Uranium (U-235), a fairly common element on Earth, undergoes spontaneous fission a small percentage of the time. U-235 is one of the few materials that can undergo induced fission. If a free neutron runs into a U-235 nucleus, the nucleus will absorb the neutron without hesitation, become unstable and split immediately.
The neutron splits into two lighter atoms and throws off two or three new neutrons. The two new atoms then emit an incredible amount of energy in the form of heat and gamma radiation as they settle into their new states. The two atoms that result from the fission, later release beta radiation and gamma radiation of their own as well. Uranium is enriched to produce more energy.
We have seen how nuclear energy can be liberated through fission in an atomic bomb. No attempt is made to control the rate of energy release in a bomb. To build a useful nuclear reactor, however, one must be very certain that the fission rate can be controlled to prevent destruction of the reactor and irradiation of people. How can this be done?
In the fission of uranium-235, the liberated neutrons have very high energy and high speed. It turns out that uranium-235 actually undergoes fission much easier if it is approached by a slow neutron that travels at much lower speed. For the bomb this fact was not important because the number of fast neutrons and number of uranium-235 nuclei were abundant enough to do the job. For the construction of a reactor, the fact that slow neutrons have a much better chance of causing fission in uranium-235, is very good news.
Why? In reactors, high energy density run-away reactions must be avoided at all cost. The reactor must be designed with a low density of uranium-235 atoms. This way the number of fission reactions per unit volume can be kept low and extremely high temperatures at any one spot can be avoided. However, if the density of uranium-235 nuclei is low, neutrons will escape without a good change of causing fission and no sustainable reaction will be possible. The system would simply be sub-critical.
If the fission neutrons can be slowed down, the chance of fission increases rapidly. This effect easily makes up for the lower density of uranium-235 and allows a sustainable chain reaction to run at low energy density. Controlling the amount of neutrons in the reactor can then control the reaction rate.
An assembly of fissionable material is said to be critical when, on the average, one of the several neutrons emitted in the fission process causes another nucleus to fission. The energy released in any given amount of time is then constant. The rest of the neutrons can either be absorbed without fission or else escape from the system. Criticality thus depends on the geometry as well as the amount and kind of material present.
If an average of more than one neutron produce fission in other nuclei, then the assembly is said to be supercritical. In this case the power output increases rapidly. If less than one fission reaction occurs per fission neutron produced, the assembly is sub-critical. The process of fission will then decrease and cease with time. This will be the case if fission neutrons have a good chance of escaping the assembly without causing further fission.