In a nuclear reactor, what is the function of the moderator (like heavy water or graphite)? MCQ with Answer and Explanation

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In a nuclear reactor, what is the function of the moderator (like heavy water or graphite)?

A. To slow down the fast-moving neutrons so they can cause further fission.

B. To absorb excess neutrons to stop the chain reaction.

C. To transport heat away from the reactor core.

D. To act as the nuclear fuel.

Answer: Option A

Solution (By JKSSB Mock Tests)

Uranium-235 undergoes fission most efficiently when struck by slow-moving (thermal) neutrons. However, fission reactions release very fast-moving neutrons. The moderator (like graphite or heavy water) collides with these fast neutrons to slow them down without absorbing them, sustaining the controlled chain reaction. Control rods (cadmium/boron) are used to absorb excess neutrons.

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The kinetic energy of a body is increased by 300%. The percentage increase in its momentum is:

A. 100%

B. 200%

C. 300%

D. 400%

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Correct Answer: Option A

Explanation:
KE ∝ p² (since KE = p²/(2m)). If KE increases by 300%, new KE = 4× original KE. Thus p_new² / p_original² = 4 ⇒ p_new / p_original = 2 ⇒ momentum doubles, i.e., 100% increase. Memory tip: 'KE ∝ p²; so %Δp = ½ %ΔKE for small changes, but for large changes use ratio'. This proportional reasoning problem tests energy-momentum relationship, frequently appearing in competitive exams. Always derive from fundamental relations when percentages are large; linear approximations fail here.

This question belongs to: Science Physics