Dr. Tsubokura's Radiation Lecture Vol.79
Author: Masaharu Tsubokura
Editors: Akihiko Ozaki M.D., Yuki Senoo
157. Control rods play an important role as a brake at nuclear reactors
Radioactive uranium produces a chain fission reaction when a neutron collides with it. Nuclear power plants generate electricity by turning a turbine with water vapor, which is obtained from heating the water using the energy released from the nuclear fission reaction of uranium 235.
This chain reaction can take place in the presence of neutrons. In nuclear reactors, there are many neutrons produced by nuclear fission. The larger the number of neutrons, the larger the chain of fission reaction, and vice versa. Moreover, when the number of neutrons decreases, the chain reaction will eventually stop.
To carry out the fusion reaction continuously, nuclear reactors have “control rods” whose function is to keep the number of neutrons at a constant level by absorbing them. These control rods function like a brake, which gradually decelerates the car when the pedal is lightly pressed and stops the car when the pedal is pressed hard.
During the earthquake that occurred in Fukushima in 2011, the seismometers were activated, and all of the control rods were inserted into the core of the reactors. This emergency braking operation successfully stopped the nuclear fission reaction in the reactor. However, even when nuclear fission stops, heat generation from the core does not stop immediately because of the various radioactive substances remaining in the core. A hydrogen explosion occurred at the Fukushima Daiichi nuclear power plant because it was unable to cool down this heat.
158. Generation of electricity in nuclear power plants requires an enormous amount of water
When a neutron collides with radioactive uranium, it produces a chain fission reaction. Nuclear power plants produce electricity by turning a turbine with water vapor, which is obtained from heating the water using the energy released from the nuclear fission reaction of uranium 235.
As explained above, the energy released from the fission of uranium transforms water into water vapor, and the energy given off from the water vapor is further converted into electric energy by turning a turbine. However, not all the energy released from the fission reaction can be effectively converted into electricity.
In fact, only about 30% of the energy generated by nuclear fission is used for power generation; the rest of the energy is unnecessary and overheats the reactor. Enormous reactor cooling systems are required because an exceedingly higher level of energy is heating the reactor compared to the maximum capacity of electricity generated by the reactor. Because a large amount of water is required for cooling the reactors, all of the nuclear power plants in Japan are located along the coast, while they are generally located along lakes and rivers in other countries.
This means when a reactor cannot be cooled down for any prolonged period of time, the temperature inside the reactor becomes extremely high, and the fuel rods start to melt. This is what happened during the nuclear accident that occurred at the Fukushima Daiichi nuclear power plant.
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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 14th and 21st January 2018 was reproduced for MRIC Global under the author's permission.