What is nuclear power generation?

Nuclear power generation produces electricity in a similar way to thermal power generation. In both cases, boil water and convert it into steam, using that steam to turn large impellers known as turbines to operate a generator and produce electricity. While thermal power generates heat by burning coal, oil and natural gas, nuclear power generation uses the heat generated by the fission of uranium fuel inside a reactor.

There are various types of nuclear reactors, and the type used in Japan is known as a light-water reactor. In these light-water reactors, regular water is used to regulate the speed of nuclear fission and to cool the heat. Additionally, light-water reactors are further divided into two types based on differences in the mechanisms used to generate steam, Boiling Water Reactors (BWR) and Pressurized Water Reactors (PWR).

One of the characteristics of nuclear power generation is that it produces a large amount of electricity from a small amount of fuel. One piece of uranium fuel (with a diameter and height of around 10 mm) can produce enough electricity to power an regular home for 6-8 months. However, since uranium fuel is a radioactive substance, strict safety controls including containment are required.

All of the substances that surround us are made up of "atoms". At the center of each atom is an atomic nuclei, made up of many protons and neutrons. The phenomenon in which an atomic nuclei splits into several parts is referred to as "nuclear fission," and "uranium" is a substance that easily undergoes this fission.

When a uranium nuclei collides with a "neutron," the nuclei splits into two and generates a large amount of energy. At this time, two or three neutrons are released, colliding with other nuclei and causing a chain reaction of nuclear fission. Nuclear power generation harnesses the immense thermal energy produced through this chain reaction of nuclear fission to produce electricity.

Naturally occurring uranium only contains 0.7% of "uranium 235," an uranium isotope that is prone to nuclear fission. The majority of uranium deposits are made up of "uranium 238," which is less prone to fission.
In nuclear power generation, the content of "uranium 235" in uranium fuel has increased to 3-5 % of the total.

Radiation refers to the energy released as particles and electromagnetic waves from radioactive materials. A property of this energy is its ability to pass through materials. Substances that give off radiation are referred to as "radioactive materials", and "radioactivity" describes its ability to emit radiation. To use a flashlight as an example, think of radiation as the light, radioactivity as the ability to produce the light, and the flashlight that emits the light as the radioactive material.

Radiation also occurs in nature, and we have always surrounded by radiation. For example, there are cosmic rays that shower down from space and radiation emitted from the ground and rocks. And there are even small amounts of radiation in the air and the food we eat that we absorb this radiation into our bodies every day.

In today's world, radiation is utilized in a wide range of fields. In the medical field, it is used for X-rays to diagnose and treat cancer, while in the industrial field, it helps inspect the inside of products. Radiation is also used in agriculture to cultivate crops and improve breeding.

There are two types of reactor currently in use in Japan, Boiling Water Reactors (BWR) and Pressurized Water Reactors (PWR). They differ in the mechanisms used to generate steam.

In a BWR, cooling water inside the reactor is boiled, and the steam that is produced is sent directly to turbines to generate electricity. Based on the Boiling Water Reactor (BWR), the Advanced Boiling Water Reactor (ABWR) has been developed with improved safety and economic efficiency.The reactor incorporates a pump to circulate cooling water inside the reactor and employs a new drive mechanisms to move the control rods. In addition, the reactor containment vessel is made of reinforced concrete integrated with the reactor building, and its earthquake resistance is improved.

BWRs are used by Tohoku Electric Power Co., Hokuriku Electric Power Comlpany, Tokyo Electric Power Company Holdings, Chubu Electric Power Co., Chugoku Electric Power Co., and the Japan Atomic Power Company.

ABWRs are used by Hokuriku Electric Power Company, Tokyo Electric Power Company Holdings and Chubu Electric Power Co., and are also expected to be introduced by Chugoku Electric Power Co., and J-Power.

There are two types of reactor currently in use in Japan, Boiling Water Reactors (BWR) and Pressurized Water Reactors (PWR). They differ in the mechanisms used to generate steam.

In a PWR, water that has been heated inside the reactor (primary cooling water) is kept from boiling by maintaining a high pressure. The heat produced is transferred to a steam generator, where a separate volume of water (secondary cooling water) is boiled to produce steam, which in turn drives a turbine to generate electricity. The major characteristic of a PWR is that the primary cooling water that comes into direct contact with the fuel rods, and the secondary cooling water that drives the turbines as steam, are completely separated.

PWRs are used by Hokkaido Electric Power Co., Kansai Electric Power Co., Shikoku Electric Power Co, Kyushu Electric Power Co., and the Japan Atomic Power Company.

Nuclear power is one of the ways to produce electricity efficiently while limiting the impact on the environment. As no carbon dioxide emissions are produced during generation, nuclear power is highly valued as a clean energy source in today's world, where we are expected to promote carbon neutrality and achieve a decarbonized society.

