Contents
The laws of Physics Topics are used to explain everything from the smallest subatomic particles to the largest galaxies.
Fission in Nuclear Power Plants
Nuclear Energy
A physical reaction which involves changes in the nucleus of an atom is called a nuclear reaction. The energy released during a nuclear reaction is called nuclear energy (because it comes from the nucleus of an atom). Nuclear energy can be obtained by two types of nuclear reactions :
- Nuclear fission, and
- Nuclear fusion.
The source of nuclear energy is the mass of nucleus. A small amount of mass of nucleus is destroyed during a nuclear reaction which gets converted into a tremendous amount of energy. The nuclear energy is released mainly in the form of heat (and some light).
The nuclear energy is also known as atomic energy because it can be considered to be coming from the atoms. We will now describe the nuclear reactions of fission and fusion in detail, one by one. Let us start with nuclear fission.
1. Nuclear Fission
The word ‘fission’ means to ‘split up’ into two or more parts. The process in which the heavy nucleus of a radioactive atom (such as uranium, plutonium or thorium) splits up into smaller nuclei when bombarded with low energy neutrons, is called nuclear fission. A tremendous amount of energy is produced in the nuclear fission process.
The sum of masses of the smaller nuclei formed in a fission reaction is a little less than that of the mass of the original heavy nucleus. So, there is a small loss of mass in the nuclear fission process which appears as a tremendous amount of energy.
Please note that nuclear fission is carried out by bombarding the heavy nuclei with low energy neutrons which are also called slow moving neutrons. We will now give an example of a nuclear fission reaction. We will use the easily fissionable isotope of uranium called uranium-235 in this example.
When uranium-235 atoms are bombarded with slow moving neutrons, the heavy uranium nucleus breaks up to produce two medium-weight atoms, barium-139 and krypton-94, with the emission of 3 neutrons. A tremendous amount of energy is produced during the fission of uranium. This fission reaction can be represented in the form of a nuclear equation as :
In the fissioning of uranium, some mass of uranium disappears (is lost), and a tremendous amount of energy is produced. An idea of the tremendous amount of energy produced during nuclear fission reaction can be had from the fact that the fission of 1 atom of uranium-235 produces 10 million times more energy than the energy produced by the burning of 1 atom of carbon from coal.
The energy produced during nuclear fission reactions is used for generating electricity at nuclear power plants. Please note that the energy produced in a fission reaction is due to the conversion of mass into energy.
If we look at the above nuclear fission reaction equation, we find that the neutrons are used up as well as produced. For example, in the nuclear fission of uranium-235 described above, 1 neutron is consumed and 3 neutrons are produced in the fission of each nucleus.
The neutrons produced in a fission process cause further fission of the heavy nuclei leading to a self-sustaining chain reaction. When all the neutrons produced during fission of uranium-235 are allowed to cause further fission, then so much energy is produced in a very short time that it cannot be controlled and leads to an explosion called atom bomb.
We can, however, control a nuclear fission reaction by using control rods made of boron. Boron has a property that it can absorb neutrons. So, when a nuclear fission reaction is carried out in the presence of boron rods, the excess neutrons produced during successive fissions of uranium-235 atoms are absorbed by boron rods and hence not available to cause further fission.
Due to this a controlled fission reaction of uranium-235 takes place liberating heat energy at a slow, steady and manageable rate which can be used for generating electricity at a nuclear power plant.
Nuclear Power Plant
A power plant in which the heat required to make steam and turn turbines (to drive generators for making electricity) is obtained by nuclear reactions, is called a nuclear power plant. The nuclear power plants use the nuclear fission reaction to generate electricity.
Most of the nuclear power plants use uranium-235 as fuel to produce heat. But uranium-235 fuel is not burnt like coal, oil or gas to obtain energy. Its energy is released by the nuclear fission process. A nuclear power plant is shown in Figure.
In a nuclear power plant, the fission of nuclear fuel uranium-235 is carried out in a steel pressure vessel V of reactor R (Reactor is a kind of nuclear furnace) (see Figure). The enriched uranium-235 rods marked A (called fuel elements) are inserted in a core made of graphite blocks inside the reactor.
Graphite is called a moderator. It slows down the speed of neutrons to make them fit for causing fission. In-between the uranium rods are inserted boron rods B. Boron rods are called control rods because they absorb excess neutrons and prevent the fission reaction from going out of control.
Boron rods can be raised or lowered in the reactor from outside (The part of boron rods which is inside the reactor absorbs neutrons). The reactor is enclosed in a concrete chamber M having thick walls to absorb the nuclear radiations (so as to protect the outside world from the dangerous nuclear radiations).
Liquid sodium (or carbon dioxide gas) is used as a ‘coolant’ to transfer the heat produced in the reactor by fission to heat exchanger (or boiler) for converting water into steam. The rest of the arrangement at a nuclear power plant is shown in Figure. We will now describe the working of a nuclear power plant.
