MEASUREMENT OF NATURAL RADIONUCLIDES AND EXTERNAL RADIATION EXPOSURE DUE TO FLY ASH FROM A COAL-FIRED POWER PLANT (SPAIN) DEPOSITED ON SOILS. COMPARISON USING TWO DIFFERENT MEASUREMENT TECHNIQUES

MEASUREMENT OF NATURAL RADIONUCLIDES AND EXTERNAL RADIATION EXPOSURE DUE TO FLY ASH FROM A... Abstract The evaluation of the radiological impact in soils due to the fly-ash ponds using both in situ techniques and laboratory based measurements is presented. In order to check the in situ techniques capabilities for monitoring this type of industries, a comparison between both techniques was performed. A characterization of external radiation exposure in the fly-ash pond and in its surrounding soils was made. The associated external radiological hazard due to the fly-ash pond has been evaluated. In situ techniques could be used to determine the radiological impact on soils due to fly-ash deposition, but its use could be limited due to the associated uncertainties. INTRODUCTION The production of electricity by coal-fired power plants (CFPP) is cataloged as a Naturally Occurring Radioactive Materials (NORM) type industry(1). This is because, depending on their origin, the fuels used have different concentrations of naturally occurring radionuclides, mainly belonging to the uranium and thorium decay series(1). Secondly, the combustion process generates by-products fly ash and slag, whose concentration in these radionuclides is enriched relative to the corresponding fuels(2). And thirdly, part of these activities may be incorporated into the environment due to the normal operation of these plants. Indeed, part of the natural occurring radionuclides contents in the fuel are volatilized as they are burnt in the boilers of these power plants and emitted into the atmosphere. Although the fly ashes generated are often used in cement manufacture, some may also be accumulated in specially designed ponds. For these reasons, there have been studies(3–7) evaluating the radiological health impact of the natural radionuclides emitted into the atmosphere and of the massive deposits of ash resulting from the operation of these power stations. The conventional procedure to determine the gamma-emitting radionuclide concentration levels in soils consists, firstly, in the selection of the appropriate spatial resolution for the required level of accuracy(8). Then, the soil samples are processed in order to match them to the particular geometry for which one has an efficiency calibration for the detector. Lastly, they are measured by gamma spectrometry using low-background Hyper Pure Germanium (HPGe) detectors. The measurement of some naturally radionuclides belonging to the uranium and thorium decay series needs some particular conditions. The plastic container must be sealed to prevent losses from 220,222Rn emanation and it must be expected to reach the secular equilibrium between 220,222Rn and its daughters. The procedure is costly in resources and time. In situ techniques could partially overcome these problems. They can be used to determine the activity and ambient dose-rate levels by means of portable HPGe detectors and dose-rate monitors. In situ measurement techniques are relatively inexpensive and involve reasonably precise methods(8, 9). Since such in situ techniques make it much quicker to measure the soil radionuclides activity levels and the ambient dose rates, one can increase the spatial resolution. However, the in situ gamma spectrometry (ISGS) measurement method are not free of technical challenges. The main difficulty is determining the geometric distribution of the radionuclides in the soils of any given natural environment with accuracy. While there are various semi-empirical and theoretical approximations to estimate these distributions(10–12), the assumptions made in some cases could potentially produce misleading results. The objectives of this study are: (i) to perform a comparison between ISGS technique and conventional lab based methods to estimate the real capability of the ISGS technique in the radiological characterization of a NORM industry; (ii) to determine if there are increase of natural radionuclides in the top soil layer due to the fly-ash deposition and evaluate their contribution to the ambient dose rate and (iii) to evaluate the external radiological hazard due to the presence fly-ash ponds. MATERIAL AND METHODS Study area and sampling strategy The Teruel CFPP (40.997056 N; 0.387512 W) has a current power capacity of 1050 MW. The coal used as fuel is a mixture of imported (bituminous) coal and local black lignite (sub-bituminous coal) in a ratio of 1:3. Its fuel consumption is around 15 000 Tm per day, and it generates about 2000 Tm of ash per day. The fly ash and slag resulting from the coal combustion are accumulated in two nearby ponds, which have a total surface area of 1 km2. The area on it is situated the Teruel CFPP is mainly formed by clay and conglomerate from the Neogene period. There are mainly poor limestone evolved soils with shallow depth. Figure 1 shows the sampling points around the CFPP and its ash ponds. Ten sampling points were selected in an area of radius <2 km from the CFPP and its ash ponds. Moreover, four sampling points were selected at different distances (2.5, 5, 7 and 10 km) from CFPP in the prevailing wind direction. A control soil profile up to 30 cm depth was taken outside the influence of CFPP. Lastly, two sample profiles were collected in the ash ponds. Figure 1. View largeDownload slide Map of the study area and sampling locations. Wind rose: Lat: 41.008 Long: −0.344. Blue: Total wind energy %. Gray: Time %(13). Figure 1. View largeDownload slide Map of the study area and sampling locations. Wind rose: Lat: 41.008 Long: −0.344. Blue: Total wind energy %. Gray: Time %(13). Experimental methods Five top layer soil samples were collected using a 30 × 30 × 4 cm frame. All of them were collected near the ash ponds (distance <2 km). Nine soil profiles were also collected in the surroundings of the CFPP and the ash ponds (distance <2 km) and at different distances of the CFPP in the prevailing wind direction. Also, a soil profile was collected at a sampling point situated outside the influence of the CFPP and the prevailing winds. All soil profile samples were collected conserving separately for assay the layers 0–2, 2–4, 4–6, 6–10, 10–15, 15–20 and 20–25 cm. At each sampling point, an ISGS measurement was made using the portable Ge detector. The method used was to place the detector face downwards at 1 m above the ground. The measurement time chosen was 3600 s. Also, at each sampling point, the FHZ600A dose-rate monitor was used to determine the ambient dose rate, taking at least 10 measurements to have good statistics, and using 180 s per measurement. All the soil samples collected for conventional gamma spectrometry in the laboratory were dried to eliminate their moisture content, and then sieved to a <2-mm grain size. This process allowed the value of the relative moisture level of each sample to be obtained. The dried soil samples were put into 191-cm3 volume Petri dishes and sealed to prevent losses from 222Rn emanation. A time of 28 days was allowed for secular equilibrium between 226Ra and its descendants to be re-established. The soil sample measurements in low-background gamma spectrometry detector were validated according to the quality requirements of ISO 17025 standard(14). The activity determination in the soil samples was performed in a low-activity laboratory. A high resolution low-background gamma spectrometer was used. It consists of a coaxial p-type HPGe detector, with a 45% relative efficiency, 1.95 keV FWHM and 64:1 peak-Compton ratio, all for the 1332.5 keV 60Co emission. The minimum detection activity values were calculated following the recommendations of ISO 11929 standard(15). For the ISGS, a portable coaxial HPGe detector was used with a 43.1% relative efficiency, 2.0 keV FWHM and 56:1 peak-Compton ratio, all for the 1332.5 keV 60Co emission. This detector was coupled to a cryostat which allowed an autonomy of ~4 days. It was calibrated using the semi-empirical techniques proposed in the literature(16, 17). For the ISGS, it was assumed that 226Ra and 232Th are also in equilibrium in the soil with their daughters, even though there is an emanation of 222,220Rn from the soil to the atmosphere(17). In both cases, the following photopeaks were systematically analyzed: 214Pb (351.9 keV), 208T1 (583.1 keV), 214Bi (609.3; 1120; 1764.5 keV), 228Ac (338.3; 911.1 keV) and 40K (1460.7 keV). The 226Ra activity was taken to be the mean