All solids, liquids and gases are composed of less than one hundred different chemical elements such as carbon, oxygen, iron and aluminum. The smallest unit of each element that still retains the characteristic properties of that element is an atom . Atoms are so small that a single airborne dust particle, that could be seen only under a powerful microscope, contains over a trillion atoms. Hydrogen, the lightest atom, and uranium, the heaviest that exists naturally to any appreciable extent, are both very important to nuclear energy.
Not all atoms of a given element are identical. For instance, hydrogen can exist as three isotopes, ordinary hydrogen, deuterium and tritium with respective masses of one, two and three atomic units. The different isotopes of other elements do not have distinct names but are identified by their masses. Thus the hydrogen isotopes would be hydrogen-1, hydrogen-2 and hydrogen-3. Uranium, as found in nature, consists of 99.28 per cent of uranium-238 and 0.71 per cent of uranium-235, with only very small amounts of other isotopes.
Differences in mass between the isotopes have virtually no effect on the atom's chemical properties. For example, all three hydrogen isotopes burn in air to form water. Some small differences in common physical properties can be detected, but these are generally of little consequence. A given volume of water produced from deuterium is about ten per cent heavier than the same volume of water from ordinary hydrogen, and is therefore known as heavy water compared with the ordinary light water . The weight difference between the uranium isotopes is only about one per cent, so that there is little noticeable effect on most physical properties. However, there can be very marked differences between isotopes in their nuclear properties, which determine how they behave in a nuclear reactor.
When an atom of uranium-235 is hit by a neutron (a sub-atomic particle that is one of the components of all atoms except hydrogen) there is a high probability of a violent reaction, but if the atom is one of uranium-238 the probability is very low. The reaction is known as nuclear fission or splitting of the atom, because the uranium atom splits into two lighter atoms, releasing energy. The two light atoms, the fission products , can be any one of about twenty atom pairs, such as iodine and silver. The uranium-235 is said to be fissile (or fissionable ).
When formed, the fission-product atoms contain excess energy, most as motion which rapidly becomes heat, and some of which is emitted over time as radiation , i.e., the atoms are radioactive . In this process, radioactive decay, individual atoms can change from one element to another, and the changes can continue until a stable element is reached. Although the decay of any given radioactive atom ( radioisotope ) is a random event, so that when the atom will decay is unpredictable, the average life of a very large number of such atoms can be measured precisely. Each radioisotope has a characteristic half-life , varying from a fraction of a second to billions of years, in which half of the original atoms present will decay: in each successive half-life half of the remaining atoms will decay. Consequently, only one thousandths (1/2 x 1/2 x 1/2 ... ten times) of the original amount of any radioactive material will remain after ten half-lives.
In addition to the fission products, two or three neutrons are emitted when an atom fissions. If one of these causes fission in another fissile atom more neutrons are emitted, one of which could possibly cause a further fission, and so on in a chain reaction . If a neutron hits a uranium-238 atom it is unlikely to cause fission. Instead, the two will probably combine and subsequently transform spontaneously into an isotope of another element, plutonium-239. Although uranium-238 is not fissile, plutonium-239 is, so uranium-238 is said to be fertile . Further neutrons can combine with the plutonium to form even heavier atoms, the transuranics , i.e., beyond uranium in ordering the elements by weight. Very small amounts of transuranics are formed but they can be significant in the details of reactor design and in the management of the wastes.
However much naturally occurring uranium ( natural uranium ) is heaped up it will not generate useful energy because there are not enough fissile atoms compared with the fertile atoms to sustain a chain reaction. The few neutrons produced are either captured by, or speed through, the much more abundant fertile atoms and so are unavailable to cause further fission.
The brute-force solution to this problem is to increase the proportion of fissile atoms artificially. One way is to enrich the uranium in uranium-235 in enrichment plants that exploit the small differences in physical properties between the two uranium isotopes. The product is enriched uranium .
A more subtle solution is to divide up the uranium into small packets that are surrounded by a moderator , a material that slows down the neutrons, that are traveling very fast when first emitted from a fissioning atom, before they hit the next packet of uranium. The reason that this solution works is that slow neutrons are much more likely to cause fission in uranium-235 than faster ones. Generally, elements with light atoms are good moderators. Ordinary water (a compound of ordinary hydrogen and oxygen) is good, but not good enough to sustain a chain reaction with natural uranium. Very pure graphite (carbon) is better, but the best is heavy water (a compound of deuterium and oxygen). Deuterium is present in all naturally occurring hydrogen, e.g., in ordinary water, to the extent of about one part in ten thousand. Heavy water is produced by enriching the deuterium content of natural water in heavy water plants .
Nuclear energy is best known as a means of generating electricity in large plants. Less well known is the fact that radioisotopes are widely used in medicine and industry.
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