The Difference between Nuclear Fission and Nuclear Fusion



The terms fission and fusion identify two types of reactions that involve the nucleus of atoms of matter. Matter consists of molecules and elements. A molecule is a chemical combination of two or more elements. For example, water is a molecule consisting of two atoms of hydrogen and one atom of oxygen, and of course, both hydrogen and oxygen are elements. One can look at the periodic table of the elements in order to view all of the currently known elements.

In the fission reaction, each atom of matter involved is one of the heavier elements such as Uranium or Plutonium (fissile material). In this reaction, each atom of fissile material is split into two lighter elements and, for reasons that will be explained later, a large amount of energy is released. Each splitting atom also releases electrically neutral subatomic particles called neutrons and they serve as bullets that can strike other atoms of fissile material. If the mass of the fissile material is as large or larger than what is called a “critical mass”, then a “chain reaction” is produced and many millions of atoms are split. Each splitting atom releases a large amount of energy which, when combined with all of the other splitting atoms, produces a very destructive energy output and is known as an atomic bomb.

In the fusion reaction, each atom involved is hydrogen, the lightest of the elements. When a large amount of heat energy (millions of degrees) is applied to a mass of hydrogen atom isotopes (the term isotope will be defined later), the atoms fuse into atoms of helium. In this process, a very large amount of additional energy is released when the hydrogen atoms fuse to form helium. In fact, this is the process that fuels the sun, and on Earth is what we call the thermonuclear bomb or the hydrogen bomb.

For the fusion reaction in the sun, the extreme heat energy (the igniter) necessary to start the reaction came from the heat of compression at the center of the sun which was caused by the force of gravity. The large ball of hydrogen gas is held together by gravity and the pressure on this gas increases toward the center. At the center of the sun, this pressure causes a large amount of heat energy to be produced because the atomic particles are colliding with great speed and force.

The igniter of early hydrogen bombs was a small fission bomb. When the fission bomb exploded, it produced the extreme heat necessary to ignite the fusion process of the thermonuclear bomb.

The size and amount of energy released by a fusion bomb is basically unlimited because we can equip this bomb with an unlimited amount of hydrogen fuel. The fission bomb, however, is limited by the size of a critical mass. Whenever we construct a critical mass of fissionable material, the mass will explode, therefore the critical mass must not be created until the moment we want the bomb to explode.


When a heavy atom of matter is split, energy is released. Also, when two atoms of hydrogen fuse to form helium, energy is released. This comes from the fact that the sum of the masses of the two lighter atoms that are products of a fission reaction is less than the mass of the heavier atom of fissile material. Therefore, mass is converted into energy according to the Einstein equation of:

E = (Ms Mp)*C2

Where Ms is the mass of the source atom and Mp is the sum of the masses of the product atoms. The constant C is of course the speed of light.

Likewise, when two atoms of hydrogen fuse, the mass of the product helium atom is less than the sum of the masses of the two hydrogen isotopes. Therefore, mass is converted into energy according to the equation listed above.


In order to understand this material thoroughly, an understanding of the following terms should be developed:

A. ATOM: The smallest particle into which an elemental mass can be divided so that the particle retains the chemical and basic physical characteristics of the original mass.
B. ELECTRON: A small negatively charged particle that circles the positively charged nucleus of an atom.
C. PROTON: A positively charged particle that forms part or the entire nucleus of an atom.
D. NEUTRON: A neutrally charged particle that is bound to the proton(s) in the nucleus of some atoms.
E. ISOTOPE: An atom of elemental matter that contains a specific number of neutrons bound to the protons in the nucleus. An atom of the same material with a different number of neutrons in the nucleus would be identified as a different isotope.
F. ATOMIC MASS: “An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of 12C” in its nuclear and electronic ground state.
G. ION: A charged atom where the number of electrons orbiting the nucleus does not equal the number of protons in the nucleus. In an electrically neutral atom, the number of electrons equals the number of protons.
H. MOLECULE: a chemical combination of two or more atoms of elements.
I. ATOMIC NUMBER: The number of protons in the nucleus of an atom.