FISSION & FUSION

Binding Energy

The nucleus of an atom  is made of protons ( + charge)  and neutrons ( No charge). Then why don’t the protons repel each other? What causes these to stick together?

In fact, the mass of the nucleus is less than the sum of the masses of the individual protons(p) and neutrons (n)  which makeup the nucleus.

The lost mass (mass defect) get changed into the energy which is necessary to bind the nucleus together. Suppose the mass defect is ∆m. Then the energy binding the nucleus together ∆E is given by:

∆E = ∆mc²    where c is the speed of light.

If  is divided it by the number of nucleons in the nucleus, then the binding energy per nucleon is obtained. The higher this value is the more stable is the nucleus.

Stable Nuclides

 Since the positive protons in the nucleus repel each other, neutrons are required to bind the nucleus together. For lighter elements, the number of protons and neutrons are approximately equal. As the number of protons increases the force required to hold the nucleus together increases and to such an extent that more neutrons are required and the ratio of neutrons / protons is greater than one. For heavier elements, the number of neutrons in the nucleus is approximately one and a half times the number of protons.

Unstable Nuclides

Whenever a   β -particle is emitted a neutron is lost and a proton gained. Therefore the neutron/proton ratio is reduced by   β- radiation.

When an  α -particle is emitted the nucleus loses two protons and two neutrons. Since there were originally more neutrons than protons it follows that the neutron/proton ratio is increased.

Nuclear Fission

Radioactive elements can undergo a number of different reactions in the process of forming a stable element. Nuclear fission is one such reaction. Fission = To divide. Elements having atomic numbers greater than 90 can undergo fission. Uranium is one such element. In nuclear fission, the nucleus of an atom splits up.

In other words, heavy unstable nuclides can be split to produce energy in a process called nuclear fission. When uranium decays naturally  α - particles and  β - particles are emitted. However, when uranium - 235 is bombarded by slow moving neutron it forms uranium – 236and turns its nucleus unstable. Uranium - 236 breaks down, splitting into two large particles and emitting three neutrons. When the exact masses of the final products are added , the sum is found to be appreciably less than the sum of the exact masses of the uranium - 235 and the original neutron. This difference in mass ∆m appears as energy given by

 ∆E = ∆mc²    where c is the speed of light.

 

Fission in uranium atoms may result in a chain reaction. A nuclear chain reaction is a series of rapid nuclear fissions. A small sample of uranium contains billions of atoms. When one U- 235 nucleus is split by a neutron, it releases three neutrons. These neutrons are used to split three more uranium nuclei. Each nuclei, causing them to split. The fission of nuclei and release of neutrons becomes a chain reaction. In a chain reaction, billions of fission reaction may occur per second.

NUCLEAR FUSION

When lighter nuclides fuse together to form a heavy nucleus the process is called nuclear fusion. In this energy is produced and mass is lost. For example, two atoms of heavy hydrogen may fuse together to form helium and a neutron:

The sum of the exact masses of the helium atom and the neutron is less than the sum of exact masses of the two heavy hydrogen atoms. This lost mass is released as energy. It is thought that the Sun's energy is produced by nuclear fusion.

Nuclear fusion is the opposite of nuclear fission. Elements with small masses combine to form elements with larger masses.

For nuclear fusion to occur, temperatures even more than one million degree Celsius must be reached. Nuclear fusion is called thermonuclear reaction. At the tremendous temperature of thermonuclear reactions, nuclear atoms no longer exist. The atoms lose their electrons (ionize) and become plasma. Then nuclei have enough energy to overcome forces of repulsion between them.

The temperature conditions for fusion exists in the sun and stars. The sun has an internal temperature of about 20,000,000^oC. In the sun, fusion occurs through a complex series of nuclear changes. During the early life of a star, four hydrogen nuclei are fused into one helium nucleus in a series of steps.

The sun is constantly losing hydrogen as it forms helium. A tremendous amount of energy is released during this fusion. The energy comes from matter that is converted to energy. The helium that is formed has a mass almost one percent less than the mass of the four hydrogen atoms. This one percent mass is converted to energy.