value of the measured 214Bi and 214Pb activities, and the 232Th activity to be the mean value of the measured 208T1 and 228Ac activities. To determine the ambient dose rate a FAG FHZ600A radiation monitor was used. This device consist of a 54.2-cm3 pressurized proportional counter with a measurement range of 0.005 μSv/h–1 mSv/h(18). The ambient dose rate readings of FAG FHZ600A dose rate monitor were corrected using the procedure detailed in a previous work(19). DISCUSSION Comparison between in situ techniques and lab based methods There are numerous previous works that detailed the good correspondence between the activity concentration measured by ISGS versus the activity values obtained from laboratory measurements(19–23). To determine the naturally occurring radionuclides concentration in soil by ISGS a good approximation is to suppose that these radionuclides are depth homogeneous distributed(17). However, in the present study one of the significant difficulty is to affirm this last hypothesis, because there is a probable accumulation of wind re-suspended fly ash on the top layer soils, increasing its naturally occurring radionuclide concentrations. In order to determine the most appropriate geometry to apply to ISGS, the multiple peaks method proposed by(10) was applied. This method is based on the relationship between the different degrees of attenuation experimented by photons emitted at different energies by the same multi-emitting radionuclide. 214Bi (609, 1120 and 1764.5 keV) and 228Ac (338 and 911 keV) radionuclides were selected to apply Sowa’s method. Table 1 shows the coefficient β (g/cm2) obtained for 214Bi and 228Ac. Although, Table 1 shows the activity values obtained from the results derived by the application of Sowa’s method and the activity values obtained assuming a depth homogeneous distribution of radionuclides. A correlation analysis between above-cited activities shows that the ratio between them are equal to 1.0, that it indicates both set of activities are statistically the same with the exception of Soil 14 which is situated near to the ash ponds. It could be affected by fly-ash deposition on its top soil layer. In fact, as shown in Figure 2, where the activity levels of 40K, 226Ra and 232Th of depth soil profile of sampling point 14 is plotted, there is a significative naturally occurring radionuclide concentration increasing in the first 5-cm top soil layer. Figure 2. View largeDownload slide Naturally occurring radionuclides concentration in soil profiles for the sampling points 14 and outside CFPP influence. Figure 2. View largeDownload slide Naturally occurring radionuclides concentration in soil profiles for the sampling points 14 and outside CFPP influence. Table 1. Activity concentrations of 214Bi and 228Ac measured by the ISGS technique with reference to the depth profile distribution. 214Bi 228Ac I.d. β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b Soil 1 20 38±8 29±4 >50 22±4 18±3 Soil 2 20 41±8 32±5 >50 44±8 36±5 Soil 3 20 37±8 34±5 >50 13±3 12±2 Soil 4 20 39±8 30±4 50 22±4 19±3 Soil 5 20 35±8 44±7 5 48±9 33±5 Soil 6 >50 22±4 29±4 >50 20±4 16±3 Soil 7 >50 19±4 26±4 >50 14±3 14±2 Soil 8 20 30±7 39±7 >50 19±4 13±2 Soil 9 >50 32±7 38±7 20 27±6 18±3 Soil 10 20 30±7 36±6 >50 20±4 15±2 Soil 11 10 70±14 45±7 5 70±14 30±5 Soil 12 20 26±7 21±3 >50 12±3 11±2 Soil 13 >50 27±7 32±4 >50 15±3 11±2 Soil 14 5 98±20 48±8 10 35±8 33±5 214Bi 228Ac I.d. β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b Soil 1 20 38±8 29±4 >50 22±4 18±3 Soil 2 20 41±8 32±5 >50 44±8 36±5 Soil 3 20 37±8 34±5 >50 13±3 12±2 Soil 4 20 39±8 30±4 50 22±4 19±3 Soil 5 20 35±8 44±7 5 48±9 33±5 Soil 6 >50 22±4 29±4 >50 20±4 16±3 Soil 7 >50 19±4 26±4 >50 14±3 14±2 Soil 8 20 30±7 39±7 >50 19±4 13±2 Soil 9 >50 32±7 38±7 20 27±6 18±3 Soil 10 20 30±7 36±6 >50 20±4 15±2 Soil 11 10 70±14 45±7 5 70±14 30±5 Soil 12 20 26±7 21±3 >50 12±3 11±2 Soil 13 >50 27±7 32±4 >50 15±3 11±2 Soil 14 5 98±20 48±8 10 35±8 33±5 aActivity determination by Sowa’s method. bActivity determination assuming homogeneous depth profile distribution. View Large Table 1. Activity concentrations of 214Bi and 228Ac measured by the ISGS technique with reference to the depth profile distribution. 214Bi 228Ac I.d. β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b Soil 1 20 38±8 29±4 >50 22±4 18±3 Soil 2 20 41±8 32±5 >50 44±8 36±5 Soil 3 20 37±8 34±5 >50 13±3 12±2 Soil 4 20 39±8 30±4 50 22±4 19±3 Soil 5 20 35±8 44±7 5 48±9 33±5 Soil 6 >50 22±4 29±4 >50 20±4 16±3 Soil 7 >50 19±4 26±4 >50 14±3 14±2 Soil 8 20 30±7 39±7 >50 19±4 13±2 Soil 9 >50 32±7 38±7 20 27±6 18±3 Soil 10 20 30±7 36±6 >50 20±4 15±2 Soil 11 10 70±14 45±7 5 70±14 30±5 Soil 12 20 26±7 21±3 >50 12±3 11±2 Soil 13 >50 27±7 32±4 >50 15±3 11±2 Soil 14 5 98±20 48±8 10 35±8 33±5 214Bi 228Ac I.d. β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b Soil 1 20 38±8 29±4 >50 22±4 18±3 Soil 2 20 41±8 32±5 >50 44±8 36±5 Soil 3 20 37±8 34±5 >50 13±3 12±2 Soil 4 20 39±8 30±4 50 22±4 19±3 Soil 5 20 35±8 44±7 5 48±9 33±5 Soil 6 >50 22±4 29±4 >50 20±4 16±3 Soil 7 >50 19±4 26±4 >50 14±3 14±2 Soil 8 20 30±7 39±7 >50 19±4 13±2 Soil 9 >50 32±7 38±7 20 27±6 18±3 Soil 10 20 30±7 36±6 >50 20±4 15±2 Soil 11 10 70±14 45±7 5 70±14 30±5 Soil 12 20 26±7 21±3 >50 12±3 11±2 Soil 13 >50 27±7 32±4 >50 15±3 11±2 Soil 14 5 98±20 48±8 10 35±8 33±5 aActivity determination by Sowa’s method. bActivity determination assuming homogeneous depth profile distribution. View Large Based on last results, all ISGS measurements had been performed assuming a depth homogeneous radionuclides in soil. With the exception of Soil 14 where it has been assumed an exponential depth distribution of 226Ra and 232Th. The obtained values had been compared with the average concentration of naturally occurring radionuclides calculated from the soil sample profiles measured in the laboratory. Figure 3 shows the correlation between them. Figure 3. View largeDownload slide Linear fit and R-squared analysis for in situ versus lab activity levels. Error bars: In situ levels: Full uncertainties. Average activity levels: 2×SD. Figure 3. View largeDownload slide Linear fit and R-squared analysis for in situ versus lab activity levels. Error bars: In situ levels: Full uncertainties. Average activity levels: 2×SD. First, there is a good correspondence between two sets of measurements for 226Ra and 232Th as the results of R-squared analysis shows. However, for 40K levels, the result of R-squared indicates a bad correlation between in situ and based on laboratory measurements, although, the slope of the linear fit shows a difference between the activity levels of 40K measured with both techniques about 25%. This is because the photon flux with energies upper than 1 MeV that reaches the HPGe detector have their origin at the first 40-cm of soil depth. CFPP is situated in a significative stony poor evolved soils area. In fact, as shows in Table 2, in some sampling points were only possible to collect sample soil profiles up to 15 cm depth. Thus, the geometry hypothesis assumed for determining the 40K levels by ISGS does not consider the presence of a rocky and poor evolved soils. Table 2. Ratio between activity concentration determined in soil depth [0–2 cm] versus activity concentration determined in the deepest soil layer. Distance (km) Depth (cm) Ratio 40 K[0–2]/[>15] Ratio 226 Ra[0–2]/[>15] Ratio 232 Th[0–2]/[>15] Soil 1 9.5 [0–15] 1.0±0.2 1.4±1.0 1.2±0.4 Soil 2 6.5 [0–15] 1.2±0.1 1.2±0.8 1.6±0.4 Soil 3 4.5 [0–15] 1.3±0.2 1.3±1.1 1.3±0.5 Soil 4 2.5 [0–15] 1.0±0.1 0.9±0.4 1.1±0.2 Soil 5 1.8 [0–25] 1.1±0.1 1.2±0.2 0.9±0.2 Soil 9 1.0 [0–10] 1.0±0.1 1.5±0.2 1.2±0.2 Soil 10 0.7 [0–30] 1.3±0.1 1.2±0.2 1.0±0.3 Soil 11 0.7 [0–20] 1.2±0.1 1.2±0.3 1.2±0.2 Soil 14 0.2 [0–25] 1.3±0.1 5.3±0.9 2.9±0.7 Control soil 10 [0–30] 0.9±0.1 1.0±0.1 1.0±0.2 Distance (km) Depth (cm) Ratio 40 K[0–2]/[>15] Ratio 226 Ra[0–2]/[>15] Ratio 232 Th[0–2]/[>15] Soil 1 9.5 [0–15] 1.0±0.2 1.4±1.0 1.2±0.4 Soil 2 6.5 [0–15] 1.2±0.1 1.2±0.8 1.6±0.4 Soil 3 4.5 [0–15] 1.3±0.2 1.3±1.1 1.3±0.5 Soil 4 2.5 [0–15] 1.0±0.1 0.9±0.4 1.1±0.2 Soil 5 1.8 [0–25] 1.1±0.1 1.2±0.2 0.9±0.2 Soil 9 1.0 [0–10] 1.0±0.1 1.5±0.2 1.2±0.2 Soil 10 0.7 [0–30] 1.3±0.1 1.2±0.2 1.0±0.3 Soil 11 0.7 [0–20] 1.2±0.1 1.2±0.3 1.2±0.2 Soil 14 0.2 [0–25] 1.3±0.1 5.3±0.9 2.9±0.7 Control soil 10 [0–30] 0.9±0.1 1.0±0.1 1.0±0.2 View Large Table 2. Ratio between activity concentration determined in soil depth [0–2 cm] versus activity concentration determined in the deepest soil layer. Distance (km) Depth (cm) Ratio 40 K[0–2]/[>15] Ratio 226 Ra[0–2]/[>15] Ratio 232 Th[0–2]/[>15] Soil 1 9.5 [0–15] 1.0±0.2 