Fusion Reaction

Nuclear fusion is the process through which nuclear reactions between light elements form heavier elements (up to and including iron). In cases where the interacting nuclei belong to elements with a low atomic number (such as hydrogen [atomic number equals to 1] or its isotopes which are deuterium and tritium), it releases significant amounts of energy.

Study material notes on Fission & Fusion Reactions

Fusion Reaction

A fusion reaction occurs when two or more smaller and lighter atoms fuse together, which creates a heavier and larger atom. A fusion reaction requires high temperatures and a high-density environment. Extremely high energy is needed to complete this reaction. But the energy released in fusion reactions is 3 to 4 times higher than the energy released in fission reactions.

Fusion reactions provide the basic source of energy for stars, including the Sun. Star evolution can be seen as passage through several stages, since thermonuclear reactions and nucleosynthesis cause compositional changes over long periods of time. The burning of hydrogen (H) initiates the source fusion energy of stars and leads to the creation of helium (He).

Production of fusion energy for practical use also depends on fusion reactions between the lightest elements which burn to create helium.

Fusion reactions between light elements, such as the fission reactions which split heavy elements, release energy due to a key property of nuclear matter which is termed as binding energy. Binding energy can be released by fusion or fission. The binding energy of the nucleus is a measure of the efficiency with which its component nucleons are bound.

Let us consider an element having Z protons and Neutrons in its nucleus. The atomic weight of element A is Z + N, and the atomic number of the element is considered as Z. The binding energy B is the energy related with the difference of mass between the Z protons and N Neutrons taken separately and the bound nucleons (Z + N) in a nucleus having mass M.

Thus, the binding energy is given as

B=(Z+N-Mc)²

Here,

mn= mass of neutron

mp= mass of proton

c = speed of light

Energy Released in Fusion Reaction

Energy is released in a nuclear reaction when the total mass of resultant particles is smaller than the mass of initial reactants. To determine this, we need to consider two nuclei, which are X and a, react to form two other nuclei which are Y and b and it is given as

X+a→Y+b

The particles a and b are nucleons which are protons or neutrons, but in general can be any nuclei. Now consider, no any particles are excited internally (that is, all are is their ground state), then the energy quantity termed as Q – value for this reaction and is given as

Q=mx+ma-mb-my)c²

Here,

 m = represents mass

c = speed of light

If energy value Q is positive, then the reaction is exoergic. If energy value Q is negative, then the reaction is endoergic (that is absorb energy). If both total number of proton and total number of neutrons are preserved before and after the reaction (like D-T reactions), then the Q-value is determined in terms of the binding energy B of each particle which are given as

Q=By+Bb-Bx-Ba

Fission Reaction

Fission reactions occur when neutrons are bombarded with unstable isotopes. This type of reaction is difficult to control. But the initial conditions are simple to achieve. In a fission reaction, an atom is split into 2 or more smaller and lighter atoms. A fission reaction produces highly radioactive particles.

Critical Energy

In principle, any nucleus can split when brought to a sufficiently high excited state. For fission to take place, the excitation energy for a given nuclide must be above a certain value. The minimum excitation energy needed for fission is called critical energy or threshold energy.

Conclusion

A fusion reaction occurs when two or more smaller and lighter atoms fuse together, which creates a heavier and larger atom.

The binding energy of the nucleus is a measure of the efficiency with which its component nucleons are bound.

Binding energy is given as

B=(Z+N-Mc)²

If energy value Q is positive, then the reaction is exoergic. 

If energy value Q is negative, then the reaction is endoergic.

Fission reactions occur when neutrons are bombarded with unstable isotopes. This type of reaction is difficult to control.

The minimum excitation energy needed for fission is called critical energy or threshold energy.