From the following materials commonly found in reactor systems, which one is used specifically as a moderator (i.e., to slow down neutrons)?

Introduction:
Moderators reduce neutron energies to the thermal range, improving the fission probability in certain fuels (e.g., U-235). Distinguishing moderators from absorbers and structural materials is a key competency in reactor design.

Given Data / Assumptions:

• Typical moderator choices: light water, heavy water, graphite.
• Absorber/control materials: cadmium, boron compounds.
• Cladding/structure: zircaloy, stainless steel.

Concept / Approach:
An effective moderator exhibits strong scattering with minimal absorption. Graphite (carbon) fulfills this role and has been used in many reactor designs. Cadmium and boron compounds are strong absorbers for control/shutdown. Zircaloy is favored for fuel cladding due to low absorption and good corrosion resistance, not as a moderator. Stainless steel is primarily structural.

Step-by-Step Solution:
Identify moderator: Graphite is a classic moderator.Exclude absorbers: Cadmium and boron carbide are control/poison materials.Exclude structural/cladding: Zircaloy and stainless steel are not used to moderate.

Verification / Alternative check:
Historical and modern reactors (e.g., graphite-moderated designs) demonstrate graphite’s moderating role.

Why Other Options Are Wrong:

• Cadmium/boron carbide: Excessive absorption kills reactivity; used for control rods.
• Zircaloy/stainless steel: Employed for mechanical reasons; not intended to slow neutrons.

Common Pitfalls:
Assuming low-absorption cladding like zircaloy is also a moderator. While it does not strongly absorb, it is not suitable for deliberate neutron slowing compared to graphite or water.

Final Answer:
Graphite