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Guide to UK nuclear power
Diagram showing how a pressurised water reactor works

How nuclear power works

Atomic energy is based on nuclear fission - which occurs when an atom splits releasing a large amount of energy.

Only certain types of atom are fissionable. The naturally occurring metal uranium (a heavy, unstable element) is most commonly used.

There are several different types - or isotopes - of uranium. Uranium-235 is the most easily fissionable, but only seven atoms in every 1,000 of naturally occurring uranium are U-235.

A uranium-235 atom not only splits when bombarded with a neutron or other sub-atomic particle, but also releases two or three more neutrons which go on to strike other U-235 atoms, triggering what is known as a chain reaction.

A nuclear power station facilitates this chain reaction under controlled conditions while harnessing the energy to generate electricity.

Rods of nuclear fuel - usually uranium with an enriched concentration of U-235 atoms - are positioned in the reactor core.

Fission is most likely to occur if the free neutrons are moving relatively slowly, so the rods are surrounded by a substance called a moderator which slows the neutrons.

In the most common type of modern reactor, the pressurised water reactor (above), water is used both as a moderator and also as a coolant.

As well as slowing the neutrons, the water carries the energy away from the reactor core, using the heat to generate steam which then turns turbines to generate electricity.

To maintain the reaction at a constant rate, for every atom that splits, only one neutron should be allowed to go on to strike another atom.

This is controlled using rods made of materials such as cadmium and boron, which absorb some of the neutrons released during the fission process. The control rods can be raised or lowered if more or less absorption is needed.

The reactor is surrounded by a heavy concrete shield, one of the few things which blocks radiation from escaping into the environment.

Modern reactors are designed in such a way that they shut themselves down if left to their own devices.

Constant intervention is needed to keep them operating - in contrast to the Chernobyl reactor, which required constant intervention to keep the reaction under control.

Graphic showing neutron striking Uranium-235 atom which then splits, releasing fission fragments and more neutrons, which go on to strike other U-235 atoms.
1. Neutron strikes unstable nucleus of Uranium 235
2. Nucleus splits releasing large amount of energy
3. Further neutrons are also released
4. New neutrons strike other nuclei, initiating chain reaction


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