Question

Heavy water is used in nuclear reactor as
(A) Coolant
(B) Moderator
(C) Both coolant and moderator
(D) Catalyst

Answer 1

Hint: Here, you are asked to find the role of heavy water in a nuclear reactor. In order to proceed, you need to consider all the properties given in the options, that are coolant, moderator and catalyst. After picturing down the definitions of the given properties, determine in what process the heavy water can be used by considering the properties of heavy water.

Complete answer:
Coolant: A coolant is passed through the core of the nuclear reactor which carries away the heat produced in it and is transferred to the electric generator.
Moderator: In a nuclear reactor, when fission takes place in the uranium, the fast-moving neutrons escape the uranium rod. In order to slow these neutrons down, a moderator is used. The neutrons collide with the moderator and are slowed down.
Catalyst: A catalyst is a substance which increases the rate of reaction without itself getting changed chemically.

Heavy water is a light molecule. Whenever there are collisions between neutrons and nuclei and if the size difference is not much appreciable, the neutron gets slowed down more efficiently. That is why lighter elements are preferred as moderators. The probability that the neutron will be absorbed by the nucleus is given by absorption cross section, smaller its value, high are chances that neutron will slow down without getting absorbed by the nuclei.

In case of heavy water, the absorption cross section is small. It slows down the neutron without absorbing it. As heavy water satisfies the requirement of being a moderator, it is used as a moderator.Hence, heavy water is used in nuclear reactors as a moderator.

Hence, option B is correct.

Note:You need to keep in mind the definitions of all the properties mentioned above. Absorption cross section is the measure of probability and it quantifies it for collisions between particles. In case if it was simply water rather than heavy water, then your answer would have been both coolant and moderator. Water is used as coolant which is passed through the nuclear core and it also acts as moderator but not as efficient as heavy water.