1.4±1.0 1.2±0.4 Soil 2 6.5 [0–15] 1.2±0.1 1.2±0.8 1.6±0.4 Soil 3 4.5 [0–15] 1.3±0.2 1.3±1.1 1.3±0.5 Soil 4 2.5 [0–15] 1.0±0.1 0.9±0.4 1.1±0.2 Soil 5 1.8 [0–25] 1.1±0.1 1.2±0.2 0.9±0.2 Soil 9 1.0 [0–10] 1.0±0.1 1.5±0.2 1.2±0.2 Soil 10 0.7 [0–30] 1.3±0.1 1.2±0.2 1.0±0.3 Soil 11 0.7 [0–20] 1.2±0.1 1.2±0.3 1.2±0.2 Soil 14 0.2 [0–25] 1.3±0.1 5.3±0.9 2.9±0.7 Control soil 10 [0–30] 0.9±0.1 1.0±0.1 1.0±0.2 Distance (km) Depth (cm) Ratio 40 K[0–2]/[>15] Ratio 226 Ra[0–2]/[>15] Ratio 232 Th[0–2]/[>15] Soil 1 9.5 [0–15] 1.0±0.2 1.4±1.0 1.2±0.4 Soil 2 6.5 [0–15] 1.2±0.1 1.2±0.8 1.6±0.4 Soil 3 4.5 [0–15] 1.3±0.2 1.3±1.1 1.3±0.5 Soil 4 2.5 [0–15] 1.0±0.1 0.9±0.4 1.1±0.2 Soil 5 1.8 [0–25] 1.1±0.1 1.2±0.2 0.9±0.2 Soil 9 1.0 [0–10] 1.0±0.1 1.5±0.2 1.2±0.2 Soil 10 0.7 [0–30] 1.3±0.1 1.2±0.2 1.0±0.3 Soil 11 0.7 [0–20] 1.2±0.1 1.2±0.3 1.2±0.2 Soil 14 0.2 [0–25] 1.3±0.1 5.3±0.9 2.9±0.7 Control soil 10 [0–30] 0.9±0.1 1.0±0.1 1.0±0.2 View Large In conclusion, the use of ISGS technique for the determination of low increments of naturally occurring radionuclides in the top soil layer may be a complicated task. It is important to emphasize that, in this study, the acquisition time in each ISGS measurement was 3600 s. If the acquisition time is increased up to 20 000 s (>5 h), the uncertainties related to Sowa’s method will decrease significantly. However, this implies the sampling strategy reconsideration. Finally, the use of the above-cited ISGS technique must be supplemented with conventional techniques using an adequate sampling strategy that minimize the time-consuming resources. Radiological environmental impact assessment The naturally occurring radionuclides concentration in fly and bottom ashes generate in a CFPP is related to the type of coal burned in the plant. In a previous work(24), Teruel CFPP fuel was studied and it was observed that the contents determined for the fuel mixes used in this CFPP were consistent with the differences in the natural radionuclide contents of the national and imported coal. The fly ash generated in the Teruel CFPP is retained in the electrostatic precipitators (99.5% of retention efficiency). Using pneumatic pipes the fly ashes are discharged in two ponds. Due to its insolubility, fly ashes settle into the bottom of the ponds and the excess water is discharged into Regallo River(25). As time goes, both ash ponds have been filled and the water was evaporated on their surface. On the dry surfaces of both ponds, two fly-ash profile samples were collected. Although, on each pond, an ambient dose rate measurement was performed. The average activity concentration of 226Ra, 232Th and 40K were 200±40, 80±20 and 340±120Bq/kg, respectively. The 226Ra and 232Th activity values are greater than those measured in soils in a 7 and 4 factors. Table 2 shows the ratio between the activity concentration in the top soil layer versus the activity concentration in the deepest layer. Firstly, control soil, sampled outside the CFPP influence, shows ratios close to 1.0. As to be expected, these ratio values are showing a depth homogeneous distribution of natural radionuclides (see Figure 2). With regards to the soils sampled downwind and far from the ash ponds, the ratio values show a higher variability, but their uncertainties do not allow to assert that there is an evidence of an increase of radionuclides in the top soil layer. Likewise, the soils which were sampled close to the ash ponds (<1 km) show ratios >1.0 showing a slight increase of natural radionuclides in the top soil layer. But from a statistical point of view, it is very difficult to assert it because the uncertainties may mask it. An exception is the soil collected at sampling point 14 that shows a significative concentration of 226Ra and 232Th in the top soil layer versus the deepest layer (see Figure 2). Finally, 226Ra and 232Th concentrations are not increasing in the top soil layer as the distance from ash ponds increases. Therefore, the radioactive soil impact is only limited to a circumscribed area close to the ash ponds. For the purpose of comparing the radiological effects of materials that contain 226Ra, 40K and 232Th by a single quantity, which takes into account the radiation hazards associated with them, common indexes Radium equivalent activity (Raeq) and external hazard index (Hex) are used in the study. Raeq and Hex can be calculated with 1 and 2 expressions(26, 27). On the other hand, absorbed dose in air (ADA) were determined from ISGS measurements using 3 expression(28). The terrestrial ambient dose rate values, H10, were determined from ADA values applying the conversion factor 0.7 Sv/Gy for energies between 60 and 2000 keV(28). Raeq=CRa+1.43·CTh+0.077·CK (1) Hex=CRa/370+CTh/259+CK/4810 (2) ADA(nGy/h)=CRa·0.462+CTh·0.604+CK·0.0417 (3) First, as shown in Table 3, there is a good correspondence between terrestrial ambient dose rate obtained from ISGS measurements, H10ISGS, and from the FAG proportional counter, H10 FAG. In fact, the correlation between both sets of values is highly satisfactory as shown the results of the linear fit: y=m·x+n; m=1.02±0.06; n=−1.3±2.4; r2=0.951. However, the correlation between H10 ISGS versus H10 lab values shows a slight difference in the slope ( m=0.91±0.08) and y-intercept ( n=7±3). As indicated in previous sections, the geometric approach used in ISGS does not include significantly rocky soils and therefore the activity values obtained were, generally, lower than those measured in the laboratory. Whatever, considering the uncertainties, the H10 values obtained from the 226Ra, 40K and 232Th activity levels both measured by ISGS and laboratory are the same. Concerning to the remarkable sampling point 14, where there is a significative accumulation of 226Ra and 232Th due to the deposition of fly-ash on the top layer soil. If the H10 value had been calculated from the activity levels determined only from the top layer of the soil sample, then the obtained result would have been greater in Factor 2. To avoid this, it is necessary to collect a depth soil profile sample. Table 3. Terrestrial ambient dose rate from ISGS, FAG proportional counter and lab measurements. Radium equivalent concentration and External hazard index. I.d. H10ISGS (nSv/h) H10 FAG (nSv/h) H10lab (nSv/h) Raeq(Bq/kg) Hex Soil 1 22±3 22±4 18±5 68 ± 8 0.18 Soil 2 33±5 24±5 30±5 103 ± 14 0.28 Soil 3 20±3 12±2 28±6 62 ± 8 0.17 Soil 4 22±3 31±6 33±3 69 ± 9 0.19 Soil 5 28±4 24±5 29±4 87 ± 11 0.24 Soil 6 20±3 28±6 27±4 61 ± 9 0.17 Soil 7 18±3 21±4 21±4 56 ± 8 0.15 Soil 8 20±3 16±3 24±4 62 ± 8 0.17 Soil 9 26±4 21±4 30±3 81 ± 11 0.22 Soil 10 23±4 18±4 22±5 71 ± 11 0.19 Soil 11 32±5 34±7 34±5 99 ± 13 0.27 Soil 12 15±2 16±3 17±4 46 ± 6 0.13 Soil 13 18±3 20±4 43±4 57 ± 7 0.15 Soil 14 30±4 27±5 33±21 93 ± 10 0.25 Control Soil – 29±4 26±5 88 ± 5 0.24 Ash Pond 110 ± 16 113 ± 24 109±3 345 ± 37 0.93 I.d. H10ISGS (nSv/h) H10 FAG (nSv/h) H10lab (nSv/h) Raeq(Bq/kg) Hex Soil 1 22±3 22±4 18±5 68 ± 8 0.18 Soil 2 33±5 24±5 30±5 103 ± 14 0.28 Soil 3 20±3 12±2 28±6 62 ± 8 0.17 Soil 4 22±3 31±6 33±3 69 ± 9 0.19 Soil 5 28±4 24±5 29±4 87 ± 11 0.24 Soil 6 20±3 28±6 27±4 61 ± 9 0.17 Soil 7 18±3 21±4 21±4 56 ± 8 0.15 Soil 8 20±3 16±3 24±4 62 ± 8 0.17 Soil 9 26±4 21±4 30±3 81 ± 11 0.22 Soil 10 23±4 18±4 22±5 71 ± 11 0.19 Soil 11 32±5 34±7 34±5 99 ± 13 0.27 Soil 12 15±2 16±3 17±4 46 ± 6 0.13 Soil 13 18±3 20±4 43±4 57 ± 7 0.15 Soil 14 30±4 27±5 33±21 93 ± 10 0.25 Control Soil – 29±4 26±5 88 ± 5 0.24 Ash Pond 110 ± 16 113 ± 24 109±3 345 ± 37 0.93 View Large Table 3. Terrestrial ambient dose rate from ISGS, FAG proportional counter and lab measurements. Radium equivalent concentration and External hazard index. I.d. H10ISGS (nSv/h) H10 FAG (nSv/h) H10lab (nSv/h) Raeq(Bq/kg) Hex Soil 1 22±3 22±4 18±5 68 ± 8 0.18 Soil 2 33±5 24±5 30±5 103 ± 14 0.28 Soil 3 20±3 12±2 28±6 62 ± 8 0.17 Soil 4 22±3 31±6 33±3 69 ± 9 0.19 Soil 5 28±4 24±5 29±4 87 ± 11 0.24 Soil 6 20±3 28±6 27±4 61 ± 9 0.17 Soil 7 18±3 21±4 21±4 56 ± 8 0.15 Soil 8 20±3 16±3 24±4 62 ± 8 0.17 Soil 9 26±4 21±4 30±3 81 ± 11 0.22 Soil 10 23±4 18±4 22±5 71 ± 11 0.19 Soil 11 32±5 34±7 34±5 99 ± 13 0.27 Soil 12 15±2 16±3 17±4 46 ± 6 0.13 Soil 13 18±3 20±4 43±4 57 ± 7 0.15 Soil 14 30±4 27±5 33±21 93 ± 10 0.25 Control Soil – 29±4 26±5 88 ± 5 0.24 Ash Pond 110 ± 16 113 ± 24 109±3 345 ± 37 0.93 I.d. H10ISGS (nSv/h) H10 FAG (nSv/h) H10lab (nSv/h) Raeq(Bq/kg) Hex Soil 1 22±3 22±4 18±5 68 ± 8 0.18 Soil 2 33±5 24±5 30±5 103 ± 14 0.28 Soil 3 20±3 12±2 28±6 62 ± 8 0.17 Soil 4 22±3 31±6 33±3 69 ± 9 0.19 Soil 5 28±4 24±5 29±4 87 ± 11 0.24 Soil 6 20±3 28±6 27±4 61 ± 9 0.17 Soil 7 18±3 21±4 21±4 56 ± 8 0.15 Soil 