Comment by dontwearitout

The energy of neutrons isn't really analogous to the energy of atoms in chemical reactions, but absolutely affects the reaction dynamics. The "cross section", or interaction probability, is a strong function of neutron speed. In bombs the neutrons don't get a chance to slow down, but in reactors you try to reduce them to "room temperature" speeds using collisions with light, inert nuclei (the moderator). Here's a diagram showing the interaction probability vs. neutron speed for a few isotopes of uranium: https://tinyurl.com/u235-cross-section