Control and Reaction States
The chain fission reaction of uranium nuclei is initiated under favorable conditions. If we do not interfere with the reaction, it is known as an uncontrolled reaction. Under certain conditions (e.g. in an atomic bomb) the reaction may propagate very rapidly, but in the majority of other cases it stops spontaneously.
For peaceful purposes, e.g. in the nuclear energy industry, we must be able to control this reaction, to increase or decrease it, and to perform startup and shutoff operations. Since the reaction intensity depends primarily on the number of free neutrons in the reactor core, the simplest way to control the chain reaction is the presence or absence of an absorber. According to the course of the reaction, there are three nuclear reactor states.
Video: Subcritical state of the nuclear reaction.
In the subcritical state, the concentration of absorbers in the reactor core is so high that more neutrons are absorbed than are produced by the fission processes. Fewer and fewer neutrons in each subsequent generation participate in the reaction, and finally, the chain reaction stops. This condition is used to decrease the reactor power or (when using a higher absorber concentration) to stop the fission at once.
Video: Critical state of the nuclear reaction.
In the critical state, the neutron concentration in the reactor core remains steady. In every generation, an approximately equal number of atoms decay, and so the chain reaction remains constant. This is the typical state of a nuclear reactor in operation.
Video: Supercritical state of the nuclear reaction.
The state is called supercritical if the number of neutrons in the reactor core increases, because there is a lack of absorbers and the majority of released neutrons cause subsequent fissions. Every new generation has more and more neutrons. In each subsequent generation, more and more neutrons enter the reaction. This condition is used during reactor startup or when an increase of the reactor power level is desired.
The first uncontrolled fission reaction was initiated about two billion years ago in Oklo, Gabon, when rainwater seeped into a uranium ore deposit and acted as a moderator.