Basic Guide | Fusion Industry Research Center, Keio University Graduate School of Media Design

What is Fusion Energy?

Fusion energy is the process that powers the sun and all the stars shining throughout the universe. When light atomic nuclei collide and fuse to form a heavier nucleus, an enormous amount of energy is released.

Among various fusion reactions, the deuterium-tritium (DT) reaction—which uses two isotopes of hydrogen, deuterium and tritium, as fuel—is considered one of the most promising candidates for humanity’s future energy source.

The Keys to Achieving Fusion Energy: “Extreme Temperature” × “High Density” × “Confinement Time”

To initiate nuclear fusion reactions, we create and confine matter in a plasma state.
Plasma is often called the fourth state of matter, beyond solid, liquid, and gas. As temperature increases, the thermal motion of particles (atoms and molecules) intensifies and matter transitions from solid to liquid to gas. Heating a gas further causes electrons to no longer remain bound to atomic nuclei; they are stripped off and move freely. The resulting ionized, quasi-neutral mixture of free electrons and ions—plasma—is the prerequisite state in which nuclear fusion can occur.

In order to induce nuclear fusion reactions by fusing atomic nuclei, it is necessary to overcome the electrostatic repulsion between their positive charges. This requires increasing the speed of the nuclei (i.e., their kinetic energy), enhancing the frequency of collisions, and maintaining the plasma for a sufficient confinement time so that the nuclei do not escape before fusing.

High-Temperature

To increase the speed of atomic nuclei, it is sufficient to raise the temperature; nuclear fusion requires ultra-high temperatures exceeding 100 million degrees Celsius.
High-Pressure condition

To increase the frequency of atomic nucleus collisions, it is necessary to create a high-density state (meaning a state where there are many particles in a given space).
A long confinement time

It is necessary to maintain the state where particles fly around at extremely high temperatures and high density, making particle collisions easy, while "not allowing heat to escape."

Why Nuclear Fusion Reactions Can Generate Enormous Energy

Fusion reactions generate an enormous amount of energy by exploiting a slight decrease in mass before and after the reaction.
In Figure 1, the numbers of neutrons (blue — actual color to be modified) and protons (red) appear unchanged, but in reality, about 0.4% of the total mass is lost during the fusion process.
This missing mass is converted into energy, producing the tremendous output characteristic of fusion reactions. The conversion of mass into energy is explained by Einstein’s equation of mass–energy equivalence, E = mc², which states that matter itself is a condensed form of energy. The equation shows that even a minute loss of mass (m) results in a vast amount of energy because it is multiplied by the square of the speed of light (c², approximately 9×10¹⁶ m²/s²).
Therefore, in fusion reactions, even a tiny change in mass leads to an immense release of energy—manifesting as heat and light.
This is the fundamental principle that enables fusion energy to generate an extraordinary amount of power from a very small amount of fuel.

Fusion vs. Fission

Difference Between Fusion and Nuclear Fission

Fusion generates energy by combining light atomic nuclei, whereas nuclear fission releases energy by splitting heavy atomic nuclei.
Although both processes extract energy from atomic nuclei through a small change in mass, as described by Einstein’s equation (E = mc²), their mechanisms and characteristics differ fundamentally.

In fusion power generation, the reaction depends on external heating and fuel supply; it does not proceed spontaneously, and the reaction stops immediately once the supply is halted. Fusion also offers distinctive advantages: it produces no long-lived high-level radioactive waste, ensures a high level of intrinsic safety, and utilizes deuterium derived from seawater as an almost inexhaustible fuel resource—features that clearly distinguish it from nuclear fission.

Fusion

Principle: Small atomic nuclei fuse

Spontaneity: Does not occur spontaneously on Earth

Chain Reaction: Chain reaction does not occur

Waste: Long-lived nuclides (≒ nuclear waste) are not generated

Fuel Resources: Fuel resources are virtually inexhaustible

Will a broken bottle spontaneously mend itself? It will not happen unless it is initiated by human effort.

Likewise, fusion reactions do not occur naturally on Earth and cannot sustain a chain reaction.

Fission

Principle: Large atomic nuclei fission

Spontaneity: Occurs spontaneously on Earth

Chain Reaction: Chain reaction occurs

Waste: Long-lived nuclides (≒ nuclear waste) are generated

Fuel Resources: Fuel resources are limited

Conversely, the reaction where large atomic nuclei split (fission) occurs naturally.

For instance, it is like a glass breaking when it falls from a table. Once the amount of fuel exceeds a certain critical level, a chain reaction is triggered, causing one fission event to trigger another in succession.

Safety

Safety of Fusion Energy

Fusion energy involves the use of tritium, a radioactive isotope of hydrogen with a relatively short half-life, as one of its fuels. During the fusion process, neutron radiation is produced, which can activate surrounding structural materials. Therefore, proper management of activated materials and equipment contaminated with tritium is required.
However, fusion reactions have inherent safety advantages. By their very nature, they cannot run away or cause chain reactions, and they do not produce highly hazardous materials that pose significant risks to human health. These characteristics make fusion a fundamentally safe and environmentally responsible energy source.