Solar and wind power generation, which are also well known sources of clean energy, suffer from unstable output due to weather. This increases the costs of integrating them into power systems, including the need for supply-demand balancing. A characteristic of nuclear power, on the other hand, is the low cost of both generation and integration. Large amounts of electricity can be supplied over the long term, and in a stable manner.

The high fuel efficiency is another major benefit. Uranium has a high energy density, which means a large amount of electricity can be produced from a small quantity. As the amount of fuel needed is much less than coal or natural gas, transportation and storage costs can also be controlled. What's more, uranium is less susceptible to fluctuating fuel prices, making it an economically stable method of power generation over the long term.

Taking lessons from the Fukushima Daiichi Nuclear Power Station accident, new regulatory standards were introduced and safety measures were significantly strengthened. Each nuclear power plant is designed to withstand the largest possible earthquakes and tsunamis, and seawalls and seal buildings from water damage are implemented.
In addition, impact assessments for other natural disasters such as volcanoes and tornadoes have been conducted, with countermeasures implemented as needed.

To take things further, the action to be taken in response to a severe accident that exceeds design criteria were also strengthened. To ensure that multiple safety devices and systems do not lose function at the same time, at least two external power sources are secured, with emergency power and water supply vehicles also deployed. Systems to prevent the dispersion of radioactive material and equipment to prevent hydrogen combustion have been installed, and remote operating systems for power plant equipment have also been introduced as anti-terrorism measures.

Additionally, the new regulatory standards require that nuclear power plant companies continually implement self-directed efforts to improve safety, even if they are deemed standards-compliant.

The electricity demand in Japan is expected to increase due to advances in digital technologies. As securing decarbonized power sources can influence Japan's economic growth and the competitiveness of its industry, it is extremely important to make the most of both renewable energy and nuclear power sources, aiming for a balanced power source mix rather than relying on specific power sources.

Given these circumstances, it will be difficult to meet Japan's capacity needs by simply restarting or extending the operating life of the nuclear power plants. Various options are therefore being explored, including the redevelopment of innovative next-generation reactors incorporating new safety mechanisms on the site of nuclear power plants scheduled for decommissioning.

Innovative next-generation reactors currently under development include innovative light-water reactors, small light-water reactors, fast reactors, high-temperature gas reactors, and nuclear fusion.

Through revised laws aimed at realizing a decarbonized society, Japan has extended limits on the operating life of nuclear power plants to beyond 60 years, while keeping the basic policy of previous standard of 40 years. In conjunction with this change, nuclear power plants that have been operating for 30 years are required to conduct technical assessments every 10 years thereafter, formulating and gaining approval for long-term facility management plans.

To enable the safe and long-term operation of nuclear power plants under this system, advanced maintenance technologies have been introduced to ensure the high-precision inspection and repair of the equipment. Nuclear reactors and other equipment subjected to high-temperature and high-pressure environments can deteriorate over time, including pipe wear and cracked insulation. IT-based continuous monitoring systems are being developed to ensure those changes over time and other abnormalities can be detected early on during operation.

Robot-based remote operating systems are also being steadily introduced to minimize the radiation exposure of workers. Accurately identifying the state of deterioration and upgrading to systems equipped with newer technologies will make it possible to maintain the overall performance of nuclear power plants.

The standard operating life of a nuclear power plant is currently set at 40 years*1. Nuclear power plants that have ceased operation due to aging or other factors need to undergo a process known as decommissioning, which is a series of measures to safely dismantle the facility and reuse the site.

Nuclear power plant decommissioning is carried out in stages. First, the spent fuel is relocated to an appropriate facility, and then decontamination operations are carried out after contamination level is assessed. Next, peripheral equipment such as turbines and piping is removed, and the dismantlement of the main nuclear reactor begins. Once the inside of the reactor has been dismantled and the removal of radioactive material has been confirmed, the entire building is finally demolished. As the decommissioning work progresses, the amount of radioactive material at the site is gradually reduced.

Decommissioning work for a nuclear power plant is a major long-term project that can last more than 30 years. In Japan, 21 reactors^*2 have been scheduled for decommissioning, and the number is expected to rise in the future. Therefore, to streamline and improve the safety of decommissioning work, progress is being made to adopt advanced technologies, including measurement technologies that utilize 3D data and dismantlement operations that use remotely controlled robots.

*1. This regulation is based on a law that was amended following the Fukushima Daiichi Nuclear Power Station accident. However, if approval is obtained from Japan's Nuclear Regulation Authority, the operating life of a nuclear power plant may be granted a one-time extension of a period not exceeding 20 years.

*2. As of April 2025, a total of 24 reactors have been scheduled for decommissioning, including three reactors scheduled to be decommissioned prior to the Fukushima Daiichi Nuclear Power Station accident, and 21 reactors scheduled after the accident. When experimental and demonstration reactors are included, the total number of reactors is 26.

1. Removal of fuel debris
Debris refers to the nuclear fuel and reactor core fragments that melted and solidified due to the accident. In November 2024, the experimental removal of fuel from Unit 2 was completed, and planning is underway to shift to full-scale removal operations based on the data that was obtained.

