Every atom has a nucleus at its core that makes up more than 99% of its mass. The nucleus is made up of neutrons and protons, which are held together by a strong nuclear force. The heavier the atom, the more protons and neutrons it has in its nucleus. These tiny components are responsible for some of the most energetic reactions on Earth, which can be both constructive and destructive. Scientists have discovered that we can split or fuse nuclei, and now we're eager to understand the energies involved in these reactions. In this article, we'll explore fission and fusion, which are the splitting and combining of nuclei.

The meaning of fission and fusion

When the nuclei of smaller atoms collide and merge, they can sometimes form heavier atoms. This is called nuclear fusion, and it releases energy. On the other hand, heavier nuclei can be broken apart into smaller ones, which generates some energy. This process is called nuclear fission.

Nuclear fission reactions

Nuclear fission is the process of splitting a heavy atom into two or more smaller nuclei, releasing a large amount of energy in the process. It is not a random process, but requires a neutron to collide with the heavy nucleus for the splitting to occur. After the neutron collides with the nucleus, it causes the nucleus to become unstable. The nucleus then splits into two smaller nuclei, that are similar in size, and releases two or three neutrons in the process along with large amounts of energy in the form of gamma rays. The smaller nuclei, known as the fission products, are usually also unstable and may release alpha or beta particles to attain stability. Part of the energy that is released is in the form of the kinetic energy of the fission products.

An example of this process is the fission of uranium-235 into barium-139 and krypton-95. When a neutron collides with the nucleus of uranium-235, the nucleus splits into barium-139 and krypton-95, releasing two or three neutrons, gamma rays, and kinetic energy in the process.^[

‍

A neutron is fired into a stable uranium-235 nucleus, making it momentarily unstable. It then splits into barium-139 and krypton-95 which are both smaller nuclei. Two neutrons are released in the process and of energy (this is equivalent to). The barium and krypton nuclei may undergo alpha and beta decay and form even smaller nuclei. The fission products (barium, krypton and nuclei) all have some kinetic energy after the fission occurs.

Nuclear fission diagram

Nuclear fission, which involves the splitting of heavy nuclei into smaller nuclei, can lead to the production of moving neutrons and an enormous amount of energy. These neutrons can be used to cause further nuclear fissions, leading to a chain reaction that can release even more energy.

In an uncontrolled chain reaction, the number of fissions grows exponentially with time, leading to a huge release of energy in a short amount of time. This principle was used to build atomic bombs, which can release an enormous amount of energy and destroy entire cities.

However, if the number of product neutrons used to initiate further fissions can be controlled, then the total amount of energy released can also be controlled. This is the principle behind nuclear fission reactors, which use uranium as a fuel source to produce controlled amounts of energy.

The fission diagram below shows the uncontrolled chain reaction that would occur from the fission of uranium-236, leading to the production of barium-144 and krypton-89. The two neutrons produced can then be used to cause the fission of two further uranium-236 nuclei, leading to a chain reaction that can release an enormous amount of energy.

It is the ability to control this chain reaction that makes nuclear fission a potentially powerful and useful energy source for the future, but also one that must be handled with great care and responsibility.

‍

Nuclear fission example

In the given example, we are told that can absorb a neutron and undergo nuclear fission, producing and three neutrons. To write the correct nuclear equation for this fission reaction, we must ensure that the mass numbers and atomic numbers balance on either side of the equation.

The reactant is , which has an atomic number of 92 and a mass number of 235. The neutron that is absorbed has an atomic number of 0 and a mass number of 1. The products are and three neutrons, with an atomic number of 56 and 36, and a mass number of 141 and 92 respectively.

Based on this information, the nuclear equation for this fission reaction can be written as:

This equation represents the balance of reactants and products, with both atomic numbers and mass numbers balanced on either side of the arrow. The atomic number on both sides is 92 and the mass number on both sides is 236, indicating that the fission reaction can indeed occur in this manner.

In summary, the balance of atomic and mass numbers is crucial in writing nuclear equations that represent fission reactions. These equations help us to understand the products and energy released in nuclear reactions and are important in fields such as nuclear energy and nuclear medicine.

