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 Table of Contents 
ORIGINAL ARTICLE
Year : 2020  |  Volume : 43  |  Issue : 1  |  Page : 36-43  

Natural radionuclides in rock and radiation exposure index from uranium mine sites in parts of Northern Nigeria


1 Department of Physics, Nasarawa State University, Keffi, Nigeria
2 Nuclear Structure Research Group (NSRG), School of Physics, University of the Witwatersrand, Johannesburg, South Africa
3 Department of Physics, Nasarawa State University, Keffi; Radiological Safety Department, Nigerian Nuclear Regulatory Authority, Abuja, Nigeria
4 Department of Physics, Federal University of Lafia, Lafia, Nasarawa State, Nigeria

Date of Submission13-Mar-2020
Date of Decision21-Mar-2020
Date of Acceptance26-Mar-2020
Date of Web Publication12-May-2020

Correspondence Address:
Samuel Odumu Ogana John
Department of Physics, Nasarawa State University, PMB 1022, Keffi
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_7_20

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  Abstract 


Radiation exposure in humans can emanate from natural radionuclides through uranium and thorium decay series as well as40K due to emitted ionising radiations. It is important to estimate the exposure of humans to the diverse sources of radiation. In this study, the activity concentrations of natural radionuclides238U and232Th in granitic rock samples from uranium mine sites in parts of Northern Nigeria were measured, by inductively coupled plasma-mass spectrometry. The highest values of238U and232Th concentrations (924.56 ± 17.13 and 21.96 ± 1.28 Bq/kg, respectively) were observed at Mika-I and Riruwai. Furthermore, the radiological exposure parameters (i.e., absorbed dose rate in air [D], annual effective dose [external], radium equivalent activity (Raeq), external exposure index (Hex), internal exposure index (Hin), and representative level index (Iγand Iα) were estimated and compared to the international recommended values. In terms of terrestrial gamma radiation from granitic rock within the study area, it does not pose any significant radiation exposure to the workers and dwellers.

Keywords: Absorbed dose, inductively coupled plasma-mass spectrometry, radioactivity, radiological exposure, uranium ore


How to cite this article:
Ogana John SO, Usman IT, Akpa TC, Abubakar SA, Ekong GB. Natural radionuclides in rock and radiation exposure index from uranium mine sites in parts of Northern Nigeria. Radiat Prot Environ 2020;43:36-43

How to cite this URL:
Ogana John SO, Usman IT, Akpa TC, Abubakar SA, Ekong GB. Natural radionuclides in rock and radiation exposure index from uranium mine sites in parts of Northern Nigeria. Radiat Prot Environ [serial online] 2020 [cited 2020 Aug 14];43:36-43. Available from: http://www.rpe.org.in/text.asp?2020/43/1/36/284233




  Introduction Top


In the universe, persons or individuals are exposed directly or indirectly to ionizing radiation. The sources of radiation exposure are the progenies of parent primordial nuclide decay series of uranium-238 (238 U) and thorium-232 (232 Th). Others include cosmic rays and artificial radioactivity from fallout in nuclear testing and industries as well as medical applications.[1],[2]

Higher concentration and higher radiation levels are associated with igneous rocks, such as granite, and lower levels with sedimentary rocks. There are exceptions, however, as some shale and phosphate rocks have relatively high content of radionuclides.[3] Granitic rocks contain natural radioactivity which includes238 U,232 Th, and40 K at various concentrations and contribute significantly to total radiation dose. The radiation dose emanates from gamma rays that are from the rocks, soils, and some building materials, and primarily depends on the geological and geographical conditions of the environment.[1],[4] The concentrations of radionuclides in the rock, soil, or water–air; the time spent outdoors; and the shielding by buildings, have been the main factors that determine the exposure rate to a given person. It was estimated by the International Atomic Energy Agency (IAEA) that natural radionuclides contributed 80%, whereas 20% was from cosmic rays and nuclear processes, of the doses to the environment.[5],[6]

Uranium is formed deep within the earth's surface through a series of geological processes, has heterogeneous distribution with widely varying concentrations in different rocks, and can be obtained through mining.[7] Upon mining and refining, it is useful as a fuel in nuclear reactor for power generation and research and has many industrial and medical applications. The mining and processing gives rise to external gamma radiation from the ores, inhalation of dusts containing long-lived alpha-emitting radionuclides, and inhalation of short-lived decay products of radon.[3] Radon decay progenies are inhaled in mine areas when poorly ventilated and can lead to exposures that exceed radiation exposure limits, resulting in the incidence of lung cancer in mine workers.[3]

