Natural uranium consists of a mix of isotopes, mostly 238U and a much smaller amount of 235U. Both of these isotopes can undergo fission when struck by a neutron of sufficient energy, and as part of this process, they will give off medium-energy neutrons. However, only 235U can undergo fission when struck by neutrons from other uranium atoms, allowing it to maintain a chain reaction. 238U is insensitive to these neutrons and it thus not fissile like 235U. While 235U is sensitive to these neutrons, the reaction rate is greatly improved if the neutrons are slowed from their original relativistic speeds to much lower energies, the so-called thermal neutron velocities.