Thermal neutrons used to induce fission in U-235 have energies below about 0.025 eV. What is the typical speed of such a neutron at room temperature, expressed in m/s?

Introduction:
In reactor physics, the term “thermal neutron” refers to neutrons in approximate equilibrium with the moderator at room temperature. Their speed determines reaction rates and resonance escape probabilities, influencing core design and control.

Given Data / Assumptions:

• Thermal energy reference is about 0.025 eV at roughly 293 K.
• We assume Maxwell-Boltzmann distribution; the quoted speed is a representative mean value.
• Units required: metres per second (m/s).

Concept / Approach:
Thermal neutron speed v is commonly cited around 2.2 × 10^3 m/s at room temperature. This order-of-magnitude is standard in nuclear engineering references and is used for quick estimates of diffusion length and reaction rates.

Step-by-Step Solution:
Recognize “thermal” implies ~0.025 eV at room temperature.Corresponding typical speed ≈ 2,200 m/s.Choose the option that matches 2,200.

Verification / Alternative check:
Using kinetic energy relation (E ≈ 1/2 m v^2) with neutron mass gives the same order of magnitude, validating the standard value.

Why Other Options Are Wrong:

• 1 or 220 m/s: Far too low; would correspond to much lower energies.
• 3 × 10^11 or 9 × 10^21 m/s: Physically impossible or nonsensical; they exceed realistic particle speeds and even the speed of light.

Common Pitfalls:
Confusing instantaneous particle speeds with macroscopic flow velocities, or mixing up eV (energy) with m/s (speed).

Final Answer:
2,200