Radioactive materials are used for therapeutic, industrial and research purposes as a result, there is always the possibility of inadvertent exposure of humans. There is no doubt that organizations using and/or transporting radioactive materials have high safety standards.

In addition to exposure to radiation during medical procedures, the most likely source of significant exposure to radioactive material is discarded or stolen sources whose dangerous properties are not recognized by the perpetrators.

Such accidents usually come to light when a radioactive source is found to have been mislaid. Prompt notification to the appropriate authorities and the use of the mass media should make the community aware of the danger. Sometimes the first indication of accidental exposure is the appearance of people with radiation injuries, e.g. radiation burns. The fact that radiation cannot be perceived by the senses, and that its first symptoms are very unspecific, compounds the problem. 

The other possibility that can lead to overexposure of persons to ionizing radiation during peacetime are accidents in nuclear power plants or research institutions dealing with radioactive materials; and undue exposure to radioactive waste or radioactive source used in industry, medicine and research laboratories. More recently, a threat of overexposure of populations to radiation due to terrorism involving nuclear facilities or the theft of radioactive substances, has become more prominent.

The medical and health response to a radiation emergency depends on its magnitude. International Nuclear Event Scale (INES) is used to inform the public about the severity of events at nuclear facilities. The scale has eight levels and are explained below.

Level 0 (deviation): An event with no safety significance.

Level 1 (anomaly): An event beyond the authorized operating regime, but not involving significant failures in safety provisions, significant spread of contamination or overexposure of workers.

Level 2 (incident): An event involving significant failure in safety provisions, but with sufficient defense in depth remaining to cope with additional failures, and/or resulting in a dose to a worker exceeding a statutory dose limit and/or leading to the presence of activity in on-site areas not expected by design and which require corrective action. 

Level 3 (serious incident): A near accident, where only the last layer of defense in depth remained operational, and/or involving severe spread of contamination on-site or deterministic effects to a worker, and/or a very small release of radioactive material off-site (i.e. critical group dose of the order of tenths of a mSv).

Level 4 (accident without significant off-site risk): An accident involving significant damage to the installation (e.g. partial core melt), and/or overexposure of one or more workers resulting in a high probability of death, and/or an off-site release such that the critical group dose is of the order of a few mSv. 

Level 5 (accident with off-site risk): An accident resulting in severe damage to the installation and/or an offsite release of activity radiologically equivalent to hundreds or thousands of TBq of 131I, likely to result in partial implementation of countermeasures covered by emergency plans. e.g. the 1979 accident at Three Mile Island, USA (severe damage to the installation), or the 1957 accident at Windscale, UK (severe damage to the installation and significant off-site release). 

Level 6 (serious accident): An accident involving a significant release of radioactive material and likely to require full implementation of planned countermeasures, but less severe than a major accident. e.g. the 1957 accident at Kyshtym, USSR (now in Russian Federation) 

Level 7 (major accident): An accident involving a major release of radioactive material with widespread health and environmental effects. e.g. the 1986 accident at Chernobyl, USSR (now in Ukraine).

In the case of a nuclear accident, the level of radiation hazard for the population depends upon the quantity and principle radionuclides released into the environment, the distance of the populated areas from the source of radioactive release, the type of buildings and the density of population, meteorological conditions at the time of the accident, season of the year, character of agricultural development in the area, water supplies and nutritional habits and status of the population.

In nuclear reactor accidents involving the release of radioactive material into the atmosphere, the following routes resulting in radiation injury to the population are expected:

1. external gamma irradiation brought by the passing radioactive cloud;
2. internal irradiation from inhaling radioactive aerosols (inhalation hazard);
3. contact radiation due to deposition of radioactive fallout on the skin and clothes;
4. total external gamma irradiation of the population due to deposition of radioactive fallout on the soil and local objects (buildings, constructions etc. ); and
5. internal irradiation resulting from water consumption and local food products contaminated by radioactive substances.

During a major nuclear accident, the following three phases are identified:

a)    Early Phase – from the threat of a serious release to the first few hours after the beginning of a release;

b)    Intermediate phase – from the first few hours after the start of the release to one or two days; and

c)    Recovery Phase – may extend from some weeks to several years.

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