## Nuclear Fusion and Differences between Fission and Fusion

Subject: Physics

#### Overview

Nuclear fusion is a process in which two or lighter nuclei combine or fuse together to form a heavier and stable nuclide. This note provides us an information on nuclear fusion and differences between fission and fusion.

#### Nuclear Fusion

Nuclear fusion is a process in which two or lighter nuclei combine or fuse together to form a heavier and stable nuclide. The difference in mass (Dm) resulting in the process is converted into energy according to Einstein’s mass-energy relation DE = Dmc2 which is released during fusion reaction. Example: The fusion of deuterium nuclei into a helium nucleus.

$$_1H^2 + _1H^2 \rightarrow _2He^4 + \text {energy}$$

The fusion Reaction

Fusion reactions constitute the fundamental energy source of stars, including the sun. Hydrogen burning initiates the fusion energy source of stars and leads to the formation of helium. Generation of fusion energy for practical use also relies on fusion reactions between the lightest elements that burn to form helium. In fact, the heavy isotopes of hydrogen-deuterium and tritium react more efficiently with each other, and when they do undergo fusion, they yield more energy per reaction than do two hydrogen nuclei.

Reaction Energy

The difference between the masses before and afte1\20r the reaction corresponds to the reaction energy, according to the mass-energy relation. If initial particles A and B interact to produce final particles C and D, the reaction energy Q is defined as

$$Q = (M_A + M_B – M_c – M_D)c^2$$

When Q is positive, total mass decreases and the total kinetic energy increases which is called an exoergic reaction. When Q is negative, total mass increases and the total kinetic energy decreases which is called an endoergic reaction.

#### Differences between Fission and Fusion

 S.N. Fission Fusion 1. Fission involves breaking up of a heavy nucleus into lighter nuclei. Fusion involves combining of two lighter nuclei into a heavy nucleus. 2. Energy released per fission is about 200 M ev. Energy released per fusion is about 24 Mev. 3. Energy released per nucleons » 0.8 MeV. Energy released per nucleons » 0.6 MeV. 4. No restriction for temperature. High temperature of the order of 107 K is needed. 5. Products of fission are radioactive. Products of fusion are not radioactive. 6. Nuclear reactor is based on controlled fission chain reaction. No controlled fusion discovered so far. 7. Uncontrolled chain reaction results in the atom bomb. Uncontrolled fusion reaction results in H-bombs. It is about 1000 times powerful than the atom bomb. 8. For fission chain reaction critical mass in needed. No limitation of critical mass/size of the fusible material. 9. The source fission is limited. Almost unlimited source. 10. Fuel is either in a solid or liquid state. Fuels are in the plasma state. 11. Fission is induced by neutrons. Fusion is induced by protons. 12. It is single stage reaction. It is a multistage reaction. 13. Fuel can be stored for any length of time. Fuel cannot be stored as no container is available for such high temperature.

Thermonuclear Reactions

The process of nuclear fusion requires a very high temperature, the nuclear fusion reactions are called thermonuclear reactions and the energy released is termed as the thermonuclear energy.

Controlled Thermonuclear Reactions

The fusion reaction can be controlled and can be used for the constructive purpose. Nuclear fusion reaction requires high temperature and ordinary metallic container vaporizes under the bombardment of such high temperature. So for controlled fusion especial container is designed to hold high temperature. At high temperature during fusion, electrons collide with each other violently and result into fully ionized gas called plasma. The plasma is an assembly of light nuclei like protons, deuterons, tritium and electrons moving rapidly in all directions. Since ordinary material wall cannot hold such plasma at a high temperature, attempts have been made to design a special type of magnetic bottle to hold such plasma matter. In such a process, the plasma is compressed and a very high temperature suitable for nuclear fusion is expected to be developed.

Nuclear fusion is suitable due to following ways:

1. Hydrogen is available everywhere on this planet in various forms.
2. The energy released per unit mass is very high.
3. It leaves no radioactive waste.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, Bhesha Raj Adhikari, Arjun Kumar Gautam, Parashu Ram Poudel.Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.

##### Things to remember

Fission involves breaking up of a heavy nucleus into lighter nuclei.

Nuclear fusion is a process in which two or lighter nuclei combine or fuse together to form a heavier and stable nuclide.

The difference between the masses before and afte1\20r the reaction corresponds to the reaction energy, according to the mass-energy relation.

The process of nuclear fusion requires a very high temperature, the nuclear fusion reactions are called thermonuclear reactions.

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