Question

Assertion :Heavy water is preferred over ordinary water as a moderator in reactors. Reason: Heavy water, used for slowing down the neutrons, has lesser absorption probability of neutrons than ordinary water.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Assertion is incorrect but Reason is correct

Answer 1

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

Heavy water is used to slow down the neutron. Fast neutrons are incapable of transforming their energy to the parent nuclei. When two bodies of the same mass make collision-exchange of velocity is most and when two bodies of different masses collide there is the least exchange of velocities. So, to reduce the velocity of a particle it should be made to collide with a body of almost equal mass. Ordinary water consisting of protons should be the best moderator but the proton-neutron collision results in the production of deuteron with the emission of $\gamma -particle$ as follows.
${}_0{n}^1{+}_1{p}^1{\to }_1{H}^2+\gamma$
This is the undesirable reaction as it results in loss of neutron. Hence heavy water is preferred.
So, the reason assigned is correct.