Question
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In a nuclear reactor, heavy water is used as a:
A). Moderator
B). Fuel
C). Shield
D). Controlling element.

Answer
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Hint: Heavy water unlike normal water contains partly or fully replaced isotopes of hydrogen-like deuterium. Heavy water is used in nuclear magnetic resonance spectroscopy, heavy water is also used in the medical field.

Complete step by step answer:
Water containing significantly more deuterium which is an isotope of hydrogen, than the natural proportions to the ordinary hydrogen atoms is known as heavy water. It is used as a moderator in some nuclear reactors because it slows down neutrons easily and effectively also and has a low chance of absorption of neutrons.
Therefore option A moderator is the correct option.

Additional Information:
There are two types of nuclear reactions one is nuclear fission another is nuclear fusion.
For the fuel, uranium is used as a radioactive substance, this type of fuel is known as radioactive fuel. The uranium is processed into small pellets and those pellets are stacked together into metal rods. These rods are known as fuel rods.
Shields are used in the nuclear reactor to protect the vessel in which the reactor is placed, it also protects different mechanisms that are inside the system. This shield is basically used to protect the device or reactor from gamma-ray absorption, if the reactor absorbs the gamma-ray it gets heated up resulting in an accident.
The controlling element is also known as control rods that are made up of silver, indium, cadmium, and boron. These rods are basically placed to control the reaction of fuels like uranium and plutonium.

Note: The reason because of which heavy water acts as a moderator in nuclear reactors is because of the unnatural proportions of deuterium in heavy water. They slow down the neutrons generally.