Uranium is the basic raw material of both civilian and military nuclear programmes.

It is extracted from either open-cast pits or by underground mining. Although uranium occurs naturally all over the world, only a small fraction is found in concentrated ores.

When certain atoms of uranium are split in a chain reaction, energy is released. This process is called nuclear fission.

In a nuclear power station this fission occurs slowly, while in a nuclear weapon, very rapidly. In both instances, fission must be very carefully controlled.

Nuclear fission works best if isotopes - atoms with the same atomic number, but different numbers of neutrons - of uranium 235 (or plutonium 239) are used. Uranium-235 is known as a "fissile isotope" because of its propensity to split in a chain reaction, releasing energy in the form of heat.

When a u-235 atom splits, it emits two or three neutrons.

When other u-235 atoms are present, these neutrons collide with them causing the other atoms to split, producing more neutrons.

A nuclear reaction will only take place if there are enough u-235 atoms present to allow this process to continue as a self-sustaining chain reaction. This requirement is known as "critical mass".

However, every 1,000 atoms of naturally-occurring uranium contain only seven atoms of u-235, with the remaining 993 being denser u-238.