It is important to estimate the exposures of humans to the different sources of radiation, both external and internal. Information obtained from such study is of interest in the area of environmental radioactivity monitoring, establishment of rules and regulation on radiation protection, and as biochemical and geochemical tracers in the environment.[8],[9]

The main objective of the present study is to determine natural radioactivity concentration in uranium ore-mined rocks from some parts of Northern Nigeria, using inductively coupled plasma-mass spectrometry (ICP-MS). In addition, it also intended to evaluate the associated radiological exposure indexes such as exposure index, representative level index, absorbed dose rate, and annual effective dose (external), which can give an idea on the onsite occupational exposure of workers in this particular uranium mine areas.


  Materials and Methods Top


Geology of the study area

The study area covers accessible locations in parts of Northern Nigeria where mining activities of uranium ores are obtainable, which includes Adamawa (Michika), Kano (Riruwai), and Taraba (Mika) areas. Global Positioning System navigator (eTrex 10 GARMIN Garmin International, Inc., Olathe, Kansas, USA) was used to register the coordinates of each sample location, which are indicated in column 2, [Table 1]. The map of sampling area of this study is shown in [Figure 1]. Riruwai mine site is located at Kaffo, 10 km east of Riruwai, in Kano state, North-western Nigeria. The area was described as a ring complex, which is composed of elliptical multi-phase granitic massif bodies with related quartz veins that intruded into Pan-African basement. As Greisenized granite Precambrian (Age 541–4000 Ma) and part of Nigerian younger granite provinces that comprise mainly of granitoids and volcanic rocks, it is endowed with 28 valid minerals, including gold, pyrochlore, coffinite, thorite, and zircon. It is chemically distinct from the basement suites of per-alkaline and sub-alkaline.[10],[11],[12]. Mika and Michika mine sites are, however, located at Taraba and Adamawa states, respectively, both in North-eastern Nigeria. Uranium mineralization in the North-eastern Nigeria is of sandstone-hosted and vein-type mineralization. Mika and Michika are of vein-type mineralogy, which occurs in deformed migmatites and granitoid.[13] The mika area is more of granites with fine-grained granite, medium-grained granite, and porphyritic granite. Both primary and secondary uranium mineralization is rhyolite and occurs at depth for pitchblende, while near surface are mainly of meta-autunite and coffinite.[14],[15] Michika is made up of mostly granitic rocks associated with rhyolite veins and brecciated, silicified, and mylonitized rocks. The area is underlain by crystalline basement rock of Precambrian age.[16],[17] The vein-type (granite-related U-deposit) mineralization occurs in the ganitoids, with the major occurrences being Gubrunde, Kanawa, Ghumchi-Michika, Mika, and Monkin-Maza deposits.
Table 1: The activity concentrations of232Thorium and238Uranium (Bq/kg) of the uranium ore rock samples

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Figure 1: Map of Nigeria showing uranium ore sample mine sites (Riruwai, Mika-I, Mika-II and Michika) for this study

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Sample collection and counting

