What is the main purpose of a neutron moderator in a reactor?

What is the main purpose of a neutron moderator in a reactor?

• A. To increase neutron speed to sustain chain reactions.
• B. To slow neutrons to thermal energies to raise fission probability.
• C. To remove neutrons from the core.
• D. To absorb neutrons to control reactivity.

Answer: (B)

Neutron energy controls how likely a fission event is in the fuel. A moderator slows the fast neutrons produced by fission down to thermal energies, where fissile isotopes like uranium-235 have a much higher fission cross section. As neutrons lose energy through elastic scattering with light nuclei in the moderator, they reach near-thermal energies (about 0.025 eV), increasing the probability they'll cause another fission before being absorbed elsewhere or leaking out. By doing this, the moderator enables the chain reaction to be sustained efficiently. In short, it’s about transforming fast neutrons into slow, or thermal, neutrons to boost the chance of fission.

Neutron energy controls how likely a fission event is in the fuel. A moderator slows the fast neutrons produced by fission down to thermal energies, where fissile isotopes like uranium-235 have a much higher fission cross section. As neutrons lose energy through elastic scattering with light nuclei in the moderator, they reach near-thermal energies (about 0.025 eV), increasing the probability they'll cause another fission before being absorbed elsewhere or leaking out. By doing this, the moderator enables the chain reaction to be sustained efficiently. In short, it’s about transforming fast neutrons into slow, or thermal, neutrons to boost the chance of fission.