Escaping radioactivity from coal-fired power plants (CPPs) due to coal burning and the associated hazards: a review

Coal, like most materials found in nature, contains trace quantities of the naturally occurring primordial radionuclides, i.e. of ⁴⁰K and of ²³⁸U, ²³²Th and their decay products. Therefore, the combustion of coal results in the released into the environment of some natural radioactivity (1.48 TBq y⁻¹), the major part of which (99 %) escapes as very fine particles, while the rest in fly ash. The activity concentrations of natural radionuclides measured in coals originated from coal mines in Greece varied from 117 to 435 Bq kg⁻¹ for ²³⁸U, from 44 to 255 Bq kg⁻¹ for ²²⁶Ra, from 59 to 205 Bq kg⁻¹ for ²¹⁰Pb, from 9 to 41 Bq kg⁻¹ for ²²⁸Ra (²³²Th) and from 59 to 227 Bq kg⁻¹ for ⁴⁰K. Fly ash escapes from the stacks of coal-fired power plants in a percentage of 3-1% of the total fly ash, in the better case. The natural radionuclide concentrations measured in fly ash produced and retained or escaped from coal-fired power plants in Greece varied from 263 to 950 Bq kg⁻¹ for ²³⁸U, from 142 to 605 Bq kg⁻¹ for ²²⁶Ra, from 133 to 428 Bq kg⁻¹ for ²¹⁰Pb, from 27 to 68 Bq kg⁻¹ for ²²⁸Ra (²³²Th) and from 204 to 382 Bq kg⁻¹ for ⁴⁰K. About 5% of the total ash produced in the coal-fired power plants is used as substitute of cement in concrete for the construction of dwellings, and may affect indoor radiation doses from external irradiation and the inhalation of radon decay products (internal irradiation) is the most significant. The resulting normalized collective effective doses were 6 and 0.5 man-Sv (GW a)⁻¹ for typical old and modern coal-fired power plants, respectively.     

Trace element and radionuclide mass balances at a coal-fired electric generating station

Coal, bottom ash, and fly ash from Milliken Station, a coal-fired 270 MW power plant, were analyzed for 20 elements (Ag, Al, As, Be, Cd, Co, Cr, Cu, Ga, Hg, Mn, Ni, Pb, Sb, Se, Sn, Te, Tl, V, and Zn) and the major natural radionuclides (²³⁸U series, ²³²Th series, and ⁴⁰K). Mass balance calculations showed that most of the Hg, and some of the Sb and Se, were unaccounted for by ash collection, suggesting their possible discharge into the atmosphere. Silver, As, Hg, Pb, Cd, and Zn were more concentrated in the fly ash than in the bottom ash while all of the other elements were equally distributed, by concentration, between the ash fractions. The radionuclides showed a 46% higher concentration in fly ash than in bottom ash, and ²¹⁰Pb was enriched in the fly ash relative to the other radionuclides by a factor of 2.4. Mass balance calculations indicated a balance within statistical error for the radionuclides, except for ²¹⁰Pb.     

Radioactive impact in sediments from an estuarine system affected by industrial wastes releases

A big fertilizer industrial complex and a vast extension of phosphogypsum piles (12 km2), sited in the estuary formed by the Odiel and Tinto river mouths (southwest of Spain), are producing an unambiguous radioactive impact in their surrounding aquatic environment through radionuclides from the U-series. The levels and distribution of radionuclides in sediments from this estuarine system have been determined. The analyses of radionuclide concentrations and activity ratios have provided us with an interesting information to evaluate the extension, degree and routes of the radioactive impact, as well as for the knowledge of the different pathways followed for the radioactive contamination to disturb this natural system. The obtained results indicate that the main pathway of radioactive contamination of the estuary is through the dissolution in its waters of the radionuclides released by the industrial activities and their later fixation on the particulate materials. Tidal activity also plays an important role in the transport and homogenization along the estuary of the radioactivity released from the fertilizer plants.     

The radiological impact from airborne routine discharges of a modern coal-fired power plant

In this paper the radiological impact from the airborne routine discharges of a modern coal-fired power plant at Langerlo (Belgium) is evaluated. Therefore, the natural radioactivity contents of the coal and the fly-ash discharged were measured. With a bi-Gaussian plume model the maximum annual values of the 226Ra concentration in the air (4.5 nBq/m3) and of the total deposition (1.5 mBq/m2) were calculated. The transfer of the radionuclides from air and soil to the biospheric media, exposing man, were modelled and the annual, individual, effective dose to the critical group, after an assumed life span of the power plant of 70 years, was evaluated at 0.05 microSv/y. This is several orders of magnitude lower than the annual doses for most power plants reported in the literature. The flue gas purification system, extended with a denitrification unit and a desulphurisation unit, was found to be the basis for this low impact.     

