TYPES OF NUCLEAR FISSION REACTION

TYPES OF NUCLEAR FISSION REACTION
The nuclear fission reactions are of two types.

1. Uncontrolled fission reactions – Atom Bomb
2. Controlled fission reactions – Nuclear reactor

ATOM BOMB
Definition
A bomb which on the principle of a fast nuclear chain reaction is called the atom bomb.
If the nuclear chain reaction is allowed to go out of control, it will lead to powerful explosion with the liberation of enormous amount of energy. When sufficient fissionable material (critical mass) is contained in a small volume, on uncontrolled explosive chain reaction occurs.
An atom bomb contains two fissionable materials (U235 or Pu239) of sub – critical mass (less than critical mass). One of these pieces constitutes wedge while the other as target. It has a mass of TNT (trinitrotoluene) in a separate pocket. When TNT is detonated, it drives one mass of U235 into other. A super critical mass (higher than critical mass) of the fissionable material is obtained. As a result of this instantaneous chain reaction, the bomb explodes with the release of enormous amount of heat energy (10 million°C) (temperature of the sun) which is much greater than that of the most powerful TNT bomb.

92 U235 + 0n1---> 56Ba140 + 36Kr93 + 30n1 + energy

The first atom bomb used in Hiroshima (Japan) on 6-8-1945 utilized U-235 as the main reacting substance and second bomb in Nagasaki on 9-8-1945 made use of Pu-239. The fission in both the cases is similar and controlled.

Nuclear Fusion
The combination of two or more nuclei is very difficult because all the nuclei are positively charged. In order to make them combine, they must move high enough to overcome the strong forces of electrostatic repulsion. The hydrogen to helium fusion reaction in the sun takes place at about 100 million degree centigrade.

Thermonuclear reaction
In order to bring about fusion reaction, temperature of high order is required. Hence the fusion reaction is called thermonuclear reaction.
The highly destructive hydrogen bomb is based on nuclear fusion reactions of isotopes of hydrogen to form helium producing large amount of energy. The very high temperature required for this uncontrolled thermonuclear reaction is obtained by the detonation of an atom bomb. It consists of a small plutonium fission bomb with a container of isotopes of hydrogen. The fission bomb produces a very high temperature at which thermonuclear reactions start resulting in the fusion of hydrogen isotopes of deuterium and tritium to form helium.

1H2 + 1H3 2He4 + 0n1 + 17.6 MeV
The explosion of hydrogen bomb is much more powerful than an atom bomb.

Differences between Nuclear Fission and Nuclear Fusion

1. In nuclear fission process, heavy nuclei split into two or more lighter nuclei with the liberation of large amount of energy.
In nuclear fusion process, two light nuclei fuse or combine to form single nuclei with the liberation of energy.

2. The atomic number and mass number of the daughter element is less than the parent element in nuclear fission process.
The atomic number and mass number of the product nuclei is greater than the starting elements in nuclear fusion process.

3. Radioactive rays are emitted in nuclear fission process.
Radioactive rays are not emitted in nuclear fusion process.

4. Nuclear fission reaction can be controlled
Nuclear fusion cannot be Controlled

5. It is a spontaneous process occurring at ordinary temperature. Occurs at very high temperature.

6. Neutrons are emitted during fission process.
Positrons are emitted during fusion process.

7. Fission will result in chain reaction
Fusion will not result in chain reaction