Differences between Nuclear Fission and Nuclear Fusion Reaction

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   Nuclear fusion may be defined as the process of combining two or more light nuclei to form a single heavy nucleus. The nuclear fusion reaction is also accompanied by liberation of a large amount of energy. It is found that the enormous amount of energy produced during nuclear fusion reaction is due to the conversion of some mass into energy. Thus, sum of the masses of products formed in nuclear fusion is slightly less than the sum of masses of reactants.

Nuclear Fusion in the Sun: The production of large amount of heat and light energy by Sun is the best example of nuclear fusion reaction. In Sun, two nuclei of deuterium (1H2) combine together to form a heavy nucleus of helium and a large amount of energy. Nuclear fusion reaction can be represented as:

    1H2      +      1H2                      2He4      +      Energy

or

    1H2       +      1H2                       2He       +      Energy

 Hydrogen Bomb

The nuclear fission reaction is used to produce highly destructible hydrogen bomb. In hydrogen bomb nuclei of tritium (1H3) and deuterium (1H2) are combined together to form a heavy nucleus of helium (2H4). During this process, very large amount of energy is released which is responsible for its destructive nature.

Advantage of Nuclear Fusion over Nuclear Fission

The first advantage of nuclear fusion process over nuclear fission process is that during this process more energy is produced as compared to nuclear fission reaction. The second advantage is that unlike nuclear fission process, the products formed at the end of nuclear fusion process are not radioactive. Thus, there is no danger of radioactive pollution. That is why, it is said that energy produced by nuclear fusion process will create less pollution problems as compared to that of nuclear fission.

Disadvantage of Nuclear Fusion Process

The main disadvantage of a nuclear fusion reaction is that it cannot be controlled in the laboratory for the production of energy.

Test Your Understanding and Answer These Questions:

1. What is nuclear fusion reaction?
2. Give an example of nuclear fusion reaction.

 In India the first nuclear power plant called “Tarapur atomic power station” was established in Maharashtra in 1969. The important nuclear power plants in India are:

1. Tarapur nuclear power plant at Tarapur in Maharashtra.
2. Rajasthan nuclear power plant at Rana Pratap Sagar near Kota.
3. Madras nuclear power plant at Kalpakkam in TamilNadu.
4. Narora nuclear power plant in Uttar Pradesh.
5. Koodankulam nuclear power plant.

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 The nuclear power plant is a device in which controlled fission reaction of U-235 is done to produce electricity. Nuclear reactor consists of nuclear fuel, coolant, moderator and controlling roads.

Construction of Nuclear Power Plant
A nuclear reactor consists of following parts:

1. Nuclear Fuel
Nuclear fuel may be defined as the fissionable material which is used in nuclear reactor to produce energy. Generally, in nuclear reactor rods of U-235 are used as fuel. But the percentage of U-235 present in natural uranium is very low (0.7 %). Thus, it becomes necessary to increase the percentage of U-235 in natural uranium so that we can use it as fuel in nuclear reactor. The percentage of u-235 in natural uranium is increased by processing natural uranium. The process of increasing the percentage of U-235 in natural uranium is called enrichment.

2. Control Rods
To control the chain reaction of nuclear fission, rods of cadmium or boron are used in the nuclear reactor. These rods are good absorber of neutrons. So, when these rods are inserted in the nuclear reactor up to a desirable length, they absorb desired number of neutrons and keep the fission reaction under control.

3. Moderator
For the fission of U-235 it is necessary that the speed of neutrons hitting the nuclei of U-235 must of slow. This objective is achieved by using moderator in nuclear reactor. A moderator is the substance which is used in the nuclear reactor to slow down the speed of neutrons. Generally, graphite or heavy water is used as moderator in nuclear reactor.

4. Coolant
Coolant may be defined as the substance which is used in the nuclear reactor to take out the heat produced during fission of U-235 nuclei. The heat energy produced during fission of U-235 in the reactor is absorbed by the coolant and passed on to the water in heat exchanger to produce steam. Usually, heavy water or liquid sodium is used as coolant in nuclear reactors.

5. Protection Shield
The whole nuclear reactor is protected by enclosing it with a 2 to 2.5 meters thick wall of concrete. This is done to avoid the harmful effects of radiations emitted during fission reaction on the person working on the reactor.

Working of Nuclear Power Plant
The working of a nuclear reactor is based on the principle of controlled fission reaction of nuclear fuel. To start the fission reaction, the control rods of cadmium or boron are pulled outward after putting the fuel rods of U-235 in the nuclear reactor. The control rods are pulled outward up to a certain length so that they absorb desired number of neutrons and keep the fission reaction under control. In this way, a manageable amount of energy is produced in the form of heat at a steady rate.

The heat energy produced in the nuclear reactor is taken out by circulating liquid sodium through the pipes embedded in the reactor. The hot liquid sodium coming out is then passed through heat exchanger to give the heat absorbed by it to water and to produce steam. This steam is passed into turbine chamber with high pressure to rotate the turbines which are connected to generators. In this way, electricity is produced. The used steam from steam chamber is converted into water by passing it through a condenser. This water is then used again to form steam by sending it back into the heat exchanger.

