Moderators Slow Neutrons for Efficient Fission

Steps:

• Recall that nuclear fission requires neutrons to be absorbed by fissile nuclei like U-235.
• Understand fast neutrons from fission are less likely to cause further fission than slow (thermal) ones.
• Identify moderators as materials that reduce neutron speed without absorbing them significantly.
• Match this to the choice describing slowing neutrons to sustain the chain reaction.

Why D is correct:

• Moderators, like water or graphite, slow fast neutrons to thermal energies (~0.025 eV) via elastic collisions, increasing fission probability per the fission cross-section law.

Why the others are wrong:

• A: Moderators do not absorb neutrons; control rods do that to stop the chain.
• B: Absorbing more neutrons halts the reaction, opposite of moderation's goal.
• C: Heat removal is the role of coolant, not moderators.

Final answer: D