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

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

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

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