Question

Water used as moderator in nuclear reactor is called
(a) heavy water
(b) hard water
(c) nuclear water
(d) critical water

Answer 1

In a nuclear reactor, a process like nuclear fission occurs, where neutrons move very fast, so, water is used as a moderator to slow down the neutrons, so that the nuclear reaction sustains for efficient output.

Complete step by step answer:
(1) The use of water as a moderator can be understood by taking the example of a pool table. When the cue ball strikes another ball at rest on the table, the head-on collision with the target ball, which is at rest, stops the cue ball, and moves the target ball with the original speed of the cue ball. In nuclear reactors, hydrogen atoms of the water play the role of the target balls, and neutrons act as the cue balls.
(2) In nuclear fission reaction, the fast-moving neutrons strike the heavy nucleus of the Uranium atom, and this bombardment leads to the formation of some new nuclei from the Uranium nucleus.
(3) When an incoming neutron strikes the nucleus of a Uranium atom to split up into smaller nuclei. Then the other neutrons are released at a very high speed. Heavy water is used here to slow down the neutrons because Uranium absorbs the neutrons more efficiently when they are moving slowly. Moreover, with slow movement of neutrons a sustained reaction is set up which allows to carry the nuclear fission at a constant rate.
(4) Heavy water is used as a moderator because it is made up of isotopes ${}_1^2H{\text{ and }}{}_8^{16}O$, whose slow down power is high.
Hence, option (a) is the correct answer, and heavy water is the type of water used as a moderator in this case.

Note:
Some materials other than water are also used as a moderator. One of them is the Graphite reactor which uses Carbon as a moderator. Chicago Pile-1 is the first artificial nuclear reactor which used graphite as a moderator.