The Difference Between Nuclear Fission and Nuclear Fusion

Subtitle: Power generation by splitting atomic nuclei and power generation by combining atomic nuclei

Nuclear fission and nuclear fusion.

These two terms are very similar.

Both contain the character for "nucleus," and both are described as reactions that produce enormous energy.

However, their mechanisms are exact opposites.

Nuclear fission is a reaction that splits heavy atomic nuclei.
Nuclear fusion is a reaction that combines light atomic nuclei.

One splits apart.
The other joins together.

Therefore, for an initial understanding, you can put it this way.

Nuclear fission extracts energy by splitting atomic nuclei.
Nuclear fusion extracts energy by combining atomic nuclei.

However, if we stop here, it is a bit shallow.

What is truly important is that in both nuclear fission and nuclear fusion, when atomic nuclei move to a more stable state, the difference appears externally as energy.

In other words, while the direction of the reaction is opposite, both nuclear fission and nuclear fusion are technologies that extract the difference in energy contained within atomic nuclei.

To be a bit more precise, the way atomic nuclei are bound changes before and after the reaction, and that mass difference or difference in binding energy appears as energy.

How is it different from thermal power generation?

For us, the most familiar form of power generation is thermal power generation.

Burning coal.
Burning oil.
Burning natural gas.

We obtain heat by burning.
We use that heat to turn water into steam.
The steam turns a turbine.
The generator turns, and electricity is produced.

Thermal power generation, to put it quite simply, is a mechanism that converts heat obtained through combustion into electricity.

What is happening here is primarily a chemical reaction.

A chemical reaction is a reaction where the way atoms are bonded to each other changes.

More specifically, it is a reaction where the relationship of the electrons outside the atoms changes.

On the other hand, in nuclear fission and nuclear fusion, the atomic nucleus itself at the center of the atom changes.

This is the decisive difference.

Thermal power generation extracts energy from the bonding of electrons.

Nuclear fission and nuclear fusion extract energy from the change of atomic nuclei.

They are dealing with different levels.

That is why nuclear energy is so immense.

It is not that fire is particularly weak.

Human civilization has long been supported by the energy of fire.

However, fire uses changes outside the atom.

Nuclear energy uses changes at the center of the atom.

This difference manifests as a difference in energy density.

What is nuclear fission?

Nuclear fission is a phenomenon where a heavy atomic nucleus splits into two or more atomic nuclei.

A representative example is the nuclear fission of uranium-235.

A neutron hits a uranium-235 nucleus.
Then, the uranium nucleus becomes unstable.
The unstable nucleus splits into about two atomic nuclei.

At that time, heat and radiation are released.

Furthermore, new neutrons are also ejected.

What is important here is those ejected neutrons.

The ejected neutrons hit other uranium nuclei.
Then, those nuclei also split.
Furthermore, more neutrons are released.
Those neutrons cause even more nuclei to split.

This is a chain reaction.

In nuclear fission power generation, which is current nuclear power generation, this chain reaction is controlled to extract heat.

The U.S. Department of Energy explains that inside a nuclear reactor, fuel rods are immersed in water, the water acts as a coolant and moderator, and control rods are used to adjust the reaction rate.

The heat generated by nuclear fission turns water into steam, and that steam turns a turbine to produce electricity.

In other words, nuclear fission does not directly produce electricity.

Heat is created by nuclear fission.
That heat turns water into steam.
Steam turns a turbine.
The turbine turns a generator.
As a result, electricity is produced.

This point is very important.

Nuclear power generation does not directly create the electricity for your wall outlet through nuclear fission.

In reality, it is steam power generation using nuclear fission as a heat source.

What is nuclear fusion?

Nuclear fusion is a phenomenon where light atomic nuclei combine to become a heavier atomic nucleus.

A representative example is the fusion of deuterium and tritium.

Deuterium and tritium are types of hydrogen.

When these nuclei fuse, a helium nucleus and a neutron are created, and a large amount of energy is released.

The sun shines because of nuclear fusion.

However, the reason nuclear fusion occurs in the sun is not simply because the sun is hot.

The sun has an enormous mass.