8 20±3 16±3 24±4 62 ± 8 0.17 Soil 9 26±4 21±4 30±3 81 ± 11 0.22 Soil 10 23±4 18±4 22±5 71 ± 11 0.19 Soil 11 32±5 34±7 34±5 99 ± 13 0.27 Soil 12 15±2 16±3 17±4 46 ± 6 0.13 Soil 13 18±3 20±4 43±4 57 ± 7 0.15 Soil 14 30±4 27±5 33±21 93 ± 10 0.25 Control Soil – 29±4 26±5 88 ± 5 0.24 Ash Pond 110 ± 16 113 ± 24 109±3 345 ± 37 0.93 View Large On the other hand, the ambient dose rate background is 29±4nSv/h. This value was measured in the sampling point situated outside the influence of the CFPP which is not affected by fly-ash deposition on its top soil layer. From the statistically point of view is not possible to assert that there are an increment of ambient dose rate due to the presence of fly-ash on the top soil layers that are situated downwind (Soil 1–5) the CFPP and those that are situated close to the ash ponds. Regarding the fly-ash ponds, the ambient dose rate is greater than the background in a Factor 5. However, subtracting the background, the annual ambient dose rate is 0.8 mSv. Using the conversion factor 0.87 to obtain effective dose rate(29) the obtained value is 0.69 mSv/y that it is below the limit of 1.0 mSv/y of annual effective dose rate due to external sources for the public members. It is clear from Table 3 that the range of levels of radium equivalent activity is 46–103 Bq/kg in the surrounding soils of the CFPP and its ash ponds. Based on the annual external dose limit of 1.0 mSv, the activity limiting terms of radium equivalent is 370 Bq/kg(2). It can be seen that the radium equivalent activity in those soils is below the recommended limit. However, in the ash ponds the maximum level of radium equivalent activity is obtained. Although this value plus its uncertainty is below the recommended limit, too. Finally, the calculated values of external hazard index for the soils studied are given in Table 3. This hazard index must be less than unity for the radiation hazard to be insignificant It can be seen in this Table that the range of values of the external hazard index is 0.15–0.93, the latter being the obtained in the fly-ash. All of them are below the unit. CONCLUSION The assessment of the radiological impact on the vicinity of the power plants appeared to be a rather complicated task. In the case of relatively modern plants like Teruel CFPP with fly-ash removal efficiency >99.5%, such impact is negligible. However, the accumulation of huge amounts of fly ash in ponds could be considered as the main source of radiological impact associated with these industries. The analytical results lead to the following conclusions: In this study a good correspondence between the 226Ra and 232Th activity levels determined both in situ techniques and conventional laboratory measurements was observed (R2 = 0.937 and R2 = 0.881, respectively). However, there is not a good correspondence between 40K activity values determined by both techniques. This is because the geometric approach used in ISGS technique consider that all photon flux coming from the soil that reaches the detector have their origin in the first 40 cm of depth. As, in previous sections was shown, the soils of the Teruel CFPP area are rocky and small deepness. And it is extremely difficult to consider them in the geometric approximation used. For that, the 40K activity values determined by ISGS technique are lower than those determined by conventional technique. In conclusion, the main drawback of ISGS technique relates to the geometric approach used to determine the naturally occurring radionuclides in soils. In fact, using Sowa’s method, it is possible to obtain, with an acceptable uncertainty, an approximate soil depth distribution of the multi-emitter radionuclides like 214Pb and 214Bi. In any case, using the depth homogeneous distributed geometric approach the obtained results would be acceptable, but it is necessary to take some soil samples at strategic points to improve the accuracy of the soil characterization of the surroundings of CFPP. The H10 values are calculated from activity levels and measured by proportional counter dose rate. A high correspondence has been observed between the H10 determined both from ISGS and laboratory based measurements versus H10 measured by proportional counter dose rate monitor (R2 = 0.943 and R2= 0.906, respectively). On the other hand, considering the uncertainties, the H10 values obtained from the 226Ra, 40K and 232Th activity levels both measured by ISGS and laboratory are the same. The results indicate that terrestrial ionizing radiation exposure associated with NORM resulting from fly ash from the Teruel CFPP could contribute and additional effective dose of up to 0.7 mSv annually. The huge amount of fly ash and bottom ash disposed in ash ponds could be a source of naturally occurring radionuclides that easily could be dispersed in surrounding soils by the wind. However, this radioactive soil impact is only limited to a circumscribed area close to the ash ponds. Finally, common indexes Raeq and Hex have been calculated. The Raeq and Hex obtained range in soils are [46–103] Bq/kg and [0.15–0.28], respectively. On the other hand, the maximum values of Raeq (346 Bq/kg) and Hex (0.93) are obtained in ash ponds. Both are below the recommended limits. FUNDING This work was made possible thanks firstly to funding received from Spain’s Nuclear Safety Council for the project entitled ‘Estudio del impacto radiológico de las centrales de carbón sobre sus entornos’, and secondly to funding from the Government of Extremadura through ‘Ayudas para el fortalecimiento de los grupos de investigación de Extremadura’. REFERENCES 1 IAEA . Extent of environmental contamination by naturally occurring radioactive material (NORM) and technological options for mitigation. Technical report ( 2003 ). 2 UNSCEAR . Sources and effects of ionizing radiation. United Nations Scientific Committee on the Effects of Atomic Radiation. Technical report ( 2000 ). 3 Rosner , G. , Bunzl , K. , Hötzl , H. and Winkler , R. Low level measurements of natural radionuclides in soil samples around a coal-fired power plant . Nucl. Instrum. Methods Phys. Res. 223 ( 2 ), 585 – 589 ( 1984 ). Google Scholar CrossRef Search ADS 4 Tso , M.-Y. W. and Leung , J. K. C. Radiological impact of coal ash from the power plants in Hong Kong . J. Environ. Radioact. 30 ( 1 ), 1 – 14 ( 1996 ). Google Scholar CrossRef Search ADS 5 Bem , H. , Wieczorkowski , P. and Budzanowski , M. Evaluation of technologically enhanced natural radiation near the coal-fired power plants in the Lodz region of Poland . J. Environ. Radioact. 61 ( 2 ), 191 – 201 ( 2002 ). Google Scholar CrossRef Search ADS PubMed 6 Papastefanou , C. Escaping radioactivity from coal-fired power plants (CPPs) due to coal burning and the associated hazards: a review . J. Environ. Radioact. 101 ( 3 ), 191 – 200 ( 2010 ). Google Scholar CrossRef Search ADS PubMed 7 Dai , L. , Wei , H. and Wang , L. 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In-situ measurement of Cs distribution in the soil . Nucl. Instrum. Methods Phys. Res. B 93 ( 4 ), 485 – 491 ( 1994 ). Google Scholar CrossRef Search ADS 13 IDAE . Atlas Eólico de España. Technical report, Instituto para la Diversificación y Ahorro de la Energía ( 2011 ). 14 EN ISO. IEC 17025 . General requirements for the competence of testing and calibration laboratories, pages 05–15 ( 2005 ). 15 ISO . Determination of the characteristic limits (decision threshold, detection limit and limits of the confidence interval) for measurements of ionizing radiation - Fundamentals and application. Technical Report ISO/DIS 11929 ( 2010 ). 16 Helfer , I. K. and Miller , K. M. Calibration factors for Ge detectors used for field spectrometry . Health Phys. 55 ( 1 ), 15 – 29 ( 1988 ). Google Scholar CrossRef Search ADS PubMed 17 ICRU . Insitu gamma-ray spectrometry in the environment. Technical report ( 1994 ). 18 FAG . FHZ600A environmental radiation monitoring system. User manual ( 1987 ). 19 Baeza , A. and Corbacho , J. A. Comparative analysis of the in and ex situ determination of environmental radiation and dosimetry levels . Radiat. Prot. Dosim. 113 ( 1 ), 90 – 98 ( 2005 ). Google Scholar CrossRef Search ADS 20 Tyler , A. N. , Sanderson , D. C. W. , Scott , E. M. and Allyson , J. D. Accounting for spatial variability and fields of view in environmental gamma ray spectrometry . J. Environ. Radioact. 