The material used to slow down the neutrons in a nuclear reactor is

Step 1: Analyze the question The question asks which material acts as the moderator in a nuclear reactor. The moderator slows fast neutrons to "thermal" neutrons so they can cause fission in uranium-235 more effectively.

Step 2: Explain the concept A nuclear reactor has these main parts:

• Fuel: uranium-235 (or plutonium), which undergoes fission.
• Moderator: slows down the fast neutrons. Common moderators are graphite, heavy water (D₂O), and ordinary water. A good moderator has light nuclei that absorb few neutrons.
• Control rods: absorb excess neutrons to control the reaction rate. They are made of boron or cadmium.
• Coolant: carries heat away, e.g. water, liquid sodium, or CO₂.
• Shielding: thick concrete to block radiation.

Step 3: Evaluate each option

• A. Boron: used in control rods to absorb neutrons, not to slow them. ✗
• B. Copper: a good conductor of heat and electricity, with no role as a moderator. ✗
• C. Graphite: a form of carbon that slows neutrons through collisions without absorbing many of them. It is a standard moderator. ✓
• D. Uranium: this is the fuel, not a moderator. ✗

Step 4: Answer The correct answer is **C.

Exam tip: Remember the pairing:

• Moderator → Graphite (or heavy water)
• Control rods → Boron or Cadmium ("absorb")

Moderators slow neutrons down, while control rods absorb them.