Naturally occurring radioactive isotopes are either primordial (e. g. 40K, 238U or 232Th; present from the creation of earth), including their decay products, or cosmogenic (formed by cosmic rays).4,29 Primordial radionuclides and the

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decay products of U and Th represent the more significant problem with regard to environmental contamination,30 but concentrations vary significantly in the environment and depend on local geology.4 Migration of and exposure to naturally occurring radionuclides can be significantly enhanced by industrial activities, such as mining and mineral processing, in particular in production of phosphate,31,32 oil production and combustion of coal (which contains trace

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quantities of radionuclide) in power stations. , , The radionuclide concentrations in different types of coals range from 12-435 Bq kg 1 for 238U; 21-309 Bq kg 1 for 226Ra; 7.5-56 Bq kg 1 for 232Th and 6-398 Bq kg 1 for 40K.34 When coal fuel is burnt in power plants the ashes generated are enriched in metals and radionuclides. The amount of ash released into the atmosphere from coal-fired power plants can vary from 10% in an old plant, to 0.5 % in modern emission-controlled power plants.35 In addition, coal burning also releases radon into the atmosphere.

Although globally the release of naturally occurring radionuclides through industrial activities is a relatively minor source of contamination, compared to civil and military nuclear programmes, it can still result in local elevated levels of contamination. Flues et al. (2002)35 found a one — to three-fold increase in the natural radionuclide concentrations (232Th, 226Ra and 210Pb) within a 1 km distance of a 10 MWe coal-fired power plant, and elevated radon levels have been reported in and around coal power plants but the dose is less than the recommended occupational exposure limit.36

1.2 Accidental Release

Accidental release of radionuclides from nuclear facilities or from other sources (e. g. industrial or medical) is a less significant source of radioactive contamina­tion in the environment and the largest releases have been due to the accidents at Chernobyl and Fukushima.4 At Chernobyl, 1.76x 1018 Bq of 131I and 8.5x 10 Bq of 137Cs were released into the atmosphere;4 at Fukushima preliminary calcula­tions estimate that 1.5×1017 Bq of 131I and 1.2×1016 Bq of 137Cs were released between 11th March and 5th April 2011 (ref: Japan Nuclear Safety Commission, press release 12th April 2011). Nuclear accidents are discussed in detail in Chapter 3. From other sources, one of the most serious incidents occurred at Goiania, Brazil, in 1987. A radiation source from a cancer therapy machine was scavenged from an abandoned clinic and sold to scrap dealers, who opened the source, containing 50.9 TBq of 137Cs in the form of a luminescent powder.37,38 The material attracted a great deal of interest and was distributed to family and friends of the scrap dealers. As a result, 249 people were contaminated, with 21 suffering acute radiation sickness or radiodermatitis; four people died and another six were in a serious condition. Seven sites covering 5000 m2 in Goiania were found to be highly contaminated and clean up involved the demolition of houses and the construction of repositories for the waste.38