Which of the following may not need a control rod?

Question

Examoogle · Accepted Answer

None of the mentioned

Explanation: Nuclear reactors require control rods to manage the chain reaction by absorbing excess neutrons. All standard fission reactors, including CANDU, liquid metal cooled, and fast breeder reactors, rely on control rods or equivalent neutron-absorbing systems to maintain criticality, control power levels, and perform emergency shutdowns (scram).

Working Principle: The reactor core maintains a self-sustaining chain reaction governed by the effective neutron multiplication factor $k_{eff}$. Control rods made of neutron-absorbing materials like Boron, Cadmium, or Hafnium are inserted into the core to adjust $k_{eff}$. If $k_{eff} > 1$, the reactor is supercritical; by inserting rods, the neutron population is reduced to achieve $k_{eff} = 1$ (criticality).

Key Formulas: $k_{eff} = \frac{P_{n+1}}{P_n}$ — where $k_{eff}$ is the effective neutron multiplication factor, $P_n$ is the number of neutrons in generation $n$.; $\rho = \frac{k_{eff} - 1}{k_{eff}}$ — represents the reactivity of the reactor core.

Key Points: Control rods are essential for reactor safety and reactivity control.; Fast Breeder Reactors (FBR) use neutron-absorbing rods to regulate the high flux of fast neutrons.; CANDU reactors use adjustable liquid poison (boron/gadolinium) in the moderator as a secondary control, but still utilize mechanical rods for shutdown.; A reactor without a mechanism to capture neutrons would be uncontrollable and potentially lead to core meltdown.

Advantages: Precise power output adjustment.; Emergency shutdown capability (Scram).

Disadvantages: Mechanical wear and tear.; Requires high-precision drive mechanisms.

Applications: Power regulation in nuclear thermal stations.; Emergency shutdown in pressurized water reactors (PWR).

Additional Info: CANDU reactors specifically use 'Liquid Injection Shutdown Systems' alongside mechanical rods, but this does not negate the need for control mechanisms.. Liquid metal cooled reactors (like FBRs) operate at higher neutron energies and require specialized neutron absorbers because standard thermal neutron absorbers are less effective.

Answer: D is correct — Every nuclear reactor design requires a system for neutron absorption to control the fission rate, making the use of control rods or equivalent poison systems universal.

Exam Tip: In competitive exams, remember that nuclear safety logic dictates that no reactor can operate without a passive or active control mechanism; 'None' is usually the answer when asked which reactor lacks a fundamental safety component.

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