Radiological characteristics and investigation of the radioactive equilibrium in the ashes produced in lignite-fired power plants

Coal- and lignite-fired power plants produce significant amounts of ashes, which are quite often being used as additives in cement and other building materials. In many cases, coal and lignite present high concentrations of naturally occurring radionuclides, such as 238U, 226Ra, 210Pb, 232Th and 40K. During the combustion process, the produced ashes are enriched in the above radionuclides. The different enrichment of the various radionuclides within a radioactive series, such as that of 238U, results in the disturbance of radioactive secular equilibrium. An extensive research project for the determination of the natural radioactivity of lignite and ashes from Greek lignite-fired power plants is in progress in the Nuclear Engineering Department of the National Technical University of Athens (NED-NTUA) since 1983. This paper presents detailed results for the natural radioactivity, the secular radioactive equilibrium disturbance and the radon exhalation rate of the fly-ash collected at the different stages along the emission control system of a lignite-fired power plant as well as of the bottom-ash. From the results obtained so far, it may be concluded that 226Ra radioactivity of fly-ash in some cases exceeds 1 kBq kg(-1), which is much higher than the mean 226Ra radioactivity of surface soils in Greece (25 Bq kg(-1)). Furthermore, the radioactivity of 210Pb in fly-ash may reach 4 kBq kg(-1). These results are interpreted in relation to the physical properties of the investigated nuclides, the temperature in the flue-gas pathway, as well as the fly-ash grain size distribution. It is concluded that towards the coldest parts of the emission control system of the power plant, the radioactivity of some natural nuclides is gradually enhanced, secular radioactive equilibrium is significantly disturbed and the radon exhalation rate tends to increase.     

Transport and fate of radionuclides in aquatic environments--the use of ecosystem modelling for exposure assessments of nuclear facilities

In safety assessments of nuclear facilities, a wide range of radioactive isotopes and their potential hazard to a large assortment of organisms and ecosystem types over long time scales need to be considered. Models used for these purposes have typically employed approaches based on generic reference organisms, stylised environments and transfer functions for biological uptake exclusively based on bioconcentration factors (BCFs). These models are of non-mechanistic nature and involve no understanding of uptake and transport processes in the environment, which is a severe limitation when assessing real ecosystems. In this paper, ecosystem models are suggested as a method to include site-specific data and to facilitate the modelling of dynamic systems. An aquatic ecosystem model for the environmental transport of radionuclides is presented and discussed. With this model, driven and constrained by site-specific carbon dynamics and three radionuclide specific mechanisms: (i) radionuclide uptake by plants, (ii) excretion by animals, and (iii) adsorption to organic surfaces, it was possible to estimate the radionuclide concentrations in all components of the modelled ecosystem with only two radionuclide specific input parameters (BCF for plants and Kd). The importance of radionuclide specific mechanisms for the exposure to organisms was examined, and probabilistic and sensitivity analyses to assess the uncertainties related to ecosystem input parameters were performed. Verification of the model suggests that this model produces analogous results to empirically derived data for more than 20 different radionuclides.     

Radionuclide transfer from soil to fruit

The available literature on the transfer of radionuclides from soil to fruit has been reviewed with the aim of identifying the main variables and processes affecting the behaviour of radionuclides in fruit plants. Where available, data for transfer of radionuclides from soil to other components of fruit plant have also been collected, to help in understanding the processes of translocation and storage in perennial plants. Soil-to-fruit transfer factors were derived from agricultural ecosystems, both from temperate and subtropical or tropical zones. Aggregated transfer factors have also been collected from natural or semi-natural ecosystems. The data concern numerous fruits and various radionuclides. Soil-to-fruit transfer is nuclide specific. The variability for a given radionuclide is first of all ascribable to the different properties of soils. Fruit plant species are very heterogeneous, varying from woody trees and shrubs to herbaceous plants. In temperate areas the soil-to-fruit transfer is higher in woody trees for caesium and in shrubs for strontium. Significant differences between the values obtained in temperate and subtropical and tropical regions do not necessarily imply that they are ascribable to climate. Transfer factors for caesium are higher in subtropical and tropical fruits, while those for strontium, as well as for plutonium and americium, in the same fruits, are lower; these results can be interpreted taking into account different soil characteristics.     

