In this blog post, let’s take a step-by-step look at what nuclear power is, how it uses nuclear fission reactions to generate heat, and how that heat is used to produce steam and generate electricity.
On March 11, 2011, a massive tsunami struck the northeastern region of Japan following an earthquake. When this tsunami engulfed the nuclear power plant in Fukushima, it caused serious problems with the power supply and cooling systems, resulting in what we now refer to as the “Fukushima disaster.” Shocked by the catastrophic outcome, many people began to question the future of nuclear power, reigniting public debate on the issue. However, because many people lack a proper understanding of nuclear power, there is no shortage of speculation and misunderstanding. To achieve effective communication and social consensus, it is essential, above all, to accurately grasp the reality of the situation and, based on that, foster empathy and understanding. Let’s explore together through this article what nuclear power actually is and how it generates electricity.
Electricity, which is indispensable to modern civilization, is produced at power plants. Power plants often generate electricity by turning turbines, and power generation methods can be broadly categorized into hydroelectric, thermal, and nuclear power, depending on the energy source and the principle used to drive the turbines. Let’s first look at hydroelectric and thermal power. The principles behind hydroelectric and thermal power are, in fact, evident from their names. Hydropower uses the force of water, while thermal power uses the heat generated when fuel is burned to produce electricity, doesn’t it? In fact, hydropower directly drives turbines using the head and flow of water stored in dams, while thermal power generates steam using the heat produced by burning fossil fuels such as oil, natural gas, and coal, and this steam is then used to drive turbines.
But what about nuclear power generation? This method seems a bit suspicious right from the name. While the term “atom” gives us a rough idea of what it means, unlike hydroelectric or thermal power, it’s not immediately clear how an “atomic reactor” actually generates electricity. To understand the principle of nuclear power generation, we must first learn a bit more about atoms in detail. An atom consists of an “atom nucleus” and “electrons.” The nucleus accounts for most of the atom’s mass and is composed of two types of particles: neutrons and protons. The type of element and its atomic number are determined by the number of protons, not the number of neutrons. In the case of hydrogen, the lightest element in nature, most hydrogen atoms have one proton and no neutrons. In contrast, uranium—the heaviest naturally occurring element—has 92 protons, and the number of neutrons varies depending on the isotope.
Now that we understand atoms, let’s explore how these “atoms” are actually used to generate electricity. The reason I mentioned earlier that the number of neutrons in uranium is not constant is because each uranium atom contains a different number of neutrons. These atoms are called isotopes. Among the uranium isotopes, the atom with 143 neutrons is called uranium-235. Here, 235 is the mass number, which is the sum of the number of neutrons and protons. Uranium-235 is prone to undergoing a nuclear fission reaction, in which the atomic nucleus becomes unstable and splits into two when it absorbs a single neutron. At this point, 2 to 3 new neutrons are emitted, and a massive amount of energy is released in the form of heat and other forms. During this process, a slight mass deficit occurs; according to Einstein’s famous equation, E=mc² (E: energy, m: mass, c: speed of light), part of the mass is converted into energy, resulting in the release of a large amount of energy.
In its natural state, uranium consists mostly of uranium-238, while uranium-235 accounts for only about 0.7 percent. Therefore, in its natural state, it is difficult for uranium-235 to sustain a chain reaction. Although uranium-238 can also participate in nuclear reactions, it is not a primary isotope capable of sustaining a chain reaction driven by slow neutrons—as uranium-235 does—in a typical light-water reactor. Therefore, to use uranium as fuel for nuclear power generation, it often undergoes an “enrichment” process to increase the proportion of uranium-235. If we think of uranium-235 as a target and neutrons as arrows, the enrichment process can be likened to enlarging a target the size of a pinhole to the size of a basketball. Unlike natural uranium, where the “target” is the size of a pinhole, in enriched uranium—where the proportion of uranium-235 is higher—the likelihood of neutrons colliding with uranium-235 is much greater, making it easier to sustain a chain reaction of fission. The fuel for commercial light-water reactors is generally enriched to a uranium-235 concentration of about 5%.
Once a fission reaction occurs, the neutrons produced in the reaction collide with other uranium-235 nuclei, triggering further fission reactions; the neutrons produced in this process then collide with yet other uranium-235 nuclei, triggering more fission reactions, and this process continues.
In this way, a chain reaction occurs. This chain reaction is called a “nuclear fission chain reaction,” and what is particularly important here is that a single uranium-235 fission can release multiple new neutrons. Some of these neutrons trigger further fissions, sustaining the chain reaction. As the reaction repeats, a very large number of fissions can occur, generating a corresponding amount of thermal energy. However, in the reactors of nuclear power plants, the number of neutrons and the reaction rate are precisely controlled to prevent this chain reaction from increasing explosively. The nuclear fission chain reaction that occurs in a nuclear power plant reactor does not proceed in the same way as the explosive nuclear fission reaction that occurs in an atomic bomb. A reactor is a device designed to continuously generate a constant level of heat while controlling the chain reaction.
After all, an explosion would certainly not be acceptable while trying to generate electricity, would it? Therefore, reactors use various methods to control the fission chain reaction so that only a portion of the neutrons produced in each reaction triggers the next fission reaction. A prime example of such a device is the “control rod.” Control rods are made of a material that absorbs neutrons, and by inserting or removing them into the reactor, the rate of the fission chain reaction is regulated. This prevents the number of reactions from increasing exponentially and ensures that the reactor remains stable at its designed output level. Consequently, energy is generated at a consistently steady rate, and we can use this heat to produce electricity.
Nuclear power generation uses the heat produced in the reactor to generate steam. This steam is then used to turn turbines and generate electricity. Specifically, the heat generated by nuclear fission is transferred to the coolant; depending on the reactor type at the power plant, this heat is used either to directly boil water to create steam or to boil water in a separate system to create steam. The steam produced in this way drives the turbine, and electricity is generated as the generator connected to the turbine rotates. Compared to thermal power generation, the basic method of producing steam to drive a turbine is the same, but as we saw earlier, there is a significant difference in how the heat is generated. Thermal power generation produces heat by burning fuel, whereas nuclear power generation produces heat through the fission of uranium.
Nuclear power generation is a method that carries the risk of accidents, and the possibility of major accidents, such as the Fukushima disaster, cannot be ignored. However, just because a chain reaction of nuclear fission occurs in a nuclear power plant does not mean it will explode like an atomic bomb. Nuclear power plants generate electricity by utilizing the heat produced through controlled nuclear fission, and in the event of an accident, various safety measures—including not only the fission reaction itself but also cooling systems, power supply, and the containment of radioactive materials—play a critical role. Furthermore, nuclear power is one of the low-carbon energy sources that emits almost no direct carbon dioxide during the power generation process, and it is currently regarded as an important method of generating electricity capable of providing a stable power supply on a large scale. On the other hand, there are clearly challenges that must be addressed, such as the management of radioactive waste, the risk of accidents, and high initial construction costs. Therefore, to determine whether nuclear power should continue, it is necessary to examine both its advantages and its risks.
