Nuclear Power Engineering
Water is a better coolant than a gas (like CO₂, He, N₂ etc.), because it

Has a better heat transfer characteristics, and it can be pressurised to attain a high temperature
Requires comparatively smaller pumps and heat exchanger for a given heat transfer rate
Is a better neutron moderator as well
All of these

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Nuclear Power Engineering
Secondary cooling circuit is a must in molten sodium cooled fast breeder reactor for

Achieving high degree of superheat in the steam
Avoiding the mixing of water with radioactive sodium, as it may cause explosion
Lowering the coolant circulation pressure
Faster heat removal rate from the core

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