How does burnup affect the effective delayed neutron fraction?

How does burnup affect the effective delayed neutron fraction?

• A. It decreases
• B. It increases
• C. It remains the same
• D. It varies unpredictably

Answer: (A)

Burnup changes the isotopic makeup of the fuel, shifting fission contributions from U-235 toward plutonium isotopes like Pu-239 and Pu-241. These plutonium isotopes have smaller delayed-neutron fractions than U-235, so when they contribute more of the fissions, the weighted average delayed-neutron fraction, Beta_eff, decreases. In short, as burnup increases, Beta_eff tends to fall because the fuel becomes dominated by isotopes with lower delayed-neutron emission per fission.

Burnup changes the isotopic makeup of the fuel, shifting fission contributions from U-235 toward plutonium isotopes like Pu-239 and Pu-241. These plutonium isotopes have smaller delayed-neutron fractions than U-235, so when they contribute more of the fissions, the weighted average delayed-neutron fraction, Beta_eff, decreases. In short, as burnup increases, Beta_eff tends to fall because the fuel becomes dominated by isotopes with lower delayed-neutron emission per fission.