Evaluation of technologically enhanced natural radiation near the coal-fired power plants in the Lodz region of Poland

Radionuclide releases together with escaping fly ashes (from 45 x 10(6) kg in previous decades to 8 x 10(6) kg annually in 1996) from the main local and several small coal-fired power plants resulted in a relatively small increase in natural radioactivity levels in the Lodz region. The natural gamma terrestrial radiation dose rates (1 m above ground level) were measured at 82 points including in the vicinity of power plants, in the center of the town and on edge of the town. The average dose rate value for the first area was 36 +/- 1.2 nGy h (-1), whereas the same dose rate for the edge of town was slightly lower 30 +/- 0.9 nGy h (-1) but this difference was statistically significant. Further confirmation of the technologically slightly enhanced exposure of the local population to natural radionuclides was achieved by gamma-spectrometry measurement of the uranium and thorium decay series radionuclides in the surface soil profiles (up to 30 cm depth). The average increase of 226Ra and 232Th radionuclides in the top layer of soil (0-10 cm) according to the 20+/-30 cm depth layer was 21% and 17%, respectively. However, due to the relatively low levels of 232Th (14.3 Bq kg (-1)) and 238U (16.8 Bq kg (-1)) in this area, the annual average effective dose from the natural terrestrial radiation for the local population is also relatively low, 0.28 mSv only.     

Critical compilation and review of plant/soil concentration ratios for uranium,thorium and lead

The transfer of natural radionuclides of the U decay series through the biosphere is important, especially for assessment of the impacts of mining and milling of U ores. The pathway from soil through plants to humans can contribute significantly to the overall dose received from these radionuclides. Element-specific concentration ratios (CRs) used to model the impact of radionuclides have been reported for U, Th and Pb but have not received the research or critical review accorded other radionuclides associated with the nuclear industry. This paper compiles and analyses many of the available data on CRs for U, Th and Pb and recommends values appropriate for environmental assessments. A brief overview of the statistical properties of CR values and the complex soil/plant processes encompassed by this ratio is also given, as well as an overview of some of the physical, chemical and biological factors likely to influence CR values.Our analysis showed that CR values decreased significantly as the corresponding soil concentrations increased. Although the variability was substantial, with ranges of 1000- to 30 000-fold, CR values did differ significantly among some soil and plant types. The overall geometric means were 0·0045, 0·0036 and 0·052 for U, Th and Pb, respectively.     

The presence of some artificial and natural radionuclides in a Eucalyptus forest in the south of Spain

Long-lived artificial radionuclides (137Cs, 90Sr) were studied in a Eucalyptus plantation located in the south-west of Spain. Radionuclide concentrations were determined in different types of samples corresponding to specific forest components (soil, trees, herbs and litter). Depth profile distributions were obtained in two selected core soils. Two layers were separately measured in three other cores. The concentration factor, defined as the ratio between the mean activity concentration in a component and the mean activity concentration in the soil, was calculated for each component. The biomass of different components was estimated in order to evaluate the total density concentration (Bq/ha) of the artificial radionuclides (137Cs, 90Sr) in the Eucalyptus plantation. The transfer of the radionuclides between the different forest components can be inferred from the results. Additionally, other naturally occurring radionuclides (40K, 226Ra, 228Ra, 228Ac) were determined for comparison. Transport of radionuclides from forest to a nearby pulp mill is also discussed.     

A probabilistic approach to obtaining limiting estimates of radionuclide concentration in biota

The US Department of Energy has developed a graded approach for evaluating radiation doses to biota. Limiting concentrations of radionuclides in water, soil, and sediment were derived for twenty-three radionuclides. Four organism types (aquatic animals, riparian animals, terrestrial animals, and terrestrial plants) were selected as the basis for method development. While environmental transfer data needed for deriving biota tissue concentrations are available for aquatic animals and terrestrial plants, less information is available for terrestrial and riparian organisms. Two methods were applied and examined for their ability to provide estimates of organism:soil or organism:water concentration factors in lieu of measured data. The kinetic/allometric approach combined with a parameter uncertainty analysis provides a needed method to estimate concentration factors across multiple species with limited input data.     

