How do fission cross-sections depend on neutron energy for U-235, and why is this important?

How do fission cross-sections depend on neutron energy for U-235, and why is this important?

• A. Independent of energy
• B. Peaks at fast neutrons
• C. High for thermal neutrons, decreases with energy
• D. Low for thermal neutrons, increases with energy

Answer: (C)

Fission cross-sections are energy-dependent: for U-235 the probability that a neutron will cause fission is highest when neutrons are slow (thermal energies around 0.025 eV) and generally decreases as the neutron energy increases. This means the fission cross-section is high at thermal energies (on the order of hundreds of barns) and falls off at higher energies, with some resonance structure in between. This trend matters because reactors rely on slowing neutrons to thermal energies to maximize fission in U-235, improving neutron economy and fuel utilization. In fast reactors, neutrons stay energetic and the fission probability for U-235 is much lower, so different reactor design considerations and fuels are involved.

Fission cross-sections are energy-dependent: for U-235 the probability that a neutron will cause fission is highest when neutrons are slow (thermal energies around 0.025 eV) and generally decreases as the neutron energy increases. This means the fission cross-section is high at thermal energies (on the order of hundreds of barns) and falls off at higher energies, with some resonance structure in between. This trend matters because reactors rely on slowing neutrons to thermal energies to maximize fission in U-235, improving neutron economy and fuel utilization. In fast reactors, neutrons stay energetic and the fission probability for U-235 is much lower, so different reactor design considerations and fuels are involved.