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With all the hubbub over the reactors in Japan and nuclear safety, I was wondering why the insertion of the control rods in Fukushima did not shut down the reactor? Shouldn't they slow the fission reaction so far that cooling is not needed? Why do they need to continue cooling the core when I would expect no more exothermic reaction to take place?

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up vote 18 down vote accepted

Per this article on the subject:

Even with rods inserted, the reactor continues to produce heat equivalent to about 3% of its full power level. This is not the same as taking a pot off the stove and letting it cool. There are still some atoms splitting and fission products decaying that produce heat. This drops off slowly and is why there needs to be layers of redundant cooling with backup power. During such an earthquake, power from outside the plant would not be expected to be available.

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Yes. A nuclear engineer came up on the news and explained that even in normal shut downs of this type of reactors, for maintenance, it takes over a week for the reactions to stop so that maintenance can take place. – anna v Mar 13 '11 at 8:26
After an earthquake and tsunami, power from inside the plant would not be available. Why aren't they designed to fail safe even in this condition? – endolith Apr 8 '11 at 15:08
@endolith - they were, there are backup generators for the cooling system. In this case something, or a series of somethings failed - the same reason aircraft crash when they are designed not to. – Martin Beckett May 12 '11 at 18:22
In particular, the backup diesel generators and their fuel supply were located much lower and closer to the coast than the plant itself and were destroyed in the tsunami. – dmckee Dec 30 '13 at 3:11
I'm surprised that an SRO would attribute a significant contribution of residual fission power to the observed decay heat. – user22620 Dec 5 '14 at 18:04

Using the Unit 1 electric power rating of 460MW and the 3% figure above give us ~14MW at scram, that is still a lot of power! So every second 14 million Joules is released(1 Watt = 1 J/S). Now it takes ~4200 joules to heat 1 kilogram of water by 1 degree C. So 14 million joules would heat 1 kg of water to ~3300 degrees C in 1s. You can see why we have a problem!

Can someone check my math, it has been a few years since physics class

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Yes, your math is fine. It's just some simple arithmetic, no? If you wanted to be specific about it, then, going off the value of 460MW, the 3% figure would be 13.8MW, and the temperature of the water would increase by 3296.7 degrees. But you knew that. Of course, all of that energy wouldn't be concentrated into 1 kilogram of water, but as you said the point is that it's a tremendous amount of energy that needs to be dissipated. – voithos May 12 '11 at 17:10
Or to put it another way, if you are allowed to discharge water with a 10C temperature rise (about the limit for going into a river) you need to pump 1/3ton per second to cool 14MW – Martin Beckett May 12 '11 at 18:27

In a nuclear power plant, it is designed so that, if the power fails, the rods will slip into place (and in fact, the problem in Chernobyl was that they didn't). The control rods DID slip into place - but the amount of damage caused to the plant was such that other cooling systems began to fail. The problem was not an ongoing chain reaction, but rather the inability to keep cool the reactors. As a result, water began to evaporate off, which meant even more radioactivity was given off.

In short, the control rods weren't the issue. Nuclear power plants have such complex safety structures, and quite right. Just turns out that Fukushima Daiichi wasn't prepared for this disaster. But then, what can you do? It's impossible to prevent all outcomes.

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Why the Japanese build nuclear plants at all knowing they live in a seismic zone? – Anixx Feb 26 '12 at 3:41
The reactivity excursion at Chernobyl is believed to have been initiated by control rod insertion. The problem is not that they weren't inserted, per se, but that the initiation of insertion caused an overpower transient which caused the fuel to explode. – user22620 Dec 5 '14 at 18:10

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