Assessment of Natural Radionuclides in Fly Ash Produced at Orji River Thermal Power Station , Nigeria and the Associated Radiological Impact Janet

Coal fired power plants produce significant amounts of ashes, which are quite often being used as additives in cement and other building materials. Coal contains trace quantities of 226Ra, 232Th and 40K. The concentrations of these radionuclides are usually low in the coal, but enriched in fly ash. The activity concentrations of natural radionuclides in fly ash and soil samples in the vicinity of Orji River thermal power station in Nigeria was determined by gamma ray spectroscopy method using NaI(Tl) detector. The mean activity concentrations of 226Ra, 232Th and 40K in the fly ash were 40.8 ± 11.6, 49.1 ± 9.3 and 321 ± 17 Bq∙kg−1, respectively. Coarse fly ash collected from the dump site had mean activity concentrations of 28.2 ± 8.3, 37.6 ± 5.0 and 335 ± 32 Bq∙kg−1, respectively for 226Ra, 232Th and 40K. Soil samples collected at about distances of 10 m from the dump site had 32.7 ± 4.3, 40.0 ± 4.2 and 298 ± 15 Bq∙kg−1, respectively. Those collected at about distances of 100 m from the dump site had 39.1 ± 11.2, 34.1 ± 5.2 and 257 ± 19 Bq∙kg−1, respectively. Occupational dose received by workers due to exposure to the fly ash ranged between 33.0 and 61.2 μSv∙y−1 with a mean value of 47.1 ± 8.4 μSv∙y−1, which is below the intervention exemption level of 1 mSv∙y−1. The radium equivalent activity concentration, external and internal hazard indices of the fly ash were below the recommended maximum values for building materials. The mean outdoor absorbed dose rate and the annual effective dose obtained in the vicinity of the plant were 49.7 ± 4.0 nGy∙y−1 and 0.30 ± 0.02 mSv (dump site), 51.7 ± 3.6 nGy∙y−1 and 0.32 mSv (soil 10 m from dump site), 49.4 ± 4.9 nGy∙y−1 and 0.30 ± 0.03 mSv (soil 100 m away from dump site), which are lower than the world average. The results obtained in this study show that there is no significant radiological impact of the fly ash on both the workers and the public from radiation protection point of view. Corresponding author. J. A. Ademola, U. C. Onyema 753


Introduction
Measurements of radioactivity in Technologically Enhanced Naturally Occurring Radioactive Materials (TE-NORMs) are important from radiation protection point of view because more than 50% of the total dose to human population from natural sources of radiation is contributed by these materials [1].Coal is technologically important material used for power generation and its fly ash is used in manufacturing of bricks, sheet, cement, land filling etc. Coal, like most materials found in nature, contains trace quantities of the naturally occurring primordial radionuclides arising from the U and Th series and 40 K. Naturally occurring radionuclides contribute most to environmental radiation.The concentration of these long lived radionuclides is usually low in the coal.When it is burnt in power plant, the fly ash that is emitted through the stack gets enriched in some of the radionuclide [2].Therefore, the combustion of coal results in the release of some natural radioactive elements into the environment and in the redistribution of these radioactive elements in the surface soil particularly in the vicinity of coal-fired power plants.This can modify ambient radiation fields and population exposures [3].Thus coal fired power plant is a major contributor towards Technologically Enhanced Natural Radiation (TENR) [4].The interest in measuring radioactivity in terms of activity concentrations of 238 U, 232 Th and 40 K concentration in fly ash is due to its health hazards and environmental pollution.The concentration of radioactivity in fly ash is extremely variable and depends on the composition of the parent coal, conditions during coal combustion and efficiency of emission control devices etc. [2].
The Oji River Power Station is a thermal power station in Oji River town in Enugu State, Nigeria.The plant reportedly last generated power in 2004.The Oji River thermal power station was effective with power generation to commercial towns and improved the industrial base of the eastern region since it was established by the British colonial administration in the country.The efficacy of its operation is based on the fact that there are constructed mechanical devices that enhance intake of water from the fast flowing Oji River into the plant house that is powered by natural coal to provide electrical energy, which is then transmitted to various towns in the former eastern region.At present, the thermal power station is not functional.However, there is fly ash in the burning chambers and dump of fly ash in the vicinity of the station.This could pose radiation risk to the workers and people in the vicinity of the thermal station.This study seeks to determine the activity concentration of natural radionuclides in fly ash and soil samples collected in the vicinity of the power station and assess the radiological implication.

Sample Preparation
A total of 20 samples of fly ash were collected from the thermal plant.In order to investigate the radioenvironmental impact due to discarded fly ash from the power plant, samples were collected from dump site where fly ash is discarded and soil samples in the vicinity of the power plant.These composed of 10 coarse fly ash samples from the dump site, 10 soil samples at distances of about 10 m away from the dump site and 10 soil samples at distances of about 100 m away from the dump site.The soil samples were collected from the surface layer (0 -10 cm depth).The samples were oven dried at a temperature of 100˚C until a constant weight was attained.The coarse fly ash from the dump site and the soil samples were pulverized.A mass of 130 g each of the fly ash and coarse fly ash and 150 g each of the soil samples were packed in cylindrical plastic containers of uniform size of base diameter 7 cm which could sit on the 7.6 cm by 7.6 cm NaI(T1) detector.The plastic containers were sealed tightly with caps and wrapped with thick cellotape around their screw necks and kept for more than four weeks in order for 226  Ra, 232 Th and their daughter products to attain secular equilibrium.

