Radiological baseline around the Barakah Nuclear Power Plant, UAE

2.1 Sampling

A total of 58 soil and sediment samples were collected in November 2014. The samples were collected using 25 × 25 × 5 cm³ stainless steel boxes. The samples were collected from various locations around the Barakah area. They were categorized into three subgroups: bottom sediment, shore and soil samples. Eighteen marine sediment samples (M1–M18), referred to as “bottom sediments”, were sampled from three areas: Sila, Barakah and Jebel Dhanna. These samples were collected by grab sampling from the water at a depth of 4–7 m. Sixteen shore samples (B1–B16) were collected along the shoreline, nearby the Barakah NPP. Twenty-four soil samples (S1–S24) were collected from sand dunes to the south of the Barakah NPP. Both shore and soil samples were taken at a depth of 0–0.5 m from the surface. Fig. 1 depicts the locations of the collected samples. The coordinates of all collected samples are provided in Table S1 of the supplementary material. The samples are collected from various zones depending on the distance of sampling points from the Barakah NPP: the restricted zone (radius: 5 km), emergency planning zone (radius: 16 km) and safety zone (radius: 30 km).

2.2 Gamma spectrometry

For low-background radio-analysis, gamma-ray spectrometry (model no. GMX40P4-76) was used. This method is simple, non-destructive and fast, which makes it suitable for collecting data for many radionuclides simultaneously (Ebaid, 2010). Prior to the gamma analysis, all of the samples were placed in a drying oven at 60 °C for 24 h. The samples were then homogenized and sieved using a 2 mm sieve (IAEA, 1989). Samples were weighted (exact masses are available in Table S2 of the supplementary material) and transferred to sealed Marinilli beakers (1L) and left for at least 4 weeks to achieve secular equilibrium between ²²⁶Ra and its daughters (NEA-OECD, 1979). Samples were analyzed using HPGe detector (Nuclear lead company, INC) with a relative efficiency of 40%. The detector was calibrated for energy and efficiency using a standard mixture of twelve gamma-emitting radionuclides certified by the National Institute of Science and Technology (NIST). The period of each run, for each of the 58 samples, in the gamma spectrometer is 24 h. The spectrometer was recalibrated after each measurement, where the energetic lines of the ¹³⁷Cs were confirmed to show at 661.6 keV and those of ⁶⁰Co show at 1173 and 1332 keV (IAEA, 2007). Background gamma-ray were measured by using empty Marinilli beaker and by acquiring spectra for 24 h.

Estimation of radionuclides activity concentrations and analysis of hazard parameters

The activity of radionuclides (A) was estimated, in Bq/kg, using the following equation: (Beretka and Mathew, 1985; El Assaly, 1981)

Both ²³⁸U (²²⁶Ra) and ²³²Th do not have intensive gamma-rays (energy lines). However, they have several daughters which have more intensive lines and activities equal to those of their parents in the state of secular equilibrium (NEA-OECD, 1979). ²²⁶Ra values were measured through the emission of the daughters along with their energy line and emission probability (IAEA, 2007): ²¹⁴Pb (351.9 keV, 35.3%) and ²¹⁴Bi (609.3 keV, 45.2%, 1120.2 keV, 14.8%, and 1764.5 keV, 15.2%). ²³²Th was measured through ²¹²Pb (238.6 keV, 43.6%), ²⁰⁸Ti (583.1 keV, 30.6%) and ²²⁸Ac (911.2 keV, 29%), while, ⁴⁰K was measured directly through its emission at 1460.8 keV, 10.7%. ¹³⁷Cs energy line (661.6 kev) was too weak to be detected in all spectra of all samples, it was thus not considered in the analysis.

The determination of the efficiency, ɛ, for the measured radionuclides involved three steps (IAEA, 2007; Gudelis et al., 2000; Knoll, 2010). First, the experimental efficiency was evaluated using standard radionuclide source with standardized activity concentrations using Eq. (1). The second step entailed constructing the efficiency-fitting curve for the given set of experimental data (energy, efficiency). Finally, the efficiency, ɛ, was determined for the different radionuclides from fit curve equations.

Radium equivalent activity (Ra_eq), in Bq/kg, is used for the assessment of radiological hazards in the environment. It is used to compare the specific activity of material containing different amounts of ²³⁸U, ²³²Th and ⁴⁰K. This radium equivalent activity represents a weighted sum of the activities of ²³⁸U, ²³²Th and ⁴⁰K radionuclides. It is based on the estimation that 1 Bq/kg of ²²⁶Ra, 0.7 Bq/kg of ²³²Th and 13 Bq/kg of ⁴⁰K produce the same radiation dose rates. It is evaluated using the following equation (Beretka and Mathew, 1985; Tufail, 2012; Ramadan et al., 2018; Mujahid et al., 2008).

UNSCEAR (UNSCEAR, 2000; UNSCEAR, 1988) provided guidelines to measure the absorbed dose rates (abs. dose) (in nGy/h) from gamma radiations in the air at 1 m above the ground surface. This is for the uniform distribution of the naturally occurring radionuclides ²³⁸U (²²⁶Ra), ²³²Th and ⁴⁰K. These UNSCEAR 1988 and 2000 guidelines were used to estimate the absorbed dose rates (UNSCEAR, 2000; UNSCEAR, 1988).

The uncertainties contributing to the results were propagated throughout by adding them all in quadrature combinations. The uncertainties in the readings could be due to sample weights, detector efficiency, geometries, gamma-ray emission probabilities and half-lives.

