How does fuel burnup affect reactor reactivity over long operation?

• A. Burnup increases fissile U-235 content, increasing reactivity.
• B. Burnup has no effect on cross sections.
• C. Burnup reduces fissile U-235 and produces isotopes that affect cross sections, typically decreasing reactivity requiring refueling or enrichment changes.
• D. Burnup only impacts cooling and has no reactivity effect.

Answer: (C)

Fuel burnup changes the neutron economy by evolving the fuel’s isotopic mix as it’s irradiated. As the reactor runs, U-235 is consumed by fission and neutron capture, while U-238 captures neutrons to form plutonium isotopes such as Pu-239 and Pu-241. These newly produced fissile isotopes can contribute to reactivity, but their buildup doesn’t fully compensate for U-235 depletion overall. At the same time, many fission products accumulate, including strong neutron absorbers like xenon-135 and samarium-149, which slow down the neutron flux. The combination of decreasing fissile content and increasing neutron absorbers shifts the cross sections the neutrons “see,” driving a steady decline in reactivity over long operation. To maintain power, refueling or enrichment changes (or adding burnable absorbers to the fuel) are used. So, burnup tends to decrease reactivity over time, not increase it.