Question

The function of moderator in nuclear reactor is to

• A. Slow the speed of neutrons
• B. Fast the speed of neutrons
• C. Slow the speed of electrons
• D. Fast the speed of electrons

Hint:

Commonly used moderators in nuclear reactors are ordinary water (H\(_2\)O), heavy water (D\(_2\)O), and graphite. Remember that the moderator's job is to 'moderate' or slow down the neutron speed, not to absorb them (that's the job of control rods, which are made of neutron-absorbing materials like Cadmium or Boron).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A nuclear reactor generates energy through a controlled nuclear chain reaction. In reactors using uranium-235, the fission process is most efficiently initiated by slow-moving neutrons, often called thermal neutrons.
Step 2: Detailed Explanation:
The fission of a uranium nucleus releases high-energy, fast-moving neutrons.
These fast neutrons are not very effective at causing fission in other uranium nuclei, as the probability of capture and fission is much lower at high energies.
To sustain the chain reaction efficiently, these fast neutrons must be slowed down to thermal energies.
The material used for this purpose is called a moderator.
The moderator consists of light nuclei (like hydrogen in water or carbon in graphite) that slow down the neutrons through a series of elastic collisions without absorbing them.
Therefore, the primary function of a moderator is to reduce the kinetic energy, or slow the speed, of fast neutrons.
Step 3: Final Answer:
Based on the explanation, the function of a moderator is to slow the speed of neutrons to sustain the nuclear chain reaction. Thus, option (A) is correct.