Nuclear fusion reactions

Nuclear fusion is a process where two light nuclei combine to form one heavier nucleus, releasing energy in the process. This reaction requires a significant amount of energy to occur and typically occurs in the core of stars like the sun, where the high temperatures and pressures enable the fusion of two hydrogen atoms into a helium nucleus.

During the fusion process, two protons are converted into neutrons, and the mass of the resulting helium nucleus is slightly less than the mass of the hydrogen atoms that fused to form it. This difference in mass is converted into energy, which is released in the form of light and heat. Nuclear fusion does not produce any radioactive waste, making it a potential clean energy source for the future.

Efforts are currently underway to harness the power of nuclear fusion on Earth through the use of fusion reactors. These reactors use a combination of hydrogen isotopes, such as deuterium and tritium, to initiate the fusion reaction. However, the high temperatures and pressures required to initiate and sustain the reaction are still challenging to achieve in a controlled way.

Despite these challenges, nuclear fusion has the potential to provide a virtually unlimited source of clean energy, with little to no greenhouse gas emissions or radioactive waste. Ongoing research and development in this field could unlock a new era of sustainable energy production for the future.

Nuclear fusion diagram

The fusion diagram below represents the fusion between two isotopes of hydrogen; deuterium and tritium. Helium is produced by this fusion, along with a neutron with kinetic energy and a significant amount of energy in the form of heat.

A diagram showing the fusion of two isotopes of hydrogen; deuterium and tritium that fuse to form helium.

‍

Nuclear fusion example

Let us consider the figure above and attempt to write and balance the nuclear equation for the reaction.

For the fusion of deuterium and tritium above we can write a nuclear equation as follows: Let us first check that the atomic numbers balance on either side of this equation.  The atomic numbers are both equal to 2, so we can now check the mass numbers. The mass number is 5 on either side of this equation and so there is a balance. This fusion reaction can indeed occur.

Differences and similarities between fission and fusion

The following table contains some of the differences and similarities between nuclear fission and nuclear fusion.

Fission and Fusion - Key takeaways Nuclear fission occurs when large, unstable nuclei split into two smaller nuclei (fission products).Two or three neutrons are also produced by nuclear fission along with a significant amount of energy. The process is initiated when a moving neutron collides with the nucleus, making it unstable. The fission products may also be unstable and decay by releasing alpha or beta particles. The fission products gain kinetic energy after the reaction. The released neutrons may initiate further fissions in a process called a chain reaction. Uncontrolled chain reactions initiate fission reactions at an exponential rate. Controlled chain reactions occur when a fission reaction initiates only one other reaction. Nuclear power stations use controlled fission reactions to generate electricity. Boron control rods are used in nuclear fission reactors to control the rate of fission reactions. Nuclear equations can be used to describe fission reactions. Nuclear fusion occurs when light nuclei fuse to become a single, heavier nucleus. Fusion requires a significant amount of energy in order to be initiated. Fusion occurs in stars when hydrogen is converted into helium due to high temperatures and pressures. Fusion is a candidate for clean energy production.

Fission and Fusion

What are fission and fusion? 

Smaller atoms can sometimes combine to form heavier atoms when their nuclei collide and merge. This process is known as nuclear fusion, during which, energy is released. Heavier nuclei can also be split in a process called nuclear fission, which produces smaller nuclei and some energy. 

What is an example of fission and fusion?

Uranium-235 can undergo fission and produce barium-139, krypton-95 and energy. Deuterium and tritium can fuse to form helium, a neutron and energy.

What are differences between fission and fusion?

Fission is the splitting of nuclei and fusion is the merging of nuclei. Fission typically involves large nuclei whereas fusion involves smaller nuclei. Fission has radioactive products and fusion does not.

Which is more powerful: fission or fusion?

A typical fusion reaction produces more energy than a typical fission reaction.

Why fusion is impossible on Earth?

Fusion is not impossible on Earth, it is simply difficult to attain the energy and temperature required to initiate fusion reactions on Earth. There are many fusion reactors around the world that conduct frequent fusion reactions at temperatures far hotter than the surface of the sun!

‍