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| style="height:20px; width:200px; text-align:left;" |As a fast moving [[neutron]] passes through a [[moderator]] it collides with [[atom]]s in the [[moderator]] passing on some of their [[energy]] and slowing down. After several collisions a [[neutron]] becomes slow enough to be captured by a [[Nuclear Fission|fissionable]] [[isotope]]. At this point it is called a [[Thermal Neutron|thermal neutron]].

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