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

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

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Nuclear Power Engineering
Commercial power generation from fusion reactor is not yet possible, because

It is difficult to initiate fusion reaction
It is difficult to control fusion reaction
The fuel required (e.g. deuterium and tritium) is scarce
Quantity of fuel required for initiating fusion reaction is prohibitively high

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