Derivation of a screening methodology for evaluating radiation dose to aquatic and terrestrial biota

The United States Department of Energy (DOE) currently has in place a radiation dose standard for the protection of aquatic animals, and is considering additional dose standards for terrestrial biota. These standards are: 10 mGy/d for aquatic animals, 10 mGy/d for terrestrial plants, and, 1 mGy/d for terrestrial animals. Guidance on suitable approaches to the implementation of these standards is needed. A screening methodology, developed through DOE's Biota Dose Assessment Committee (BDAC), serves as the principal element of DOE's graded approach for evaluating radiation doses to aquatic and terrestrial biota. Limiting concentrations of radionuclides in water, soil, and sediment were derived for 23 radionuclides. Four organism types (aquatic animals; riparian animals; terrestrial animals; and terrestrial plants) were selected as the basis for development of the screening method. Internal doses for each organism type were calculated as the product of contaminant concentration, bioaccumulation factor(s) and dose conversion factors. External doses were calculated based on the assumption of immersion of the organism in soil, sediment, or water. The assumptions and default parameters used provide for conservative screening values. The screening methodology within DOE's graded approach should prove useful in demonstrating compliance with biota dose limits and for conducting screening assessments of radioecological impact. It provides a needed evaluation tool that can be employed within a framework for protection of the environment.     

Enhancement of population doses due to production of electricity from brown coal in Poland

Changes in natural radioactivity as a result of the operations of three Polish brown-coal-fired power plants have been assessed via calculations based on the results of radio-spectrometric measurements of coal and fly ash, information on local climatic conditions and data on atmospheric releases from the plants. Calculations were performed using a computer code derived from solution of the Pasquille equation. Transfer coefficients and conversion factors of absorbed radioactivity to effective dose equivalent (EDE) were selected using literature data. Values for EDE due to inhalation, ingestion and external gamma radiation from radionuclides deposited on the ground were calculated for the population inhabiting the most polllated area and also for the whole population. The individual maximum EDE for the most ‘radioactive’ power plant was estimated at 10·4 μSv y⁻¹. This EDE is less than 1% of the total natural radiation burden of 2 mSv y⁻¹. EDE estimates for the vicinities of the other two power plants are 0·5 and 4·2 μSv y⁻¹. The collective EDE resulting from the production of electrical energy from brown coal was calculated to be 104 man-Sv y⁻¹, i.e. 14 man-Sv per GW-year. It is concluded that some of the brown-coal-fired power plants in Poland should improve their fly ash control and that the manner in which existing transfer coefficients are reported in the literature does not lead to unequivocal results when applied to EDE calculations.     

Radiological impact of coal-fired power generation

A radiological impact assessment was carried out at the 4000 MW Nanticoke Thermal Generating Station on the north shore of Lake Erie and at an ash disposal site in Metropolitan Toronto, both operated by Ontario Hydro. Analyses were performed on feed coal, fly ash and bottom ash and on samples of air filters, precipitation, water, soil and vegetation in the vicinity of the generating station. The measured radionuclide levels in the vicinity of the generating station showed no evidence of enhancement from station emissions. There were no indications that leaching of radionuclides or emanation of radon gas from ash disposal sites would be a problem. There was evidence, however, of higher radionuclide concentrations on the finer ash particles. Furthermore, the use of fly ash in building materials could lead to enhanced radiation levels. Atmospheric dispersion and radiation dose calculations were also carried out. Predicted concentrations in air were all less than 1% of background values. The committed dose equivalent from one year of operation was estimated to be 0·27 μSv at the site boundary and 0·11 μSv at 10 km east of the site.     

Uptake of uranium and thorium by native and cultivated plants

Large part of available literature on biogeochemistry of uranium and thorium refers to the studies performed either in highly contaminated areas or in nutrient solutions that have been artificially ‘spiked’ with radionuclides. Effects of background levels of natural radioactivity on soil-grown plants have not been studied to the same extent. In this paper, we summarised results of greenhouse and field experiments performed by the author from 2000 to 2006. We examined some of the factors affecting transfer of U and Th from soil to plants, differences in uptake of these radionuclides by different plants, relationships between U and Th in soil and in plants, and temporal variations of U and Th in different plant species. Concentrations of radionuclides (critical point for experimental studies on biogeochemistry of U and Th - rare trace elements in non-contaminated regions) and essential plant nutrients and trace elements were determined by instrumental neutron activation analysis.     

