Moderators slow neutrons to thermal energies to increase the fission probability of fissile material. What is the direct effect of this on reactor operation?

• A. Slowing neutrons decreases fission probability
• B. Slowing neutrons increases fission probability
• C. Slowing neutrons eliminates neutrons from the core
• D. Slowing neutrons has no effect on fission

Answer: (B)

Thermal neutrons have a much higher probability of causing fission in fissile materials like U-235. When a moderator slows neutrons to thermal energies, the fission cross-section for these isotopes increases, so each neutron is more likely to trigger fission. That directly raises the rate of fission reactions in the reactor and, if not limited by control systems, increases the reactor’s power. The moderator’s role is to keep neutrons at energies where fission is most probable, sustaining the chain reaction. The ideas that slowing neutrons decreases fission probability, eliminates neutrons, or has no effect don’t fit the shown energy dependence of fission cross-sections.