Can you think of some sources used for energy? Plants use sunlight as a source of energy for preparing food. At home, we use coal, kerosene, LPG to prepare food. This food, then after digestion, gives us energy. We also use electricity for using various machines like fan, computer, mixer, heater, refrigerator in our homes. Can you name the sources of energy used in industry? Coal, petroleum, natural gas, solar, tidal, nuclear are few sources of energy used in industry. Coal, petroleum, natural gas are called non-renewable sources of energy and solar, tidal, are called renewable sources of energy as they will not finish off. Can coal, petroleum, natural gas be prepared by man? No. Coal, petroleum, natural gas cannot be prepared by man. They have been formed in nature by burial of forests and sea animals. This has taken millions of years to form. Coal forming process is still continuing. Then why do we say that energy resources like coal and petroleum will finish off? After the industrial revolution, the requirement of energy increased many times compared to previous years. Coal and petroleum was used indiscriminately and people realized that these non-renewable energy sources coal and petroleum may soon finish off. This fear and further need of energy for development of countries scientists started to look for greener options of energy, which were renewable causing less harm to environment. Nuclear energy is one such energy, which causes less harm to environment as no burning or formation of CO2 or other polluting gases takes place. Nuclear reactor is a machine that generates electricity on large scale by using nuclear energy. In a nuclear reactor, the nuclear energy in an atom is released by nuclear reactions on the nuclear fuel. Let us see how these nuclear reactions take place. Nuclear reactions are of two types, fusion reaction, fusion reaction. In both types, tremendous heat energy is generated, which is then converted into electricity. In the fusion reaction, the nucleus of radioactive element is split into two or more nuclei of different elements by bombarding neutrons or similar other particles. In the fusion reaction, two or more nuclei of same or different elements are forced to come together under certain conditions to form a nucleus of new element. Most of the nuclear reactors work on the principle of a nuclear reaction called the controlled fission reaction. The fission of 1 kilogram of uranium, which is the nuclear fuel, produces energy equivalent to burning 3 million tons of coal. You can imagine how much coal can be saved if these thermal power plants are converted to nuclear reactors. Nuclear power is the fifth largest source of electricity in India after thermal, hydroelectric and renewable sources of energy. India has 22 nuclear reactors in operation at 8 sites producing 6,780 megawatt of power. 11 more reactors are under construction to generate 8,100 megawatt additional power. Apsara is the first nuclear reactor built by Bhaba Atomic Research Center, popularly known as BARC or BARC. Apsara became functional that is critical on 4th August 1956. Now let us see the principle on which the nuclear reactor works. Control chain reaction is the principle on which nuclear reactor works. Let us study this chain reaction. In the reactor nuclear fuel like uranium, plutonium and thorium is used. Let us take the nucleus of the isotope uranium 235. The nucleus of uranium 235 is bombarded with slow neutron. This splits the uranium nucleus into two different elements, Krypton 92 and Barium 141 and 2 to 3 neutrons. This 2 to 3 neutrons further bombard the uranium nuclei and distributes a chain reaction which is uncontrolled. As the uranium 235 nucleus splits into Krypton 92 and Barium 141 it is called Fission reaction. A large amount of energy is released during this chain reaction of Fission. Uncontrolled chain reaction can cause explosion. This Fission chain reaction is controlled by absorbing the excess neutrons and only one neutron is allowed to further bombard the nucleus of 92 uranium 235 instead of 3 nuclei. This way Fission chain reaction continues in a controlled manner which can be used for power generation. That is use of nuclear energy for peaceful purposes. To control chain reaction in the nuclear reactor, moderators are used for reducing the speed of neutrons. Graphite or heavy water is used as a moderator for reducing the speed of neutrons. As only one neutron per nucleus for Fission has to be allowed for Fission chain reaction to continue. The excess neutrons are absorbed by controllers which are rods of boron and cadmium or beryllium etc. This generates tremendous amount of heat by which water used as coolant is converted into steam. This steam rotates the turbines in the generator and electricity is generated. In these nuclear reactors, nuclear fuel like uranium 235, thorium and plutonium is used instead of coal. This helps in saving coal as well as it is environment friendly. India has large reservoirs of thorium through 32 therefore Indian scientists have developed a future plan for nuclear reactors which can use thorium through 32 which is converted to isotopes U233. Nuclear energy is also generated by fusion reaction. In fusion reaction, 4 hydrogen nuclei fuse to form helium nucleus and a lot of energy is released. This fusion reaction takes place in the sun. This nuclear fusion reaction requires a very high temperature for reaction to take place. Work is going on to see if fusion nuclear reactors can be used for producing energy on the earth as it is a more cleaner source. It does not produce any radioactive byproducts. Fission reaction produces radioactive byproducts whose disposal is still a problem. Some of these are isotopes of iodine, calcium, strontium, xenon and barium. Right now, radioactive waste is dumped deep into sea which again disturbs the environment of sea. Inhaling large concentrations of uranium can cause lung cancer. Ingesting of uranium can cause kidney damage. Its radioactive decay can cause several health hazards. We need to develop proper ways of disposal of radioactive waste then only in two sense energy produced by nuclear reactors will be environment friendly. A major accident took place in nuclear reactor at Chernobyl on 26 April 1986. The area prepared is not habitable and even now high amount of radiation is still present. Accident was triggered by tsunami in nuclear reactor at Fukushima Daiichi on 11 March 2011. The radiation was so high that people had to be evacuated. Both the places still have high amount of radiation making it impossible for people to stay. Controlled chain reaction is the principle on which a nuclear reactor works. One kg of uranium produces energy equivalent burning 3 million tons of coal. As no burning takes place, emission of greenhouse gases like carbon dioxide is zero making it environment friendly. But we need effective method for disposing radioactive waste which are generated in the process. Challenge, do the following activity. Prepare a list of nuclear power plants in India and then locate them on the map of India.