Explain Nuclear Fusion and Controlled thermonuclear fusion

Definition of Nuclear Fusion :

Nuclear fusion is the process in which two light nuclei combine to form a heavier nucleus. During this process, a large amount of energy is released because the resulting nucleus is more tightly bound.

Examples of Fusion Reactions

1. Proton-Proton Reaction

¹H + ¹H → ²H + e⁺ + ν + 0.42 MeV

Two hydrogen nuclei (protons) combine to form deuterium, a positron, and a neutrino, releasing 0.42 MeV energy.

2. Deuterium-Deuterium Reaction

²H + ²H → ³He + n + 3.27 MeV

Two deuterium nuclei combine to form helium-3 and a neutron, releasing 3.27 MeV energy.

3. Another Deuterium Fusion Reaction

²H + ²H → ³H + ¹H + 4.03 MeV

Two deuterium nuclei combine to form tritium and a proton, releasing 4.03 MeV energy.

Why High Temperature is Required for Fusion?

For fusion to occur, two nuclei must come very close so that the attractive nuclear force can act between them. Nuclear force is a short-range force and acts only at very small distances.

Since both nuclei are positively charged, they repel each other due to electrostatic force. This repulsive barrier is known as the Coulomb Barrier.

Therefore, nuclei must possess very high kinetic energy to overcome the Coulomb barrier and come close enough for fusion.

Coulomb Barrier and Required Temperature

For two protons, the Coulomb barrier energy is approximately 400 keV.

Using the relation: (3/2)kT = K

The required temperature is: T ≈ 3 × 10⁹ K

Thermonuclear Fusion

When fusion is achieved by raising the temperature of the system so that nuclei have sufficient kinetic energy to overcome the Coulomb barrier, the process is called Thermonuclear Fusion.

Thermonuclear fusion is the source of energy in stars.

Energy Production in the Sun

The temperature at the interior of the Sun is approximately: 1.5 × 10⁷ K

This temperature is much lower than the estimated temperature required for fusion of average-energy protons. However, some protons possess energies much higher than the average energy, allowing fusion reactions to occur in the Sun.

The Sun produces energy through the Proton-Proton (p-p) Cycle.

Proton-Proton (p-p) Cycle

Step (i)

¹H + ¹H → ²H + e⁺ + ν + 0.42 MeV

Step (ii)

e⁺ + e⁻ → γ + γ + 1.02 MeV

Step (iii)

²H + ¹H → ³He + γ + 5.49 MeV

Step (iv)

³He + ³He → ⁴He + ¹H + ¹H + 12.86 MeV

For the fourth reaction to occur, the first three reactions must occur twice so that two helium-3 nuclei are produced.

Net Reaction of the Proton-Proton Cycle

4¹H + 2e⁻ → ⁴He + 2ν + 6γ + 26.7 MeV

Thus, four hydrogen atoms combine to form one helium atom and release 26.7 MeV of energy.

Formation of Heavier Elements in Stars

Helium is the first element synthesized in the interior of a star.

As hydrogen in the core gets depleted and is converted into helium, the core contracts under its own gravity, causing the temperature to rise.

When the temperature reaches about 10⁸ K, helium nuclei begin to fuse and form carbon nuclei.

In massive stars, fusion continues and produces heavier elements.

However, elements more massive than those near the peak of the binding energy curve cannot be produced through fusion because such reactions no longer release energy.

Future of the Sun

Present age of the Sun ≈ 5 × 10⁹ years.
The Sun contains enough hydrogen fuel for approximately another 5 billion years.
After hydrogen burning stops, the Sun will begin to cool and contract.
The outer layers of the Sun will expand.
The Sun will eventually become a Red Giant.

Controlled Thermonuclear Fusion

Scientists are attempting to reproduce the natural fusion process of stars on Earth. This process is called Controlled Thermonuclear Fusion.

The aim is to generate steady electrical power by heating nuclear fuel to extremely high temperatures.

Conditions Required

Temperature of about 10⁸ K.
Fuel exists as plasma (a mixture of positive ions and electrons).
The plasma must be confined using special techniques.

Main Challenge

No material container can withstand a temperature of 10⁸ K. Therefore, magnetic confinement methods are used to control the hot plasma.

Advantages of Controlled Fusion

Produces enormous energy.
Fuel is abundantly available.
Environment-friendly energy source.
Less radioactive waste than nuclear fission.
Potential source of nearly unlimited energy.

Key Points for Revision

Nuclear fusion combines light nuclei to form a heavier nucleus.
Fusion releases energy because the final nucleus is more tightly bound.
The Coulomb barrier opposes fusion and requires very high temperatures.
Thermonuclear fusion powers stars.
The Sun generates energy through the Proton-Proton Cycle.
Four hydrogen nuclei form one helium nucleus and release 26.7 MeV energy.
Helium is the first element synthesized inside stars.
The Sun will eventually become a Red Giant.
Controlled thermonuclear fusion is a promising future source of clean energy.

Frequently Asked Questions (FAQs)

1. What is Nuclear Fusion?

Nuclear fusion is the process in which two light nuclei combine to form a heavier nucleus with the release of a large amount of energy.

2. Why does Nuclear Fusion release energy?

Fusion releases energy because the resulting nucleus is more tightly bound and has a higher binding energy per nucleon.

3. What is the source of energy in the Sun?

The Sun derives its energy from thermonuclear fusion occurring in its core through the Proton-Proton Chain.

4. What is the Proton-Proton Chain?

The Proton-Proton Chain is a sequence of fusion reactions in which hydrogen nuclei combine to form helium while releasing energy.

5. What is the Coulomb Barrier?

The Coulomb Barrier is the electrostatic repulsive force between two positively charged nuclei that must be overcome for fusion to occur.

6. Why is very high temperature required for fusion?

High temperature provides nuclei with enough kinetic energy to overcome the Coulomb Barrier and come close enough for the nuclear force to act.

7. What is Thermonuclear Fusion?

Thermonuclear fusion is nuclear fusion that occurs at extremely high temperatures.

8. What is the temperature at the core of the Sun?

The temperature at the core of the Sun is approximately 1.5 × 10⁷ K.

9. How much energy is released when four hydrogen nuclei form one helium nucleus?

Approximately 26.7 MeV of energy is released.

10. What is Controlled Thermonuclear Fusion?

Controlled thermonuclear fusion is the artificial production of fusion reactions on Earth to generate useful electrical power.

11. What is Plasma?

Plasma is a high-temperature state of matter consisting of free electrons and positive ions.

12. Why is plasma confinement necessary?

The plasma temperature is extremely high, so it cannot be contained by ordinary materials and must be confined using magnetic fields.

13. What is the future of the Sun?

After exhausting its hydrogen fuel, the Sun will expand and become a Red Giant.

14. What is the main advantage of Nuclear Fusion?

Fusion provides enormous energy from a small amount of fuel and produces less radioactive waste than fission.

15. Can Nuclear Fusion provide unlimited energy?

If controlled fusion becomes commercially successful, it could provide a nearly unlimited and clean source of energy for humanity.