Control rod
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A control rod is a rod made of a chemical element capable of absorbing many neutrons without decaying themselves. They are used in nuclear reactors to control the rate of decay of uranium and plutonium.
Control rods stand vertically within holes through the pile, and are lifted partially out to allow a chain reaction to occur. The number of control rods lifted and the distance by which they are lifted can be varied to control the rate of the reaction.
In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage. This means that automatically in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods will fall, under gravity, fully into the pile to stop the reaction. Quickly shutting down a reactor in this way is called scramming the reactor.
Control rods are commonly composed of silver, indium and cadmium. Other elements that can be used include boron, carbon, cobalt, hafnium, gadolinium and europium. ja:制御棒