Nuclear reactor kinetics: A reactor is said to be 'critical' when the neutron population in the core is ________ with time (i.e., power level is steady).

Difficulty: Easy

Correct Answer: constant

Explanation:


Introduction / Context:
Reactor criticality is a foundational concept in nuclear power. It describes the balance between neutron production (via fission) and losses (via absorption and leakage). Understanding the terms subcritical, critical, and supercritical is essential for safe operation and control.



Given Data / Assumptions:

  • Thermal or fast reactor with steady-state consideration.
  • Reactivity changes are small and control systems are active.
  • We use the point-kinetics perspective for clarity.


Concept / Approach:
A reactor is critical when the effective multiplication factor keff = 1. At this condition, each generation of neutrons produces exactly one subsequent generation of the same size, so neutron population and hence reactor power remain constant over time.



Step-by-Step Solution:

Define keff: ratio of neutrons in successive generations.If keff > 1 → supercritical → neutron population grows with time.If keff < 1 → subcritical → neutron population decays with time.If keff = 1 → critical → neutron population is constant (steady power).


Verification / Alternative check:
Examine reactor power trends: during long steady operation, heat removal (steam generation) balances fission heat production, indicating constant neutron flux and critical state.



Why Other Options Are Wrong:

  • Rapidly increasing → indicates supercritical conditions, not critical.
  • Decreasing → subcritical operation.
  • Reduced to zero → shutdown with no chain reaction.


Common Pitfalls:
Equating “critical” with “dangerous”. In nuclear engineering, “critical” simply means steady-state chain reaction, not instability.



Final Answer:
constant

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