1. Criticality Accidents and Explosions

Fusion is not a chain reaction like nuclear fission. Therefore, phenomena such as criticality accidents or nuclear runaway are fundamentally impossible in fusion systems.

Fusion reactions occur only when specific conditions—adequate fuel supply, extremely high temperature, and sufficient confinement—are simultaneously achieved. If any of these conditions are lost, the reaction stops immediately. In other words, a fusion reaction cannot sustain itself without continuous external control and energy input, and it automatically ceases in the event of any anomaly. This is one of the key safety characteristics of fusion energy.
In contrast, in nuclear fission, when a neutron collides with a fissile atom such as uranium, the atom splits and releases additional neutrons. These neutrons can trigger further fission events in neighboring nuclei, creating a self-sustaining chain reaction.
When the amount of fissile material exceeds a certain critical mass, this spontaneous chain reaction (known as criticality) continues without external input. If not properly controlled, such reactions can lead to a criticality accident.
Fusion, on the other hand, does not undergo spontaneous reactions, regardless of how much fuel is present. Thus, fusion systems are inherently free from the risks of nuclear runaway or criticality accidents associated with fission. Naturally, there is no possibility of explosion in a fusion reaction.

2. Tritium Release to the Environment

It is technically unavoidable that trace amounts of tritium are released from fusion plants into the environment. However, multiple containment and purification systems are employed to minimize such releases, ensuring that emissions remain well below regulatory limits, thereby posing no impact on the environment or human health.

Tritium is a substance that is chemically difficult to completely contain or remove, and as such, a small amount of tritium release from fusion facilities is inevitable each year.
To address this, fusion plants are equipped with multi-stage tritium recovery and removal systems that strictly limit emissions to levels below the regulatory thresholds designed to guarantee environmental and biological safety.
The current Japanese regulatory limits are as follows:

Tritium concentration in liquid effluents: 60,000 Bq/L
Tritium concentration in gaseous emissions: 5 Bq/L

Even if an individual were to drink 2 liters of water per day or continuously breathe air containing tritium at these maximum concentrations, the annual radiation dose would remain below 1 millisievert, the internationally recognized public safety limit.
The annual tritium release from a fusion plant is expected to be well below 1 gram, which is slightly higher than that from an operating nuclear fission plant in Japan, but less than one-tenth of the estimated annual release from the Rokkasho Reprocessing Plant.
Furthermore, even in the highly unlikely event of external incidents such as an aircraft crash or terrorist attack causing the forced release of stored tritium, the total inventory within a fusion facility would be only several tens of kilograms—1/1,000 to 1/10,000 of the mass typically found in nuclear fission plants.
Since most of this tritium would disperse and dilute rapidly in the atmosphere, the resulting radiological impact would remain extremely small—less than one hundredth of that associated with conventional nuclear power plants.

3. Neutron Generation from Fusion Reactions

Neutrons produced in fusion reactions are almost completely shielded by the reactor structure and surrounding concrete walls, ensuring no impact on the external environment.

In a fusion reactor, most of the neutrons generated are absorbed by a device called a blanket, which surrounds the plasma and captures neutrons to produce heat and tritium. Any remaining neutrons are effectively blocked by the thick concrete walls that encase the reactor, resulting in negligible leakage outside the facility.
According to one estimation (Takeuchi, Journal of the Atomic Energy Society of Japan, 1980), even if a worker were to stand continuously for one year at a location just 180 meters from the center of a fusion reactor, the annual radiation dose from fusion-generated neutrons would be only 0.066 millisieverts (mSv).
For comparison, natural background radiation during the same period would be approximately 2.1 mSv, about 30 times higher, and the regulatory annual dose limit for radiation workers is 50 mSv.
These findings indicate that neutron leakage from fusion plants is virtually zero, and the radiation is almost completely contained within the shielding structures.

4. Management of Activated Materials

Fusion energy plants do not produce high-level radioactive waste, and thus generate no so-called “nuclear waste” that requires geological disposal over tens of thousands of years.

However, components inside the reactor become activated by exposure to high-energy neutrons during operation, resulting in low-level radioactive waste that must be properly managed within the plant site for a limited period.
Unlike nuclear fission reactors, fusion systems use no uranium or plutonium fuel, and therefore do not generate long-lived radioactive isotopes requiring deep geological storage.
Nevertheless, neutron irradiation from fusion reactions causes structural components such as the blanket and divertor to become activated over time.
According to one estimate (Prototype Reactor Joint Safety Design Team, Waste Management Subgroup, 2024, unpublished), the annual amount of activated material generated is approximately:

Blanket: ~2,700 tons
Divertor: ~1,500 tons

All of this material is classified as low-level radioactive waste. It is first stored in dedicated facilities for about 5 to 15 years to allow radioactivity to decay, then retained on-site for up to 50 years, after which it can be safely disposed of by near-surface burial or recycled.
In summary, fusion plants do not produce any waste requiring deep geological disposal hundreds of meters underground for millennia.
All radioactive materials generated from fusion operations can be safely managed, recycled, or disposed of through shallow burial, reflecting fusion’s fundamental advantage in waste safety and sustainability.

