Fast breeder reactor physics: which moderator is used in a fast breeder reactor to slow down neutrons?

Introduction / Context:
Neutron spectrum control is central to reactor design. Thermal reactors rely on moderators to slow neutrons, whereas fast reactors are designed to maintain a fast (high-energy) spectrum to improve breeding of fissile material from fertile isotopes and to exploit fast-fission characteristics.

Given Data / Assumptions:

• We consider fast breeder reactors (FBRs), which aim for breeding ratios above 1.
• Options list common moderator materials and a “No” option.
• Question asks what moderator is used to slow down neutrons in an FBR.

Concept / Approach:
Fast reactors intentionally avoid moderators because moderating collisions would soften the spectrum, reducing breeding gains and altering cross sections unfavorably. Designs therefore employ coolants with low moderating power, such as liquid sodium, lead, or lead–bismuth, and minimize hydrogenous materials in the core region.

Step-by-Step Solution:
Recognize that the defining feature of an FBR is a fast neutron spectrum.Moderators like light water, heavy water, graphite, or beryllium would slow neutrons, contrary to FBR goals.Hence, the correct choice is that no moderator is used.

Verification / Alternative check:
Operational and prototype FBRs (e.g., sodium-cooled designs) exclude moderators and are configured to keep leakage and spectral softening minimal.

Why Other Options Are Wrong:
Graphite/heavy water/beryllium: all are moderator materials used in thermal reactors; using them would defeat the fast spectrum purpose.

Common Pitfalls:
Assuming every reactor needs a moderator; confusing reflector materials with moderators.

Final Answer:
No