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Uranium in its state is found in quantities that average about 4 grammes to every ton of rock. It is an extremely process to extract this mineral, from the rocks that contain it even when the deposits are relatively rich.

The best material for nuclear fission is Uranium 235, but natural uranium has only one atom of this structure for every has been extracted from rocks, the element has to be further processed to get the portion with the atomic structure needed for nuclear reactors.

Once the atomic reaction has been set in motion, the energy which is released mostly takes the form of heat. This heat led to a type of boiler where it generates steam that is later put to several uses. One kilogramme of uranium yields as much energy as 3 million kilogrammes of coal.

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We speak of nuclear reaction whenever the nucleus of an atom undergoes any change in its properties any change in properties. For example, this could be the loss of one or more protons or other particles from within the nucleus, which in turn is possibly caused by the impact of other particles. In nature this process can take place spontaneously in certain substances and gives rise to radioactivity.

Radioactivity was discovered in 1896 by the French scientist Henri Becquerel who proved that pitchblende, a mineral that contains uranium, could darken photographic plates even if they were wrapped in dark paper. It became evident to Becquerel that a very penetrating from of radiation was involved.

We now know that this radiation consists of alpha particles and that radioactive materials also give out two other types of radiation: beta and gamma. Alpha particles are not very powerful and they can be stopped by a thickness of a few sheet of paper or by a few centimeters of air. Beta rays are more penetration but can be stopped by thick cardboard, a few meters of air or thin sheet metal. Gamma rays, like X-rays, are extremely penetrating and can be very dangerous to plant and animal life. To stop them several centimeters of metal thickness is needed to reduce gamma radiation to an acceptable level.

It was not simple to produce these rays artificially and it took many years of difficult research and complicated experiments. In the end the scientists succeeded. They bombarded the atoms of certain materials with particles taken from naturally radioactive material. By increasing or decreasing this bombardment, the scientists were able to break aparkthe protective shell of electrons and reach the nucleus of an atom.

In this way nuclear fission, or the splitting of the atom, was achieved. Under such bombardment to atomic nucleus splits into two smaller nuclei. As this happens, some neutrons are rejected by the splitting atomic nucleus and collide with the nuclei of neighbouring atoms. This sets off a chain reaction, releasing enormous quantities of energy which can go out of control with disastrous results.

Nuclear reactors are complicated structures in which the chain reaction from atomic fission can be set off continued and kept under control. In this way, an atom can be split without the risk of a terrible destructive explosion. Instead, the process is done gradually and a large amount of energy is produced.

Nuclear reactors are fuelled in different ways. Nuclear fuel must always be substances which cab set off a chain reaction when bombarded with neutrons. The most commonly used elements in fueling reactors are uranium, plutonium and thorium.

At the heart of the reactor there is the moderator which is a substance that slows down the speed of the neutrons and regulates their flow. The reactor is called fast if it uses fast neutrons and thermal if the neutrons have been slowed down, thereby transferring much of their energy to the moderator.

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Everything is made up of atoms which are the smallest parts of an element still possessing the chemical properties of that element. It is difficult to realize how small atoms are. They have a diameter of about one-hundred-millionth of a centimeter. At one time scientists believed atoms were little spheres that could not be broken, but we know that atoms are composed of other particles which are even smaller. Each atom is like a miniature solar system: at the centre it has a nucleus which consists of protons and neutrons around which electrons revolve.

The atom consists almost entirely of empty space and its entire size is that of the orbit of its outer electron, which revolves at extremely high velocity, forms an impenetrable shield. A propeller going round very fast will give us an idea of an electron. The electron seems to be at every point of its orbit at the same time because it goes round the nucleus so fast. That is why we say the atom consists mostly of empty space. The spherical shield formed by the revolving electrons prevents the emptiness between their orbits and the nucleus from being filled in normal circumstances.

The nucleus and the electrons each have a diameter of about one-tenth of a millionth part of millionth part of one centimeter. Nearly all the mass of the atom is contained within the nucleus. The electrons are very light compared with the protons and the neutrons which are 1,837 times heavier than the electrons.

Electrons have a negative charge and they are fixed to the atom and cannot break away from their orbits through centrifugal force because protons have an equivalent positive charge and the two balance each other. Neutrons have no electrical charge.

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