What does temperature affect in reactor reactivity?

• A. Only the moderator density
• B. Only the fuel's absorption cross sections
• C. The density of the moderator and the neutron absorption cross sections of the fuel
• D. Has no effect on reactivity

Answer: (C)

Temperature affects reactivity through two interrelated effects: changes in moderator density and changes in the fuel’s absorption cross sections.

As the temperature rises, the moderator expands and its number density decreases. This reduces the likelihood of neutrons scattering and slowing down in the moderator, which shifts the neutron spectrum and alters the balance of reactions in the core. In other words, the way neutrons slow down and how often they interact with the fuel changes because the moderator is less dense.

At the same time, the fuel’s absorption cross sections depend on energy in a sharp, resonance-driven way. Higher fuel temperature causes Doppler broadening of these resonances, making them wider and more likely to absorb neutrons at a given energy. This increase in resonance absorption inside the fuel tends to reduce reactivity (a negative Doppler coefficient).

Putting these together, temperature influences reactivity both by changing the moderator density and by altering the neutron absorption behavior of the fuel. That’s why the correct understanding is that temperature affects the density of the moderator and the neutron absorption cross sections of the fuel.