What is the function of moderators in nuclear reactor?

Question

Examoogle · Accepted Answer

Option B: Slow down the secondary neutrons

Explanation & Concepts:
Moderators are materials used in nuclear reactors to slow down fast neutrons produced during nuclear fission. By reducing the kinetic energy of these neutrons through elastic scattering, the moderator increases the probability of these neutrons being captured by fissile nuclei, thereby sustaining the chain reaction.

Working Principle: In a nuclear fission event, neutrons are emitted with high kinetic energy (approx. 2 MeV), termed 'fast neutrons'. To sustain a self-sustaining chain reaction using $U^{235}$, these neutrons must be 'thermalized' (slowed down to roughly 0.025 eV). The moderator consists of light nuclei (like Hydrogen in water or Carbon in graphite); when neutrons collide with these nuclei, they lose a fraction of their kinetic energy per collision until they reach thermal equilibrium.

Key Formulas: $\Delta E = E_{initial} - E_{thermal}$ — The energy reduction required during moderation; $P = \frac{1}{1 + B^2 L^2}$ — Non-leakage probability influenced by moderator properties

Key Points: Moderators must have a high scattering cross-section and low absorption cross-section.; Commonly used moderators: Ordinary water ($H_2O$), Heavy water ($D_2O$), Graphite, and Beryllium.; Heavy water is the most efficient moderator as it has a very low neutron absorption cross-section.; The process of slowing down neutrons is formally called 'Thermalization'.

Advantages: Enables the use of natural or low-enriched uranium fuel.; Provides safety by controlling the rate of the chain reaction.

Disadvantages: Can absorb some neutrons, reducing the neutron economy if not chosen correctly.; Requires additional shielding and volume in the reactor core.

Applications: Pressurized Water Reactors (PWR); Pressurized Heavy Water Reactors (PHWR); Gas-Cooled Reactors (GCR)

Additional Info: Option A is incorrect because moderators should avoid absorbing neutrons; control rods are responsible for neutron absorption.. Option C is incorrect because control rods, not moderators, are used for regulating the intensity of the chain reaction.. The ideal moderator has a high scattering cross-section ($\sigma_s$) and a low absorption cross-section ($\sigma_a$).

Answer: B is correct — Moderators are essential for slowing down fast-moving secondary neutrons to thermal energy levels to facilitate efficient fission.

Exam Tip: Always distinguish between 'Moderators' (which slow neutrons) and 'Control Rods' (which absorb neutrons) to avoid common confusion in nuclear power plant questions.

Examoogle · Answer

Option A: Absorb the secondary neutrons

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Option C: Control the chain reaction

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Option D: None of the mentioned

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