Nuclear Reactions: Fission, Fusion, and Radioactivity

Nuclear Fusion

Nuclear fusion is a reaction in which two light nuclei combine to form a heavier one. This process releases energy because the weight of the heavier nucleus is less than the sum of the weights of the lighter nuclei. This mass defect is converted to energy, as related by the formula E = mc². Although the mass defect is very small, and the gain per atom is also very small, it must be borne in mind that energy is very concentrated; a small amount of fuel yields a large amount of energy.

Not all fusion reactions produce the same energy; it always depends on the nuclei that bind and the reaction products. The easiest reaction to achieve is the fusion of deuterium (one proton plus one neutron) and tritium (one proton and two neutrons) to form helium (two protons and two neutrons) and a neutron, releasing 17.6 MeV of energy.

Fusion is a virtually inexhaustible energy source since deuterium is found in seawater, and tritium is easy to produce from the neutron escaping from the reaction.

Nuclear Fission

Nuclear fission is the breakup of the nucleus of an atom of higher atomic weight (e.g., uranium-235) into other lighter nuclei by bombardment with subatomic particles, such as neutrons. This process releases heat energy and more neutrons.

These neutrons released can, in turn, split other atoms and generate a chain reaction. When this process is controlled, and nuclear fission energy is released slowly in a nuclear reactor, it can be transformed into electrical energy. In contrast, if this reaction is not controlled, the energy is released instantaneously, resulting in a tremendous and violent explosion, as occurs in nuclear weapons.

Natural Radioactivity

Radioactivity is the property that certain substances, called radioactive substances, have to emit radiation capable of penetrating opaque bodies, ionizing the air, impressing photographic plates, and exciting the fluorescence of certain substances.

• Alpha radiation consists of helium nuclei composed of two protons and two neutrons.
• Beta radiation consists of fast electrons resulting from neutron decay in the nucleus, giving rise to a proton and an electron.
• Gamma radiation is electromagnetic radiation, more energetic than X-rays. Exposure to high doses of radiation increases the rate of cancer, and other disorders can cause genetic changes. The most dangerous radiation to humans when the source is outside the body is gamma radiation, and when the source is internal to the body, it is alpha radiation.

Radioactivity also has many uses in different fields: medicine, industry, chemistry, agriculture, engineering, etc.

Mass Defect and Binding Energy

Binding energy is the energy released when a number of isolated nucleons come together to form a nucleus. In this process, the nucleons lose some of their mass. The mass defect is:

The binding energy is related to the mass defect:

Soddy's Law

A helium nucleus (alpha particle), consisting of two protons and two neutrons, leaves the parent nucleus. Thus, the mass number decreases by four units, and the atomic number decreases by two units.

Fajans' Law

A neutron of the parent nucleus becomes an electron (beta particle), a proton, and an antineutrino (a particle with no mass and no electric charge). The mass number remains unchanged, and the atomic number increases by one unit.