Based on the geological features and mineralization of the sampling location, about 2.0 kg each of the uranium ore rock samples was collected at open pit-mining sites and immediately transferred into a zip-lock plastic bag to avoid contamination. [Figure 1] illustrates the study area. ICP-MS analytical technique requires that the sample be introduced in liquid or aerosol form. Hence, the rock samples were crushed and milled into powder texture, using Dickie and Stockler TS-250, Johannesburg, crushing and pressurized milling machines. Digital Microwave Digestive System (Anton Paar Multiwave, PerkinElmer Germany) was used to dissolve the samples. About 0.5 g of the sample was weighed into a microwave digester vial and 2.0 mL of distilled water was added, with 2.0 mL of concentrated HNO3, 6.0 mL of concentrated HCl, and 0.5 mL H2O2. The vials containing the aliquot of sample are sealed into a rotor, placed into the microwave reactor system, and then heated at 180°C for a maximum period of 50 min. An aqueous solution in acidified matrix was then obtained. The dissolved samples were then filtered using polyvinylidene fluoride 0.22 μm. A diluent, composed of 5% (50 mL) HNO3 and 1% (10 mL) HCl acids, was added to the filtrate in two sets by 10% and 50%. Both the prepared and blank samples, with the calibration standards (5 ppb, 20 ppb, 50 ppb, 100 ppb, 500 ppb, and 1000 ppb), and quality control samples (20 ppb and 100 ppb) were placed into the autosampler, for ICP-MS measurement and calibration [Figure 2]. Prior to the upload of the samples into the low-flow sample introduction system, nitric acid was used to clean the tubing, to avoid washing of residual memory. The cleaning and rinsing of the tubing was repeated after each sample to avoid cross contamination. The samples were introduced into a nebulizer containing plasma at atmospheric pressure, with an extremely high temperature of 10,000°C, which makes for easy transition of most elements from atoms to ions. Mostly, single-charged positive ions are formed and because argon plasma is used, it becomes relatively simple for mass spectra to be generated and interpreted. The plasma ionizes the samples and then through a vacuum interface, transfers to the mass analyzer – A hyperbolic quadrupole mass analyzer, which is sequential, thereby measures elements in sequence. Due to ICP-MS's abundant sensitivity, superior adjacent peak separation was achieved.[18] The separated ions are then detected by the ICP-MS detection system – a fast simultaneous dual-mode detector (of nine orders dynamic range), giving rise to data acquisition. Fifty elements were measured/identified including238 U,232 Th, trace elements, and rare earth elements, upon completion of measurement. The process is automatic and computerized. The control is performed on the computer system via an installed software-ICP-MS (MassHunter Workstation software). ICP MS (The Agilent 7700 series, Agilent Technologies, Inc. USA) was used for sample analysis with rhodium internal standard (ISTD), at environmental analytical chemistry Laboratory, University of the Witwatersrand, Johannesburg, South Africa.
Figure 2: A graph of inductively coupled plasma-mass spectrometry calibration for thorium and uranium versus ppb of the samples in this study

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Determination of activity concentrations of238 uranium and232 thorium

The activity concentration of uranium (238 U) and thorium (232 Th) was obtained by converting concentration from ppb to ppm and then to Bq/kg, using equation (1),[19] which is consistent with the recommended conversion factors (CFs) from the IAEA.[20]



Where A is the specific activity concentration of the isotope in Bq/kg, NA is Avogadro's number (6.023 × 1023 mol-1), CI is the measured isotope concentrations in ppm and t1/2 is the half-life of the isotope in seconds, and MI is atomic masses (kg/mol).

Absorbed dose rate and annual effective dose

The determination of the average absorbed dose rate (D) due to terrestrial sources of gamma radiations in air 1.0 m above the ground surface for uniform distribution of naturally occurring radionuclides (238 U and232 Th) was calculated based on the guidelines from UNSCEAR, presented in equation (2).[1],[3],[6],[21] However, the settings and ISTDs for the ICP-MS, in this work, used40 K as correction isotope, therefore, AK was subtracted from equation (2) and other ones applicable to40 K. Therefore, the present work focused on the dose, which emanates from uranium and thorium as considered under the covered area of study.



Where D is the absorbed dose rate in nGy/h, and are the concentrations of uranium, thorium, and potassium in Bq/kg, respectively, the coefficients 0.462, 0.604, and 0.042 are the activity concentration to dose rate CFs of AU,ATH and AK, respectively, in nGy/h per Bq/kg.[21] The absorbed dose rate has a global average value of 55 nGy/h.[3]

The net external gamma radiation dose rate (D) was used to estimate the annual effective dose (external), by taking into account the outdoor occupancy factor 0.2, and the CF from absorbed dose rate in air to effective dose 0.7 Sv/Gy for adults. Equation (3) is used to compute the Eext as proposed by the UNSCEAR.[3],[22]



Where Eext is the external annual effective dose in mSv/a.

Determination of radiological index parameters

Radium equivalent dose

Due to nonuniform distribution of primordial radionuclides in rocks and soils, the radium equivalent activity (Raeq) in Bq/kg has been used as a single index that describes the gamma output from the mixture of238 U,232 Th, and40 K from samples. The use of rocks and soils from mining sites to build houses resulting in the gamma radiation exposure to human can, therefore, be assessed.[6],[9] The concentration of Raeq exceeding 370 may result in radiation exposure elevation. The exposure due to radiation in terms of radium equivalent is calculated using Equation (4).[3],[22],[23],[24]



Where ARa, ATh, and AK represent the activity concentration of226 Ra,232 Th, and40 K, in Bq/kg, respectively.

