In \(1939\), German scientists Otto Hahn and F. Strassman discovered the process of nuclear fission. When a uranium nucleus is bombarded with a neutron, it splits up into two smaller nuclei of equivalent mass, emitting a few neutrons and some amount of energy.
Nuclear fission is the process of splitting or breaking up a heavy nucleus into two smaller nuclei with the release of a huge quantity of energy and a few neutrons.
Example: Nuclear fission of a uranium nucleus (\(U^{235}\))
\(_{92}U^{235}\) \(+\) \(_{0}n^{1}\) \(\rightarrow\) \(_{56}Ba^{141}\) \(+\) \(_{36}Kr^{92}\) \(+\) \(3\) \(_{0}n^{1}\) \(+\) \(Q\) (\(energy\))
The average energy released in each process of nuclear fission is about \(3.2 \times{10^{-11}}\) \(J\).
Fissionable materials:
A fissionable material or a fissile material is a radioactive element, which undergoes fission in a sustained manner when it absorbs a neutron.
Natural uranium consists of \(99.28\) \(\%\) of \(_{92}U^{238}\) and \(0.72\) \(\%\) of \(_{92}U^{235}\). Out of these two, \(U^{238}\) does not undergo fission, whereas \(U^{235}\) undergoes fission. Hence, \(U^{235}\) is a fissionable material, and \(U^{238}\) is non-fissionable material.
Fertile materials:
Fertile materials are radioactive elements that are converted into fissionable materials.
Uranium (\(U^{238}\))
Thorium (\(Th^{232}\))
Plutonium (\(Pu^{240}\))
Critical mass:
The minimum mass of a fissile material required to sustain the chain reaction is called 'critical mass (\(m_c\))'. It depends on the nature, density and size of the fissile material.
If the mass of the fissile material is less than the critical mass, it is termed as 'subcritical'. If the mass of the fissile material is more than the critical mass, it is termed as 'supercritical'.
Generally, two types of chain reactions are possible. They are
(i) Controlled chain reaction
(ii) Uncontrolled chain reaction
(i) Controlled chain reaction
(ii) Uncontrolled chain reaction
Controlled chain reaction:
The number of neutrons released in a controlled chain reaction is always one. This is accomplished by using a neutron absorber to absorb the excess neutrons released during the process, leaving only one neutron to produce further fission. As a result, the reaction is kept under control.
UnControlled chain reaction:
The quantity of neutrons multiplies endlessly in an uncontrolled chain reaction, causing fission in a huge mass of fissile material. This causes a massive amount of energy to be released in a fraction of a second. This chain reaction is employed in an atom bomb to create an explosion. It is calculated that \(10^{23}\) uranium atoms undergo fission in \(one\ minute\) in an uncontrolled chain reaction.
Atom bomb:
The atom bomb works on the principle of uncontrolled chain reaction. In an uncontrolled chain reaction, the number of neutrons and fission processes multiply in a nearly geometrical progression. This chain reaction causes an explosion by releasing a large amount of energy in a short period. The bomb that was dropped over Nagasaki was named 'Fat man'. It was an explosion type bomb with a plutonium core.
Nuclear fusion:
The process in which two lighter nuclei combine to form a heavier nucleus is termed nuclear fusion. Nuclear fusion occurs only at a very high temperature of the order of \(10^7\) to \(10^9\ K\), along with a very high pressure to fuse the hydrogen nuclei. This type of reaction is known as the Thermonuclear reaction.
Atomic mass unit:
Atomic mass unit (u) is defined as \(1/12\)\(^{th}\) of the mass of a neutral carbon atom \(_{6}C^{12}\) (carbon – 12 atom). This unit is used to measure the mass of an atom.
Mass defect:
The mass of the daughter nucleus formed during a nuclear reaction (fission and fusion) is lesser than the sum of the masses of the two-parent nuclei. This difference in mass is called the mass defect.
Unit of energy:
In nuclear physics, the electron volt (\(eV\)) is the unit used to measure the energy of tiny particles. It is the energy of an electron when it is accelerated by an electric potential of one volt.
Mass - energy equivalence:
According to Einstein, mass and energy are not independent but are mutually convertible. A body that changes its energy '\(E\)' undergoes a change in its mass, '\(m\)'. For example, if the energy of a particle is increased, its mass also increases.
Binding energy:
Binding energy holds the nucleons (protons and neutrons) of a nucleus together, even from the loss in the total mass of the nucleons (mass defect). It is measured in \(MeV\).
Stellar energy:
Our Sun, like other stars, emits some amount of energy in the form of light and heat. This energy is known as Stellar energy. Fusion reaction that takes place in the cores of the Sun and other stars results in an enormous amount of energy called Stellar energy.
Hdrogen bomb:
The hydrogen bomb works on the principle of nuclear fusion. A hydrogen bomb contains an inbuilt atom bomb for an explosion since it provides the high temperature and pressure required for fusion. Thus, the hydrogen bomb is a fission-fusion bomb.
Radio isotopes and uses:
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Radio isotopes
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Symbol
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Uses
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Radio sodium
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\(Na^{24}\)
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Effective functioning of the heart and detects blood clots in the body
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Radio iron
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\(Fe^{59}\)
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Diagnose anaemia and provide treatment
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Radio phosphorous
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\(P^{32}\)
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Treatment of skin diseases
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Radio iodine
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\(I^{131}\)
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Tests the functioning of the thyroid gland
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Radio cobalt
Radio gold
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\(Co^{60}\)
\(Au^{198}\)
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Treatment of skin cancer and to kill cancer cells
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A dosimeter is a device that detects the amounts of ionising radiation exposure.
1. Radioactive materials should be stored in a lead container with a thick wall.
2. While working with harmful radioactive materials, lead-coated aprons and gloves should be worn.
3. One should avoid eating while working with radioactive materials.
4. Only tongs or a remote control device should be used to handle radioactive materials.
5. Users should use dosimeters to monitor their exposure to radiation.
A nuclear reactor is a device in which the nuclear fission reaction takes place in a self-sustained and controlled manner to produce electricity.
Components of a nuclear reactor:
(i) Fuel
(ii) Moderator
(iii) Control rod
(iv) Coolant
(v) Protection wall
The nuclear reactors that are used in different places are
- Breeder reactor
- Fast breeder reactor
- Pressurized water reactor
- Pressurized heavy water reactor
- Boiling water reactor
- Water-cooled reactor
- Gas-cooled reactor
- Fusion reactor
- Thermal reactor
Nuclear power plants in India:
In August \(1948\), the Department of Indian Scientific Research committee founded the Indian Atomic Energy Commission (\(AEC\)) in Bombay (Mumbai), Maharashtra. It serves as the central hub for all atomic energy research.
The first chairman of the Indian Atomic Energy Commission was Dr Homi Jahangir Bhaba. Nowadays, it is well-known as Bhaba Atomic Research Centre (\(BARC\)).
Nuclear power is India's fifth most powerful source of energy. India's first nuclear power plant or station is the Tarapur Atomic Power Station (\(420\ MW\)). There are currently seven power plants in operation, one each in Maharashtra, Rajasthan, Gujarat, Uttar Pradesh, and two plants in Tamilnadu. In Tamilnadu, the nuclear power plants are located in Kalpakkam and Kudankulam. A nuclear reactor named Kamini is located at Kalpakkam.
Some other operating nuclear reactors are
• Cirus (\(100\ MW\))
• Dhuruva (\(100\ MW\))
• Purnima (zero energy fast breeder)
• Cirus (\(100\ MW\))
• Dhuruva (\(100\ MW\))
• Purnima (zero energy fast breeder)