The foundation of nuclear physics has been established with the historical α-scattering experiment by E. Rutherford using Geiger’s gas counter in 1906 and the term ‘nucleus’ was introduced to the world, as a positively charged massive central part of the atom in 1912 [1]. Till today, a century has been passed and nuclear physics has come a long way with various modern techniques that are advanced to elaborate the properties of atomic nuclei from nucleus-nucleuscollisions to nucleon-nucleon collisions and has reached to the study of quark gluon plasma; diverging its branches towards particle physics.

Understanding a stable nucleus in its ground state in absence of any external potential is not possible. The only way is to excite the nucleus with some external energy and visualizing its behaviour during the de-excitation. The process of projecting energetic nuclei on the target nuclei is called nuclear reaction and it possesses various stages of interaction between two colliding nuclei. The different kinds of nuclear reactions (based on their incident energies from few KeV to TeV) and type of nuclei exhibit their importance innuclear structure studies, nuclear astrophysics, transmutation of nuclear waste, medical application, nuclear accelerators, nuclear power plants, nuclear technology R&D, reactor design and safety consideration etc.  [2].

Nuclear energy is a result of heat released during the nuclear reaction. The amount of energy released is represented as the Q-value of the reaction which decides the type of the reaction. For reactions in which there is a decrease in the kinetic energy of the products, Q-value isnegative. The negative Q-value reactions are said to be endothermic (or endoergic). However the reactions for which the Q-value is positive are said to be exothermic. The examples of fusion and fission reactions are as follows:

1. Fusion reaction
2. H  + ³H    →⁴He  +  ¹n    (Q-value = 17.5893 MeV)
2. Fission reaction
235. U +  ¹n → ¹⁴⁴Ba + ⁹⁰Kr + 2¹n (Q value= 183MeV)

The energy released during a nuclear reaction is more in fission as compared to fusion. A series of fission reactions is called a chain reaction. If enough uranium is used under the right conditions, a continuous chain reaction occurs which is called a self-sustaining chain reaction. A self-sustaining chain reaction creates a great amount of heat, which can be used to generate electricity. Although nuclear energy is considered as clean energy yet its inclusion in the renewable energy list is a subject of major debate.

On the other hand, fusion reactions are basically helpful to make new radioactive element which are further helpful for the medical treatment. The radioactive rays are subjected to particular infected area to destroy the tumour. If the energy released in a nuclear reaction is very low i.e. few keV, these types of reactions are helpful to study the structure of a nucleus and its ground state properties i.e. liquid state of nuclear matter. In thenuclear reaction at very high incident energies (few GeV to TeV), the nuclear matter become very hot and dense and it produces hot quark gluon plasma which is again useful for the understanding of gas phase of nuclear matter. The branches where nuclear physics has its most impact are in defense and electricity in which the nuclear fusion and fission reactions are mainly involved.

Nuclear energy for defense:

Explosion is release ofa large amount of energyin a very small volume.The nuclear explosion is million times more powerful than the conventional detonation. The nuclear bombs or weapons are device which releases a huge amount of energy through nuclear fission, fusion or combination of both processes. Fission based weapons are called atom bombs and fusion or combination based weapons are called hydrogen bomb/thermonuclear bombs. The elements mostly involved are Uranium, Plutonium, Tritium and Deuterium. The amount of energy released due to explosion caused by nuclear weapon containing 1 kg of Plutonium is same as produced by 10⁶ kg of TNT (Trinitrotoluene). One ton of TNT (4.184×10⁹ Joules) is an energy unit that was invented to describe the destructive potential. Thus,a nuclear explosion caused by fission reaction of 1kg Uranium-235 and Plutonium-239 produces 15000 tons of TNT and 21000 tons of TNT respectively. Approximately 50% of energy of nuclear weapon produces air blast (the shock, the expansion of intensely hot gases at extremely high pressures) and 35% of energy is emitted in the form of light and heat, generally referred to as thermal energy. The remaining 15% of the energy is released as various type of nuclear radiation. Out of 15%, 5% are prompt nuclear radiation produced within the one minute of explosion (gamma rays and neutrons) and 10% are delayed nuclear radiation, which is emitted over a period of time due to the radioactivity of the product.The thermal energy is capable of causing skin burns and eye injuries and starting fires of combustible material at considerable distances. The shock wave, arriving later, may spread fires further. In addition, an immediate source of destruction is the electromagnetic pulse which leads to the impairment of electronic devices, including those needed for health services. The explosion can causethe destruction and collapse of several buildings as well as damages the fertility of soil and climate.  These weapons also cause long lasting health effects like cancer, genetic damage, non-healing wounds, suppurating extensive burns, skin infestations, gastrointestinal infections and psychic trauma.

