Energy of thermal neutrons used to fission U-235: which range best represents their kinetic energy at room temperature?

Introduction / Context:
In thermal reactors, neutrons are moderated to low energies where the fission cross-section of U-235 is high. Recognizing the approximate energy scale of “thermal” neutrons at room temperature is essential for interpreting cross-section data, diffusion lengths, and reactor kinetics.

Given Data / Assumptions:

• Room temperature near 293 K is assumed.
• Thermal equilibrium of neutrons with the moderator.
• Energy distribution is approximately Maxwellian.

Concept / Approach:
The characteristic energy of thermal neutrons at 293 K corresponds to about kT, where k is Boltzmann’s constant. Numerically, kT ≈ 0.025 eV at room temperature. Therefore, typical thermal neutrons have energies around 0.025 eV or less, much lower than epithermal (eV to keV) or fast (~MeV) neutron energies.

Step-by-Step Solution:

Verification / Alternative check:
Cross-section charts for U-235 show a pronounced 1/v dependence at low energies, peaking effectiveness in the thermal range near 0.025 eV at room temperature.

Why Other Options Are Wrong:

• 1–25 eV and > 1 eV: epithermal energies, not thermal.
• > 200 eV and ~1 MeV: fast neutron ranges, orders of magnitude too high for thermal neutrons.

Common Pitfalls:
Confusing average energy (≈0.025 eV) with a sharp value; thermal spectra are distributions centered around kT.

Final Answer:
< 0.025 eV