Which neutron energy range is most effective for sustaining fission in U-235 in a thermal reactor?

• A. Fast neutrons in the MeV range
• B. Thermal (slow) neutrons in the meV range
• C. Cold neutrons in the meV range
• D. Epithermal neutrons in the eV range

Answer: (B)

Neutrons at thermal energies are most effective for sustaining fission in U-235. In a thermal reactor, neutrons are slowed by a moderator until their energies are around 0.025 eV (roughly meV range). At these thermal energies, the fission cross-section for U-235 is at its highest, so each neutron has a much higher probability of causing fission when it collides with U-235. Slowing fast neutrons to thermal energies greatly increases the fission rate and helps maintain the chain reaction efficiently. Fast neutrons, while capable of fissioning U-235, interact with a much lower probability at MeV energies, so fewer fissions occur per neutron. Epithermal or cold neutrons have lower or less favorable fission probabilities in typical reactor conditions, making them less effective for sustaining the reaction in a standard thermal reactor.