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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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Which of the following material can be used as a moderator ?
Graphite
Heavy water
Beryllium
Any of the above
Any of the above
A moderator in a nuclear reactor is a substance used to reduce the speed of fast neutrons produced by fission, turning them into thermal neutrons. Graphite, heavy water (D2O), and beryllium all possess low atomic mass and low neutron absorption cross-sections, making them effective moderators.
A moderator in a nuclear reactor is a substance used to reduce the speed of fast neutrons produced by fission, turning them into thermal neutrons. Graphite, heavy water (D2O), and beryllium all possess low atomic mass and low neutron absorption cross-sections, making them effective moderators.
ΔE=21mv2 — kinetic energy of neutrons
ξ=ln(E2E1) — logarithmic energy decrement, representing average energy loss per collision
The principle relies on elastic scattering. When high-energy neutrons collide with moderator nuclei of comparable mass, they lose kinetic energy rapidly through collisions. By reaching thermal equilibrium with the surrounding medium, these slow neutrons have a much higher probability of inducing fission in U-235 isotopes.
The moderator must have a high scattering cross-section and low absorption cross-section.
Graphite was used in the first nuclear reactors (e.g., Chicago Pile-1).
Heavy water is a highly efficient moderator that allows the use of natural uranium as fuel.
Beryllium is effective but expensive and toxic, limiting its widespread use.
Increases the chain reaction probability by thermalizing neutrons.
Allows efficient power generation using controlled fission.
Materials like Beryllium are costly and difficult to manufacture.
Moderators occupy space within the reactor core, increasing overall size.
Nuclear Power Generation (Pressurized Heavy Water Reactors - PHWRs).
Research reactors for neutron scattering experiments.
Graphite was used in the Chernobyl-type reactors (RBMK).
Light water (H2O) is also a common moderator, but it absorbs neutrons, requiring enriched uranium.
D is correct — Graphite, heavy water, and beryllium are all standard materials used as moderators in nuclear reactors to slow down fast-moving neutrons.
In exams, remember that a moderator must have low atomic mass to be effective; heavy elements would not absorb neutron energy efficiently through collisions.