Energy resources: An introduction to energy resources
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# 5 Nuclear energy

Einstein's famous equation E = mc 2 shows that mass (m) and energy (E) are proportional to one another. The constant c 2 linking the two is the square of the speed of light c (3 × 108 m s−1). Implicit in the equation is that mass can be converted into energy, and vice versa, although the conversion of energy into mass occurs only in very powerful particle accelerators. The conversion of matter into energy is the basis of nuclear energy. When unstable nuclei split (nuclear fission) the sum of the masses of the isotopes that are produced is slightly less than that of the original fissile isotope. That tiny mass deficit is converted into a relatively huge amount of energy, because c 2 is a very large number (9 × 1016 m2 s−2). When isotopes fuse at immense temperatures in the interiors of stars, the product again has lower mass than the original isotopes. So both nuclear fission and fusion potentially produce energy that can be exploited, but in both cases the phenomena have to be artificially induced.

The 'fuel' for nuclear fission occurs naturally in the Earth's crust in the form of unstable isotopes of uranium and thorium, added to by other isotopes that are formed artificially inside nuclear reactors. Note that uranium and thorium isotopes naturally break down by emission of various particles (helium nuclei and electrons), as a result of which their 'daughter' isotopes have lower atomic masses and numbers. Some of the 'daughter' isotopes are also unstable and undergo radioactive decay, which eventually results in the formation of several stable isotopes of the element lead. Such radioactive decay is not the same as nuclear fission and occurs at a constant pace. Radioactive decay also releases energy, but at an amount far lower per decayed atom than in nuclear fission. That energy continually heats the Earth's interior (Section 3.3) and is the source of geothermal energy.

Potential 'fuel' for nuclear fusion also occurs naturally in the form of an isotope of hydrogen that includes a neutron as well as a proton in its nucleus, instead of the single proton. This isotope (2H or deuterium) occurs in tiny amounts in water, and was produced originally by processes early in the history of the Universe.

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