Fission and Fusion

Nuclear fission reactions

Some heavy atoms are unstable and they maintain stability by emitting radioactive particles such as alpha and beta particles. This process occurs spontaneously and randomly. Large nuclei can also become more stable by splitting into two smaller nuclei. This process is known as nuclear fission and is not random. It 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. The example below shows how fission occurs.

Nuclear fission diagram

Moving neutrons are responsible for fission to occur and the fission products include neutrons with kinetic energy. These neutrons that are produced may then be used to cause further nuclear fissions. This process is called a nuclear chain reaction. The example above showed uranium-235 undergoing fission to form barium-139 and krypton-95 and two neutrons. These two neutrons can cause two further uranium nuclei to undergo fission producing four neutrons which may cause four fissions to occur. The number of fissions will therefore grow exponentially with time. This is called an uncontrolled chain reaction and can release an enormous amount of energy in a short amount of time. It is this principle that was used to build the atomic bomb. Just a small amount of uranium undergoing fission has the potential to destroy an entire city.

If we could control the number of product neutrons used to initiate further fissions then we could control the total amount of energy that is released. If only one product neutron is used to cause the fission of one nucleus, then the chain reaction is controlled. This is how nuclear fission reactors are created using uranium. The fission diagram below shows the uncontrolled chain reaction that would occur from the fission of uranium-236.

Uranium-236 undergoes fission to produce barium-144 and krypton-89. The two neutrons produced are then used to cause the fission of two further uranium-236 nuclei. The process continues in an uncontrolled chain reaction, Wikimedia Commons CC BY-SA 4.0

Nuclear fission example

We must first recall that the symbol for any neutral atom$X$is written in the following way:

${}_Z{}^{A}X$

where the atomic number$Z$represents the number of protons in the nucleus of this atom and the mass number$A$represents the number of protons and neutrons in the nucleus. Now, if we consider the previous example of the splitting of uranium-235 into barium-139 and krypton-95, we can write an equation for the reaction to represent the balance of the reactants and products. This is called a nuclear equation and the equation for the example described looks as follows. The mass and atomic numbers on both sides of a nuclear equation must balance for the nuclear reaction to occur. Note that this equation contains a rightward-facing arrow rather than the equality symbol.

${}_0{}^1\mathrm{n}+{}_{92}{}^{235}\mathrm{U}\to {}_{56}{}^{139}\mathrm{Ba}+{}_{36}{}^{95}\mathrm{Kr}+2{}_0{}^1\mathrm{n}$

The reactants are on the left side of this arrow and the products are on the right. The atomic numbers and mass numbers on either side of this equation balance, which means that the fission can indeed occur in this manner. The example below shows how another nuclear equation can be written.

Nuclear fusion reactions

If two light nuclei merge, they can form one heavier nucleus and release energy in the process. This is known as nuclear fusion. The interaction requires a significant amount of energy to occur. Nuclear fusion occurs in stars, like the sun, and is the main source of their energy. The temperatures and pressures on stars are great enough to fuse two hydrogen atoms (two protons each) into a helium nucleus (two protons and two neutrons). In the process, two protons are converted into neutrons. The mass of the helium nucleus is less than the mass of the hydrogen atoms that fused to form it, so the remainder of the mass is released in the form of energy after the fusion reaction. Nuclear fusion creates no radioactive products and this makes it a good candidate as a clean energy source for the future.

Nuclear fusion diagram

The fusion diagram below represents the fusion between two isotopes of hydrogen; deuterium${}_1{}^2\mathrm{H}$and tritium${}_1{}^3\mathrm{H}$. 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. A neutron is released in the process along with a significant amount of energy, Wikimedia Commons CC BY-SA 3.0

Nuclear fusion example

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

Differences and similarities between fission and fusion

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