A heavy isotope of U-235 or plutonium - 239 can undergo chain reaction yielding vast amounts of energy. The Energy released during a nuclear process like nuclear fission is called as Nuclear energy. Tremendous amounts of energy are released in nuclear fission reactions. There reactions, when done at a controlled pace, can be very useful power/electricity source. Nuclear energy is also calculated as Kilo calories.
The energy produced during a fission or a fusion reaction is called the nuclear energy.
Fission of U-235 or Pu - 239 occurs instantaneously, producing incomprehensible quantities of energy in the form of heat and radiation. If the reaction is uncontrolled, it is accompanied by explosive violence and can be used in atom bombs.
It is a very useful source of energy and is trapped for many purposes.
When a nucleus is bombarded with some sub-atomic particles such as α - particles, neutrons, protons etc
, these particles are captured by the target nucleus, which then disintegrates. The new element formed has mass either slightly greater or slightly smaller than the parent element.
The process of splitting of a heavier nucleus (like that of U235) into a number of fragments of much smaller mass, by suitable bombardment with sub-atomic particles is called nuclear fission.
Of the three natural isotopes of uranium
nucleus undergoes nuclear fission when bombarded with slow neutrons.
is formed which being unstable, further breaks up in several
23592U + 01n → 23692U → 14056Ba + 9436Kr + 2 01n
The tremendous amount of energy released during nuclear fission is
because of the loss in mass. The sum of the masses of the fragments
produced and neutrons released as a result of fission is less than the
sum of the masses of target 235
U and bombarding neutron. The loss in
mass gets converted into energy according to Einstein equation
E = mc2
The mass loss in Uranium fission is of the order of 0.2 amu, which
corresponds to 186 million electron volt (MeV) per Uranium atom
fission or about 4.3 x 109
k cal per mole of Uranium atoms fission. This represents tremendous amount of release of energy.
For example, we can calculate the loss of mass when types of nuclear reactions - nuclide splits up into 144Ba and 90Kr along with the release of two neutrons.
This is one of the nuclear reaction equations. Dm, the mass defect or the mass converted into energy is given by,
Dm = 236.127 - 235.846 = 0.281 amu
1 amu = 931.48 MeV
Energy released = 0.281 amu = 931.48 x 0.281
= 261.75 MeV
The neutrons emitted from the fission of first uranium atom hit other uranium nuclei and cause their fission resulting in the release of more neutrons, which further continue the fission process. In this way, a nuclear chain reaction sets up releasing tremendous amount of energy
The fission process is complicated by the fact that a given nucleus undergoing fission may split in a variety of alternative ways. Over 30 pairs are known. Following are some possible ways of fission of Uranium nucleus.
Let us consider the nuclear fission reaction of U-235 nucleus brought about by neutron capture. Although this fission process occurs in a large number of ways, in each case a large amount of energy is released and more than one neutron emitted.
23592U + 01n → 23692U → 9037Rb + 14455Cs + 2 01n
Two neutrons are emitted for every neutron that initiates the fission process. The neutrons emitted during fission may be further absorbed by other U-235 nuclei thus causing further fission and emission of more neutrons. Thus, a chain reaction is initiated in this way and it releases a tremendous amount of energy. If the amount of the parent nuclei, U-235 is small, most of the neutrons will escape from the surface and the reaction stops. Therefore, a certain mass of Uranium, called critical mass is necessary in order to start and sustain a chain reaction. The critical mass is mostly found to be between 1 to 100 kilograms.
Another type of nuclear reaction that generates even larger amount of energy is the fusion reaction. For example, when two deuterium nuclei are made to fuse together to form a helium nucleus:
12H + 12H → 24He
5.5 x 108
K Calories per mole of Helium formed is released. Following are the examples of fusion reactions together with the energies released.
||Mass loss (amu)
||Energy released (K cal per mole He)
|12H + 12H → 24He
||5.5 x 108
|12H + 13H → 24He + 01n
||4.10 x 108
|411H → 24He + 210e
||6.2 x 108
So, a nuclear fusion is a type of nuclear reaction where energy is released when two smaller nuclei combine to form a heavier nuclei.To bring about such fusion reactions, the reactants have to be initially at very high temperatures of order of 107 to 108 degree Celsius. The energy released by the sun results from a series of nuclear fusion reactions. The overall reaction consists of the fusion of four hydrogen nuclei to form helium nucleus.
The principle underlying nuclear reactions like nuclear fission has been employed in harnessing vast amounts of energy in nuclear reactors. In these reactors, the fission reaction is made to occur at a controlled rate.
Essentially, a reactor consists of lumps of U-235 separated from each other by blocks of graphite or heavy water, D2
O, which by slowing the neutrons, help in controlling the chain reaction. The large amount of energy in the form of heat, which is released during the reactions is converted into electrical energy. Many such nuclear power plants are set up all across the world.
There are a lot of benefits of nuclear energy. Some of them are
- Light-water nuclear power plants: There are many commercial power plants, called light water power plants, where U-235 nuclei fuel rods are submerged in water, and undergo fission reactions. These light water reactors produce a large amount of electricity and are used world wide.
- Nuclear energy is also used in the production of atom bombs and hydrogen bombs.
- Fusion reaction. occurring in the sun is also a major source of energy, called as solar energy.
- Fusion is supposed to be a source of energy in 21st century. A fusion reactor can supply and operate much better than a fission reactor for generating electricity and the raw material needed, the deuterium is also easily available.
There are many disadvantages of nuclear reactions, like its effect on the environment. For example,
- The fusion bomb produces high temperature required fro nuclear fusion and triggers the Hydrogen bomb. The explosion of such a bomb is more powerful than that of a fission bomb or the atomic bomb. If they are used in a warfare, it may mean the end of civilization on earth.
- Disposal of nuclear reactor waste poses another hazard. The waste produced from such reactors contain Ba-139 and Kr- 92, which are itself radioactive. They emit radiations for several hundred years.
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- Radioisotopes as such are used for many purposes. So is the nuclear fission and fusion reactions which produce nuclear energy.
- It is possible to control fission reaction of U-235 so that energy is released slowly at a usable rate.
- Controlled fission is carried out in a specially designed plant called a nuclear power reactor. A nuclear reactor can continue to function and supply power for generations.
- It is believed that fusion process can also be controlled like that of fission, can produce a lot of energy.