The sun and stars are seemingly inexhaustible sources of energy. That energy is the result of nuclear reactions, in which matter is converted to energy. We have been able to harness that mechanism and regularly use it to generate power. Presently, nuclear energy provides for approximately 16% of the world's electricity.

Unlike the stars, the nuclear reactors that we have today work on the principle of nuclear fission. Scientists are working around the clock to make fusion reactors, which have the potential of providing more energy with fewer disadvantages compared to fission reactors.

The figures show a traditional nuclear power plant, and the chamber of a Tokamak fusion reactor.

Atomic nuclei release excess energy when they have a means to do so. For example, a radioactive isotope may spontaneously release energy by undergoing radioactive decay. However, sometimes a nucleus needs an external stimulus to release energy.

Nuclear Fission (the splitting of nuclei) and Nuclear Fusion (the joining of nuclei) are nuclear processes that both result in the release of energy that is no longer needed by the resulting nucleus, after Nuclear Fission or Nuclear Fusion has occurred.

The nuclear energy that is released in a nuclear process can be calculated from the difference in mass between the original nucleus and its reaction products. Einstein’s famous energy-mass relationship, E=mc², allows us to calculate the change in the energy of nucleus ∆E, when we measure the change in the mass of the nucleus ∆m.

The term “fission” means “splitting apart”, so in nuclear fission the splitting of atomic nuclei produces typically two or three smaller nuclei. We find that when nuclear fission occurs, the mass of the reaction products is less than the original mass of the nucleus or reacting particles, resulting in the release of the energy that was used to bind the original nucleus together. This is the case of elements with heavy nuclei (such as Uranium).

In the same context, the term “fusion” means combining nuclei together. In nuclear fusion the total mass of the reaction product (also called the daughter nucleus) is still less than the original mass of the nucleus or reacting particles, even though two nuclei are now combined. This is because it takes less energy for atoms with lighter nuclei (from elements such as Helium) to exist fused together, rather than exist individually. Therefore, energy is released when fusion of lighter nuclei takes place. Nuclear fusion is more common that fission in nature and is most easily obtained using lighter elements such as Hydrogen, Helium and Carbon.

In general, if a nucleus is formed through “gluing” nucleons together, its mass is smaller than the mass of the original free nucleons. This effect is known as the mass defect.

Nuclear energy is released in the form of kinetic energy of the produced particles, and also as electromagnetic radiation (gamma rays). The high energy particles collide with atoms in the surrounding material, slowing them down as they transfer their energy to other particles they collide with. This heats up the surrounding material and is the reason that a lump of radioactive material is generally warmer than its surroundings.

The IS unit of energy, the Joule (J), is too large to measure the energy released by a single nucleus. By convention we use the MeV (million electron volt) for this, where 1MeV = 10⁶eV and 1eV = 1.602177x10^-19J.

A nuclear process that releases a large amount of energy is the fission of a heavy nucleus. For example, when a single ²³⁵U nucleus undergoes fission, about 200MeV is released. This is a lot of energy as can be seen from some comparisons:

1. the energy released when a single carbon atom is burnt in air is about 4eV (NOT MeV), about fifty million times smaller!
2. the energy released in Alpha or Beta decay is typically a few MeV
3. the energy released in nuclear fusion is of the order of 20MeV

The most significant comparison is the one between atomic or molecular energies and nuclear energies. The former are always about a million times smaller than nuclear energies. This is the reason why we can get about a million times as much energy out of Uranium as from the same weight of coal.