Moderator in a nuclear reaction

Question

Examoogle · Accepted Answer

Option C: Reduces the speed of fast-moving neutrons

Explanation & Concepts:
A moderator in a nuclear reactor is a substance used to slow down fast-moving neutrons produced during fission. This process is essential because slow (thermal) neutrons are significantly more likely to induce further fission in $U^{235}$ nuclei than fast neutrons.

Working Principle: The moderator works through the process of elastic scattering. When fast neutrons collide with the light nuclei of the moderator (like hydrogen in water or carbon in graphite), they transfer a portion of their kinetic energy to the moderator nuclei. Through repeated collisions, the neutron's energy is reduced to thermal equilibrium with the reactor environment, increasing its probability of causing a chain reaction.

Key Formulas: $E_{th} = \frac{3}{2} k_B T$ — Represents the thermal energy of neutrons after moderation at equilibrium temperature T

Key Points: Moderators must have a low atomic mass and low neutron absorption cross-section.; Common materials used as moderators include Heavy Water ($D_2O$), Graphite, and Ordinary Water ($H_2O$).; Heavy water is the most efficient moderator as it has a very low probability of neutron absorption compared to light water.

Advantages: Sustains the chain reaction by optimizing the neutron energy spectrum.; Allows the use of naturally occurring uranium in specific reactor designs like CANDU.

Disadvantages: Additional cooling or structural material adds complexity to the reactor core.; Some moderators, like graphite, can be prone to Wigner energy buildup if not managed.

Applications: Thermal Nuclear Reactors (PWR, BWR, CANDU).; Research reactors for isotope production.

Additional Info: Control rods are used to absorb neutrons and stop the chain reaction, making Option B the function of control rods.. Cooling agents remove heat from the core to generate steam; while some moderators (like water) act as coolants, their primary role as a 'moderator' refers to slowing neutrons.

Answer: C is correct — The primary function of a moderator is to slow down high-velocity fission neutrons to thermal speeds to maintain the chain reaction.

Exam Tip: Always distinguish between moderators (slow neutrons) and control rods (absorb neutrons to stop/regulate the reaction).

Examoogle · Answer

Option A: Stop the chain reaction

Examoogle · Answer

Option B: Absorbs neutrons

Examoogle · Answer

Option D: Acts as a cooling agent

Correct Answer

Discussion (0)

Correct Answer

Related Questions

Discussion (0)