Avoiding "resonant captures"
In the core of a reactor, the neutrons below 10 keV enter in the region of uranium-238 resonances. The figure gives a partial picture of this domain where capture probabilities are important for certain energies, and almost nil elsewhere. If the aim is to get slow neutrons to operate the reactor, neutrons must remain as least time as possible in this resonances area. In the presence of a water moderator, a collision on a proton of the hydrogen allows a neutron to make great leaps above turbulences by halving its energy on average. Without a moderator, the neutron loses its energy by small jumps and has a high probability of being captured by hitting upon on a resonance.
IN2P3 (Source JANIS)