Nuclear reactor fundamentals: Which reactor type may operate without a moderator (i.e., does not require neutron moderation)?

Introduction / Context:
Moderators slow down fast neutrons to thermal energies to increase fission probabilities in most power reactors. However, not all reactor concepts need moderation. This question checks understanding of which reactor operates on fast neutrons.

Given Data / Assumptions:

• Standard reactor types: CANDU, fast breeder, homogeneous, and PWR.
• Definition: a moderator slows neutrons via elastic scattering (e.g., heavy water, light water, graphite).

Concept / Approach:
Fast breeder reactors are designed to use fast neutrons to breed fissile fuel (e.g., convert fertile U-238 to Pu-239). They therefore do not employ moderators and often use liquid metal coolants (e.g., sodium) to avoid moderation. By contrast, CANDU uses heavy water as moderator, PWRs use light water, and homogeneous reactors also rely on moderation within the fuel-moderator mixture.

Step-by-Step Solution:
Identify reactor requiring no moderation → fast breeder reactor.Other listed types explicitly employ moderators (heavy or light water).Hence, select the fast breeder reactor.

Verification / Alternative check:
Reactor physics texts categorize fast reactors as unmoderated, relying on fast neutron spectra for breeding and high conversion ratios.

Why Other Options Are Wrong:
CANDU: heavy-water moderated. PWR: light-water moderated. Homogeneous reactor: fuel mixed with moderator (historical designs). All require moderation.

Common Pitfalls:

• Confusing coolant type with moderator; some coolants (e.g., sodium) avoid moderation.
• Assuming “heavy water” reactors are “fast”; they are thermal spectrum.

Final Answer:
Fast breeder reactor