Question

The graphite rods used in a nuclear reactor:

• A. React with uranium to release energy
• B. Produce neutrons
• C. Undergo combustion that triggers fission
• D. Convert fast neutrons into thermal neutrons

Hint:

Graphite acts as a moderator in a nuclear reactor, not a fuel.

Answer 1

The Correct Option is D

Step 1: In a nuclear reactor, fission of U-235 releases fast moving neutrons along with energy. These fast neutrons have too much kinetic energy and are not easily captured by other U-235 nuclei.

Step 2: To sustain a controlled chain reaction, the fast neutrons must be slowed down to thermal, that is slow, speeds, because slow neutrons are captured by U-235 much more effectively.

Step 3: A material used for this slowing down is called a moderator. Graphite, in the form of rods or blocks, is one of the common moderators used in nuclear reactors, along with heavy water.

Step 4: The graphite rods do not react chemically with uranium, do not produce neutrons themselves, and do not burn to trigger fission. Their only job is to slow down the fast neutrons through repeated elastic collisions with carbon atoms.

Answer: Graphite rods convert fast neutrons into thermal neutrons, option D.