Power Plant Engineering
Where reactor operation is designed with fast neutrons such as in reactors using highly enriched fuel, the moderator used is

No moderator is needed
Heavy water
Carbon dioxide
Graphite

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Power Plant Engineering
A nuclear unit becoming critical means

It is capable of generating much more than rated capacity
There is danger of nuclear spread
It is generating power to rated capacity
Chain reaction that causes automatic splitting of the fuel nuclei has been established

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