What is Fission?
Fission is a nuclear reaction in which the nucleus of a heavy atom, such as uranium, breaks down into two particles that are of about the same size and of medium mass.
Atoms with heavy nuclei that can undergo fission include plutonium, uranium and thorium.
A great deal of energy is released as a result of the process of the reaction. This energy takes the form of both neutrons and electromagnetic radiation. The electromagnetic radiation can take the form of gamma rays.
A chain reaction can be set up where each neutron that is released triggers fission in nearby atoms. This is referred to as self-sustained fission. Nuclear power plants take advantage of and use these chain reaction events to generate large amounts of electric power.
Chain reactions can occur only if a critical mass is present. If the mass is insufficient to self-perpetuate reactions it is said to be subcritical, while if the mass causes an increasing rate of fission, it is said to be supercritical.
Nuclear power plants generate energy by means of controlled nuclear fission reactions. World-wide nuclear power plants generate about 16% of the worlds’ energy.
Nuclear fission reactors use uranium-235. They add a neutron to the uranium-235 which then becomes uranium-236 which becomes very unstable and thus breaks down to produce more fissionable fragments. Fission reactors do not require high energy neutrons to catalyze the reaction.
Nuclear reactors do have the problem of generating a great deal of chemical waste that is often radioactive.
Heavy atoms are inherently unstable but they still have a fission barrier that has to be overcome before fission can occur.
How likely it is for a fission reaction to occur depends on whether or not the nucleus has an even or odd number of neutrons.
If it has an even number of neutrons (e.g. uranium-238) then it is less easy for the fission reaction to occur. Those with an odd number can undergo fission more quickly and easily if a neutron is added (e.g. uranium-235).
There are nuclear bombs that use only fission reactions. Fission bombs can produce a great deal of energy, for example a 10 kiloton bomb can produce as much as 4.2 x 1013 Joules of energy.
What is Fusion?
Nuclear fusion describes the reaction when two or more atomic nuclei come together and fuse to form a larger heavier atom. In the process a great deal of energy is released as mass decreases during this process.
For fusion to occur Coulombs repulsion forces have to be overcome. This is also known as the Coulomb barrier. This can be achieved by accelerating nuclei or using the kinetic energy from ions of gasses. Accelerating nuclei may be necessary to enable the atoms to overcome the repulsion forces.
Fusion occurs more readily with lighter atoms since the Coulombs repulsion is not too great, thus nuclear attraction and fusion can occur.
Electrostatic Coulombs repulsion is related to the number of protons, so elements with lower atomic numbers will more readily fuse since fewer protons are present and so less attractive forces are present.
This explains why fusion tends to occur among the lighter elements or their isotopes, such as deuterium or tritium, both isotopes of hydrogen atoms.
In order to overcome these repulsion forces and for fusion to occur a high enough temperature has to be attained for a certain amount of time known as the confinement time.
Energy production by the sun is from fusion reactions that occur deep in the core of the sun. The sun and other stars are powered by nuclear fusion reactions.
When the reaction occurs the hydrogen atoms become fused into helium (this is known as a proton-proton chain reaction). Neutrinos are released in the process and energy in the form of electromagnetic radiation is also released.
Fusion reactions also release energy like fission reactions because atoms have binding energy. This is the energy associated with the difference in mass between the protons and neutrons.
There exist two types of fusion reactions:
- one in which the number of protons and neutrons changes after the reaction, which occurs in star burning, and
- one where the same number of protons and neutrons are present at the end of the reaction as at the start of the reaction.
Fusion reactions have yet to be successfully harnessed to produce electric energy.
What are the similarities between Fission and Fusion?
- They both involve a change in the atomic nucleus.
- Both fission and fusion results in the release of large amounts of energy.
- One or more atoms of a different weight to the reactant atom is formed in both fission and fusion reactions.
- There is a change in weight between the reactant(s) and product(s) in both fission and fusion reactions.
- The atom(s) that are formed as products are different to the reactant atom(s).
- Both nuclear reactions can be used in nuclear weaponry.
Fission and Fusion in a Nutshell: Summary:
- Fission describes a nuclear reaction when a heavy atom splits into two atoms that are of a similar size and similar mass.
- Fusion describes a nuclear reaction when two or more light atoms come together to form a larger heavier atom.
- In both fusion and fission reactions there is thus a change in mass from reactant(s) to product(s).
- Fission can occur spontaneously in some cases in some heavy atoms. Even so there is a fission barrier that has to be overcome before the reaction can proceed.
- Fission can result in a chain reaction of self-perpetuating fission reactions.
- Fusion occurs among atoms of low mass such as hydrogen atoms.
- Hydrogen atoms fuse to form helium in the sun. Electromagnetic radiation is released as a consequence of this reaction.
- A result of both fusion and fission reactions is the release of huge amounts of energy due to mass changes.
- Nuclear fission is used in nuclear reactors to generate electricity.
- Both fission and fusion reactions can be used in the development of powerful nuclear weaponry in the form of nuclear bombs.
Author: Dr. Rae Osborn
Dr. Rae Osborn holds Honours Bachelor of Science degrees in Zoology and Entomology, and Masters of Science in Entomology from the University of Natal in South Africa. She has received a PhD in Quantitative Biology from the University of Texas at Arlington. She was a tenured Associate Professor of Biology at Northwestern State University in Louisiana for 10 years. She also completed an AAS Degree in Information Network Specialist and an AAS in Computer Information Systems, at Bossier Parish Community College in Louisiana.