33 ( 3 ), 213 – 235 ( 1996 ). Google Scholar CrossRef Search ADS 21 Kastlander , J. and Bargholtz , C. Efficient in situ method to determine radionuclide concentration in soil . Nucl. Instrum. Methods Phys. Res. A 547 ( 2 ), 400 – 410 ( 2005 ). Google Scholar CrossRef Search ADS 22 Thummerer , S. and Jacob , P. Determination of depth distributions of natural radionuclides with in situ gamma-ray spectrometry . Nucl. Instrum. Methods Phys. Res. A 416 ( 1 ), 161 – 178 ( 1998 ). Google Scholar CrossRef Search ADS 23 Shebell , P. et al. . An in situ gamma ray spectrometry intercomparison . Health Phys. 85 ( 6 ), 662 – 677 ( 2003 ). Google Scholar CrossRef Search ADS PubMed 24 Baeza , A. , Corbacho , J. A. , Guillén , J. , Salas , A. , Mora , J. C. , Robles , B. and Cancio , D. Enhancement of natural radionuclides in the surroundings of the four largest coal-fired power plants in Spain . J. Environ. Monit. 14 ( 3 ), 1064 – 1072 ( 2012 ). Google Scholar CrossRef Search ADS PubMed 25 Baeza , A. , Corbacho , J. A. , Guillén , J. , Salas , A. and Mora , J. C. Analysis of the different source terms of natural radionuclides in a river affected by NORM (Naturally Occurring Radioactive Materials) activities . Chemosphere 83 ( 7 ), 933 – 940 ( 2011 ). Google Scholar CrossRef Search ADS PubMed 26 Beretka , J. and Mathew , P. J. Natural radioactivity of Australian building materials, industrial wastes and by-products . Health Phys. 48 ( 1 ), 87 – 95 ( 1985 ). Google Scholar CrossRef Search ADS PubMed 27 Tahir , S. N. A. , Jamil , K. , Zaidi , J. H. , Arif , M. , Ahmed , N. and Ahmad , S. A. Measurements of activity concentrations of naturally occurring radionuclides in soil samples from Punjab province of Pakistan and assessment of radiological hazards . Radiat. Prot. Dosim. 113 ( 4 ), 421 – 427 ( 2005 ). Google Scholar CrossRef Search ADS 28 UNSCEAR . UNSCEAR 2008 report Vol. I: sources of ionizing radiation ( New York : United Nations ) ( 2010 ). 29 ICRP . Data for use in protection against external radiation. Technical report ( 1987 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

MEASUREMENT OF NATURAL RADIONUCLIDES AND EXTERNAL RADIATION EXPOSURE DUE TO FLY ASH FROM A COAL-FIRED POWER PLANT (SPAIN) DEPOSITED ON SOILS. COMPARISON USING TWO DIFFERENT MEASUREMENT TECHNIQUES

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Abstract

Abstract The evaluation of the radiological impact in soils due to the fly-ash ponds using both in situ techniques and laboratory based measurements is presented. In order to check the in situ techniques capabilities for monitoring this type of industries, a comparison between both techniques was performed. A characterization of external radiation exposure in the fly-ash pond and in its surrounding soils was made. The associated external radiological hazard due to the fly-ash pond has been evaluated. In situ techniques could be used to determine the radiological impact on soils due to fly-ash deposition, but its use could be limited due to the associated uncertainties. INTRODUCTION The production of electricity by coal-fired power plants (CFPP) is cataloged as a Naturally Occurring Radioactive Materials (NORM) type industry(1). This is because, depending on their origin, the fuels used have different concentrations of naturally occurring radionuclides, mainly belonging to the uranium and thorium decay series(1). Secondly, the combustion process generates by-products fly ash and slag, whose concentration in these radionuclides is enriched relative to the corresponding fuels(2). And thirdly, part of these activities may be incorporated into the environment due to the normal operation of these plants. Indeed, part of the natural occurring radionuclides contents in the fuel are volatilized as they are burnt in the boilers of these power plants and emitted into the atmosphere. Although the fly ashes generated are often used in cement manufacture, some may also be accumulated in specially designed ponds. For these reasons, there have been studies(3–7) evaluating the radiological health impact of the natural radionuclides emitted into the atmosphere and of the massive deposits of ash resulting from the operation of these power stations. The conventional procedure to determine the gamma-emitting radionuclide concentration levels in soils consists, firstly, in the selection of the appropriate spatial resolution for the required level of accuracy(8). Then, the soil samples are processed in order to match them to the particular geometry for which one has an efficiency calibration for the detector. Lastly, they are measured by gamma spectrometry using low-background Hyper Pure Germanium (HPGe) detectors. The measurement of some naturally radionuclides belonging to the uranium and thorium decay series needs some particular conditions. The plastic container must be sealed to prevent losses from 220,222Rn emanation and it must be expected to reach the secular equilibrium between 220,222Rn and its daughters. The procedure is costly in resources and time. In situ techniques could partially overcome these problems. They can be used to determine the activity and ambient dose-rate levels by means of portable HPGe detectors and dose-rate monitors. In situ measurement techniques are relatively inexpensive and involve reasonably precise methods(8, 9). Since such in situ techniques make it much quicker to measure the soil radionuclides activity levels and the ambient dose rates, one can increase the spatial resolution. However, the in situ gamma spectrometry (ISGS) measurement method are not free of technical challenges. The main difficulty is determining the geometric distribution of the radionuclides in the soils of any given natural environment with accuracy. While there are various semi-empirical and theoretical approximations to estimate these distributions(10–12), the assumptions made in some cases could potentially produce misleading results. The objectives of this study are: (i) to perform a comparison between ISGS technique and conventional lab based methods to estimate the real capability of the ISGS technique in the radiological characterization of a NORM industry; (ii) to determine if there are increase of natural radionuclides in the top soil layer due to the fly-ash deposition and evaluate their contribution to the ambient dose rate and (iii) to evaluate the external radiological hazard due to the presence fly-ash ponds. MATERIAL AND METHODS Study area and sampling strategy The Teruel CFPP (40.997056 N; 0.387512 W) has a current power capacity of 1050 MW. The coal used as fuel is a mixture of imported (bituminous) coal and local black lignite (sub-bituminous coal) in a ratio of 1:3. Its fuel consumption is around 15 000 Tm per day, and it generates about 2000 Tm of ash per day. The fly ash and slag resulting from the coal combustion are accumulated in two nearby ponds, which have a total surface area of 1 km2. The area on it is situated the Teruel CFPP is mainly formed by clay and conglomerate from the Neogene period. There are mainly poor limestone evolved soils with shallow depth. Figure 1 shows the sampling points around the CFPP and its ash ponds. Ten sampling points were selected in an area of radius <2 km from the CFPP and its ash ponds. Moreover, four sampling points were selected at different distances (2.5, 5, 7 and 10 km) from CFPP in the prevailing wind direction. A control soil profile up to 30 cm depth was taken outside the influence of CFPP. Lastly, two sample profiles were collected in the ash ponds. Figure 1. View largeDownload slide Map of the study area and sampling locations. Wind rose: Lat: 41.008 Long: −0.344. Blue: Total wind energy %. Gray: Time %(13). Figure 1. View largeDownload slide Map of the study area and sampling locations. Wind rose: Lat: 41.008 Long: −0.344. Blue: Total wind energy %. Gray: Time %(13). Experimental methods Five top layer soil samples were collected using a 30 × 30 × 4 cm frame. All of them were collected near the ash ponds (distance <2 km). Nine soil profiles were also collected in the surroundings of the CFPP and the ash ponds (distance <2 km) and at different distances of the CFPP in the prevailing wind direction. Also, a soil profile was collected at a sampling point situated outside the influence of the CFPP and the prevailing winds. All soil profile samples were collected conserving separately for assay the layers 0–2, 2–4, 4–6, 6–10, 10–15, 15–20 and 20–25 cm. At each sampling point, an ISGS measurement was made using the portable Ge detector. The method used was to place the detector face downwards at 1 m above the ground. The measurement time chosen was 3600 s. Also, at each sampling point, the FHZ600A dose-rate monitor was used to determine the ambient dose rate, taking at least 10 measurements to have good statistics, and using 180 s per measurement. All