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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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A moderator ______ the neutrons.
Accelerates
Slows down
Blocks
Stops
Slows down
Quick Summary: 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.
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.
E=21mv2 — Kinetic energy of a neutron
σf(E)∝v1 — Fission cross-section is inversely proportional to velocity for thermal neutrons
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.
Thermal neutrons have much higher fission cross-sections than fast neutrons.
Common moderators include light water (H2O), heavy water (D2O), 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.
Enables chain reaction with natural or low-enriched uranium fuel.
Improves neutron economy by reducing non-fission capture.
Adds complexity to the core design.
Moderators like graphite can pose fire risks; water requires high-pressure vessels.
Pressurized Water Reactors (PWR)
Boiling Water Reactors (BWR)
CANDU Reactors (Heavy Water)
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.
B is correct — A moderator slows down high-energy neutrons to thermal energies to sustain the nuclear chain reaction.
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.