Note also that daughter nuclide226 Ra is replaced by the parent nuclide238 U, even though there may be disequilibrium given as 1.03 between the parent and daughter nuclides, according to the UNSCEAR.[3],[6]

External and internal exposure index

The external exposure index (Hex) is used as a radiation exposure index to assess the indoor radiation dose rate with respect to external exposure to gamma radiation from the natural radionuclides. This is to limit such exposures from building materials in which rocks and soils are part. Internal exposure index (Hin), on the other hand, seeks to assess the internal exposure to radon and its short-lived progenies, which are hazardous to the respiratory organs.[6],[9],[23]

The values for the Hex and Hin are calculated using equations (5) and (6), and must be less than unity for radiation hazard to be negligible.[3],[23]





Where ARa, ATh, and AK are the respective activity concentration of226 Ra,232 Th, and40 K, in (Bq/kg).

Representative gamma and alpha index

Inside the human body, radiation is being monitored using the representative gamma index (Iγ) and also to determine the level of risks as a result of human exposure to external annual effective doses from gamma radiation of radioactive nuclides sources in rock samples, as proposed by the European Commission.[25],[26] Equation (7) was applied to estimate the (Iγ) gamma, such that if (Iγ) ≤2, the annual effective dose increases by 0.3 mSv, but will increase by 1 mSv for 2≤ (Iγ) ≤6.[25] Hence, safe materials for the purpose of construction can be identified by Iγ.



Where ARa, ATh,AK are the respective specific activities of226 Ra,232 Th, and40 K, in Bq/kg.

The representative alpha index is the exposure index (Iα )used to estimate, in a given sample, the level of α-radiation exposure due to natural radionuclides and radon exhalation as presented in equation (8).[9],[24]



Where ARa is the specific activity of226 Ra, in Bq/kg.


  Results and Discussion Top


The activity concentrations (Bq/kg) of the naturally occurring232 Th and238 U radionuclides were determined in the uranium-ore rock samples collected from some parts of Northern Nigeria, and the results are presented in [Table 1]. As shown in [Table 1], the activity concentrations of the rock samples have highest values at 21.9 ± 1.3 Bq/kg for232 Th and 924.6 ± 17.1 Bq/kg for238 U. The lowest values were 0.22 ± 0.03 Bq/kg and 1.1 ± 0.03 Bq/kg, respectively. The results obtained are illustrated in [Figure 3], which showed that the highest concentration for232 Th radionuclide was detected from sample UO01, whereas for238 U was from UO02, which were collected from Riruwai and Mika-I. The values from this study are below those reported by El-Taher and Uosif and Ademola et al.[1],[6] and the world average concentration of 30 and 40 Bq/kg for232 Th and238 U,[3],[21] except for238 U value of sample UO03 that is extremely higher by a factor of 26.4 and sample UO04 higher by a factor of 2.8. The geological formations of the elements explain the reason for the variations in the natural radioactivity.[7]
Figure 3: Comparison of the238Uranium activity concentration (Bq/kg) and232Thorium activity concentration (Bq/kg) present in the collected samples in the study area

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The absorbed dose rates were estimated using Equation (2), and the result is presented in column 2 of [Table 2] and displayed in [Figure 4]. The absorbed dose rate ranged from 0.62 ± 0.03 to 422.4 ± 7.8 nGy/h and was found to be below the world average value of 55 nGy/h,[3] except for sample U003 whose value is higher by a factor of 7.6. [Figure 5]a and [Figure 5]b shows the correlation between the absorbed rate and activity concentrations of232 Th, found to be least significant (R = 0.999). This implied a major portion of dose contributed due to the uranium content of the rock. However, Rangaswamy et al.[9] had major portion of the dose contributed from thorium, in their studies, which could be attributed to variations in geological locations of the samples studied.
Table 2: Results of the absorbed dose rate, radium equivalent activity, and annual effective dose estimated from uranium ore rock samples

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Figure 4: Absorbed dose rates (nGy/h) and radium equivalent activity (Bq/kg) due to232Thorium and238Uranium concentrations present in the study area

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Figure 5: Correlation between (a) the absorbed dose rate and232Thorium activity concentration, (b) the absorbed dose rate and238Uranium activity concentration, (c) radium equivalent and232Thorium activity concentration, and (d) radium equivalent and238Uranium activity concentration of the rock samples in this study

