Nuclear reactor physics — meaning of critical size and critical mass The critical size of a reactor refers to the size at which a self-sustaining chain reaction can be initiated with just enough fissionable material (the critical mass). Is this definition correct?

Introduction / Context:
In neutron chain reactions, geometry and material composition determine whether the neutron population grows, decays, or remains steady. The term critical size connects the physical dimensions of a reactor to the concept of critical mass for a given configuration and set of materials.

Given Data / Assumptions:

• Homogeneous or heterogeneous reactor with specified fuel, moderator, and reflector.
• Steady-state analysis with effective multiplication factor k-effective.
• Leakage and absorption determine criticality for a given geometry.

Concept / Approach:
Criticality occurs when k-effective equals 1. At this condition, one fission on average causes exactly one subsequent fission, maintaining a steady neutron population. For a given set of materials, there exists a minimum configuration (critical size) that provides sufficient moderation and reflection to limit leakage so that k-effective reaches unity with a corresponding amount of fissile material known as the critical mass.

Step-by-Step Solution:
Define criticality: k-effective = 1, neutron production equals losses.Recognize geometric effect: as the system grows, surface-to-volume ratio decreases, reducing leakage.For fixed materials, there is a minimum size at which leakage is low enough for k-effective to reach 1 with a particular fissile content.Hence, the critical size corresponds to the configuration where the chain reaction is just self-sustaining with the critical mass.

Verification / Alternative check:
Reflectors reduce the required critical size and critical mass by returning escaping neutrons. This practical design strategy confirms the tight link between geometry and the amount of fissile material needed to achieve criticality.

Why Other Options Are Wrong:
Calling the definition incorrect would ignore the standard usage of critical size and critical mass in reactor physics and shielding design.

Common Pitfalls:
Assuming critical mass is a single number independent of geometry and reflectors. It varies strongly with configuration and materials.

Final Answer:
Correct