Reactor Physics – Fuel Enrichment and Core Compactness Consider a thermal nuclear reactor. If the fuel is richer in fissionable material (higher fraction of fissile atoms), can the critical reactor core be made more compact compared to fuel with a lower fissile fraction?

Introduction / Context:
Reactor core size and geometry are governed by neutron economy. The balance between neutron production, absorption, and leakage determines whether a system reaches criticality. Fuel enrichment—raising the fraction of fissile isotopes like U-235—directly influences this balance and therefore affects how compact a core can be while remaining critical and controllable.

Given Data / Assumptions:

• Thermal reactor with a suitable moderator (e.g., light water).
• Comparison at similar power density constraints and materials.
• Goal is to understand critical size trends with fissile content.

Concept / Approach:

Increasing fissile atom density raises the probability that a thermalized neutron causes fission rather than being captured non-productively or leaking out. As the macroscopic fission cross-section grows, the required neutron path length to sustain the chain reaction shrinks. This permits a reduction in geometric buckling requirements and allows a smaller core to achieve k_eff = 1 with acceptable margins, provided heat removal and structural limits are respected.

Step-by-Step Solution:

Verification / Alternative check:

Practical designs move from natural uranium (large cores with specific moderators) to low-enriched uranium (smaller cores). Research reactors with high enrichment achieve very compact cores, illustrating the principle.

Why Other Options Are Wrong:

Moderator removal in thermal reactors would worsen moderation; enrichment does not eliminate the need for moderation. Fast reactors are a different regime; the statement applies broadly, not exclusively to fast systems. Leakage certainly changes with size; enrichment helps counteract leakage by boosting fission probability, so saying leakage is unaffected is incorrect.

Common Pitfalls:

Ignoring thermal-hydraulic limits: even if criticality allows a compact core, heat flux and material limits may constrain minimum size. Criticality alone does not set all design dimensions.

Final Answer:

Agree