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The Raeq was calculated using Equation (4), and the result is shown in column 3, [Table 2], and illustrated in [Figure 4]. The Raeq from this study varies from 1.4 ± 0.06 to 914.4 ± 16.9 Bq/kg, with the highest value of 914.4 ± 16.9 Bq/kg reported from sample UO03, whereas the lowest value was from sample UO02. It can be seen that the values of Raeq in all the measured samples are lower than the UNSCEAR-recommended value of 370 Bq/kg except the highest value. [Figure 5]c and d shows the correlation between Raeq with232 Th and238 U whose values are R = 0.1003 and R = 0.9989, respectively, which indicated good and strong correlation between Raeq and238 U, implying that contribution is mostly from238 U.

The annual effective dose (external) (Eext) was determined using equations (3) and (4), to test the effect of the absorbed dose rates, and presented in column 4, [Table 2]. As shown in [Table 2], the external annual effective dose ranges from 0.001 ± 3.202E-05 to 0.518 ± 0.009 mSv/a. The values are within the safety limit (1.0 mSv/a) proposed by the UNSCEAR[3],[21] and lower than that reported by Ademola et al.[6]

Equations (5–8) were used to calculate Hex, Hin, and representative gamma and alpha indexes (Iγ and Iα), and their respective results are presented in columns 2–5, in [Table 3]. It is shown for the study locations that Hex and Hin have values that vary from 0.004 to 2.47 and 0.007 to 4.94, respectively, which were lower than unity as deserved, except for the highest value from UO03 that exceeded.
Table 3: External exposure, the internal exposure, gamma index, and alpha index for the study

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Both Iγ and Iα from [Table 3] have values that ranged from 0.0046 to 3.05 and 0.0052 to 4.57, respectively. The criterion indicates that, for Iγ ≤2 the annual effective dose increase by 0.3 mSv, but will increase by 1 mSv for 2 ≤ Iγ ≤ 6[25] and applies for the samples in this study. For Iα it is recommended that the activity concentration of226 Ra at upper limit be 200 Bq/kg. However, radon the progeny of226 Ra has <200 Bq/m3 whereby Iα< l[9],[25]. The values in this study are below the recommended limit, suggesting that radon exhalation from the rock samples will cause indoor concentration <200 Bq/m3, except for sample UO03 that exceeded the limit.


  Conclusions Top


The level of natural radioactivity in uranium-ore rock samples collected from some parts of Northern Nigeria was analyzed using ICP-MS. The results obtained showed the activity concentrations of232 Th in the samples from the mining sites as 0.22 ± 0.03 to 21.9 ± 1.3 Bq/kg and238 U as 1.05 ± 0.03 Bq/kg to 16.5 ± 0.72 Bq/kg to be lower than the world average values identified by the UNSCEAR.[3] However, activity concentration values of238 U from sample locations Mika-II and Michika, 924.6 ± 17.1 and 98.2 ± 3.7 Bq/kg, respectively were much higher than the recommended value. The estimated values due to232 Th and238 U, of mean absorbed dose rate (D), annual effective dose (external), Raeq, Hex, Hin. Others include representative gamma and alpha indexes (Iγ and Iα), which were lower than the world average values except for Mika-II location. It can be concluded from the results in this study that the study areas investigated are radiologically safe for mining activity except for the sample location with higher values. The radiation dose from this study (maximum value being 0.52 ± 0.009 mSv/a, for external) does not generalize for the entire Nigeria, but deals particularly with the occupational radiation exposure for workers in the respective sample locations, where adequate protective measures should be emphasized. The values are far below the worldwide-allowed dose of 20 mSv/a for workers.[27]

Acknowledgments

The authors would like to thank the Petroleum Technology Development Fund, Abuja, Nigeria, for Ph.D scholarship under the Local Scholarship Scheme covering research visit to Wits University where this research was made possible. The authors also wish to thank Ms. Andelina M of Environmental Analytical Chemistry, University of the Witwatersrand, Johannesburg, for support in the analysis of the samples. The authors also gratefully acknowledge the Nuclear Structure Research Group of the School of Physics as well as the School of Geosciences for their meaningful contributions during the research.

Financial support and sponsorship

This work was supported by the PTDF through 2016 LSS scholarship – PTDF/TR/LSS/PHD/NSU/Vol.1.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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