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Dr. Tsubokura's Radiation Lecture Vol.81

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

161. When a fission chain reaction is stabilized, this is called "criticality."

Neutrons play an important role in inducing a continuous fission chain reaction of uranium-235, which is required for nuclear power generation. When nuclear fission of uranium-235 occurs, two or three neutrons are released. These neutrons produced from the fission reaction of uranium will then collide with another uranium-235 atom, which is followed by another fission reaction. In this manner, self-sustaining chain fission reactions can take place, and new neutrons are repeatedly produced.

As mentioned above, two or three neutrons are released as a result of the fission reaction of uranium-235. Moreover, when "all" of these neutrons collide with another uranium-235 atom, the number of neutrons will increase rapidly in a short time period, and an enormous amount of energy will be generated.

On the other hand, if only one neutron initiates the next fission reaction and the remaining free neutrons are absorbed by control rods, the number of neutrons in the reactor core will remain constant. In this way, nuclear fuel could sustain a fission chain reaction. This reactor state is called "criticality." When the number of neutrons within the reactor core is controlled, and when a chain reaction is operated in a main