Which of the following is used in a nuclear reactor to slow down fast moving neutrons?

Step 1: Analyze the question The question asks which material slows down fast-moving neutrons in a nuclear reactor. That material is called a moderator.

Step 2: The concept In a fission reactor, uranium-235 splits and releases fast neutrons. Fast neutrons are not very good at causing further fission in U-235. Slow (thermal) neutrons are much more effective. A moderator slows the neutrons down by making them collide with its light atoms, so they lose kinetic energy without being absorbed.

Common moderators are graphite (carbon), heavy water, and ordinary water.

Step 3: Examine each option

• A. Graphite rods: Graphite is a classic moderator. It slows neutrons by collisions and absorbs very few of them. ✔
• B. Carbon dioxide gas: This is used as a coolant in some reactors (e.g. the British Magnox and AGR designs). It carries heat away and is not the moderator. ✘
• C. Liquid sodium metal: This is a coolant in fast breeder reactors. ✘
• D. Concrete shield: This protects people from radiation. ✘
• E. Boron rods: These are control rods. They absorb neutrons to regulate or stop the chain reaction, and they do not slow neutrons down. ✘

Step 4: Answer The correct option is A, graphite rods.

Tip: Remember the reactor parts by their jobs:

• Moderator → slows neutrons (graphite, heavy water)
• Control rods → absorb neutrons (boron, cadmium)
• Coolant → removes heat (CO₂, water, liquid sodium)
• Shield → blocks radiation (concrete, lead)