Dr. Tsubokura's Radiation Lecture Vol.80

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

159. The reactor continues to cool after the termination of a nuclear fission reaction

Nuclear power plants generate electricity by turning a turbine via water vapor, which is obtained from heating the water using the energy released from the nuclear fission reaction of uranium 235.

Of the energy generated by nuclear fusion reaction at nuclear power plants, approximately 30% is used for power generation, while the rest is unnecessary and overheats the reactor. Therefore, nuclear power plant operation requires a huge amount of cooling water. However, this does not mean that the reactor cooling system can be stopped when the nuclear reactor operation is suspended and nuclear fission chain reactions stop.

This is because radioactive substances originating from the nuclear fission reaction of uranium will continue to produce heat as they emit radiation. This phenomenon is called "decay heat." Within a day following the reactor shutdown, the production of heat will immediately decrease to less than 1% of that produced during normal operation. However, after that, the amount of the produced heat does not quickly decline, and the reactor will continue to generate a low amount of heat for a long time.

For this reason, the reactor will still produce a certain amount of heat, and it is still imperative to check the temperature of the reactor core constantly and continue running the cooling system of the reactor.

160. A set of large and small fragments produced by a nuclear fission reaction

Nuclear power plants generate electricity by using the energy released from the nuclear fission reaction of uranium 235.

As repeatedly discussed in previous articles, all materials in the world are composed of small invisible particles called protons, neutrons, and electrons. Uranium 235 contains a total of 235 protons and neutrons at its core.

When a neutron collides with uranium 235 and causes a nuclear fission reaction, the uranium 235 does not split into exact halves. It has been found that uranium splits into two fragments centered on atomic mass numbers 95 and 140, with slight differences between the cases. This reaction will, in turn, generate another fission reaction as several neutrons are released.

Since various kinds of fission reactions involving several combinations of large and small fragments are possible, a variety of radioactive substances are produced by the fission reaction of radioactive uranium; common examples of large fragments are iodine-131 and cesium-137, while an example of a small fragment is strontium-90.

You may be surprised to hear that there are various kinds of radioactive substances produced by nuclear fission reactions at nuclear reactors. However, the amount, half-life, and the spreading methods differ depending on the radioactive substance. As a result, of the many radioactive substances produced from nuclear reactors, we still have to pay the most attention to cesium.

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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 28th January and 4th of February 2018 was reproduced for MRIC Global under the author's permission.

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