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Why didn't the control rods in Fukushima shut down the reactor?

They say that pressure rises and that this can be dangerous. But why does this happen if the reactor is shutdown ?

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marked as duplicate by mbq Mar 14 '11 at 11:52

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
This question is almost identical to : physics.stackexchange.com/questions/6768/… At least the answers there cover this question completely. –  Georg Mar 14 '11 at 10:50
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Because even if reactor is not-critical, there are lots of radioactive materials in the fuel which decays and produce some heat. You cannot stop that process. Although it gives much less heat than nuclear fission, it is still significant and can cause meltdown.

The same reason is why spent nuclear fuel is stored under water for few years - it just selfheat and can melt.

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