the soil samples collected for conventional gamma spectrometry in the laboratory were dried to eliminate their moisture content, and then sieved to a <2-mm grain size. This process allowed the value of the relative moisture level of each sample to be obtained. The dried soil samples were put into 191-cm3 volume Petri dishes and sealed to prevent losses from 222Rn emanation. A time of 28 days was allowed for secular equilibrium between 226Ra and its descendants to be re-established. The soil sample measurements in low-background gamma spectrometry detector were validated according to the quality requirements of ISO 17025 standard(14). The activity determination in the soil samples was performed in a low-activity laboratory. A high resolution low-background gamma spectrometer was used. It consists of a coaxial p-type HPGe detector, with a 45% relative efficiency, 1.95 keV FWHM and 64:1 peak-Compton ratio, all for the 1332.5 keV 60Co emission. The minimum detection activity values were calculated following the recommendations of ISO 11929 standard(15). For the ISGS, a portable coaxial HPGe detector was used with a 43.1% relative efficiency, 2.0 keV FWHM and 56:1 peak-Compton ratio, all for the 1332.5 keV 60Co emission. This detector was coupled to a cryostat which allowed an autonomy of ~4 days. It was calibrated using the semi-empirical techniques proposed in the literature(16, 17). For the ISGS, it was assumed that 226Ra and 232Th are also in equilibrium in the soil with their daughters, even though there is an emanation of 222,220Rn from the soil to the atmosphere(17). In both cases, the following photopeaks were systematically analyzed: 214Pb (351.9 keV), 208T1 (583.1 keV), 214Bi (609.3; 1120; 1764.5 keV), 228Ac (338.3; 911.1 keV) and 40K (1460.7 keV). The 226Ra activity was taken to be the mean value of the measured 214Bi and 214Pb activities, and the 232Th activity to be the mean value of the measured 208T1 and 228Ac activities. To determine the ambient dose rate a FAG FHZ600A radiation monitor was used. This device consist of a 54.2-cm3 pressurized proportional counter with a measurement range of 0.005 μSv/h–1 mSv/h(18). The ambient dose rate readings of FAG FHZ600A dose rate monitor were corrected using the procedure detailed in a previous work(19). DISCUSSION Comparison between in situ techniques and lab based methods There are numerous previous works that detailed the good correspondence between the activity concentration measured by ISGS versus the activity values obtained from laboratory measurements(19–23). To determine the naturally occurring radionuclides concentration in soil by ISGS a good approximation is to suppose that these radionuclides are depth homogeneous distributed(17). However, in the present study one of the significant difficulty is to affirm this last hypothesis, because there is a probable accumulation of wind re-suspended fly ash on the top layer soils, increasing its naturally occurring radionuclide concentrations. In order to determine the most appropriate geometry to apply to ISGS, the multiple peaks method proposed by(10) was applied. This method is based on the relationship between the different degrees of attenuation experimented by photons emitted at different energies by the same multi-emitting radionuclide. 214Bi (609, 1120 and 1764.5 keV) and 228Ac (338 and 911 keV) radionuclides were selected to apply Sowa’s method. Table 1 shows the coefficient β (g/cm2) obtained for 214Bi and 228Ac. Although, Table 1 shows the activity values obtained from the results derived by the application of Sowa’s method and the activity values obtained assuming a depth homogeneous distribution of radionuclides. A correlation analysis between above-cited activities shows that the ratio between them are equal to 1.0, that it indicates both set of activities are statistically the same with the exception of Soil 14 which is situated near to the ash ponds. It could be affected by fly-ash deposition on its top soil layer. In fact, as shown in Figure 2, where the activity levels of 40K, 226Ra and 232Th of depth soil profile of sampling point 14 is plotted, there is a significative naturally occurring radionuclide concentration increasing in the first 5-cm top soil layer. Figure 2. View largeDownload slide Naturally occurring radionuclides concentration in soil profiles for the sampling points 14 and outside CFPP influence. Figure 2. View largeDownload slide Naturally occurring radionuclides concentration in soil profiles for the sampling points 14 and outside CFPP influence. Table 1. Activity concentrations of 214Bi and 228Ac measured by the ISGS technique with reference to the depth profile distribution. 214Bi 228Ac I.d. β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b Soil 1 20 38±8 29±4 >50 22±4 18±3 Soil 2 20 41±8 32±5 >50 44±8 36±5 Soil 3 20 37±8 34±5 >50 13±3 12±2 Soil 4 20 39±8 30±4 50 22±4 19±3 Soil 5 20 35±8 44±7 5 48±9 33±5 Soil 6 >50 22±4 29±4 >50 20±4 16±3 Soil 7 >50 19±4 26±4 >50 14±3 14±2 Soil 8 20 30±7 39±7 >50 19±4 13±2 Soil 9 >50 32±7 38±7 20 27±6 18±3 Soil 10 20 30±7 36±6 >50 20±4 15±2 Soil 11 10 70±14 45±7 5 70±14 30±5 Soil 12 20 26±7 21±3 >50 12±3 11±2 Soil 13 >50 27±7 32±4 >50 15±3 11±2 Soil 14 5 98±20 48±8 10 35±8 33±5 214Bi 228Ac I.d. β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b Soil 1 20 38±8 29±4 >50 22±4 18±3 Soil 2 20 41±8 32±5 >50 44±8 36±5 Soil 3 20 37±8 34±5 >50 13±3 12±2 Soil 4 20 39±8 30±4 50 22±4 19±3 Soil 5 20 35±8 44±7 5 48±9 33±5 Soil 6 >50 22±4 29±4 >50 20±4 16±3 Soil 7 >50 19±4 26±4 >50 14±3 14±2 Soil 8 20 30±7 39±7 >50 19±4 13±2 Soil 9 >50 32±7 38±7 20 27±6 18±3 Soil 10 20 30±7 36±6 >50 20±4 15±2 Soil 11 10 70±14 45±7 5 70±14 30±5 Soil 12 20 26±7 21±3 >50 12±3 11±2 Soil 13 >50 27±7 32±4 >50 15±3 11±2 Soil 14 5 98±20 48±8 10 35±8 33±5 aActivity determination by Sowa’s method. bActivity determination assuming homogeneous depth profile distribution. View Large Table 1. Activity concentrations of 214Bi and 228Ac measured by the ISGS technique with reference to the depth profile distribution. 214Bi 228Ac I.d. β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b Soil 1 20 38±8 29±4 >50 22±4 18±3 Soil 2 20 41±8 32±5 >50 44±8 36±5 Soil 3 20 37±8 34±5 >50 13±3 12±2 Soil 4 20 39±8 30±4 50 22±4 19±3 Soil 5 20 35±8 44±7 5 48±9 33±5 Soil 6 >50 22±4 29±4 >50 20±4 16±3 Soil 7 >50 19±4 26±4 >50 14±3 14±2 Soil 8 20 30±7 39±7 >50 19±4 13±2 Soil 9 >50 32±7 38±7 20 27±6 18±3 Soil 10 20 30±7 36±6 >50 20±4 15±2 Soil 11 10 70±14 45±7 5 70±14 30±5 Soil 12 20 26±7 21±3 >50 12±3 11±2 Soil 13 >50 27±7 32±4 >50 15±3 11±2 Soil 14 5 98±20 48±8 10 35±8 33±5 214Bi 228Ac I.d. β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b β (g/cm2) Act. (Bq/kg)a Act. (Bq/kg)b Soil 1 20 38±8 29±4 >50 22±4 18±3 Soil 2 20 41±8 32±5 >50 44±8 36±5 Soil 3 20 37±8 34±5 >50 13±3 12±2 Soil 4 20 39±8 30±4 50 22±4 19±3 Soil 5 20 35±8 44±7 5 48±9 33±5 Soil 6 >50 22±4 29±4 >50 20±4 16±3 Soil 7 >50 19±4 26±4 >50 14±3 14±2 Soil 8 20 30±7 39±7 >50 19±4 13±2 Soil 9 >50 32±7 38±7 20 27±6 18±3 Soil 10 20 30±7 36±6 >50 20±4 15±2 Soil 11 10 70±14 45±7 5 70±14 30±5 Soil 12 20 26±7 21±3 >50 12±3 11±2 Soil 13 >50 27±7 32±4 >50 15±3 11±2 Soil 14 5 98±20 48±8 10 35±8 33±5 aActivity determination by Sowa’s method. bActivity determination assuming homogeneous depth profile distribution. View Large Based on last results, all ISGS measurements had been performed assuming a depth homogeneous radionuclides in soil. With the exception of Soil 14 where it has been assumed an exponential depth distribution of 226Ra and 232Th. The obtained values had been compared with the average concentration of naturally occurring radionuclides calculated from the soil sample profiles measured in the laboratory. Figure 3 shows the correlation between them. Figure 3. View largeDownload slide Linear fit and R-squared analysis for in situ versus lab activity levels. Error bars: In situ levels: Full uncertainties. Average activity levels: 2×SD. Figure 3. View largeDownload slide Linear fit and R-squared analysis for in situ versus lab activity levels. Error bars: In situ levels: Full uncertainties. Average activity levels: 2×SD. First, there is a good correspondence between two sets of measurements for 226Ra and 232Th as the results of R-squared analysis shows. However, for 40K levels, the result of R-squared indicates a bad correlation between in situ and based on laboratory measurements, although, the slope of the linear fit shows a difference between the activity levels of 40K measured with both techniques about 25%. This is because the photon flux with energies upper than 1 MeV that reaches the HPGe