Japanese Tsunami

[Platapus]

Originally Posted by UnderseaLcpl

Here's a question: Why weren't the control rods designed to simply drop into the core in the event of a power failure to stop the reaction?

In a BWR the control rods are raised from the bottom instead of dropping from the top.

a full application of control rods should be enough to stop any reaction the plant is capable of generating.

Under normal circumstances, yes, the application of the control rods will prevent further fission. However, it will have no affect on the heat being present or even continuing after the fission is stopped. The problem is that if the reactor has a large void (negitive) coefficient , the increased heat (when the cooling system fails), may overcome the absorption effects of the control rod.

http://en.wikipedia.org/wiki/Void_coefficient

In plain words, the hotter the reactor gets, the less effective the moderator will be. Given enough heat, the control rods may be of minimal use.