Assertion: heavy water's neutron | /r/CapableOf | sustain a chain reaction
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text: And so using ordinary water as a moderator will easily absorb so many neutrons that too few are left to sustain a chain reaction with the small isolated 235U nuclei in the fuel, thus precluding criticality in natural uranium.
text: The main advantage of heavy-water moderator over light water is the reduced absorption of the neutrons that sustain the chain reaction, allowing a lower concentration of active atoms (to the point of using unenriched natural uranium fuel).
text: More neutrons are released than are needed to maintain the chain reaction; when uranium-238 absorbs just the excess, plutonium is created, which helps to make up for the depletion of uranium-235.
text: Most nuclear reactors are hence operated in a prompt subcritical, delayed critical condition: the prompt neutrons alone are not sufficient to sustain a chain reaction, but the delayed neutrons make up the small difference required to keep the reaction going.