= Nuclear reactor
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A <nuclear reactor> sustains and controls a <nuclear chain reaction> to produce heat or a useful <neutron> field. <Nuclear fission> supplies fresh <neutrons>; absorption and <neutron leakage> remove them. A steady <neutron> population requires a balance between production and losses, the condition of <nuclear criticality>.
The functions of its components are different. Fuel supplies fissioning <atomic nuclei>; <control rods> change <neutron> absorption; coolant transports heat. In a thermal reactor, a <neutron moderator> reduces <neutron> <energies> before subsequent reactions. Slowing <neutrons> and cooling the fuel are separate physical functions even when the same material performs both.
<Chicago Pile-1> demonstrated a controlled self-sustaining chain reaction in 1942 using a uranium–graphite assembly and neutron-absorbing controls. The decisive observation was the <neutron> population's response to changing absorption, rather than generation of useful electrical power.<The first nuclear reactor, explained by University of Chicago>[Chicago Pile-1 demonstration on 2 December 1942 and qualitative experimental apparatus]{ref} A power reactor adds heat-transfer and conversion equipment; <decay heat> persists after the chain reaction is stopped.
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