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A nuclear reactor needs to maintain enough of reaction to generate heat, but not allow the core to become super critical and melt down. To do this control rods, which are made of a neutron absorbing material, are placed into the core and are literally raised and lowered to tweak the reaction – if you need to generate more heat, you raise the rods out of the core to let more neutrons split more atoms. To curb the reaction, you lower the rods into the core to absorb more of the neutrons before they have a chance to come in contact with the uranium. In emergency cases (like recently in Japan), the rods are automatically shoved into the core using gravity, hydraulics or a mechanical spring, causing the chain reaction to stop. This is called “SCRAMing” the reactor.