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

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Nuclear Power Engineering
The half life period of a radioactive substance is best determined by counting the number of alpha particles emitted per second in a Geiger Muller counter from its known quantity. If the half life period of a radioactive substance is one month, then

3/4th of it will disintegrate in two months
L/8th of it will remain intact at the end of four months
It will completely disintegrate in two months
It will completely disintegrate in four months

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