The controlled fission of uranium-235 in the nuclear reactor produces a lot of heat energy. Liquid sodium (or carbon dioxide gas) is pumped continuously through the pipes embedded in reactor by using a pump P (see Figure 37). Sodium absorbs the heat produced in the reactor.
This extremely hot sodium is then passed into the coil of the heat exchanger H containing water. Water absorbs heat from hot sodium and boils to form steam. The hot steam at high pressure is introduced into a turbine chamber C having a turbine T. The pressure of steam makes the turbine rotate. The shaft S of turbine is connected to a generator G. When the turbine rotates, its shaft also rotates and drives the generator. The generator produces electricity.
The spent steam coming out from the turbine chamber is passed through a condenser D. The condenser cools the spent steam to form water. This water is again sent to the heat exchanger for forming fresh steam. A nuclear power plant can work day and night for two to three years with the same uranium fuel.
Please note that the waste materials (called nuclear wastes) which are produced by the fission of uranium-235 during the generation of electricity at a nuclear power plant are radioactive and hence extremely harmful to all living beings, plants as well as animals, including human beings.
Nuclear Power Plants in India
There are a total of six nuclear power plants in India at present. These nuclear power plants are located at :
- Tarapur in Maharashtra,
- Rana Pratap Sagar near Kota in Rajasthan,
- Kalpakkam in Tamil Nadu,
- Narora in Uttar Pradesh,
- Kaprapur in Gujarat, and
- Kaiga in Karnataka.
At present only about 3% of the total electrical power produced in India is obtained from nuclear power plants (or nuclear reactors). Please note that in some of the industrialised countries like France, Germany and Japan, etc., more than 30% of their total electrical power comes from nuclear power plants.
Nuclear Bomb (or Atom Bomb)
The highly destructive nuclear bomb (or atom bomb) is based on the nuclear fission reactions of uranium-235 or plutonium-239. In the nuclear bomb, the fission reaction of uranium-235 (or plutonium-239) is deliberately allowed to go out of control so as to produce an enormous amount of energy in a very short time.
This energy causes destruction all around. The atom bombs based on the fission of uranium- 235 and plutonium-239 were dropped on the Japanese cities of Hiroshima and Nagasaki, respectively in 1945 during the second world war. Both these atom bombs caused a great loss of human life and property. About 1.54 lakh people were killed in these two atom bomb attacks (see Figure).
Einstein’s Mass-Energy Relation
Einstein said that mass and energy are equivalent, and are related by the equation :
E = mc2
where E is the amount of energy produced if mass m is destroyed, and c is the speed of light (in vacuum).
Since the speed of light is very, very large, so an extremely large amount of energy is produced even if a small mass gets destroyed. The destruction of mass happens in nuclear reactions (like nuclear fission and fusion) with the liberation of tremendous amount of energy.
Please note that in the mass-energy equation, if we put the mass in kilograms (kg), and the speed of light in metres per second (m/s), then the energy will come in joules (J). For example, if a mass of 1 kg of any matter could be destroyed in a nuclear reaction, then the amount of energy produced would be given by Einstein’s equation as :
E = mc2
E = 1 × (3 × 108)2
E = 9 × 1016 J
Thus, 1 kg mass produces a huge amount of energy of 9 × 1016 joules.
Energy Units for Expressing Nuclear Energy
We know that the common unit of energy is joule (J). But the energy released in nuclear reactions is expressed in the units of ‘electron Volt’ (eV) or ‘Million electron Volt’ (MeV). 1 electron volt is the amount of energy acquired by an electron (having a charge of 1.602 × 10-19 coulombs) when accelerated through a potential difference of 1 volt.
1 electron volt – 1.602 × 10-19 joules
or 1 eV = 1.602 × 10-19 J
The electron volt is a small unit, so the energy released in nuclear reactions is usually expressed in terms of a bigger unit called million electron volts. We can obtain the value of million electron volt (MeV) by multiplying the above value of electron volt (eV) by 1 million (which is 106). Thus,
1 million electron volts = 1.602 × 10-19 × 106 joules
or 1 million electron volts = 1.602 × 10-13 joules
or 1 MeV = 1.602 × 10-13 J
The Value of Atomic Mass Unit in Terms of Energy
We can calculate the value of atomic mass unit (u) in terms of energy in joules by using the Einstein’s mass-energy equation : E = me2. Now, the absolute mass of atomic mass unit is 1.66 × 10-27 kg and the exact value of speed of light is 2.998 x 108 m/s. So, by putting these values in the Einstein’s equation, we will find that:
1 atomic mass unit = 1.492 ×10-10 joules
or 1 u = 1.492 × 10-10 J
Thus, 1 atomic mass unit (u) is equivalent to 1.492 × 10-10 joules of energy. We can convert this energy from joules to million electron volts (MeV) by using the relation : 1.602 × 10-13 J = 1 MeV.
This will give us the following value of atomic mass unit:
1 atomic mass unit = 931 million electron volts
or 1 u = 931 MeV
Thus, 1 atomic mass unit (u) is equivalent to 931 MeV of energy.