Homogeneous nuclear reactor: Choose the statement that best defines a homogeneous reactor configuration.

Introduction / Context:
Nuclear reactors are categorized by how fuel and moderator are spatially arranged. Homogeneous designs differ fundamentally from heterogeneous designs in which solid fuel elements are separated from moderator regions. Understanding this definition is important in reactor physics and engineering.

Given Data / Assumptions:

• The term “homogeneous” refers to fuel–moderator mixture at the microscopic scale.
• Typical examples: uranyl sulfate solutions in water (aqueous homogeneous reactors).
• Coolant identity is not, by itself, the defining criterion.

Concept / Approach:
A homogeneous reactor mixes fuel and moderator into a single phase or uniform suspension, enabling uniform neutron moderation throughout the medium. By contrast, using the same substance as moderator and coolant (option b) can be true in some heterogeneous designs (e.g., D2O-moderated, D2O-cooled CANDU), so it is not definitive of homogeneity. Statement (a) is imprecise and refers to distribution within the “reactor mass” rather than the specific fuel–moderator mixture concept.

Step-by-Step Solution:

Verification / Alternative check:
Classic aqueous homogeneous reactors (AHRs) used uranyl salts dissolved in water, confirming that mixing defines homogeneity.

Why Other Options Are Wrong:

• (a) Vague; does not necessarily imply fuel–moderator homogeneity.
• (b) Same substance for cooling and moderation is possible in heterogeneous systems.
• (d) Cannot be correct because (a) and (b) are not true definitions of homogeneous reactors.

Common Pitfalls:
Equating coolant identity with reactor classification; the spatial relationship between fuel and moderator is the key differentiator.

Final Answer:
The fuel and the moderator are mixed to form a single homogeneous material (solution or slurry).