The strong fission cross-section of U-235 for thermal neutrons implies reactors optimize for what condition?

The strong fission cross-section of U-235 for thermal neutrons implies reactors optimize for what condition?

• A. Fast neutron spectrum with no moderator
• B. Very high temperature operation
• C. Highly moderated, thermal neutron spectrum
• D. No control rods needed

Answer: (C)

The crucial idea is that U-235 is far more likely to fission when it encounters slow, thermal neutrons than fast ones. By moderating (slowing down) the neutrons produced by fission, reactors create a thermal neutron spectrum where the fission cross-section for U-235 is high. This boosts the probability that each neutron will cause another fission, helping sustain a controlled chain reaction. That’s why reactors are designed with a moderator material (like water or graphite) to keep the neutron population in the thermal energy range. Fast-neutron operation, high temperatures, or omitting control mechanisms would not optimize this high fission probability.

The crucial idea is that U-235 is far more likely to fission when it encounters slow, thermal neutrons than fast ones. By moderating (slowing down) the neutrons produced by fission, reactors create a thermal neutron spectrum where the fission cross-section for U-235 is high. This boosts the probability that each neutron will cause another fission, helping sustain a controlled chain reaction. That’s why reactors are designed with a moderator material (like water or graphite) to keep the neutron population in the thermal energy range. Fast-neutron operation, high temperatures, or omitting control mechanisms would not optimize this high fission probability.