Due to that mass, the sun's own gravity acts toward its center.

The outer material crushes the inner material.

Because of that compression, the center of the sun becomes extremely high-temperature and high-pressure.

In this high-temperature, high-pressure environment, atoms cannot remain in their original form.

Electrons separate from atomic nuclei, resulting in a plasma state where nuclei and electrons move independently.

Within that extreme plasma, light atomic nuclei move around violently.

Then, some atomic nuclei that overcome the repulsion between them fuse together.

This is the nuclear fusion occurring in the sun.

In other words, the sun's nuclear fusion is supported by gravity.

However, the current sun is not collapsing infinitely due to gravity.

The sun continues to shine stably because the inward-pushing gravity and the outward pressure generated by nuclear fusion are in balance.

Enormous gravity pushes material inward.
The compressed center becomes high-temperature and high-pressure.
Plasma is created within the high-temperature and high-pressure environment.
Atomic nuclei fuse within the plasma.

This is the flow.

Therefore, causing nuclear fusion on the ground is also an attempt to artificially create conditions close to the center of the sun within a device on Earth.

Of course, there is no gravity on Earth as massive as that of the sun.

That is why humanity is trying to confine ultra-high-temperature plasma using magnetic fields, lasers, and the like, rather than gravity.

Herein lies the difficulty of nuclear fusion power generation.

The sun confines plasma with gravity.
Humans are trying to confine plasma with devices.

Nuclear fusion power generation is not just a technology for causing a nuclear fusion reaction.

It is a technology that attempts to artificially reproduce on the ground the extreme environment that the sun creates naturally through gravity.

And there is a major barrier to that artificial reproduction.

Atomic nuclei have a positive charge.

Positive charges repel each other.

In other words, atomic nuclei do not approach each other on their own.

To fuse deuterium and tritium, one must create a high-temperature state sufficient to overcome the repulsion between atomic nuclei.

What is needed for that is plasma.

Plasma is a high-temperature state where electrons have separated from atoms, and atomic nuclei and electrons are moving independently.

ITER explains that at extreme temperatures, electrons separate from atomic nuclei, gas becomes plasma, and this plasma becomes an environment where nuclear fusion of light elements occurs.

However, it is not enough just to make it high-temperature.

Create ultra-high-temperature plasma.
Confine that plasma.
Continue the reaction stably.
Make the extractable energy greater than the input energy.

Furthermore, make it economically viable as a power plant.

Only when this is achieved does it become nuclear fusion power generation.

Nuclear fusion is often spoken of as an ideal energy.

But being ideal is different from being easy to put into practical use.

Currently, nuclear fusion is at a stage where research and development are progressing, and it is not widely put into practical use as a commercial power generation method.

ITER also positions its own goal as "achieving nuclear fusion output at a power plant scale and demonstrating nuclear fusion reactor technology."

In other words, nuclear fusion is a technology with future potential.

But its technological maturity is completely different from nuclear fission power generation, which already supports society.

Why does humanity aim for nuclear fusion?

So, why is humanity aiming for nuclear fusion power generation?

There is already thermal power generation.
There is also nuclear power generation.
There are also solar power and wind power generation.

Even so, researchers around the world are trying to realize nuclear fusion.

The reason is that nuclear fusion is not just a new power generation method, but has the potential to simultaneously overcome multiple limitations currently faced by energy issues.

First, nuclear fusion is anticipated as a power generation method that emits almost no carbon dioxide during power generation.

Thermal power generation burns coal, oil, and natural gas.
If you burn them, carbon dioxide is emitted.
Carbon dioxide is one of the causes of global warming, and it has become an unavoidable issue in modern energy policy.

Of course, there is also renewable energy.

Solar power and wind power generation create electricity without burning fuel.

In that sense, they are very important technologies.

However, solar power cannot generate electricity at night.
Wind power generation drops if the wind is weak.

Because they are dependent on natural conditions, mechanisms such as storage batteries, power grids, and supply-demand adjustment are necessary to use them as stable large-scale power sources.

Therefore, nuclear fusion appears as another possibility.

Nuclear fusion can become a large-scale power source that is less dependent on weather or time of day.