Advantage

Advantages of Fusion Energy

Fusion energy is a next-generation, high–energy-density power source that uses abundant fuel derived from seawater, emits no greenhouse gases during operation, can be easily controlled and safely shut down, and produces no long-lived high-level radioactive waste.

Abundant Fuel Resources

Fuel can be extracted from seawater, enabling a stable and sustainable energy supply for the long term.
Environmental Compatibility

Fusion power generation emits no greenhouse gases during operation, contributing to global decarbonization efforts.
Intrinsic Safety

Fusion reactions do not occur spontaneously and will immediately cease if fuel or power input is interrupted, minimizing the risk of runaway reactions or accidents.
Ease of Radioactive Waste Management

Fusion does not produce high-level radioactive waste that requires storage for tens of thousands of years; radioactivity from materials decays to safe levels within about 100 years.
High Energy Efficiency

Fusion yields about four times more energy per gram of fuel than conventional nuclear fission, offering exceptional energy density and efficiency.

Mechanism

How Fusion Energy Generates Power

Fusion energy is produced by fusing the atomic nuclei of deuterium and tritium under conditions of extremely high temperature and density—exceeding 100 million degrees Celsius. Through this fusion process, a small amount of mass is converted into a tremendous amount of energy, according to Einstein’s equation (E = mc²). About 80% of this energy is carried by neutrons, and the remaining 20% by helium nuclei (alpha particles).
The high-energy neutrons generated in the reaction escape from the plasma and strike the surrounding wall, called the blanket (literally meaning “a covering layer”). As neutrons collide with the blanket, they are slowed down, converting their kinetic energy into heat.
The heat accumulated in the blanket is then transferred to a coolant—such as water, gas, or liquid metal—which carries the heat outward to produce steam. This steam drives a turbine generator, thereby generating electricity.
Meanwhile, the helium produced in the reaction contributes to plasma heating within the reactor and is eventually exhausted from the system.
To sustain the fusion reaction, it is essential to maintain the plasma at sufficiently high temperatures using external plasma heating systems while continuously supplying deuterium and tritium fuel. The self-heating effect from helium (alpha heating) also plays an important role in maintaining the reaction.
Among the two fuel components, deuterium is abundant in seawater, while tritium is almost nonexistent in nature. Therefore, tritium must be bred within the reactor blanket using neutrons that react with lithium.
The design makes use of the neutrons generated by the fusion reaction itself, along with neutron-multiplying materials such as beryllium or lead, to efficiently produce additional tritium. Lithium can be obtained from salt lakes, mineral deposits, or even seawater, making the fuel cycle highly sustainable.
In this way, fusion energy aims to achieve steady-state operation by creating a self-sustaining cycle: extracting heat from neutrons for power generation, while simultaneously using those same neutrons to breed tritium fuel.
The key technologies enabling this cycle are:

Efficient heat recovery through the blanket,
Self-sufficient tritium breeding, and
Stable high-temperature plasma confinement.

When these three elements work in concert, fusion energy can reliably generate clean and sustainable electricity.

Mechanism of Tokamak-type Nuclear Fusion Power Generation

Toward Fusion

Toward a Prosperous Society Empowered by Fusion Energy

Uniting Global Knowledge to Create Energy for Peace

Fusion energy is a form of clean energy that recreates the reactions of the Sun, enabling a society free from constraints of limited energy resources and greenhouse gas emissions.
As an inexhaustible “sun on Earth,” fusion energy represents humanity’s collective challenge to illuminate the future—for ourselves and for the generations to come. Around the world, nations are working together to make this dream a reality.
Our goal is to create a stable and accessible energy source for all—one that reduces conflicts over resources and contributes to peace and sustainable development.
Achieving this vision requires more than overcoming physical and engineering challenges. It demands an integrated approach that also considers economic, policy, and community perspectives, allowing fusion energy to evolve into an energy system truly rooted in society.
The Fusion Industry Research Center approaches fusion energy from the combined perspectives of industry and society.
Through evidence-based and impartial analysis, we aim to shape institutions, policies, and market frameworks, while serving as a bridge between industry, government, academia, local communities, and citizens.
As a hub for social implementation, we are committed to building a trustworthy foundation for the energy transition.
By bringing together knowledge and expertise from across the world, we strive to realize energy for peace.
Through the collaboration of science, policy, and practice, we aim to lead the creation of a prosperous and sustainable future powered by fusion energy.

Fusion — The Dream of a Clean Energy Future.