Activity Concentration Measurement
The activity concentration of natural radionuclides 226  Ra, 232 Th and 40 K were measured with gamma ray spec-trometry system having a 7.6 cm by 7.6 cm NaI(T1) detector (Model No. 802 series, Canberra Inc.) shielded in a lead.The detector is coupled through a preamplifier base to Canberra series 10 plus multichannel analyzer (MCA) (Model No.1104) and has a resolution of about 8% at energy of 0.662 MeV ( 137 Cs).This is capable of distinguishing the gamma ray energies considered during the measurements.The uranium-238 and thorium-232 activities were determined indirectly through the activities of their daughter products.The choice of the reference nuclides for their activity determination was made based on the fact that NaI(Tl) detector has a poor resolution, hence, the peaks of interest to be considered would be sufficiently discriminated and intense.The 226 Ra and 232 Th contents were determined from 1.760 MeV ( 214 Bi) and 2.614 MeV ( 208 Tl), respectively.The potassium content was determined from 1.461 MeV photopeak of 40 K.
The energy calibration of the counting system was done as described by Ademola and Olatunji (2013) [5].The detection efficiency of the detector was determined using a reference source of known activity concentrations of each of the radionuclides prepared from Rocketdyne Laboratories, California USA, which is traceable to a mixed standard gamma source (No. 48772-356) by Analytic Inc., Atlanta, Georgia.Each of the sample containers was placed on top of the detector and counted for 10 hours.Background radiation was corrected for all the samples.

Occupational Dose Assessment
Occupational exposure could be external, internal or both.For the assessment of the occupational dose at the power plant station due to the fly ash, the following exposure pathways were considered: 1. external exposure to gamma radiation

inhalation of dust
The external exposure to gamma radiation is calculated using the equation [6] , where A R is the activity concentration of radionuclide R (Bq•kg −1 ), DC ext,R is the effective dose coefficient for radionuclide R in Sv•h −1 per Bq•kg −1 and T is the annual exposure time assumed to be 2000 h•y −1 .The inhalation of contaminated dust was calculated using the relation given in Weiss et al. (2005) [7]; where V is the breathing rate, g inh,R is the inhalation dose coefficient for radionuclide R (Sv•Bq −1 ) and C R is the ambient activity concentration of radionuclide R (Bq where k is the dust loading factor in kg•m −3 .

Radiological Hazard Assessment of Fly Ash as Building Material
Fly ash has been shown to be a very good substitute of soil for geo-technical applications, substitute of cement for mortar and concrete and substitute of clay for manufacture of bricks and various ceramic products [8].The radiological hazard of materials used as building materials is assessed by the radium equivalent activity concentration (Ra eq ), external (H ext ) and internal (H int ) hazard indices.Ra eq , H ext and H int are calculated using the following relations, respectively [9]- [11]; where A Ra , A Th and A K are the activity concentrations of 226 Ra, 232 Th and 40 K in Bq•kg −1 , respectively.Radium equivalent activity concentration is based on the estimation that 1 Bq•kg −1 226 Ra, 0.7 Bq•kg −1 232 Th or 13 Bq•kg −1 40 K produce the same gamma dose rate [12].For safe use of building materials as far as radiation is concerned, the maximum value of Ra eq in the materials must be less than 370 Bq•kg −1 .H ext is obtained from the Ra eq expression by assuming that its maximum value allowed (equal to unity) corresponds to the upper limit of Ra eq (370 Bq•kg −1 ).H int takes into consideration the radiation hazard to respiratory organs.The numerical quantities at the denominator of Equations ( 5) and ( 6) are in the units of Bq•kg −1 ; hence the indices are dimensionless quantities.The indices should be less than unity for safe use of building material.

Outdoor Radiation Dose
The outdoor exposure due to gamma radiation from soil in the vicinity of the plant was calculated using the relation [13] [14]; ( ) where A Ra , A Th and A K are the activity concentrations of 226 Ra, 232 Th and 40 K, respectively in Bq•kg −1 .This equation accounts for external absorbed dose rate at 1 m above ground surface.The annual effective dose was estimated taking into the conversion coefficient (CC), from absorbed dose to effective dose, 0.7 Sv•Gy −1 and occupancy factor (OF) [13] [15]; ( )

Activity Concentrations
The activity concentrations of the samples examined are presented in   Ra, 232 Th and 40 K, respectively.When compared with those of the world population weighted value for soil, 226 Ra in the fly ash and soil samples collected 100 m from the dump site are higher than the world population weighted value of 33 Bq•kg −1 .This could be due to the fact that uranium compounds are soluble and the enhanced level in the fly ash could be partially leached from the dump site to the soil [16].The other two radionuclides in all the samples are within the world population weighted value of 45 Bq•kg −1 for 232 Th and 420 Bq•kg −1 for 40 K [13].They are also lower than the global average for building material, 50 Bq•kg −1 for 226 Ra, 50 Bq•kg −1 for 232 Th and 500 Bq•kg −1 for 40 K [15].

