5 Nuclear Energy

Nuclear Energy

• Energies involved in conventional energy sources are of the order of electron volts per atom, but energy involved in nuclear processes are million times larger.

Fission

• When neutrons bombard various elements, new radioactive elements are produced.
• When a neutron is bombarded on a uranium target, the uranium nucleus breaks into nearly equal fragments releasing great amount of energy.

• Fission of uranium does not always produce Ba and Kr. It can produce any other pair also.
• Fragment nuclei produced in fission are highly neutron rich and unstable. Therefore, they emit beta particle until they reach a stable end product.
• Enormous amount of energy released in an atom bomb comes from uncontrolled nuclear fission reaction.

Nuclear Reactor

• When undergoes fission, it produces neutron, which initiates another nucleus to undergo fission.
• When this chain reaction is controlled suitably, we can obtain a steady energy output.
• This phenomenon is used in a nuclear reactor.
• Average energy of a neutron produced in fission of  is 200 MeV. Unless slowed down, it will escape from the reactor without interacting with the uranium nuclei.

• Light nuclei called moderator is provided in the reactor along with the nuclei for slowing down fast neutrons.
• Commonly used moderators are water, heavy water (D₂O), and graphite.
• Moderator helps to increase the ratio of number of fission produced by a given generation of neutrons to the number of the proceeding generation. This ratio is called multiplication factor (K).
• The factor K should be brought close to unity to avoid explosion.
• Reaction rate is controlled through control rods made of neutron absorbing material such as cadmium.
• Safety rods are also added to reduce K rapidly.
• Core of the reactor is the site for nuclear fission.
• Core is surrounded by reflector to reduce leakage.
• Energy released in fission is continuously removed by a suitable coolant.
• Whole assembly is shielded to check harmful radiation from coming out.
• Wastes produced by nuclear reactors are hazardous.

Nuclear Fusion

• Two light nuclei fuse to form a larger nucleus, and energy is released in the process.

• Two nuclei must come close enough so that attractive short range nuclear force is able to affect them.
• When fusion is achieved by raising the temperature, it is called thermonuclear fusion.
• Fusion reaction also takes place in sun in which the hydrogen is burnt into helium.
• Reaction occurring in sun is

Four hydrogen atoms combine to form a atom with the release of 26.7 MeV of energy.

• After 5 billion years, the hydrogen burning will stop and the sun will begin to cool. The outer envelope of the sun will expand, turning it into the so called ‘red giant’.