Notes On Nuclear Fusion - ICSE Class 10 Physics

The sun and the other stars that we see in the sky emit energy in the form of light and other electromagnetic radiations. These radiations are the main source of energy for all of us on the earth. Nuclear fusion reactions are responsible for the energies of the sun and the stars.


Nuclear reaction
A nuclear reaction in which lighter nuclei combine to form a heavy nucleus with the release of an enormous amount of energy is ca
lled “nuclear fusion".
In the nuclear fusion process, two lighter nuclei combine to form a heavier nucleus releasing a large amount of energy.
Example: Consider the nuclear reaction of two deuterium nuclei combining to form a light 
isotope of helium. In this fusion reaction, in addition to the isotope of helium, one neutron and 3.27 MeV of energy is also released.
12H +
12 → 23He + 3.27 MeV

In any nuclear fusion reaction, the mass of the heavier nucleus formed is less than the combined mass of the two lighter nuclei that combine.

When a light helium isotope is formed by the fusion of two deuterium nuclei the combined mass of the two deuterium nuclei is greater than the total mass of the products

Mass of the reactants > Mass of the products

Hence, there is loss of mass in the reaction and the mass loss appears as energy. According to Einstein’s theory of mass energy equivalence, E = mc2

When applied to the nuclear fusion reaction, if “m” is the mass loss in the fusion reaction then the energy released (E) = mc2
Energy is released in a nuclear fission reaction also.


Comparison between nuclear fusion reaction and nuclear fission reaction
If
nuclear fission and fusion reactions are compared, the energy released per nucleon is much higher in a fusion reaction than in a fission reaction.
Energy released per nucleon in fusion > Energy released per nucleon in fission
In fact, the energy released per nucleon in a fusion reaction is about seven times that released in a fission reaction.n

Energy released per nucleon in fusion ≈ 7 × Energy released per nucleon in a fission reaction.
Nuclear reactors based on nuclear fission harness the energy released and use it for generating electricity. 


Advantages of Nuclear reactor that works on nuclear fusion over the reactor that works on nuclear fission reaction

  • If a nuclear reactor that works on nuclear fusion were built it would be much more efficient than a reactor that works on nuclear fission. It would also have many other advantages.
  • The raw material normally used in a fission reactor is “uranium”. The availability of this mineral is limited.
  • However, a nuclear fusion reactor would use smaller elements like hydrogen as the raw material. These lighter elements are abundant on the surface of the earth.
  • Unlike nuclear fission reactions, nuclear fusion reactions don’t release any radioactive waste.
  • Hence, nuclear fusion energy would be clean and green energy.

Because of these advantages many scientists world over are working on developing a nuclear fusion reactor that can produce electricity.
But it is very difficult and the main problem is that nuclear fusion reactions can take place only at very high temperatures. In all nuclear fusion reactions, two positively charged smaller nuclei combine to form a larger nucleus.The two nuclei fuse when they are brought very close to each other. When they are very close, the strong nuclear attractive forces fuse the two nuclei into one.

But bringing the two positively charged nuclei so close is very difficult because of the coulomb repulsive force between them. In order to overcome the coulomb repulsive force, the nuclei participating in the fusion reaction must have very high kinetic energies.
The minimum kinetic energy that each nucleus must have for a successful fusion reaction is about 0.01 million electron volts.


Particle accelerator
In a laboratory environment, devices known as particle accelerators are used to accelerate charged particles.
The light nuclei are accelerated to very high speeds using particle accelerators. When they attain the minimum energy necessary, the nuclei are made to collide with each other to cause a nuclear fusion reaction.

The nuclear fusion reactions in the laboratory are useful for scientific studies. But, they would not work for nuclear fusion reactors. The nuclei participating in a fusion reaction acquire the necessary kinetic energies at very high temperatures. The approximate temperature at which the lighter nuclei can gain the required kinetic energy is of the order of 107 K. 


Thermo - nuclear reactions
Nuclear fusion reactions that take place at very high temperatures are called “thermo-nuclear reactions”.

Generating very high temperatures that can initiate a nuclear fusion reaction is the most difficult task in the development of a nuclear fusion reactor.
In a hydrogen bomb, an uncontrolled thermonuclear fusion reaction takes place. The high temperature required to start a fusion reaction is generated by blasting an atom bomb.
The most important thermonuclear reactions take place in the stars. The continuous release of energy from the stars is due to the thermonuclear reactions that occur in them regularly.
The process that produces energy in the stars can take place in sequences of fusion reactions.One such important sequence of fusion reactions is known as the “proton–proton cycle”.


Proton-proton cycle
The “proton-proton” cycle consists of three nuclear fusion reactions.
In the first nuclear fusion reaction, two protons are fused to form a deuterium. The reaction also produces two more particles positron and neutrino.

11H + 11H    → 12H   ++10 e + 00v

In the second fusion reaction, the deuterium is involved in another fusion reaction with a hydrogen nucleus and produces a lighter isotope of helium.

12H + 11H → 23He 

In the third reaction, two such lighter isotopes of helium collide and fuse to form a regular helium isotope and two protons.

23He  +  23He  → 24He + 211H

To sum up, in a proton-proton cycle, four protons fuse to form a helium nucleus, two positrons and one neutrino. In addition, there is a huge release of energy to the extent of 26.7 MeV

11H → 24He + 2 +10 e + 00v +  26.7 MeV



These thermonuclear reactions power the sun and the other stars.

Summary

The sun and the other stars that we see in the sky emit energy in the form of light and other electromagnetic radiations. These radiations are the main source of energy for all of us on the earth. Nuclear fusion reactions are responsible for the energies of the sun and the stars.


