Moderator in nuclear reactor
A material in a nuclear reactor that slows down (moderates) neutrons without absorbing them significantly, increasing their probability of causing fission in fuel nuclei. Common moderators are water, heavy water, and graphite.
Real World
Ordinary water acts as the moderator in the pressurised water reactors used by EDF's Hinkley Point C, slowing fast fission neutrons down to thermal energies so they are far more likely to trigger further U-235 fissions.
Exam Focus
Explain moderation using elastic collisions — maximum energy transfer occurs when neutron and nucleus have similar mass, so name hydrogen/graphite specifically.
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