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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 (D2тАЛO), 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 (D2тАЛO), and beryllium all possess low atomic mass and low neutron absorption cross-sections, making them effective moderators.
╬ФE=21тАЛmv2 тАФ kinetic energy of neutrons
╬╛=ln(E2тАЛE1тАЛтАЛ) тАФ 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 (H2тАЛO) is also a common moderator, but it absorbs neutrons, requiring enriched uranium.
D is correct тАФ 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.