Test Your Understanding and Answer These Questions:

1. What is nuclear power plant?
2. What is nuclear fuel?
3. What is enrichment of uranium?
4. Name control rods of a nuclear reactor.
5. Give name of moderator in nuclear reactor.
6. Give name of a coolant in a nuclear reactor.

The process of nuclear fission can be understood on the basis of liquid drop model proposed by Yakov Frenkel, Neils Bohr and John Wheeler. In this model, nucleus of U-235 is treated as a positively charged, non-compressible drop of a liquid which is enclosed by a thin elastic membrane. When the nucleus is in stable state, it is spherical in shape. Inside this nucleus two types of forces are operating. These forces are called electrostatic force and nuclear force. The electrostatic force is the force which is exerted by one charge particle on other charged particle. As we know that positively charged protons are present in the nucleus, so every proton present in the nucleus tends to repel other protons due to electrostatic force of repulsion. But the electrostatic force of repulsion is balanced by nuclear force of attraction between the constituents of the nucleus.
Now, when a slow moving neutron hits the nucleus of U-235, it disturbs the balance of forces in the nucleus. Due to collision of neutron with nucleus of U-235 the magnitude of electrostatic force of repulsion increases which results in breaking the nucleus into small fragments with emission of a large amount of energy.

Test Your Understanding and Answer These Questions:

1. Who proposed liquid drop model of nuclear fission?
2. What is nuclear theory?
3. What is definition of electrostatic force?
4. What is definition of nuclear force?

 If the nuclear reaction is made to take place in a controlled manner, then the energy released during this reaction can be used for peaceful purposes. A nuclear reaction can be controlled by using rods of cadmium and boron. We know that when a slow moving neutron hits a nucleus of uranium-235 atom, it produces smaller nuclei of barium and krypton, three neutrons and a lot of energy. Now, let’s remove two neutrons out of three neutrons produced by absorbing them in rods of cadmium or boron. Then we will be left with only one neutron, which can cause the fission of only one more uranium atom and release small amount of energy. Now, if we continue to remove two neutrons at each step then only one uranium atom will undergo fission at a time. This reaction is also known as critical fission reaction. The energy produced during controlled fission reaction can be used to produce electricity in nuclear power stations.

Test Your Understanding and Answer These Questions:

1. What is controlled nuclear fission reaction?
2. What is critical fission reaction?

We have already discussed that when a slow moving neutron hits a nucleus of U-235, three neutrons are produced. These three neutrons can strike with three more nuclei of U-235 to produce 9 neutrons, which in turn can bring about fission of nine nuclei of U-235 to produce 27 neutrons and so on. Thus, a continuous chain of nuclear reactions called nuclear chain reaction will be set up which will lead to production of a large amount of energy. If this reaction is allowed to continue endlessly, a very large amount of energy will be produced in a very short time which will lead to explosion. That is why; this reaction is used to make highly destructible atom bombs. This reaction can be represented as:

Test Your Understanding and Answer These Questions:

1. What is nuclear chain reaction?
2. Draw diagram of nuclear chain reaction.

Nuclear fission is the process of splitting of a heavy nucleus into a number of lighter nuclei with the liberation of a large amount of energy. For example, when uranium 235 is bombarded with slow moving neutrons, it breaks up into two light-weight atoms barium 141 and krypton 92 with emission of three neutrons and a great amount of energy. This reaction can be represented as:

   ₉₂U²³⁵  +  ₀n¹      ₅₆Ba¹⁴¹  +  ₃₆kr⁹²  +  ₃₀n¹  +  Energy
Uranium    Neutron                           Barium    Krypton   Neutrons

The energy produced during nuclear fission (splitting of an element) is used to produce electricity. It should be noted that a large amount of energy is produced during nuclear fission reaction because some mass disappears in this process i.e. sum of the masses of products formed in nuclear fission is slightly less than the sum of masses of reactants. This loss of mass is converted into energy. The loss of mass of reactants is called nuclear decay.

The energy released in the nuclear reaction is expressed in the unit of Million

electron Volts (MeV). One MeV is equal to 1.602 × 10^-13 J. On an average about 200 MeV energy is released during fission of one nucleus of U-235.

Test Your Understanding and Answer These Questions:

1. What is nuclear fission?
2. What is nuclear decay?
3. What is splitting of an atom?
4. What amount of energy is released during nuclear fission reaction?

At present we are aware about 118 elements. Out of these 118 elements, first 92 elements i.e. the elements up to uranium are found in nature. This means that elements from Hydrogen (having atomic number 1) to Uranium (having atomic number 92) are naturally occurring elements. While the elements which come after uranium, i.e. the elements which have more atomic number (Z > 92) as compared to uranium have been prepared in laboratory by artificial transmutation. Such elements which have higher atomic number as compared to uranium are called transuranium elements or alternatively we can say that the elements which come after uranium in the periodic table are called transuranium elements. For example, neptunium, plutonium, americium etc. have atomic number more than uranium so all of these elements are called transuranium elements.

Test Your Understanding and Answer These Questions:

1. What are transuranium elements?
2. Name some naturally occurring elements.