A moderator ______ the neutrons.

Question

Examoogle · Accepted Answer

Slows down

Explanation: A moderator is a material in a nuclear reactor that reduces the kinetic energy of high-speed (fast) neutrons to lower (thermal) energy levels. By slowing these neutrons down, the moderator significantly increases the probability of fission occurring in U-235 fuel.

Working Principle: Fast neutrons produced during fission move at very high speeds and are likely to escape the fuel or be captured by U-238 without causing fission. The moderator consists of light nuclei (like Hydrogen in water or Carbon in graphite); when neutrons collide elastically with these nuclei, they transfer a fraction of their kinetic energy, effectively slowing them down until they reach thermal equilibrium with the surroundings.

Key Formulas: $E = \frac{1}{2}mv^2$ — Kinetic energy of a neutron; $\sigma_f(E) \propto \frac{1}{v}$ — Fission cross-section is inversely proportional to velocity for thermal neutrons

Key Points: Thermal neutrons have much higher fission cross-sections than fast neutrons.; Common moderators include light water ($H_2O$), heavy water ($D_2O$), and graphite.; A good moderator must have a high scattering cross-section and low absorption cross-section.; If neutrons are not slowed down, a self-sustaining chain reaction is difficult to achieve in thermal reactors.

Advantages: Enables chain reaction with natural or low-enriched uranium fuel.; Improves neutron economy by reducing non-fission capture.

Disadvantages: Adds complexity to the core design.; Moderators like graphite can pose fire risks; water requires high-pressure vessels.

Applications: Pressurized Water Reactors (PWR); Boiling Water Reactors (BWR); CANDU Reactors (Heavy Water)

Additional Info: Fast neutrons typically have energies around 1-2 MeV, while thermal neutrons are around 0.025 eV.. Option B is the correct choice because the primary function of a moderator is to maximize collision probability through kinetic energy reduction.

Answer: B is correct — A moderator slows down high-energy neutrons to thermal energies to sustain the nuclear chain reaction.

Exam Tip: Remember that while a moderator slows neutrons, a control rod (like Boron or Cadmium) absorbs them to stop the reaction. Do not confuse these two functions.

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