detector have their origin at the first 40-cm of soil depth. CFPP is situated in a significative stony poor evolved soils area. In fact, as shows in Table 2, in some sampling points were only possible to collect sample soil profiles up to 15 cm depth. Thus, the geometry hypothesis assumed for determining the 40K levels by ISGS does not consider the presence of a rocky and poor evolved soils. Table 2. Ratio between activity concentration determined in soil depth [0–2 cm] versus activity concentration determined in the deepest soil layer. Distance (km) Depth (cm) Ratio 40 K[0–2]/[>15] Ratio 226 Ra[0–2]/[>15] Ratio 232 Th[0–2]/[>15] Soil 1 9.5 [0–15] 1.0±0.2 1.4±1.0 1.2±0.4 Soil 2 6.5 [0–15] 1.2±0.1 1.2±0.8 1.6±0.4 Soil 3 4.5 [0–15] 1.3±0.2 1.3±1.1 1.3±0.5 Soil 4 2.5 [0–15] 1.0±0.1 0.9±0.4 1.1±0.2 Soil 5 1.8 [0–25] 1.1±0.1 1.2±0.2 0.9±0.2 Soil 9 1.0 [0–10] 1.0±0.1 1.5±0.2 1.2±0.2 Soil 10 0.7 [0–30] 1.3±0.1 1.2±0.2 1.0±0.3 Soil 11 0.7 [0–20] 1.2±0.1 1.2±0.3 1.2±0.2 Soil 14 0.2 [0–25] 1.3±0.1 5.3±0.9 2.9±0.7 Control soil 10 [0–30] 0.9±0.1 1.0±0.1 1.0±0.2 Distance (km) Depth (cm) Ratio 40 K[0–2]/[>15] Ratio 226 Ra[0–2]/[>15] Ratio 232 Th[0–2]/[>15] Soil 1 9.5 [0–15] 1.0±0.2 1.4±1.0 1.2±0.4 Soil 2 6.5 [0–15] 1.2±0.1 1.2±0.8 1.6±0.4 Soil 3 4.5 [0–15] 1.3±0.2 1.3±1.1 1.3±0.5 Soil 4 2.5 [0–15] 1.0±0.1 0.9±0.4 1.1±0.2 Soil 5 1.8 [0–25] 1.1±0.1 1.2±0.2 0.9±0.2 Soil 9 1.0 [0–10] 1.0±0.1 1.5±0.2 1.2±0.2 Soil 10 0.7 [0–30] 1.3±0.1 1.2±0.2 1.0±0.3 Soil 11 0.7 [0–20] 1.2±0.1 1.2±0.3 1.2±0.2 Soil 14 0.2 [0–25] 1.3±0.1 5.3±0.9 2.9±0.7 Control soil 10 [0–30] 0.9±0.1 1.0±0.1 1.0±0.2 View Large Table 2. Ratio between activity concentration determined in soil depth [0–2 cm] versus activity concentration determined in the deepest soil layer. Distance (km) Depth (cm) Ratio 40 K[0–2]/[>15] Ratio 226 Ra[0–2]/[>15] Ratio 232 Th[0–2]/[>15] Soil 1 9.5 [0–15] 1.0±0.2 1.4±1.0 1.2±0.4 Soil 2 6.5 [0–15] 1.2±0.1 1.2±0.8 1.6±0.4 Soil 3 4.5 [0–15] 1.3±0.2 1.3±1.1 1.3±0.5 Soil 4 2.5 [0–15] 1.0±0.1 0.9±0.4 1.1±0.2 Soil 5 1.8 [0–25] 1.1±0.1 1.2±0.2 0.9±0.2 Soil 9 1.0 [0–10] 1.0±0.1 1.5±0.2 1.2±0.2 Soil 10 0.7 [0–30] 1.3±0.1 1.2±0.2 1.0±0.3 Soil 11 0.7 [0–20] 1.2±0.1 1.2±0.3 1.2±0.2 Soil 14 0.2 [0–25] 1.3±0.1 5.3±0.9 2.9±0.7 Control soil 10 [0–30] 0.9±0.1 1.0±0.1 1.0±0.2 Distance (km) Depth (cm) Ratio 40 K[0–2]/[>15] Ratio 226 Ra[0–2]/[>15] Ratio 232 Th[0–2]/[>15] Soil 1 9.5 [0–15] 1.0±0.2 1.4±1.0 1.2±0.4 Soil 2 6.5 [0–15] 1.2±0.1 1.2±0.8 1.6±0.4 Soil 3 4.5 [0–15] 1.3±0.2 1.3±1.1 1.3±0.5 Soil 4 2.5 [0–15] 1.0±0.1 0.9±0.4 1.1±0.2 Soil 5 1.8 [0–25] 1.1±0.1 1.2±0.2 0.9±0.2 Soil 9 1.0 [0–10] 1.0±0.1 1.5±0.2 1.2±0.2 Soil 10 0.7 [0–30] 1.3±0.1 1.2±0.2 1.0±0.3 Soil 11 0.7 [0–20] 1.2±0.1 1.2±0.3 1.2±0.2 Soil 14 0.2 [0–25] 1.3±0.1 5.3±0.9 2.9±0.7 Control soil 10 [0–30] 0.9±0.1 1.0±0.1 1.0±0.2 View Large In conclusion, the use of ISGS technique for the determination of low increments of naturally occurring radionuclides in the top soil layer may be a complicated task. It is important to emphasize that, in this study, the acquisition time in each ISGS measurement was 3600 s. If the acquisition time is increased up to 20 000 s (>5 h), the uncertainties related to Sowa’s method will decrease significantly. However, this implies the sampling strategy reconsideration. Finally, the use of the above-cited ISGS technique must be supplemented with conventional techniques using an adequate sampling strategy that minimize the time-consuming resources. Radiological environmental impact assessment The naturally occurring radionuclides concentration in fly and bottom ashes generate in a CFPP is related to the type of coal burned in the plant. In a previous work(24), Teruel CFPP fuel was studied and it was observed that the contents determined for the fuel mixes used in this CFPP were consistent with the differences in the natural radionuclide contents of the national and imported coal. The fly ash generated in the Teruel CFPP is retained in the electrostatic precipitators (99.5% of retention efficiency). Using pneumatic pipes the fly ashes are discharged in two ponds. Due to its insolubility, fly ashes settle into the bottom of the ponds and the excess water is discharged into Regallo River(25). As time goes, both ash ponds have been filled and the water was evaporated on their surface. On the dry surfaces of both ponds, two fly-ash profile samples were collected. Although, on each pond, an ambient dose rate measurement was performed. The average activity concentration of 226Ra, 232Th and 40K were 200±40, 80±20 and 340±120Bq/kg, respectively. The 226Ra and 232Th activity values are greater than those measured in soils in a 7 and 4 factors. Table 2 shows the ratio between the activity concentration in the top soil layer versus the activity concentration in the deepest layer. Firstly, control soil, sampled outside the CFPP influence, shows ratios close to 1.0. As to be expected, these ratio values are showing a depth homogeneous distribution of natural radionuclides (see Figure 2). With regards to the soils sampled downwind and far from the ash ponds, the ratio values show a higher variability, but their uncertainties do not allow to assert that there is an evidence of an increase of radionuclides in the top soil layer. Likewise, the soils which were sampled close to the ash ponds (<1 km) show ratios >1.0 showing a slight increase of natural radionuclides in the top soil layer. But from a statistical point of view, it is very difficult to assert it because the uncertainties may mask it. An exception is the soil collected at sampling point 14 that shows a significative concentration of 226Ra and 232Th in the top soil layer versus the deepest layer (see Figure 2). Finally, 226Ra and 232Th concentrations are not increasing in the top soil layer as the distance from ash ponds increases. Therefore, the radioactive soil impact is only limited to a circumscribed area close to the ash ponds. For the purpose of comparing the radiological effects of materials that contain 226Ra, 40K and 232Th by a single quantity, which takes into account the radiation hazards associated with them, common indexes Radium equivalent activity (Raeq) and external hazard index (Hex) are used in the study. Raeq and Hex can be calculated with 1 and 2 expressions(26, 27). On the other hand, absorbed dose in air (ADA) were determined from ISGS measurements using 3 expression(28). The terrestrial ambient dose rate values, H10, were determined from ADA values applying the conversion factor 0.7 Sv/Gy for energies between 60 and 2000 keV(28). Raeq=CRa+1.43·CTh+0.077·CK (1) Hex=CRa/370+CTh/259+CK/4810 (2) ADA(nGy/h)=CRa·0.462+CTh·0.604+CK·0.0417 (3) First, as shown in Table 3, there is a good correspondence between terrestrial ambient dose rate obtained from ISGS measurements, H10ISGS, and from the FAG proportional counter, H10 FAG. In fact, the correlation between both sets of values is highly satisfactory as shown the results of the linear fit: y=m·x+n; m=1.02±0.06; n=−1.3±2.4; r2=0.951. However, the correlation between H10 ISGS versus H10 lab values shows a slight difference in the slope ( m=0.91±0.08) and y-intercept ( n=7±3). As indicated in previous sections, the geometric approach used in ISGS does not include significantly rocky soils and therefore the activity values obtained were, generally, lower than those measured in the laboratory. Whatever, considering the uncertainties, the H10 values obtained from the 226Ra, 40K and 232Th activity levels both measured by ISGS and laboratory are the same. Concerning to the remarkable sampling point 14, where there is a significative accumulation of 226Ra and 232Th due to the deposition of fly-ash on the top layer soil. If the H10 value had been calculated from the activity levels determined only from the top layer of the soil sample, then the obtained result would have been greater in Factor 2. To avoid this, it is necessary to collect a depth soil profile sample. Table 3. Terrestrial ambient dose rate from ISGS, FAG proportional counter and lab measurements. Radium equivalent concentration and External hazard index. I.d. H10ISGS (nSv/h) H10 FAG (nSv/h) H10lab (nSv/h) Raeq(Bq/kg) Hex Soil 1 22±3 22±4 18±5 68 ± 8 0.18 Soil 2 33±5 24±5 30±5 103 ± 14 0.28 Soil 3 20±3 12±2 28±6 62 ± 8 0.17 Soil 4 22±3 31±6 33±3 69 ± 9 0.19 Soil 5 28±4 24±5 29±4 87 ± 11 0.24 Soil 6 20±3 28±6 27±4 61 ± 9 0.17 Soil 7 18±3 21±4 21±4 56 ± 8 0.15 Soil 8 20±3 16±3 24±4 62 ± 8 0.17 Soil 9 26±4 21±4 30±3 81 ± 11 0.22 Soil 10 23±4 18±4 22±5 71 ± 11 0.19 Soil 11 32±5 34±7 34±5 99 ± 13 0.27 Soil 12 15±2 16±3 17±4 46 ± 6 0.13 Soil 13 18±3 20±4 43±4 57 ± 7 0.15 Soil 14 30±4 