The speed of fast-moving neutrons in a nuclear reactor can be reduced by using?

Analyze the question: The question asks which material slows down (reduces the speed of) fast neutrons in a reactor. The material that does this is called a moderator.

Concept:

• Moderator: slows fast neutrons to thermal speeds so they can cause further fission in uranium-235. Examples are graphite, heavy water (D₂O), and ordinary water.
• Control rods: absorb neutrons to control the reaction rate. Examples are boron and cadmium.
• Shield: thick concrete (often with lead) absorbs radiation to protect workers.

Going through the options:

• A. Graphite rods: graphite is a classic moderator. Neutrons collide with carbon atoms and lose kinetic energy without being absorbed much. ✔
• B. Concrete shield: it protects against radiation and does not control the neutron speed in the core. ✘
• C. Iron rods: iron is a structural material and not used as a moderator. ✘
• D. Boron rods: boron absorbs neutrons and acts as a control rod, which regulates the number of neutrons rather than slowing them. ✘

Answer: The correct option is A. graphite rods.

Exam tip: Remember "Moderator = Slows, Control rod = Absorbs". Graphite and heavy water slow neutrons down, while boron and cadmium absorb neutrons and stop the reaction.