Radionuclides in coal
Coal is largely composed of organic matter, but some trace elements in coal are naturally radioactive. These radioactive elements include uranium (U), thorium (Th), and their numerous decay products, including radium (Ra) and radon (Rn). During coal combustion most of the uranium, thorium and their decay products are released from the original coal matrix and are distributed between the gas phase and solid combustion products. Almost all radon gas present in feed coal is transferred to the gas phase and is lost in stack emissions. In contrast, less volatile elements such as thorium, uranium, and the majority of their decay products are almost entirely retained in the solid combustion wastes. Therefore, the concentration of radioactive elements in solid combustion wastes will be approximately 10 times the concentration of the original coal.
The possible risk from radiation and the prediction of the mobility of radioactive elements during the coal fuel-cycle, depends as well on the distribution as on the form of radioactive elements in coal and fly ash: most uranium in coal is found in both the mineral and organic fractions of coal. In fly ash, the uranium is more concentrated in the finer sized particles, more particularly within the glassy component of fly ash particles.
Radioactive elements from coal and fly ash may come in contact with the general public when they are dispersed in air or water, when they are dispersed on cultivated soils, or when they are included in commercial products that contain fly ash (concrete building products, disposal in engineered surface impoundments and landfills, …). The leachability of radioactive elements from fly ash in disposal sites is a potential for groundwater contamination. Leachability of radioactive elements is critically influenced by the pH that results from reaction of water with fly ash. Extremes of either acidity (pH<4) or alkalinity (pH>8) can enhance solubility of radioactive elements.
Moreover, industrial products based on coal waste material will also be enriched in radioactive elements.

In many coals, the concentration of uranium is low. In Europe, the average specific activity in coal is generally around 20 (5-300) Bq.kg^-1 for both U-238 and Th-232. Activity levels in lignite (brown coal) are generally much lower. In general, the radionuclide enhancement factor for uranium and thorium in the ash is about 10. Consequently, the activity in ash remains rather low, as is the impact on the public. The leaching from fly ash is low and therefore there are hardly any restrictions for landfill and road construction. The use of fly-ash for buildings leads to an additional radiological burden by direct radiation and by exhalation of radon. Dumping may increase the radiation level around the dumping site. The most significant exposure route identified is from the re-suspension of ash from disposal sites and the radionuclides - Th-230, Th-232, Po-210 and Pb-210 - are the most significant in the ingestion and inhalation exposure pathways. The operation of coal-fired plants over several decades is reported to result in an increase in natural background of less than 1%.

There should consequently be no concern about the impact of coal and coal combustion products when low uranium-bearing coal is used. Although enriched, the uranium and thorium in the coal by-products should not cause concern because they are mostly in insoluble forms at concentration levels similar to most soils.
The impact for health of population or environment from the presence of radionuclides in the industrial wastes of industrial plants consuming coal – as well as in the recycled products - is of concern when coal is used that contains appreciable amounts of uranium bearing minerals as accessories. This is the case in the Western Balkans countries. For example, in Croatia, in the dump of a coal-fired power plant , the U-238 and Ra-226 activity in the coal-slag and ash are up to 18640 and 6200 Bq.kg^-1, respectively. In Slovenia, in the old coal mining district of Kocevje, daily average outdoor radon levels were recorded of 80 Bq.m^-3 with hourly maxima of 150 Bq.m^-3. Indoor radon levels are up to some thousands of Bq.m^-3. Cancer statistics show the highest incidence of lung cancer in Slovenia occurring in this town.
In Bosnia-Herzegovina, the coal fired power plant of Kakanj uses coal from the coal mine of Tusnica which shows a high level of activities of U-238 and Ra-226.
In Albania, in the coal mine of Mollas in the Prisk District, the uranium content in coal grades from 60 mg/kg to 200 mg/kg. In the dumps the radiation levels are from 0.2 µSv/h to 0.4 µSv/h.