Scientific American asks NRC how Japan will cool their reactors and amt of time before meltdown

[snagglepuss]

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Scientific American spoke with Scott Burnell, public affairs officer at the U.S. Nuclear Regulatory Commission (NRC), the government agency charged with monitoring the safety of the 104 nuclear reactors in the U.S., about what it takes to cool down a reactor.



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SciAmer: How much time is there before a meltdown?

NRC: It depends on the plant. It depends on whether it's a boiling-water reactor or a pressurized-water reactor. Basically, you have the benefit of natural forces such as convection. There is a coolant loop no matter what, so you end up to some degree cooling the core because the heated water rises and colder water gets pulled in. But that's not as effective as a pump bringing in cool water. Just to speak very broadly, you have many hours to restore power to the system to get normal cooling going. It's really not possible to get more specific than "many hours."

SciAmer: But generally, less than 24 hours.

NRC: That's fair to say.



SciAmer: What happens when a reactor gets disconnected from the grid?

NRC: There are emergency diesel generators. You also have a battery system to keep instruments running, but that can also provide power to safety systems . It's all meant to provide defense in depth. First you rely on the grid. If the grid is no longer available, you use diesel generators. If there is an issue with the diesels, you have a battery backup. And the batteries usually last long enough for you to get the diesels going.






http://www.scientificamerican.com/article.cfm?id=how-to-cool-a-nuclear-reactor