Nuclear reaction
A nuclear reaction in which lighter nuclei combine to form a heavy nucleus with the release of an enormous amount of energy is ca
lled “nuclear fusion".
In the nuclear fusion process, two lighter nuclei combine to form a heavier nucleus releasing a large amount of energy.
Example: Consider the nuclear reaction of two deuterium nuclei combining to form a light 
isotope of helium. In this fusion reaction, in addition to the isotope of helium, one neutron and 3.27 MeV of energy is also released.
12H +
12 → 23He + 3.27 MeV

In any nuclear fusion reaction, the mass of the heavier nucleus formed is less than the combined mass of the two lighter nuclei that combine.

When a light helium isotope is formed by the fusion of two deuterium nuclei the combined mass of the two deuterium nuclei is greater than the total mass of the products

Mass of the reactants > Mass of the products

Hence, there is loss of mass in the reaction and the mass loss appears as energy. According to Einstein’s theory of mass energy equivalence, E = mc2

When applied to the nuclear fusion reaction, if “m” is the mass loss in the fusion reaction then the energy released (E) = mc2
Energy is released in a nuclear fission reaction also.


Comparison between nuclear fusion reaction and nuclear fission reaction
If
nuclear fission and fusion reactions are compared, the energy released per nucleon is much higher in a fusion reaction than in a fission reaction.
Energy released per nucleon in fusion > Energy released per nucleon in fission
In fact, the energy released per nucleon in a fusion reaction is about seven times that released in a fission reaction.n

Energy released per nucleon in fusion ≈ 7 × Energy released per nucleon in a fission reaction.
Nuclear reactors based on nuclear fission harness the energy released and use it for generating electricity. 


Advantages of Nuclear reactor that works on nuclear fusion over the reactor that works on nuclear fission reaction

  • If a nuclear reactor that works on nuclear fusion were built it would be much more efficient than a reactor that works on nuclear fission. It would also have many other advantages.
  • The raw material normally used in a fission reactor is “uranium”. The availability of this mineral is limited.
  • However, a nuclear fusion reactor would use smaller elements like hydrogen as the raw material. These lighter elements are abundant on the surface of the earth.
  • Unlike nuclear fission reactions, nuclear fusion reactions don’t release any radioactive waste.
  • Hence, nuclear fusion energy would be clean and green energy.

Because of these advantages many scientists world over are working on developing a nuclear fusion reactor that can produce electricity.
But it is very difficult and the main problem is that nuclear fusion reactions can take place only at very high temperatures. In all nuclear fusion reactions, two positively charged smaller nuclei combine to form a larger nucleus.The two nuclei fuse when they are brought very close to each other. When they are very close, the strong nuclear attractive forces fuse the two nuclei into one.

But bringing the two positively charged nuclei so close is very difficult because of the coulomb repulsive force between them. In order to overcome the coulomb repulsive force, the nuclei participating in the fusion reaction must have very high kinetic energies.
The minimum kinetic energy that each nucleus must have for a successful fusion reaction is about 0.01 million electron volts.


Particle accelerator
In a laboratory environment, devices known as particle accelerators are used to accelerate charged particles.
The light nuclei are accelerated to very high speeds using particle accelerators. When they attain the minimum energy necessary, the nuclei are made to collide with each other to cause a nuclear fusion reaction.

The nuclear fusion reactions in the laboratory are useful for scientific studies. But, they would not work for nuclear fusion reactors. The nuclei participating in a fusion reaction acquire the necessary kinetic energies at very high temperatures. The approximate temperature at which the lighter nuclei can gain the required kinetic energy is of the order of 107 K. 


Thermo - nuclear reactions
Nuclear fusion reactions that take place at very high temperatures are called “thermo-nuclear reactions”.

Generating very high temperatures that can initiate a nuclear fusion reaction is the most difficult task in the development of a nuclear fusion reactor.
In a hydrogen bomb, an uncontrolled thermonuclear fusion reaction takes place. The high temperature required to start a fusion reaction is generated by blasting an atom bomb.
The most important thermonuclear reactions take place in the stars. The continuous release of energy from the stars is due to the thermonuclear reactions that occur in them regularly.
The process that produces energy in the stars can take place in sequences of fusion reactions.One such important sequence of fusion reactions is known as the “proton–proton cycle”.


Proton-proton cycle
The “proton-proton” cycle consists of three nuclear fusion reactions.
In the first nuclear fusion reaction, two protons are fused to form a deuterium. The reaction also produces two more particles positron and neutrino.

11H + 11H    → 12H   ++10 e + 00v

In the second fusion reaction, the deuterium is involved in another fusion reaction with a hydrogen nucleus and produces a lighter isotope of helium.

12H + 11H → 23He 

In the third reaction, two such lighter isotopes of helium collide and fuse to form a regular helium isotope and two protons.

23He  +  23He  → 24He + 211H

To sum up, in a proton-proton cycle, four protons fuse to form a helium nucleus, two positrons and one neutrino. In addition, there is a huge release of energy to the extent of 26.7 MeV

11H → 24He + 2 +10 e + 00v +  26.7 MeV



These thermonuclear reactions power the sun and the other stars.

Videos

Activities

Activity 1
Visionlearning.com  has created a wonderful simulation on “Nuclear Fusion”. This simulation explains nuclear fusion of hydrogen in a nuclear reactor.
Go to Activity

Activity 2
Unl.edu has created a wonderful animation on “Nuclear Fusion”. This animation shows the process of nuclear fusion in hydrogen atoms.
Go to Activity

References

Previous
Next