Nuclear Power Engineering
Ordinary water is not used as a moderator because, it

Absorbs neutrons
Has a low scattering cross-section
Does not absorb neutrons
Has a low absorption cross-section

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Nuclear Power Engineering
Molten sodium (as a coolant in fast breeder reactor)

Is not at all corrosive, even at a higher temperature
Can't attain high temperature at normal pressure
Is highly radioactive at elevated temperatures and can cause explosion, when it comes in contact with air or water
None of these

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