2. Removal of fuel from spent fuel pools
All of the fuel has been removed from Units 3 and 4, and preparations are currently underway for Units 1 and 2. (*As of April 1, 2025)

3. Measures to deal with contaminated water
Water used to cool the nuclear reactor and groundwater mix with radioactive materials and produce contaminated water. To address this, an impermeable land-side wall and groundwater bypass were installed. The water that was already contaminated is being decontaminated using the Advanced Liquid Processing System (ALPS) and is managed after a significant amount of radioactive material has been removed.

4. Disposal of ALPS-treated water
The process to release the treated water into the sea was started in 2023. Before being released, the water was decontaminated to below the regulatory limits, and diluted in accordance with international standards.

5. Treatment and disposal of waste, dismantling of the nuclear reactor building, etc.
Highly radioactive waste is sealed in special containers and strictly managed. Some metal and concrete confirmed to be safe is expected to be reused. Efforts are aimed at ultimately dismantling the buildings and facilities and restoring the site to a state free from the effects of radiation.

Decommissioning work on the Fukushima Daiichi Nuclear Power Station requires long-term planning from a 30-to-40-year timeframe. While this is a difficult task without precedent anywhere else in the world, steady progress is being made while prioritizing safety under the banner of "balancing revitalization with decommissioning."

The nuclear fuel cycle is the process for reusing fuel that has been used at nuclear power plants (spent nuclear fuel). Plutonium and uranium reprocessed from spent nuclear fuel are mixed and processed into mixed oxide (MOX) fuel, which is then reused for power generation in nuclear reactors.

Nuclear power generation produces high-level radioactive waste from the use of fuel. This radioactive waste can be disposed of as-is, but if reprocessed, it not only facilitates the effective use of resources, but also significantly reduce the volume of waste. In addition, high-level radioactive waste emits higher levels of radiation for long periods, but the nuclear fuel cycle has a number of potential benefits, such as reducing the impact of harmful components by separating them through reprocessing.

Nuclear fuel cycle facilities are currently being constructed in Rokkasho Village, Aomori Prefecture. The facilities include a uranium enrichment plant, reprocessing plant, low-level radioactive waste disposal center, and high-level radioactive waste storage management center.

Spent nuclear fuel from nuclear power generation contains reusable uranium and plutonium, which will be recoverd and reutilized throughout the nuclear fuel cycle in Japan. However, high-level radioactive waste which cannot be reused is also generated.

This high-level radioactive waste is immobilized together with molten glass, forming what is called "vitrified waste". As glass is resistant to dissolving in water and is chemically stable, it is well suited to the long-term containment of radioactive material. However, as it takes a very long time for radioactive levels to decrease sufficiently, a solution known as geological disposal is employed, where the waste is contained deep underground in stable bedrock, isolated from the environments in which people live.

Where exactly is this radioactive waste buried? The vitrified waste is sealed in metal containers which, after being covered with clay, are buried in stable bedrock at a depth of more than 300 meters. The bedrock and clay form natural barriers, preventing the dispersion of radioactive materials. The construction of facilities for geological disposal is already underway in Finland and Sweden, and surveys are also being conducted to select appropriate final disposal sites in Japan.

Nuclear fusion is the process in which two light atomic nuclei combine to form a heavier nucleus. During this process, some of the mass is lost, releasing an enormous amount of energy in the form of heat and light. Research is underway to reproduce this mechanism artificially and use it sustainably.

The greatest advantage of nuclear fusion is its remarkable energy efficiency and environmental performance. For example, it is possible to obtain an amount of heat equivalent to about 8 tons of oil from just 1 gram of fuel. In addition, the fuel can be extracted almost inexhaustibly from seawater, which eliminates concerns about resource depletion. It also emits no carbon dioxide and produces very little radioactive waste, making it an ideal energy source for combating global warming.

To achieve nuclear fusion technologies, the fuel must be heated to over 100 million degrees to be in a state of plasma. At this extreme temperature, conventional containers cannot hold the plasma, so methods such as magnetic confinement and inertial confinement have been developed.

Accelerator is a device that accelerates particles (electrons, protons, ions, etc.) to high energies and is used in a wide range of fields, including physics, materials science, medicine, and energy research.

In particular, various types of accelerators have been developed for research purposes. For example, there are high-energy accelerators that accelerate particles to nearly the speed of light and collide them to study elementary particles, the universe, and the origin of matter. At the European Organization for Nuclear Research (CERN) near Geneva, Switzerland, the Higgs boson was observed for the first time in the Large Hadron Collider (LHC), the world's largest and highest-energy particle accelerator with a circumference of 27 km.

The high-intensity beam accelerator is a device that directs a powerful proton beam at a target and uses the resulting secondary particle beams (neutrons, muons and neutrinos, etc.) for research. At the Japan Proton Accelerator Research Complex (J-PARC), the world's most powerful high-energy proton beams are produced, enabling a wide range of research from fundamental studies to industrial applications.