![]() As I've described, this is due to mechanisms that vaguely fit the intuition you had. So if you change the moderator temperature, you will change the reaction rate. But practically, the operators control the reaction rate with engineered methods (control rods), and that is set by regulatory concerns. So if you change reactivity, then ultimately that will result in a change in the fission reaction rate. ![]() I should note that our reactors will come to equilibrium given enough time (provided we build them stable). The fuel has absorption peaks between the fast and thermal energies, and the fuel temperature affects the efficiency of these peaks through a more complicated effect.Moderator temperature affects that above thermal peak of neutrons, and the location of the peak affects reactivity.In commercial LWRs, the reactivity is related to temperature via two major interplays: Two Mechanisms for Temperature Dependence Even though the protons (in water) and neutrons are about the same mass, the neutron energy distribution is shifted toward the higher energies, because they downscatter from higher energies to begin with, and they are preferentially absorbed by the fuel at lower energies. This exists around the temperature of the reactor moderator. You can see that there is a "thermal flux" hump. In the lower part of the neutron energy spectrum, in fact, the neutrons are relatively well thermalized to the moderator temperature, but it depends on the reactor design. In fact, for light water reactors (LWRs), the neutrons are in direct momentum exchange with protons that exist in the moderator. There's no theoretical difficulty in applying the same relationship between particle energy and temperature as we do for gases ($ T $). The fuel is in Kelvin $K$, whereas the neutrons are most often stated in electron-volts, most commonly $MeV$. We don't often measure the temperature of neutrons in the same units as the fuel. They are generated at extremely high energies, and absorbed at much lower energies on average. Neutrons in a fission reactor are really in a transient state - moving from genesis to absorption again. Temperature of the neutrons has abundant qualifiers on it. In a fission reactor, we can talk about the temperature of the fuel, and we can also talk about the temperature of the neutrons.
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