The significance of agricultural vs. natural ecosystem pathways in temperate climates in assessments of long-term radiological impact

Recent developments in performance assessment biosphere models have begun to emphasise the importance of natural accumulation pathways. In contrast to the agricultural pathways, the database for natural ecosystem pathways is less well developed, leading to a mismatch in quality of representations of the two types of system. At issue is the lack of reliable soil-plant and animal ingestion transfer factors for key radionuclides in natural ecosystems. The relative importance of the agricultural vs. natural ecosystem pathways is investigated here, in the context of a temperate site in present day, Eastern France. The BIOMASS Candidate Critical Group (CCG) methodology has been applied to map a set of eight candidate critical groups derived from the present-day societal context onto physical locations within a simple model of a river catchment system. The overall assessment model has been implemented using the Aquabios code. Annual individual dose to each of the CCGs has been calculated for each of the key radionuclides (79Se, 94Nb, 99Tc, 129I, 135Cs and 237Np) released to the valley aquifer and river. In addition to the traditional agricultural pathways, lifestyle groups exploiting natural habitats are explicitly addressed. Results show the susceptibility of different candidate critical groups to different radionuclides. A reference database typical of those employed in long-term performance assessment models is employed. Doses from external exposure (94Nb) and dust inhalation (237Np) are shown to dominate agricultural food consumption by factors of more than six, but, with the reference data set, foodstuffs obtained from natural ecosystems do not contribute significantly to critical group dose and, at most, show similar exposures to the agricultural pathways. This may lead to the conclusion that natural food can be ruled out of consideration in performance assessment models. However, systematic parametric sensitivity studies carried out on soil-plant and animal ingestion transfer factors restrict the validity of this observation and demonstrate the limitations of existing databases. Remaining uncertainties can be reduced by improving structural models for performance assessment and by better characterisation of sources of locally obtained foods. Improved characterisation of radionuclide accumulation in natural ecosystems in temperate as well as alternative future climate states should complement the modelling approach.     

Sector field inductively coupled plasma mass spectrometry, another tool for plutonium isotopes and plutonium isotope ratios determination in environmental matrices

The transfer of radio nuclides into the different compartments of the environment are widely studied and leads to the elaboration of transfer models in order to evaluate potential impact onto the environment and humans. Accurate experimental data are needed to validate these models for all types of matrices (air, water, sediments, soils, biota and food...). Among these radionuclides, 238Pu, 239Pu, 240Pu and 241Pu, are often mentioned. They have been released into the environment by nuclear weapon tests, nuclear facilities, reactors or satellite accidents. These different sources have different 240Pu/239Pu ratios and therefore this ratio is used to provide information on the source of contamination into the environment. The most conventional analytical tools used for plutonium isotope determination are liquid scintillation and alpha spectrometry, and thermal ionisation mass spectrometry (TIMS) is still considered as the primary method for determination of plutonium isotope ratios. During the last decade, mass spectrometers equipped with plasma ion sources and sector field analysers were developed and can offer now another alternative method for the accurate determination of isotope content and ratios of long-lived radionuclides in environmental samples. This paper presents and discusses the results obtained for 239Pu, 240Pu and 241Pu content and isotope ratios by sector field ICP-MS in different environmental matrices.     

A dynamic model for assessing radiological consequences of routine releases in the Loire river: parameterisation and uncertainty/sensitivity analysis

A dynamic model for assessing the transfer of several radionuclides ((58)Co, (60)Co, (110 m)Ag, (134)Cs, (137)Cs, (54)Mn and (131)I) in a food-chain was applied on the Loire river, where 14 nuclear power plants situated on five different sites operate. The model considers the following potential exposure pathways: (i) transfer of radionuclides through the aquatic food chain and the subsequent internal exposure of humans due to ingestion of contaminated water and/or fish; (ii) use of river water for agricultural purposes (irrigation), transfer of radionuclides through the terrestrial food chain and the subsequent internal exposure of humans due to ingestion of contaminated foodstuffs; (iii) internal exposure due to inhalation of dust originating from resuspension of contaminated soil particles; (iv) external exposure from radionuclides present in the river or deposited on the river sediments or the soil. For each of the parameters introduced in this model, a probability density function, allowing further uncertainty and sensitivity analysis, was proposed. Uncertainty/sensitivity analysis were performed to: (i) compare calculations to empirical data; (ii) determine a confidence interval for the mean annual dose to critical groups; and (iii) identify the parameters responsible for the uncertainty and subsequent research priorities.