the current crop of reactors in this country were designed. Comparing Fukushima and Three Mile Island era reactors to modern reactors is like comparing Edsels to modern day cars. Modern reactors are fundamentally better designed.
Nobody is proposing we build more Edsels.

Besides pebble bed reactors, molten salt reactors can also be designed to be completely passively safe. Requiring no power or even water for cooling. You can just turn them off and walk away from them and they will be fine indefinitely. They also can't get out of control as the basic physics of the thermal expansion from increasing temperature shuts down the reaction automatically with no intervention.


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