Short Answer
To slow neutrons to thermal energies to sustain the chain reaction is the best answer.
Nuclear and particle questions tend to hinge on conservation laws, decay rules, interaction types, or scale. A clean answer usually comes from identifying the process before the details.
Moderators (water, graphite) slow fast neutrons to thermal energies where fission cross-section of ²³⁵U is large enough to sustain a chain reaction.
Why This Answer Is Correct
This is a Medium-level question in Nuclear & Particle Physics. The prompt is really testing whether you can connect the concept to its defining physical relationship instead of picking a nearby-but-wrong term.
Track conserved quantities carefully: energy, momentum, charge, lepton number, baryon number, and spin-like constraints when relevant.
Choices At A Glance
- A. To absorb neutrons
- B. To slow neutrons to thermal energies to sustain the chain reaction
- C. To prevent coolant leaking
- D. To convert heat to electricity
When similar options appear on an exam, eliminate the ones that break the core law, use the wrong units, or confuse a definition with a consequence.
Study Next
Review the related formula library
review the theoretical physics hub
Topic Snapshot
Topic: Nuclear & Particle Physics
Difficulty: Medium
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.