Overview: Nuclear Fission and Nuclear Fusion : Sources of Energy

Nuclear fission is a process in which a heavy nucleus, after capturing a thermal neutron (having energy 0.027 eV), splits up into two lighter nuclei of comparable masses.

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Nuclear fission is a disinteg.ration process, in which a heavier nucleus gets split up into two lighter nuclei, with the release of a large amount of energy.

• U²³⁵ undergoes fission by slow neutrons, forming an unstable U²³⁶ nucleus which splits into two lighter nuclei, releasing 2–3 neutrons and large energy (~200 MeV).
• The energy released per fission is about 200 MeV, due to the increase in binding energy per nucleon of the fission fragments.
• Fission of 1 g of U²³⁵ releases about 8.2 × 10¹⁰ J, which is equivalent to the explosion of about 20 tons of TNT.

A nuclear chain reaction is a process in which neutrons produced in one fission reaction cause further fissions, leading to a self sustaining series of nuclear reactions.

An uncontrolled chain reaction is a chain reaction in which more than one neutron from each fission causes further fissions, leading to a rapid increase in reactions and a violent release of energy (as in a nuclear bomb).

A controlled chain reaction is a chain reaction in which, on average, only one neutron from each fission causes further fission, so the reaction proceeds at a steady rate (as in a nuclear reactor).

• Fast neutrons produced in fission are mostly absorbed by U²³⁸, so moderators like graphite or heavy water are used to slow them down and help continue the reaction.
• The uranium must have a minimum size (critical mass); otherwise, neutrons escape and the chain reaction stops.

A chain reaction once started in a fissionable material, will remain steady, decreases or increases, depends on a parameter known as multiplication or reproduction factor (K).

K = \[\frac{\text{Number of neutrons present at the begining of present generation}}{\text{Number of neutrons in the begining of previous generation}}\]

A nuclear reactor is a device in which a self-sustaining controlled chain reaction is produced in a fissionable material.

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Nuclear reactor is a device which works on the principle of nuclear fission by sustained chain reaction, to release nuclear energy at a constant rate.

When two or more very light nuclei moving at very high speeds are fused together to form a single nucleus, then the process is known as ‘nuclear fusion’.

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When two or more light nuclei combine to form a heavier stable nuclide, part of the mass disappears and is converted into energy. This phenomenon is called nuclear fusion. 

Since very high temperatures are needed for the fusion of nuclei, the process is called a 'thermonuclear reaction', and the energy released is called as 'thermonuclear energy'.

“Nuclear holocaust means large-scale destruction and devastation that would result by the use of nuclear weapons.”

Stellar energy is the energy obtained continuously from the sun and the stars.