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nuclear power reactorsNuclear Fusion vs Fission: What's The Difference? (Includes Examples)


Fission and fusion are nuclear reactions that produce energy. These two nuclear processes use the binding energy of the protons and neutrons in the nucleus of atoms to release an enormous amount of energy.

The main difference is that fission is the splitting of a heavy, unstable nucleus into two smaller nuclei, while fusion reactions involve combining two light nuclei together.

To understand more about the differences, we will look at each process in more detail…


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What Is Fission?

Nuclear fission involves the splitting of atoms to release the binding energy of the atomic nuclei. This energy is released as heat and radiation, with the heat being used by a nuclear power plant to boil water into steam to turn a turbine and drive generators to produce electricity. As the process uses uranium rather than fossil fuels to generate the heat, there are no carbon emissions with the nuclear fission process.

The process of splitting an atom at a power plant involves placing uranium in sealed metal cylinders inside a steel reactor vessel. Neutrons are then fired at the uranium atoms, causing them to split and release more neutrons that hit other atoms, creating a chain reaction that splits more atoms, releasing energy as heat and radiation. For example, uranium-235 atoms split into nuclei of krypton and barium along with three extra neutrons that create fission chain reactions by hitting other uranium-235 atoms.

What Is Fusion?

Nuclear fusion is the process of combining atomic nuclei rather than splitting them (as with fission) to produce energy. This process occurs naturally in the centre of stars like the Sun and creates no long-term radioactive waste or greenhouse gases.

Fusion power plants operate in a similar manner to fission plants, using the heat generated by the atomic reaction to heat water, produce steam, drive turbines and generate electricity, but it has been a challenge to create the required conditions in a fusion reactor without consuming more energy than is produced.

While still being developed, a fusion reactor (known as a tokamak) uses a gas – usually a hydrogen isotope that can be extracted from seawater called deuterium. When subjected to high heat and pressure electrons are forced away from the deuterium atoms to create a plasma. This plasma is a superheated, ionised gas that needs to be contained by strong magnetic fields as it can reach temperatures of 100,000,000°C or more. These temperatures are ten times that found at the core of the Sun, but are needed for the process as it is impossible to create the gravitational pressure within the Sun instead.

The energised plasma particles collide and heat up as auxiliary heating systems increase the temperature to the required levels for fusion (150-300 million °C). These conditions allow the highly energised particles to overcome their natural electromagnetic repulsion as they collide, fusing them together and releasing huge amounts of energy.

Other, alternative reactors being tested use lasers to heat and compress hydrogen fuel to create fusion.

What Are The Key Differences?

Although both fusion and fission use atomic energy, there are a number of key differences between the two processes:

  • Fission releases energy when atoms are split, while fusion releases energy when atoms are joined
  • The fusion reaction releases more energy than fission
  • Fusion doesn’t produce harmful long-term radioactive waste as a by-product like fission does
  • Fusion needs more energy to accomplish than fission does. The energy required for fusion has been a barrier to its widespread use for energy generation



Most nuclear power reactors use uranium-235 as the target nucleus into which a neutron is accelerated to split the atom into two smaller isotopes (called ‘fission products’) as well as three more neutrons, releasing a large amount of energy. The neutrons that are released create further fission reactions that continue the process with other uranium-235 atoms.  The energy that is produced is used to heat water into steam, producing electricity by turning turbines to power a generator.


Fusion occurs when two low-mass isotopes combine under conditions of extreme heat and pressure. This typically occurs with the hydrogen isotopes tritium (hydrogen-3) and deuterium (hydrogen-2), which combine to create a helium isotope and a single extra neutron. This fusing of isotopes releases several times as much energy as the fission process, without producing long-term radioactive by-products.


What is the Difference between Fusion and Fission?

Fusion is where two light atomic nuclei combine and release energy, while fission is the process of splitting two heavy, unstable atomic nuclei into two lighter nuclei, also releasing energy – although less than with fusion.

Is Fission or Fusion more Powerful?

Fusion releases several times the energy generated by fission, making it a far more powerful process.

What are 3 Differences between Fission and Fusion?

Three important differences between fission and fusion include:

  1. Fusion is the joining of atomic nuclei and fission is the splitting of atomic nuclei
  2. Fusion produces far more energy than that created by fission
  3. Fusion, unlike fission, does not create harmful radioactive by-products that need to be stored for thousands of years

Why is Nuclear Fusion not used?

Nuclear fusion is not currently used for power generation as it is a difficult process to recreate and control, as well as being expensive. This is due to the temperatures required to overcome the strongly repulsive electrostatic forces between positively charged nuclei in order to allow them to collide and fuse. However, work is ongoing to overcome these challenges.

Are Fusion and Fission Similar?

Both are nuclear reactions that produce energy, but fusion and fission are not the same. Fusion involves combining two light nuclei to form a larger nucleus and fission involves the splitting of a heavy nucleus into two lighter ones.

Is Fusion or Fission More Dangerous?

Nuclear fission is more dangerous than fusion as it produces harmful weapons-grade radioactive waste in the fuel rods that need to be stored safely away for thousands of years.

Is Fusion or Fission Safer?

While safety measures mean that the dangers of fission are greatly reduced, fusion is inherently safer as a process.

Does Fusion or Fission Produce More Energy?

Fusion produces more energy than fission, but there have been challenges around the energy expended on creating the conditions required for fusion, as it has been more than the amount of energy returned as a result. However, when these challenges are fully solved, fusion has the capacity to produce several times the amount of energy created by fission.

Does Fusion or Fission Occur in the Sun?

As a main-sequence star, the Sun generates energy through the nuclear fusion of hydrogen nuclei into helium. The nuclear fusion that occurs in the Sun is a combined result of the high temperatures and extreme pressures in the core of the Sun, which fuses 500 million metric tons of hydrogen each second.

Does Fusion or Fission Produce Radioactive Waste?

Both fusion and fission produce some level of radioactive waste. However, fission power plants generate unstable nuclei, some of which are radioactive for thousands of years while fusion doesn’t create any long-term nuclear waste. The typical fusion reaction creates the inert gas helium while also producing and consuming tritium within the plant. As a beta emitter, tritium is radioactive, but it has a short half-life and is only used in very small amounts, so does not pose a serious danger.

Does Fusion or Fission Require More Energy?

Because it is impossible to recreate the type of pressure that allows fusion to occur naturally in the core of the Sun, fusion power plants instead require more heat. Producing the required levels of heat to allow fusion to take place (150-300 million °C) requires more energy than the fission process. Reducing the energy required for fusion is one of the biggest challenges faced by the nuclear industry.

How are Fusion and Fission Similar?

Both fusion and fission involve nuclear reactions that release large amounts of energy that can be used to produce electricity. However, fission is the splitting of atoms, while fusion joins them together.


Fusion and fission are both nuclear processes that can be used to produce energy. Fission is where a large, unstable nucleus is split in two and fusion is where two smaller nuclei are joined to create a larger nucleus.

Fission is much easier to achieve than fusion although fission produces long-term radioactive by-products that are not created with fusion.

However, while fusion occurs naturally in the core of the Sun, it is difficult to create the required conditions on Earth, meaning that it has yet to be adopted for power generation, although it has been described as the ‘holy grail of clean energy.’

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