27±5 33±21 93 ± 10 0.25 Control Soil – 29±4 26±5 88 ± 5 0.24 Ash Pond 110 ± 16 113 ± 24 109±3 345 ± 37 0.93 I.d. H10ISGS (nSv/h) H10 FAG (nSv/h) H10lab (nSv/h) Raeq(Bq/kg) Hex Soil 1 22±3 22±4 18±5 68 ± 8 0.18 Soil 2 33±5 24±5 30±5 103 ± 14 0.28 Soil 3 20±3 12±2 28±6 62 ± 8 0.17 Soil 4 22±3 31±6 33±3 69 ± 9 0.19 Soil 5 28±4 24±5 29±4 87 ± 11 0.24 Soil 6 20±3 28±6 27±4 61 ± 9 0.17 Soil 7 18±3 21±4 21±4 56 ± 8 0.15 Soil 8 20±3 16±3 24±4 62 ± 8 0.17 Soil 9 26±4 21±4 30±3 81 ± 11 0.22 Soil 10 23±4 18±4 22±5 71 ± 11 0.19 Soil 11 32±5 34±7 34±5 99 ± 13 0.27 Soil 12 15±2 16±3 17±4 46 ± 6 0.13 Soil 13 18±3 20±4 43±4 57 ± 7 0.15 Soil 14 30±4 27±5 33±21 93 ± 10 0.25 Control Soil – 29±4 26±5 88 ± 5 0.24 Ash Pond 110 ± 16 113 ± 24 109±3 345 ± 37 0.93 View Large Table 3. Terrestrial ambient dose rate from ISGS, FAG proportional counter and lab measurements. Radium equivalent concentration and External hazard index. I.d. H10ISGS (nSv/h) H10 FAG (nSv/h) H10lab (nSv/h) Raeq(Bq/kg) Hex Soil 1 22±3 22±4 18±5 68 ± 8 0.18 Soil 2 33±5 24±5 30±5 103 ± 14 0.28 Soil 3 20±3 12±2 28±6 62 ± 8 0.17 Soil 4 22±3 31±6 33±3 69 ± 9 0.19 Soil 5 28±4 24±5 29±4 87 ± 11 0.24 Soil 6 20±3 28±6 27±4 61 ± 9 0.17 Soil 7 18±3 21±4 21±4 56 ± 8 0.15 Soil 8 20±3 16±3 24±4 62 ± 8 0.17 Soil 9 26±4 21±4 30±3 81 ± 11 0.22 Soil 10 23±4 18±4 22±5 71 ± 11 0.19 Soil 11 32±5 34±7 34±5 99 ± 13 0.27 Soil 12 15±2 16±3 17±4 46 ± 6 0.13 Soil 13 18±3 20±4 43±4 57 ± 7 0.15 Soil 14 30±4 27±5 33±21 93 ± 10 0.25 Control Soil – 29±4 26±5 88 ± 5 0.24 Ash Pond 110 ± 16 113 ± 24 109±3 345 ± 37 0.93 I.d. H10ISGS (nSv/h) H10 FAG (nSv/h) H10lab (nSv/h) Raeq(Bq/kg) Hex Soil 1 22±3 22±4 18±5 68 ± 8 0.18 Soil 2 33±5 24±5 30±5 103 ± 14 0.28 Soil 3 20±3 12±2 28±6 62 ± 8 0.17 Soil 4 22±3 31±6 33±3 69 ± 9 0.19 Soil 5 28±4 24±5 29±4 87 ± 11 0.24 Soil 6 20±3 28±6 27±4 61 ± 9 0.17 Soil 7 18±3 21±4 21±4 56 ± 8 0.15 Soil 8 20±3 16±3 24±4 62 ± 8 0.17 Soil 9 26±4 21±4 30±3 81 ± 11 0.22 Soil 10 23±4 18±4 22±5 71 ± 11 0.19 Soil 11 32±5 34±7 34±5 99 ± 13 0.27 Soil 12 15±2 16±3 17±4 46 ± 6 0.13 Soil 13 18±3 20±4 43±4 57 ± 7 0.15 Soil 14 30±4 27±5 33±21 93 ± 10 0.25 Control Soil – 29±4 26±5 88 ± 5 0.24 Ash Pond 110 ± 16 113 ± 24 109±3 345 ± 37 0.93 View Large On the other hand, the ambient dose rate background is 29±4nSv/h. This value was measured in the sampling point situated outside the influence of the CFPP which is not affected by fly-ash deposition on its top soil layer. From the statistically point of view is not possible to assert that there are an increment of ambient dose rate due to the presence of fly-ash on the top soil layers that are situated downwind (Soil 1–5) the CFPP and those that are situated close to the ash ponds. Regarding the fly-ash ponds, the ambient dose rate is greater than the background in a Factor 5. However, subtracting the background, the annual ambient dose rate is 0.8 mSv. Using the conversion factor 0.87 to obtain effective dose rate(29) the obtained value is 0.69 mSv/y that it is below the limit of 1.0 mSv/y of annual effective dose rate due to external sources for the public members. It is clear from Table 3 that the range of levels of radium equivalent activity is 46–103 Bq/kg in the surrounding soils of the CFPP and its ash ponds. Based on the annual external dose limit of 1.0 mSv, the activity limiting terms of radium equivalent is 370 Bq/kg(2). It can be seen that the radium equivalent activity in those soils is below the recommended limit. However, in the ash ponds the maximum level of radium equivalent activity is obtained. Although this value plus its uncertainty is below the recommended limit, too. Finally, the calculated values of external hazard index for the soils studied are given in Table 3. This hazard index must be less than unity for the radiation hazard to be insignificant It can be seen in this Table that the range of values of the external hazard index is 0.15–0.93, the latter being the obtained in the fly-ash. All of them are below the unit. CONCLUSION The assessment of the radiological impact on the vicinity of the power plants appeared to be a rather complicated task. In the case of relatively modern plants like Teruel CFPP with fly-ash removal efficiency >99.5%, such impact is negligible. However, the accumulation of huge amounts of fly ash in ponds could be considered as the main source of radiological impact associated with these industries. The analytical results lead to the following conclusions: In this study a good correspondence between the 226Ra and 232Th activity levels determined both in situ techniques and conventional laboratory measurements was observed (R2 = 0.937 and R2 = 0.881, respectively). However, there is not a good correspondence between 40K activity values determined by both techniques. This is because the geometric approach used in ISGS technique consider that all photon flux coming from the soil that reaches the detector have their origin in the first 40 cm of depth. As, in previous sections was shown, the soils of the Teruel CFPP area are rocky and small deepness. And it is extremely difficult to consider them in the geometric approximation used. For that, the 40K activity values determined by ISGS technique are lower than those determined by conventional technique. In conclusion, the main drawback of ISGS technique relates to the geometric approach used to determine the naturally occurring radionuclides in soils. In fact, using Sowa’s method, it is possible to obtain, with an acceptable uncertainty, an approximate soil depth distribution of the multi-emitter radionuclides like 214Pb and 214Bi. In any case, using the depth homogeneous distributed geometric approach the obtained results would be acceptable, but it is necessary to take some soil samples at strategic points to improve the accuracy of the soil characterization of the surroundings of CFPP. The H10 values are calculated from activity levels and measured by proportional counter dose rate. A high correspondence has been observed between the H10 determined both from ISGS and laboratory based measurements versus H10 measured by proportional counter dose rate monitor (R2 = 0.943 and R2= 0.906, respectively). On the other hand, considering the uncertainties, the H10 values obtained from the 226Ra, 40K and 232Th activity levels both measured by ISGS and laboratory are the same. The results indicate that terrestrial ionizing radiation exposure associated with NORM resulting from fly ash from the Teruel CFPP could contribute and additional effective dose of up to 0.7 mSv annually. The huge amount of fly ash and bottom ash disposed in ash ponds could be a source of naturally occurring radionuclides that easily could be dispersed in surrounding soils by the wind. However, this radioactive soil impact is only limited to a circumscribed area close to the ash ponds. Finally, common indexes Raeq and Hex have been calculated. The Raeq and Hex obtained range in soils are [46–103] Bq/kg and [0.15–0.28], respectively. On the other hand, the maximum values of Raeq (346 Bq/kg) and Hex (0.93) are obtained in ash ponds. Both are below the recommended limits. FUNDING This work was made possible thanks firstly to funding received from Spain’s Nuclear Safety Council for the project entitled ‘Estudio del impacto radiológico de las centrales de carbón sobre sus entornos’, and secondly to funding from the Government of Extremadura through ‘Ayudas para el fortalecimiento de los grupos de investigación de Extremadura’. REFERENCES 1 IAEA . Extent of environmental contamination by naturally occurring radioactive material (NORM) and technological options for mitigation. Technical report ( 2003 ). 2 UNSCEAR . Sources and effects of ionizing radiation. United Nations Scientific Committee on the Effects of Atomic Radiation. Technical report ( 2000 ). 3 Rosner , G. , Bunzl , K. , Hötzl , H. and Winkler , R. 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A. , Jamil , K. , Zaidi , J. H. , Arif , M. , Ahmed , N. and Ahmad , S. A. Measurements of activity concentrations of naturally occurring radionuclides in soil samples from Punjab province of Pakistan and assessment of radiological hazards . Radiat. Prot. Dosim. 113 ( 4 ), 421 – 427 ( 2005 ). Google Scholar CrossRef Search ADS 28 UNSCEAR . UNSCEAR 2008 report Vol. I: sources of ionizing radiation ( New York : United Nations ) ( 2010 ). 29 ICRP . Data for use in protection against external radiation. Technical report ( 1987 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Radiation Protection DosimetryOxford University Press

Published: May 17, 2018

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