Nuclear reactor classification: a heterogeneous reactor is one in which the moderator and the fuel are present in different phases or distinct regions. Which option best matches this definition?

Introduction / Context:
Reactors are often categorised as homogeneous or heterogeneous based on how fuel and moderator are arranged. In a heterogeneous reactor, fuel is typically in discrete elements (rods, plates, pebbles) embedded in or surrounded by a moderator, yielding spatially separate regions and often different physical phases.

Given Data / Assumptions:

• “Heterogeneous” refers to spatial/phase separation between fuel and moderator.
• Examples include solid fuel rods in graphite or heavy/light water moderators.
• Coolant may be the moderator or a separate fluid; this does not define heterogeneity by itself.

Concept / Approach:
The defining feature is that fuel and moderator are not uniformly mixed at the microscopic level. Homogeneous systems (e.g., solutions of fuel salts in molten salt reactors) contrast with solid-fuel rods in a fluid moderator. Thus, an option explicitly stating fuel and moderator are in different phases/regions captures the definition most directly.

Step-by-Step Solution:
Identify the key terms: “moderator” and “fuel.”Check which option states they are in different phases/regions.Select the option with explicit example: solid uranium with liquid heavy water.

Verification / Alternative check:
Graphite-moderated, gas-cooled reactors (solid fuel, solid moderator) are still heterogeneous because fuel elements and moderator blocks are distinct regions; phase difference is a common but not exclusive indicator of heterogeneity—distinct regions suffice.

Why Other Options Are Wrong:

• Moderator and coolant different materials: Common but not definitional.
• Moderator and coolant different phases: Irrelevant to heterogeneity between fuel and moderator.
• None of these: Incorrect since (c) matches the definition.
• All same phase requirement: Opposite of heterogeneity.

Common Pitfalls:
Equating “heterogeneous” with “different materials anywhere”; the distinction is between fuel and moderator arrangements, not coolant identity alone.

Final Answer:
Moderator and fuel are present in different phases (e.g., solid uranium fuel with liquid heavy water moderator)