Moreover, since it does not burn fossil fuels, there is a possibility that carbon dioxide emissions during power generation can be significantly suppressed.

Next, nuclear fusion is also attractive in terms of fuel.

In typical nuclear fusion reactions, deuterium and tritium are used.

Deuterium is an isotope of hydrogen and also exists in seawater.

Tritium does not exist in large quantities in the natural world, but there is a concept to produce it from lithium inside a nuclear fusion reactor.

In other words, if nuclear fusion is put into practical use, there is a possibility that constraints on fuel resources can be significantly eased.

This is different from energy that is dependent on the uneven distribution of resources, like oil and natural gas.

If energy resources are concentrated in certain regions, dependency relationships between nations and geopolitical risks arise.

Where do you buy fuel from?
What do you do if the price goes up?
What if transport routes are stopped?
What if supply is stopped due to war or sanctions?

Energy is not just science and technology.

It is national security, the foundation of the economy, and the continuity of society itself.

That is why nuclear fusion is sometimes called "dream energy."

That does not mean that infinite energy can be easily obtained.

Rather, it means that if nuclear fusion is realized, there is a possibility that dependency on fossil fuels, carbon dioxide emissions, uneven distribution of resources, and long-term energy anxiety can be reduced simultaneously.

Furthermore, nuclear fusion has safety characteristics different from nuclear fission.

In nuclear fission power generation, control of the chain reaction is important.
One must stably cool the reactor, manage the reaction, and handle radioactive waste for a long period.

On the other hand, in nuclear fusion, extremely special conditions are required to continue the reaction.

If ultra-high-temperature plasma cannot be maintained, the nuclear fusion reaction will not continue.

If the conditions collapse, the reaction stops.

Of course, nuclear fusion also has challenges such as management of activated reactor materials, handling of tritium, and equipment safety.

Therefore, it is not accurate to call nuclear fusion a "perfectly safe technology."

However, it is certain that it has a safety structure different from nuclear fission.

The reason nuclear fusion is aimed for lies here.

Humanity does not just want large energy.

We want large energy.
But we want to reduce carbon dioxide.
We also want to reduce the scramble for resources.
We also want to reduce waste that remains for a long time.
We also want a stable power source that is not too dependent on weather.
We also want to ensure national energy security.

Nuclear fusion exists in the place where these multiple demands overlap.

In other words, nuclear fusion is not simply a "technology to bind atomic nuclei."

It is an energy technology that humanity is trying to move to next, after seeing the limitations of thermal power generation, the weight of nuclear fission power generation, and the instability of renewable energy.

The reason for aiming for nuclear fusion is not just to create future electricity.

It is so that civilization does not depend only on burning anymore.

And it is so that we do not continue to carry the weight of splitting.

Humanity has burned fire, split atomic nuclei, and now, we are trying to bind atomic nuclei.

The meaning of nuclear fusion lies within that flow.

Organizing the two reactions

The difference between nuclear fission and nuclear fusion can be organized as follows.

Nuclear fission is a reaction that splits heavy atomic nuclei.
Heavy atomic nuclei like uranium-235 or plutonium-239 split, and a large amount of energy is released at that time.

Nuclear fusion is a reaction that binds light atomic nuclei.
Light atomic nuclei like deuterium and tritium fuse and become heavier atomic nuclei such as helium. Energy is released in that process.

What is used in current nuclear power generation is nuclear fission.

In nuclear power plants, heat generated by nuclear fission is used to turn water into steam, and that steam is used to turn turbines to generate electricity.

Nuclear fusion power generation has not yet been generally put into practical use as commercial power generation.

Although research is progressing, there are many challenges to be solved, such as control of ultra-high-temperature plasma, durability of reactor wall materials, securing tritium, and power generation costs.

In nuclear fission, control of the chain reaction is important.
Once nuclear fission occurs, new neutrons fly out, and they split other atomic nuclei. Stably controlling this chain reaction becomes the central technology of a nuclear reactor.

In nuclear fusion, confinement of ultra-high-temperature plasma is important.

Because atomic nuclei have a positive charge, they repel each other and cannot approach on their own.

