How do nuclear fuel control rods work?

The answer is surprisingly (and somewhat unnervingly) simple. First, a quick recap:

Nuclear reactors work by using the heat generated by nuclear fission to produce steam that powers a turbine to produce electricity. Fission is when the nucleus of an atom (in most cases Uranium-235) splits in two, creating heat and expelling free neutrons. When these neutrons collide with other U-235 atoms, its causes more fission; this creates a nuclear chain reaction. If left unchecked the chain reaction will grow exponentially and result in a nuclear meltdown.

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.

How this for an explanation:

“Scram is usually cited as being an acronym for safety control rod axe man, however the term is probably a backronym. The actual axe man at the first chain-reaction was Norman Hilberry who was ushered to the balcony rail, handed a well sharpened fireman’s ax and told that was it, “if the safety rods fail to operate, cut that manila rope.” The safety rods, needless to say, worked, the rope was not cut”

Science is fucking brilliant.

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