In a nuclear reactor, which material is commonly used as a moderator to slow down fast neutrons so that they can efficiently cause further fission?

Introduction / Context:
This question relates to nuclear reactor design and the role of moderators. In many types of reactors, neutrons produced in fission are fast and must be slowed down to thermal energies to efficiently cause further fission in fuel such as uranium 235. The substance used to slow down these neutrons without capturing too many of them is called a moderator.

Given Data / Assumptions:

• We are asked which material slows down neutrons in a nuclear reactor.
• Options include heavy water, ordinary water, zinc rods, molten caustic soda and liquid sodium metal.
• We are considering typical commercial and research reactors discussed in general science courses.
• A good moderator should slow neutrons effectively while having low neutron absorption.

Concept / Approach:
Moderators reduce the kinetic energy of fast neutrons through elastic scattering collisions without capturing too many of them. Light nuclei such as hydrogen and deuterium are effective for this purpose. Ordinary water (H2O) can act as a moderator, but it absorbs more neutrons than heavy water. Heavy water (D2O), which contains deuterium instead of hydrogen, is an excellent moderator with a very low neutron absorption cross section. For this reason, some reactor types, especially heavy water reactors, use D2O as both coolant and moderator.

Step-by-Step Solution:
1) Identify that the question is about slowing neutrons, which is the task of a moderator, not of a coolant or control rod alone.2) Zinc rods are metals and are not used as standard moderators; they can capture neutrons and are not efficient for slowing down fast neutrons.3) Molten caustic soda is a corrosive medium and is not a typical moderator in reactor designs.4) Liquid sodium metal is used as a coolant in some fast breeder reactors because it transfers heat efficiently but does not serve as a neutron moderator in thermal reactors.5) Ordinary water can act as a moderator in light water reactors, but the textbook answer that appears most often in general science questions is heavy water, particularly in the context of heavy water reactors.6) Therefore, heavy water (D2O) is the correct choice in this question as the moderator that slows down neutrons.

Verification / Alternative check:
Descriptions of pressurised heavy water reactors (PHWRs) used in several countries state clearly that heavy water acts as a moderator to thermalise neutrons, allowing natural uranium fuel to sustain a chain reaction. While some reactors use ordinary water as both coolant and moderator, heavy water is unique because it slows down neutrons efficiently while absorbing relatively few, making it ideal when natural or low enriched fuel is used. This widely discussed role of heavy water confirms it as the expected answer.

Why Other Options Are Wrong:
Ordinary water (H2O): It can be a moderator in light water reactors, but the classic exam answer for this question is heavy water, especially in the context of moderators highlighted in school level textbooks.
Zinc rods: These are not used as moderators; they would absorb neutrons and are not suited to slow down neutrons efficiently.
Molten caustic soda: Highly corrosive and not used as a reactor moderator.
Liquid sodium metal: Used primarily as a coolant in some fast reactors, not as a thermal neutron moderator.

Common Pitfalls:
Students sometimes pick ordinary water because they remember that water plays an important role in reactors. It is true that light water acts as a moderator in many designs, but heavy water is often emphasised in exam syllabi as the specialised moderator used in certain reactor types, especially for natural uranium fuel. To avoid confusion, remember that both H2O and D2O can moderate, but heavy water is the classic, low absorption moderator often highlighted in general knowledge questions.

Final Answer:
In many nuclear reactors, especially heavy water reactors, the speed of neutrons is slowed down by heavy water (D2O) acting as a moderator.