Occupational Dose Assessment
The total dose to workers is the summation of the external dose and the internal dose that is Equations ( 1) and (2).The following parameters were used for the calculation of the occupational dose: 1.The effective dose equivalents for contaminated surface, 9.929, 0.003 and 1.175 nSv•h −1 per Bq•g −1 for 226 Ra, 232 Th and 40 K, respectively [6]. 2. Inhalation dose coefficients, 2.2 × 10 −6 , 2.9 × 10 −5 and 3.0 × 10 −9 Sv•Bq −1 , respectively for 226  Ra, 232 Th and 40 K.These are the values for the activity median aerodynamic diameter (AMAD) of dust particle 5 µm for normal working condition.The conversion factor corresponds to M (moderate residence time in lungs) for 226 Ra and 232 Th and F (short residence time in lungs) for 40 K [17].3. The breathing rate for industrial scenario was taken as 1.5 m 3 •h −1 [18].4. Dust loading factor for industrial activity, 10 mg•m −3 was used [19].
The mean activity concentrations of the three radionuclides in the fly ash had been used to estimate the occupational exposure using Equations ( 1) and ( 2), and the parameters stated above.The results obtained are presented in Table 3.The effective dose due to external exposure varied from 1.2 to 2.1 µSv•y −1 and that of internal exposure varied from 31.6 to 59.1 µSv•y −1 , showing a higher inhalation dose.The total annual effective dose varied between 33.0 and 61.2 µSv•y −1 , with a mean value of 47.1 ± 8.4 µSv•y −1 .The mean of the estimated annual effective doses is below the intervention exemption level (1.0 mSv•y −1 ) given in the International Commission on Radiological Protection Publication 82 [20].However, it is recommended that adequate protective means should be ensured in order to minimize the occupational dose.

Radiation Hazard of Fly Ash as Building Material
Equations ( 4)-( 6) had been used to determine the suitability of the fly ash for use as building material as far as radiation is concerned and the results are shown in Table 4.The value of Ra eq ranged from 107 to 180 Bq•kg −1 .The values of the hazard indices ranged between 0.29 and 0.49 for H ext and 0.36 and 0.66 for H int .The mean values of Ra eq , H ext and H int are 136 ± 19 Bq•kg −1 , 0.37 ± 0.05 and 0.48 ± 0.08, respectively.The errors quoted are the standard deviations from the mean values.All the results obtained are lower than the maximum recom- mended limits for building materials; 370 Bq•kg −1 for Ra eq , and 1 (unity) for H ext and H int .This is an indication that the use of the fly ash as building material does not pose any significant hazards as far as radiation is concerned.

Outdoor Radiation Dose
The outdoor radiation dose at 1 m above the ground was calculated using the mean activity concentrations of 226 Ra, 232 Th and 40 K in the fly ash from the dump site and soil samples in the vicinity of the plant and Equation (7).The results obtained are presented in Table 5.The absorbed dose rate was 49.7 ± 4.0 nGy•h −1 for coarse fly ash collected from the dump site.Those of the soil samples were 51.7 ± 3.6 and 49.4 ± 4.9 nGy•h −1 , respectively for 10 m and 100 m distances.All are below world population weighted mean of 60 nGy•h −1 [13].
The annual effective doses had been calculated assuming an occupancy factor of 0.2 in Equation ( 8).The results are within the range of 0.30 ± 0.02 and 0.32 ± 0.02 mSv, which is lower than the annual 1 mSv recommended for the public by the International Commission on Radiological Protection [21].

Conclusion
Ashes produced in thermal power plants may contain high levels of natural radioactivity and constitute a potential health hazard to the power plant personnel, and to the population living in the vicinity, due to fly-ash releases, fly ash depositions and fly-ash industrial utilization.The radioactivity of fly ash samples and soil samples in the vicinity of the thermal plant was determined using gamma ray spectroscopy method.The activity concentrations of 226 Ra, 232 Th and 40 K in the fly ash were higher than other samples.However, they were below the world average for soil and building materials.The occupational dose obtained was very low compared to the recommended occupational dose.The radium equivalent activity, the external and internal hazard indices of the fly ash were below the recommended limit.The annual effective doses estimated were within the recommended limit of 1 mSv for the public.From radiological point of view, it can therefore be concluded that workers and the people in the vicinity of the thermal plant are not at risk to significant radiological hazard.

Table 1 .
Activity concentrations of natural radionuclides in fly ash.

Table 4 .
Radium activity concentrations (Ra eq ), external (H ext ) and internal (H int ) hazard indices of fly ash.

Table 5 .
Outdoor absorbed dose rate (D) and annual effective dose (H).