The effects of an accident may reach a wide area only if a large amount of radioactive gaseous substances and radioactive particles are released into the air.
Radioactive substances are transported in the air along with the wind. The wind velocity determines the speed of the cloud. The wind direction determines the area to be contaminated. Weather determines when and where it is necessary to shelter. People should stay indoors when the cloud moves over their area. In this way they would avoid inhaling radioactive air and being exposed to the direct radiation caused by the cloud. If people act effectively, they will have enough time to protect themselves. For example, if the wind velocity is 9 metres per second, it will take the cloud 3 hours to advance 100 kilometres.
The radioactive cloud does not continue its journey indefinitely. As the cloud keeps expanding, its radioactivity decreases. You cannot see or smell a radioactive cloud in the air. It can be detected solely with a radiometer.
When the cloud has passed the area, there are no longer radioactive substances in the air. Instead, there will be fallout on the ground and on the surfaces of buildings. There may be great local differences in the amount of fallout. For instance, rain increases the number of particles falling on the ground. In the most contaminated urban areas it will be necessary to wash the roofs and walls of buildings, among other things. Inhabitants may be requested to move elsewhere while decontamination is carried out.
It can take a long time before the radioactive substances remaining in the environment have disappeared. Fortunately, the concentration of radioactive substances in the environment decreases considerably during the first year. In order to limit the radiation dose, it is ensured that the foodstuffs for sale are clean. If necessary, restrictions on the consumption of mushrooms and berries are imposed.
Health hazards of radiation
An unprotected person at the site of the accident or in close neighbourhood may in a short time receive such a high radiation dose that it has an immediate effect on the person's health. A high radiation dose destroys a lot of cells. It may cause a radiation sickness. In a severe nuclear power plant accident, there would be symptoms within some twenty-kilometre radius of the nuclear power plant, at the most. As a consequence of the use of a nuclear weapon, unprotected people can get radiation sickness as far as within a few hundred-kilometre radius from the location of the nuclear explosion.
Several years after a severe accident, an increase in cancer cases and possibly in hereditary disorders can be discovered. Even in the severest accidents, health hazards can be avoided if appropriate measures against radiation are taken.