Everything in the world is made up of atoms. Some atoms are unstable and change or decay, until they become more stable, emitting their surplus energy as radiation in the process. This is radioactivity.

Radioactivity has always been a part of our environment. The substances in the world around us all contain unstable atoms that emit energy as radiation when they decay to more stable atoms. Such radioactive atoms are also called radionuclides. The air that we breathe, the water we drink, the food we eat, and our own bodies all contain radionuclides. Many radionuclides were formed long before our own solar system came into existence, while some are continually being created in the Earth’s atmosphere by the action of cosmic rays.

Although radioactivity is a natural phenomenon which is almost as old as the universe itself, it has only been within the past 100 years that we have discovered it and given it a name. Man's use of radioactive material, for example in power generation and nuclear weapons, has added to the natural levels of radiation in our environment.

When living cells are exposed to radiation, they absorb some of its energy and can be damaged. There are internationally agreed procedures for assessing the effects of radiation on human health and for limiting radiation exposures. Around 85 per cent of our average annual radiation dose comes from natural sources, with 14 per cent from medical procedures. The remaining 1% comes from a combination of man-made sources. (See the page on monitoring of radioactivity for an explanation of ‘dose’.)

Radioactive materials are used in industry for a variety of purposes, including:

• measuring the thickness and integrity of structures and materials
  
• preserving food
  
• diagnose disease and injury and, in larger doses, to treat diseases like cancer
  
• defence, in nuclear weapons and submarine propulsion.
  
  
  
  

Radioactive waste

About 90% of the volume of radioactive waste produced is described as low level, 10% is intermediate level and about 0.1% is high level. However, although a relatively small volume, high-level waste contains over 95% of the total radioactivity of all nuclear wastes. In the UK, most low-level waste is disposed of by burying it at Drigg, a site near Sellafield in Cumbria. Some similar disposals were made at Dounreay. It is government policy that higher-level wastes should be disposed of deeper underground once a suitable repository is developed. The Nuclear Industry Radioactive Waste Executive (NIREX) is developing this proposal. High level and intermediate level waste is consequently stored at nuclear installations for the time being. In the past, some intermediate level waste was disposed of in a disposal shaft at Dounreay.

Some spent fuel from nuclear reactors is reprocessed at Sellafield, separating potentially re-usable uranium and plutonium from radioactive waste.

The four Scottish nuclear power installations dispose of around 600m³ of solid low-level waste per year, as well as making liquid and gaseous discharges. These disposals result in increased levels of radioactivity in the environment, and a very small increased exposure of the population to radiation.