Nuclear Fission and Fusion

Nuclear Fission and Fusion are the core topics of Modern physics.  They assume great importance because these topics are not very tough and fetch quite a few questions in the JEE. Both Nuclear Fission and Fusion are reactions but they are quite opposite to each other. Various numerical are also formed on these topics and even they are not very difficult. It is vital to master this topic to remain competitive in the JEE.

Various topics covered under this head include:

  • Nuclear Fission

  • Nuclear Fusion

  • Solved Examples on Fission and Fusion

  We discuss these heads in brief here as they have been covered in detail in the coming sections. We start with the definition of Nuclear Fission and Fusion.
Nuclear Fission

In simple words, fission refers to the process of breaking of an atom into smaller parts. When a nucleus undergoes fission, it breaks or splits into various smaller fragments. These fragments are termed as fission products and are equal to about half of the original mass of the atom. The emission may also include two to three neutrons.
Once a neutron is captured, fission may take place in any of the heavy nuclei. But the low energy neutrons which are slow are capable of causing fission only in those isotopes of uranium and plutonium whose nuclei have an odd number of neutrons. The most appropriate examples of this are U-233, U-235, and Pu-239. Thermal fission may take place in some other transuranic elements whose nuclei contain odd numbers of neutrons. But in case of nuclei containing an even number of neutrons, fission can only occur if the incident neutrons have energy above about one million electron volts (MeV).
Nuclear Fission
let us now consider an example of fission of Uranium-235.

Uranium-235 has a massive nucleus and hence, when it fissions, there is a net yield of energy. As stated above, the sum of the masses of the fragments i.e. the smaller parts is less than the original uranium nucleus. The fission of U-235 in reactors is prompted by the absorption of a low energy neutron, often termed a "slow neutron" or a "thermal neutron". Other isotopes which can be induced to fission by slow neutrons are plutonium-239, uranium-233, and thorium-232.
A uranium-235 nucleus may capture a slow neutron, making it unstable towards nuclear fission. A single fission event can yield over 200 million times the energy of the neutron which triggered it!
Fission of Uranium-235

Nuclear Fusion:

Fusion is quite opposite to nuclear fission. As the name suggests, fusion refers to the process in which two or more atoms combine to form a larger atom. The best possible example of the fusion reaction is the sun. The light and the warmth emitted by sun is the result of fusion reaction. The hydrogen nuclei collide and fuse into heavier helium atoms and this process leads to the emission of huge amounts of energy. The gravitational forces in the stars of the universe have created favorable conditions for fusion. Over the last billions of years, the hydrogen clouds of the early universe have been converted into stellar bodies through gravity. And it is in their cores that fusion occurs as a result of extreme density and temperature.  
The figure given below clearly demonstrates the difference between nuclear fission and fusion.
Nuclear Fission and Fusion
You may view this video to know more on Fission and Fusion

askIITians offers study material on nuclear reaction for IIT JEE. It is important for the JEE aspirants to take various tests and exams on nuclear fission and fusion to be well versed with the concepts. They must also practice numerical on these topics as they are usually asked in JEE.

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