Nuclear fission and nuclear fusion
A physical reaction which causes change in the nucleus of an atom is called nuclear reaction and the energy released during this reaction is called nuclear energy.
The mass of nucleus serves as the source of nuclear energy which is released mainly in the form of heat. There are two types of nuclear reaction. They are:
i) Nuclear Fission
ii) Nuclear Fusion
The heavy nucleus of radioactive atoms like uranium, plutonium or thorium is bombarded with low energy neutrons which split the nucleus into smaller nuclei. This process is called nuclear fission. For example when uranium-235 atoms are bombarded with neutrons then the heavy uranium nucleus splits to produce barium-139 and krypton-94 with the emission of three neutrons. A lot of energy is also produced in this reaction because mass is converted into energy.
Also, in a nuclear fission reaction neutrons are used up and produced as well. The neutrons produced in the nuclear fission reaction leads to further fission of heavy nuclei and cause chain reaction. If all the neutrons produced during fission of uranium-235 produce further fission, then so much energy will be produced that it will not be controlled and leads to explosion called atom bomb. However, nuclear fission reaction can be controlled by using boron rods as boron can absorb neutrons.
Nuclear fission reactions are done to generate electricity at nuclear power plants.
Nuclear power plant
Nuclear power plants use nuclear fission reaction to generate electricity and the fuel used for this purpose is uranium-235.
In a nuclear power plant fission reaction is carried out in a steel pressure vessel and inside is a nuclear reactor. In a nuclear reactor uranium-235 rods are inserted in graphite core. Graphite is called the moderator as it helps in slowing down the speed of neutrons so that a proper fission reaction takes place. In between the uranium-235 rods are placed boron rods as they help in absorbing excess neutrons and prevent nuclear fission reaction to of out of control. Boron rods are called control rods. The nuclear rods can be raised inside or pulled outside the reactor as the demand is. The nuclear reactor is enclosed in a concrete chamber which has thick wall so that it can absorb the nuclear radiations.
A Nuclear power plant or Atomic power plant
Now the heat produced due to fission reaction in the reactor is cooled by using liquid sodium or carbon dioxide gas which also helps it transfer to heat exchanger. Here with the help of a coolant water is converted into steam. The steam produced is used to turn turbines and run generator.
Tremendous amount of heat energy is produced when controlled fission reaction takes place in nuclear reactor. That is why liquid sodium is pumped continuously through the pipes attached to the reactor. Sodium helps in absorbing the heat produced in the reactor. Then through pipes extremely hot sodium is passed through water in the heat exchanger. Water absorbs heat from hot sodium and boils to form steam. This steam is then passed at high pressure into turbine chamber having turbine. This steam then rotates the turbine which is further attached to its shaft and the generator. So, when turbine rotates, its shaft also rotates and drives generator. This generator helps in generating electricity.
The spent steam coming out of turbine chamber is passed through condenser which contains water and this water helps in cooling the steam. This steam then converts into water and through pipes is again sent to heat exchanger. The waste material produced in the nuclear fission reaction of uranium-235 is radioactive and extremely harmful for the environment.
Nuclear power plants in India
At there are seven nuclear power plants in India. They are:
i) Tarapur Atomic Power Station, Maharashtra
ii) Rajasthan Atomic Power Station, Rajasthan
iii) Madras Atomic Power Station, Tamil Nadu
iv) Kaiga Atomic Power Station, Karnataka
v) Kudankulam Atomic Power Station, Tamil Nadu
vi) Narora Atomic Power Station, Uttar Pradesh
vii) Kakrapar Atomic Power Station, Gujarat
Nuclear bomb is based on the nuclear fission reaction of uranium-235 and plutonium-239. The fission reaction is deliberately allowed to go out of control so as to produce large amount of energy in a very short while.
The atom bombs based on nuclear fission of uranium-235 ad plutonium-239 were dropped on Japanese cities of Hiroshima and Nagasaki in 1945 during the Second World War. This caused tremendous loss of human life.
Einstein’s Mass-Energy Relation
According to Einstein mass is equal to energy.
E = mc2
E is the amount of energy produced
M is mass destroyed
C is the speed of light in vacuum
Since the speed of light is large so an extremely large amount of energy is produced even if small amount of mass is destroyed. Also, if mass is taken in kilograms (kg) and speed of light in meters per second (m/s) then energy will come in joules (J).
Therefore, if one kg of mass of any matter is destroyed in nuclear reaction than the amount of energy produced is:
E = mc2
E = 1 * (3 * 108)2
E = 9 * 1016 J
Energy units for expressing nuclear energy
The SI unit of energy released in nuclear reactions is electron volt (eV) or million electron volt (MeV). And,
1 electron volt = 1.602 * 10-19 joules
1 million electron volt = 1.602 * 10-19 * 106 joules
1 MeV = 1.602 * 10-13 J
Value of atomic mass unit in terms of energy
Since absolute mass of atomic mass unit is 1.66 * 10-27 kg and the exact value of speed of light is 2.998 * 108m/s. When we put these values in Einstein’s equation we get,
1 atomic mass unit (u) = 1.492 * 10-10 J
1 atomic mass unit (u) = 931 MeV
The meaning of fusion is to join or to combine. Therefore, the process in which two nuclei of light electrons are combined to form a heavy nucleus is nuclear fusion. In the process of nuclear fusion also tremendous amount of energy is released.
The nuclei of atoms are positively charged and thus they repel each other. So in order to combine or fuse these two nuclei to form one heavy nucleus a lot of heat energy and high pressure is required. This shows that nuclear fusion is carried out by heating lighter atoms to an extremely high temperature at high pressure. Some mass is also lost in this process which gives tremendous amount of energy.
For example, when deuterium atoms are heated to an extremely high temperature under high pressure than two deuterium nuclei combine to form helium which has a heavy nucleus, a neutron is emitted and a lot of energy is liberated.
Nuclear fusion reaction is opposite of nuclear fission reaction. The energy produced in nuclear fusion reaction has not been controlled yet and is much more than nuclear fission reaction.
Nuclear reactions which occur at extremely high temperature are called thermonuclear reactions. This reaction is used in producing hydrogen bomb which causes mass destruction. Isotopes of hydrogen, deuterium (2H) and tritium (3H), along with an element lithium-6 is used in making a hydrogen bomb. The explosion of hydrogen bomb is done by using an atom bomb. This is because when atom bomb is exploded then its fission reaction produced a lot of heat which raises the temperature of deuterium and tritium in a few microseconds. Thus the fusion reaction takes place and hydrogen bomb is exploded producing enormous energy. Hydrogen bomb causes destruction of life.
Advantages of nuclear energy
- It produced tremendous energy from a small amount of fuel (Uranium-235).
- There is no need to put the fuel again and again in nuclear reactor. One the fuel (Uranium-235) is put in the reactor it can function for two to three years at a stretch.
- It does not produce gases like carbon dioxide or sulphur dioxide.
Disadvantages of nuclear energy
- The waste products of nuclear reactors are radioactive and keep on emitting harmful radiations.
- Risk of an accident in nuclear reactors which may cause leakage of radioactive material.
- The availability of fuel uranium is limited.
- High installation cost of nuclear power plant.
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