Although the explosion through nuclear warheads affects the health and climate drastically; however, the presence of theseweaponswork as a shield to the security of a Nation. In 1945, United States became the first country with nuclear power and till today there are seven declared and one undeclared countries with nuclear power. After the first confirmed and successful nuclear bomb test in Pokhran (Rajasthan) by Indian Army under the supervision of several key Indian Generals, India becameanuclear power stateon 18 May 1974. According to SIPRI (Stockholm International Peace Research Institute), a total of 14,465 nuclear weapons (or warheads) have been developedworldwide from which 3,750 were deployed with operational forces. The countries with nuclear power are Russia (>45%), United States (>45%), United Kingdom (>2%), France (>2%), China (>2%), India (>1%), Pakistan (>1%)and North Korea (<1%). Israel (<1%) claims to have nuclear weapons, however there is no evidence and no test performed by them.

Nuclear energy for electricity:

Since, the uncontrolled nuclear fission chain reaction is the basis of large explosion and destruction; however, on the other hand, the controlled nuclear fission chain reaction is used to produce electricity. The nuclear power plant/station/reactor runs on the thermal energy released during the controlled nuclear fission reactions. The thermal energy further raises the temperature of the water to make steam which drives large turbine connected to a generator which produces electricityand is further subjected to operate a large range of electrical machines for commercial as well as non-commercial use. The fissile elements mostly used in these power stations are Uranium (238 and 235) and Plutonium (239).Current research is being done to investigate how Thorium-232 can be used as a fuel. One nuclear fuel enriched pellet (as big as a fingertip) contains as much energy as produced by approximately 150 Gallon of oil. A nuclear reactor core contains a dozen or more fuel rods bundled with a large number of nuclear fuel pellets.

As per the data provided by IAEA (International Atomic Energy Agency), there are 454 nuclear reactors worldwide in 31 countries. Approximately 17% of electricity worldwide is produced by nuclear power plants. However, the future of nuclear energy depends on safe and efficient reactor designs.  The nuclear power plant is the fifth largest source of electricity in India. There are 22 nuclear reactors in operation at seven sites in India: Tarapur Atomic Power Station in Maharashtra, Kakrapar Atomic Power Station in Gujarat, Kudankulam Nuclear Power Plant and Madras Atomic Power Station in Tamil Nadu, Kaiga Nuclear Power Plant in Karnataka, Rajasthan Atomic Power Station in Rajasthan and Narora Atomic Power Station in Uttar Pradesh. The total installed capacity of these nuclear reactors is 6780 MW. In addition to these, four nuclear power plants (total capacity of 5300 MW) are under construction and eleven are planned projects which will cover a capacity of approximately equal to 42400 MW.Since, India has the world’s largest reserve of Thorium, therefore with the development in nuclear research, the Thorium-based nuclear reactors will become the largest source of electricity production. Moreover, using the stores of Thorium, India has become an expert at developing fast breeder reactors, which maximize Plutonium production and can produce more fuel than they consume. Presently, India produces 3.6% of its total electricity by nuclear fuel and stands at 13^th number in top 17 countries (through global statistics provided by the UN's International Atomic Energy Agency) which produces electricity through nuclear power plants. However, this percentage will rise to 25% by 2050. These achievements will glorify India as a strong and independent nuclear power state.

References:

1. E. Rutherford and H. Geiger, Proc. R. Soc. London A 81, 162 (1908).
2. S. Kailas, Physics Reports 284, 381-416 (1997).