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 maintained manner where the rate of the fission reaction does not increase, it is said to be that "the reactor has reached criticality."
162. Unintentional criticality can lead to an accident.
As explained in the previous article, "criticality" refers to the state where the number of neutrons is constant, which sustains the fission chain reactions at a constant rate. During the operation of a nuclear reactor, it is said that "the reactor has reached criticality” when the number of neutrons within the reactor core remains constant, and the fission chain reactions are sustained in a controlled manner.
On the other hand, sometimes a fissionable material, such as uranium-235, unintentionally goes into criticality or initiates a fission chain reaction beyond the critical state triggered by operation misconduct. This is called a criticality accident.
In September 1999, a criticality accident occurred during the preparation of a uranium fuel solution at a nuclear reprocessing plant in Tokai-mura, Ibaraki Prefecture. Unlike nuclear reactors, nuclear reprocessing plants are not the kinds of facilities equipped to reach the criticality of uranium. However, a sustained fission chain reaction was unintentionally initiated, and radiation, including neutron rays and gamma rays, was spread to the surrounding environment. As a result, of the three workers exposed to the neutron radiation rays at close range, two died, and one was severely injured. The accident also caused a large number of people across the planet to be exposed to radiation.
The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 11th and 18th of February 2018 was reproduced for MRIC Global under the author's permission.