To fuse them, it is necessary to create a very high-temperature plasma state and confine it stably.

The main challenges of nuclear fission are radioactive waste, accident risk, fuel management, and safety management.

In nuclear fission reactions, radioactive materials that require long-term management are produced. Also, it is important to continue cooling the nuclear reactor stably.

The main challenge of nuclear fusion is technical feasibility.
Although it is theoretically very attractive, many technical challenges still remain to operate it stably as a power plant.

And both nuclear fission and nuclear fusion ultimately convert heat into electricity.

This is surprisingly easy to overlook.

In both nuclear fission and nuclear fusion, the reaction itself does not directly become electricity.

Energy is born through nuclear reactions.
That energy is extracted as heat.
That heat is used to warm water or coolant.
The power of steam or fluid turns a turbine.
The turbine turns a generator.
As a result, electricity is born.

In other words, the difference is not in the final stage of power generation.

The difference is on the side of the nuclear reaction that creates heat.

Nuclear fission obtains heat by splitting atomic nuclei.
Nuclear fusion obtains heat by binding atomic nuclei.

The direction is opposite, but both are extracting the difference when atomic nuclei move to a more stable state as energy.

Power generation is the translation of heat

Here, let's look at power generation a bit more broadly.

We think of electricity as something very modern.

Charging a smartphone.
Operating a computer.
Turning on lights.
Cooling a refrigerator.
Running a train.

Electricity is like the blood of modern civilization.

But much of that electricity is made with quite classical mechanisms.

Create heat.
Boil water.
Create steam.
Turn a turbine.
Turn a generator.

This structure does not change much between thermal power generation, nuclear power generation, and the concept of nuclear fusion power generation.

ITER also explains that a nuclear fusion power plant, like conventional power plants, creates steam from heat generated by nuclear fusion reactions and generates electricity using turbines and generators.

In other words, power generation is a technology that translates natural energy into a form that humans can use.

Thermal power generation translates the heat of chemical reactions into electricity.
Nuclear power generation translates the heat of nuclear fission into electricity.
Nuclear fusion power generation is trying to translate the heat of nuclear fusion into electricity.

Looking at it this way, the difference between nuclear fusion and nuclear fission is no longer just a difference in scientific terminology.

It is a difference in which layer of natural phenomena humanity is trying to extract energy from.

Combustion uses the molecular layer.
Nuclear fission uses the instability of heavy atomic nuclei.
Nuclear fusion uses the force that binds light atomic nuclei.

Humanity started with fire.

We burned wood, burned coal, burned oil, and burned natural gas.

Eventually, we came to obtain heat by splitting atomic nuclei.

And now, we are trying to obtain heat by binding atomic nuclei.

The direction of civilization is expressed here.

Humans have tried to extract larger energy by descending to deeper layers of nature.

But each time, new control problems appear.

Fire spreads.
Nuclear fission must control chain reactions.
Nuclear fusion must confine ultra-high-temperature plasma.

Energy is not something that should just be extracted.

To what extent can we control the extracted energy?

The maturity of civilization is expressed there.

Conclusion

Nuclear fission and nuclear fusion are reactions in opposite directions.

Nuclear fission splits heavy atomic nuclei.
Nuclear fusion binds light atomic nuclei.

However, their essence is common.

Both are extracting the difference in energy that occurs when atomic nuclei move to a more stable state.

And power generation is a mechanism that receives that difference as heat and converts it into electricity that can be used in society through steam, turbines, and generators.

Nuclear fission is "splitting nuclear energy" that has already been put into practical use.

Nuclear fusion is "binding nuclear energy" that is on the way to practical use.

The direction is opposite.
But what humanity is doing is the same.

Human civilization is borrowing as energy the difference that occurs when the natural world moves toward stability.

In that sense, both nuclear fission and nuclear fusion are technologies that extract heat from deep layers of nature.

And power generation is the act of translating that heat into electricity, which is the language of civilization.

Final Proposition

Nuclear fission and nuclear fusion have opposite reaction directions, but both are technologies where humans extract the difference when the natural world moves toward stability as heat and translate it into electricity.

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