Answer: Slows down (moderates) fast neutrons to thermal energies.
- A Absorbs neutrons permanently, removing them from the reaction
- B Slows down (moderates) fast neutrons to thermal energies
- C Controls the overall reaction rate by being inserted or withdrawn
- D Cools the reactor core by directly carrying away heat
Correct answer: B. Slows down (moderates) fast neutrons to thermal energies
Explanation: Moderator (heavy water, graphite): slows fast neutrons to thermal energies (~0.025 eV) where U-235 fission cross-section is highest.
Binding energy per nucleon rises sharply for light nuclei, peaks around iron-56 (the most stable nucleus), then slowly declines for heavier nuclei - which is exactly why fusing light nuclei or splitting heavy nuclei both release energy: each moves the products toward this stability peak.
Concept context
Radioactivity, nuclear reactions, fission, fusion, and binding energy.