Critical mass in fission: What does the term “critical mass” specifically mean in reactor physics and nuclear chain reactions?

Introduction / Context:
In nuclear fission systems, the concept of critical mass is central to whether a self-sustaining chain reaction can occur. It has precise physical meaning unrelated to economics or plant scale, and depends on material, geometry, density, and presence of reflectors and moderators.

Given Data / Assumptions:

• Fissile material considered (e.g., U-235, Pu-239).
• Neutron economy determined by production, absorption, leakage.
• Geometry and reflectors influence effective mass required.

Concept / Approach:
Critical mass is the minimum mass (under specified conditions) of a fissile assembly at which the effective multiplication factor keff equals 1. At keff = 1, each generation of neutrons produces exactly one subsequent generation on average, so the reaction is self-sustaining (critical). Subcritical (keff < 1) dies away; supercritical (keff > 1) increases in power. This definition is purely physical and does not mandate commercial-scale power production or economic viability.

Step-by-Step Solution:

Verification / Alternative check:
Standard reactor physics texts define critical mass via keff = 1 and show dependence on reflectors, enrichment, and density.

Why Other Options Are Wrong:

• Commercial scale or economic power: These are engineering/economic outcomes, not the physics definition of critical mass.
• None of these: Incorrect because option (a) is precisely correct.

Common Pitfalls:
Confusing critical mass with criticality at operating power; criticality can occur at any power level once control rods set reactivity to zero.

Final Answer:
Sustainment of chain reaction.