To improve P f i s s i o n and enable a chain reaction, natural or low enrichment uranium-fueled reactors must include a neutron moderator that interacts with newly produced fast neutrons from fission events to reduce their kinetic energy from several MeV to thermal energies of less than one eV, making them more likely to induce fission. This is because U has a larger cross section for slow neutrons, and also because U is much less likely to absorb a thermal neutron than a freshly produced neutron from fission.