Atoms contain three fundamental particles namely electrons, protons and neutrons. Protons and neutrons constitute the nucleus whereas electrons are present in the extra nuclear region of the atom.
Chemical changes are caused by rearrangements of outer electrons while nuclei retain their identity. Consequently the identity of atoms remains the same.
In the beginning of 20th century, another kind of transformation, which involves changes in the atomic nucleus was observed by scientists. Certain elements were found to undergo spontaneous changes in their nuclei, resulting in the formation of atoms of different elements. These transformations are called nuclear reactions.
The nuclear reactions have become tremendous source of energy all over the world, both for peaceful and destructive purposes. Nuclear chemistry is the branch of chemistry, which deals with the study of atomic nucleus and nuclear changes.
The knowledge gained in the fields of nuclear and radio-chemistry has provided valuable information about the structure of atom, it has unlocked the secret to the process of transforming one element into another, and has gifted mankind a new source of energy - the nuclear energy.
The vast amount of energy released from nuclear reactions like nuclear fission and nuclear fusion makes up the nuclear energy.
The amount of energy released in a nuclear fission can be calculated by the mass defect method which is based on the fact that in a nuclear fission reaction, the total mass of the products is always less than that of the reacting nuclei or the parent nuclei.
This decrease in mass is called Mass defect which is actually converted into energy, according to the mass energy relationship given by Einstein's equation
E = mc2
In the early days of the investigation of radioactivity, many
radioactive materials were found to give off heat. At the time this was
very hard to explain. The energy that was being released seemed to come
from nowhere; but this was of course, known to be impossible.
The first measurement of the amount of heat energy generated by a
radioactive substance was performed by Pierre Curie
in March 1903 and
showed that 1 gram of Radium produced about 450J every hour.
So, it was realized that substances like Radium had the potential of
being an almost free and inexhaustible source of energy. And this paved
way for the utilization of Nuclear energy for useful purposes.Nuclear reaction in which one heavier nucleus is split up into two lighter nuclei of almost equal size with the release of a huge amount of energy is called as nuclear fission or atomic fission.
- In 1938, German radio chemists, Otto Hahn and his associates, Meitner and Strassman bombarded U - 235 with slow moving neutrons and observed that U-235 nucleus, after capturing neutron, gave U-236 nucleus which was radioactive.
- It was called as compound nucleus. This compound nucleus, U-236 broke up to form a pair of stable, fission products consisting of Ba- 141 and Kr- 92 and liberated three neutrons, which were ultimately called as secondary neutrons.
- This was also accompanied by a release of large amount of energy.
- Immediately after the announcement of the discovery of nuclear fission, physicists in many laboratories repeated and confirmed these experiments. Within two years the same process was confirmed for thorium and pro-actinium.
- In addition to the two fragments, on the average 3 neutrons and about 200MeV energy were released per fission. The energy was much larger than that released in any of the exoergic nuclear reactions.
- Nuclear fusion was first observed by Eddington in 1926. He suggested that the energy in sun is due to the nuclear fusion reaction in which hydrogen is converted into helium.
The process of creation of nuclear energy involves either of the nuclear reactions, nuclear fission or fusion. The way nuclear energy is made can be shown with the help of individual reactions.
1. Nuclear Fission
- The natural Uranium consists of three isotopes of atomic masses 238, 235 and 234 respectively. The abundance of U-235 is only 0.17% of the natural Uranium which consists of 99.29% of U-238 and an extremely small percentage of U-234.
- U-238 can be split up be very fast moving neutrons while U-235 can be fissioned by much lower neutrons. And, fission of U-235 is much more violent.
- When a neutron strikes the nucleus of U- 235 it is captured and a highly unstable nucleus U-236, is produced which at once breaks up into two large parts with the emission of two or three neutrons. A large variety of products are possible.
- The three neutrons released will in turn, cause the fission of three other U-235 nuclei and will liberate three more neutrons in the fission of three U-235 nucleus.
- Now, the nine neutrons produced will repeat the same process, causing fission in further nine U-235 and thus the process continues.
- Consequently, a self-propagating or self-sustaining chain reaction, also called as auto-catalytic reaction, starts in which the number of neutrons keeps multiplying rapidly, in geometric progression.
- This chain reaction continues till the whole of U-235 has undergone fission. This chain reaction is accompanied by a large amount of energy called as Fission energy or atomic energy, or generally referred to as the nuclear energy.
This energy can be either very fast resulting in a violent explosion as in atom bomb, or can be controlled, as in a nuclear reactor. Thus, nuclear energy can be made from a fission reaction by controlled means.
2. Nuclear Fusion
"When lighter nuclei moving at a high speed are fused together to form a heavy nucleus, the processes is called nuclear fusion".
In nuclear fusion, the mass of the product nuclei is less than the sum of the masses of reacting nuclei.
The difference between the masses of nuclei of reactants and products is converted into energy according to the Einstein's mass energy relationship. The fusion reaction cannot be carried out at a lower temperature. It is not spontaneous as fission reaction. It requires a temperature of 108K. the energy produced in a fusion reaction is 1000 times more than the nuclear Bomb.
Nuclear energy produced from nuclear fission or fusion reactions are used in production of electricity. It is a major power source in many countries.
Energies from controlled fusion and fission reactions are tapped and used for many useful purposes.
Some examples of nuclear energy are :
1. Stellar energy
Stellar energy is the energy of the sun and other stars of the main sequence. Stellar energy is due to a series of nuclear reactions involving carbon-Hydrogen-Nitrogen cycle, which operates through following steps.
612C + 11H → 713 N + 00γ + Energy -----------------1
713N → 613 C + 10e + Energy -----------------2
613C + 11H → 714 N + 00γ + Energy -----------------3
714C + 11H → 815 O + 00γ + Energy -----------------4
815O → 715 N + 10e + Energy -----------------5
715N + 11H → 612 C + 24He + Energy -----------------6
Thus stellar energy is produced due to the fusion of four Hydrogen atoms or protons.
2. Atom bomb
The energy is from the fission of U-235 isotope. Nuclear energy released from an uncontrolled fission reaction results in atom bomb.
3. Hydrogen Bomb
The phenomenon of nuclear fusion gives rise to the formation of hydrogen bomb.
Hydrogen bomb is based in the fusion of lighter nuclei such as hydrogen to form heavier nuclei such as Helium.
Though U-235 is exploited world wide in all the nuclear reactors, it is available in plenty and is replenished by eroding earth from river surfaces. So nuclear energy is often defined as renewable energy
- There are many fission reactors set up world wide. They use many fissionable isotopes.
- The waste obtained also contain radioisotopes, which continuously emit radiations. Storing them and disposing them is a real struggle for the scientists.
- Hydrogen bomb is considered to be very powerful bomb which can destroy the whole world if used.
- Leakages from nuclear power plants due to natural calamities like the one occurred in Japan in 2011 can have disastrous results worldwide.
It is believed that since most of the natural resources like petroleum, coal, etc are reaching their tethers, the vast amount of energy required for future populations will come from controlled fission and fusion reactions, or the nuclear energy reactions.
Nuclear energy is created by making use of the radioactive reactions undergone by radioactive isotopes of Uranium, Thorium, etc. The reactions are discussed separately.
- Advantages of nuclear energy over other power resources is that energy created by other power resources depletes these resources, like hydro power plant, coal plants, etc.
- Nuclear energy, on the other hand, is non depleting. Thus, this poses a major advantage over other power resources.