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In a nuclear reactor, the control rods are of a highly neutron absorbing element. When the reactor is shutdown, the rods are fully down in the reactor core essentially capturing the free neutrons. As they're raised, neutrons from the radioactive fuel begin to react with the fuel itself. By react I mean a fuel atom captures a free neutron and undergoes fission, releasing energy. Each atom that undergoes fission produces more free neutrons, so the reaction increases.
Reactors also use what are called moderators. A moderator is an element that slows down the neutron (a slow neutron is easier captured than a fast one). The neutron "bounces" off a moderator atom and imparts (loses) energy.
The thing to remember is that elements below iron on the periodic table release energy during fusion, but need energy to split (ie fission). Hydrogen releases the most energy, helium the next ..... upto iron. This is why main sequence stars sit fusing hydrogen into helium in their core, but once the hydrogen begins to run out, the core begins to collapse until it reaches a hot enough temp to fuse the helium ..... massive stars follow this down to iron cores ... no more fusion ... supernova. Smaller stars only fuse down to helium and carbon and leave a white dwarf behind, which is the "dead" stellar core.
Once you get past iron, it's fission that produces energy, with the far end of the periodic table near uranium producing the most energy.
Andrew.
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