Fission and Fusion


In nuclear fission, a large atomic nucleus splits into 2 smaller nuclei and sometimes several Neutrons. The smaller nuclei and neutrons that are produced gain large amounts of Kinetic Energy, we interpret this energy as heat.


In nuclear fusion, 2 small atomic nuclei combine to form a larger nucleus. Other small particles (such as Neutrons) may also be left over. The larger nucleus and other particles produced gain large amounts of Kinetic Energy, which we interpret as heat.

The video below summarises both Fission and Fusion as well as linking to the Chemistry course equivalent.

Fission may be either:

Spontaneous - The large atomic nucleus splits up by itself at random - There is no "outside influence".

Stimulated - Neutron Bombardment causes the large atomic nucleus to split. 

For the Fission process used in Nuclear power stations, we must look more closely at Stimulated Fission. 

Stimulated Fission

A neutron is "fired" at a uranium nucleus, causing the uranium nucleus to split. Smaller daughter particles are produced as well as three further neutrons, all of which have a great deal of Kinetic Energy. Each of these neutrons can go on to cause more Uranium atom to split, causing an ever increasing energy chain reaction.

Note that the mass numbers (top) and atomic numbers (bottom) must be conserved.


Fusion occurs when two light nuclei combine to form a nucleus of larger mass number.

The Fusion reaction that we will look at is the fusion of Hydrogen into Helium.

Hydrogen has a symbol form of :-

Helium has a symbol form of :- 

As before, Mass and Atomic numbers must be conserved. 

There is NO way to combine the above Hydrogen in order to get a stable Helium atom, so how does Fusion occur?

We must use Isotopes of Hydrogen in order to combine correctly:-

Hydrogen (1 Proton + 1 Electron)

Deuterium (1 Proton + 1 Neutron + 1 Electron)

Tritium ( 1 Proton + 2 Neutrons + 1 Electron )

By fusing Deuterium and Tritium together, we can create a stable Helium nucleus, with an additional free Neutron. 

Lost Mass and Energy

In the above equations, there is no mention of Energy.

Obviously, nuclear fission and fusion both release a large amount of energy, so where does this energy come from?

In both nuclear fission and nuclear fusion reactions, the mass of the products formed is always less than the mass of the starting species - Mass is lost during the reaction.

The "lost mass" is converted into kinetic energy of the products, in accordance with Albert Einstein's famous equation:-

Example 1 :-

For the following formula, calculate the Mass lost, and hence, the Energy released in this reaction.

Lost Mass = Total Mass Before - Total Mass After

Total mass Before = 3.901x10-25 + 0.017x10-25 = 3.918x10-25 kg

Total Mass After    = 2.221x10-25 + 1.626x10-25 + (4 x 0.017x10-25) = 3.915x10-25 kg

Lost Mass = 3.918x10-25 - 3.915x10-25 = 0.003x10-25 kg

Energy released = Lost Mass x (Speed of Light 2)

Energy Released = 0.003x10-25 x (3x108)2

                            = 2.7x10-11 J

This value seems very small but when you take into account that in 1 kg of uranium there are 2.56x1023 atoms, each capable of undergoing fission and releasing this Energy. 

Total energy released by 1 kg of Uranium undergoing fission = 6.91x1012 J = 6.91 TJ

Total energy released by 1 kg of burning Anthracite (Coal)    = 2.56x10 J  = 2.56 MJ

Nuclear fission releases ~ 2.7 million times the energy of burning Coal.

The video below shows a debate giving both sides of the Nuclear power debate from a scientific standpoint.