The much more common U 238 nucleus also attracts free neutrons, but not so strongly, and it survives neutron capture without fission. A neutron has to be moving very slowly to be captured by U 235 . On the other hand, when U 235 fissions the emitted neutrons are moving very fast. To get a chain reaction in a nuclear reactor, the main problem is to slow down the emitted neutrons before they leave the reactor. However, the neutrons bounce off the heavy uranium nuclei with almost no energy loss to get a particle to lose much of its energy in a collision, it must collide with something approximately its own size.
Water is often used, the neutrons lose energy bouncing off the protons in the hydrogen atoms.
Unfortunately, the neutrons often stick to the protons, forming deuterons, so to keep the reaction going the uranium must be enriched the U 235 content boosted to one to four percent. Of course, care must be taken to ensure that the chain reaction isn t too successful moderators, which absorb neutrons, are put into the reactor if the rate of fission increases beyond the planned level. Needless to say, moderators are not used in military applications of this concept.