2.3 Alpha spectrometry

In order to determine the isotopic composition of uranium, alpha spectrometry (model no. 7401VR) was used. Alpha spectrometry is a destructive method for measuring alpha particle emitting radionuclides (Alamelu and Kumar, 2016). Based on the gamma radiation results, the ten samples that had the highest activity concentration of ²³⁸U were analyzed using alpha spectrometry in the Egyptian Nuclear and Radiological Regulatory Authority in Cairo, Egypt. Generally, alpha spectrometry is used to measure the concentration of isotopes of Am, Pu and U. It is based on the response of an electronic counting system to an incident alpha particle (Ide et al., 1989). The nuclide of interest was separated from the sample matrix by anion exchange and electroplated on a stainless-steel disc (Gautier et al., 1986). Uranium was separated by Eichrom UTEVA resin (Maxwell III, 2006; Pimble et al., 1992) then electrodeposited on stainless steel disc to complete the measurements by alpha spectrometry. The detailed analytical procedure is provided by Eichrom (https://www.eichrom.com/wp-content/uploads/2018/02/09-mcalister-eichrom-method-and-application-note-updates-rrmc2014-final.pdf). Fig. 2 is a schematic diagram of the procedure followed by Eichrom for the radiochemical separation of uranium in soil. Uranium isotopes (²³⁴U, ²³⁵U and ²³⁸U) were extracted from the soil digestion solution by coprecipitation with calcium phosphate. They were then separated from other actinides and purified using extraction chromatography (Horwitz et al., 1992). To minimize the experimental error, quality assurance was achieved by analyzing samples of known concentration activity from the IAEA and doubly deionized water (DDW) spiked with known activity of ²³²U.

Close

Fig. 2

Schematic procedure of soil digestion, uranium separation and alpha spectrometry measurements.

Export to PPT

3.1 Radionuclide activity concentrations

The activity concentrations of radionuclides (in Bq/kg), radium equivalent (in Bq/kg) and absorbed dose rates (in nGy/hr) in the shore, soil and bottom sediment samples are provided in Table S3 of the supplementary material. The statistical analysis of the measured radionuclides is summarized in Table 1 and illustrated in Fig. 3. The average activity concentrations of ²³⁸U (²²⁶Ra) are 4.43 ± 1.12, 13.54 ± 4.16 and 4.73 ± 3.01 Bq/kg in shore, soil and bottom sediment samples, respectively. The radioactive activity ranking is as follows: shore samples < bottom sediments < soil samples. The activity in the shore samples is much lower than that in soil (see Fig. 3) due to tidal fluctuations and wave currents (Buesseler et al., 2011). All radioactive activities reported lie below the world average threshold (33 Bq/kg) set by UNSCEAR (UNSCEAR, 2000), except in S11 (64.82 Bq/kg) which is already excluded as an outlier. The high activity concentration of ²²⁶Ra in S11 is high enough not to be correlated to ²³⁸U but rather to the transport of ²²⁶Ra from depositional systems to the surface soil (Lidman, 2005; Faure and Mensing, 2005).

Close

Fig. 3

Average values of the radiological activities (in Bq/kg) and radium equivalent (in Bq/kg) and absorbed dose rates (in nGy/hr).

Export to PPT

The estimated average activity concentrations of ²³²Th are 1.68 ± 0.49, 8.31 ± 3.87 and 1.83 ± 1.67 Bq/kg in shore, soil and bottom sediment samples, respectively. The activity concentrations of ²³²Th for all samples are lower than the world average activity concentration (45 Bq/kg, UNSCEAR (2000)). The activity concentrations of ⁴⁰K show wide variations (high standard deviation values): they range from 40.71 to 240.91 Bq/kg, with an average of 106.30 ± 50.68 Bq/kg in shore; from 141.35 to 611.16 Bq/kg with an average of 349.72 ± 107.16 Bq/kg in soil; and from 7.81 to 544.12 Bq/kg with an average of 105.23 ± 130.14 Bq/kg in bottom sediment samples. The maximum value measured in the bottom sediments is in M17, which is located to the east where Jebel AlDhannah port is located. Apart from the anthropologic factor (harbor activities in this case), the muddy texture of M17 sample could lead to the adsorption of radionuclides in lattice defects or onto crystal and grain boundaries (Baeza et al., 1995).

Activity concentrations are in the following order: ⁴⁰K > ²³⁸U (²²⁶Ra) > ²³²Th in all sampling sites, except for soil samples S19 and S21 where ²³²Th activity is slightly higher than ²³⁸U (²²⁶Ra). High ⁴⁰K activity concentrations in all samples, compared to other radionuclides considered in this study, indicate high percentage of potassium levels that correlate to the high levels of natural K-feldspar which are found in many types of sedimentary rocks (Harvey and Robert, 1996). Despite the wide variations in the activity concentrations of ⁴⁰K in the studied area, all averages are below the world average value (420 Bq/kg). As shown in Fig. 3, soil samples show the highest activity concentrations. The relatively higher activity concentrations measured in soil samples are likely related to geological factors such as weathering and erosion of the older rocks near the study area (Beretka and Mathew, 1985; Montes et al., 2012). These older rocks are dominated by cross-bedded quartz sandstones, fossiliferous mudstones, cross-bedded carbonate eolianites and evaporites of Late Miocene to Holocene Epoch (Whybrow et al., 1999; Alsharhan and Kendall, 2003). The activity concentrations can also vary with the grain size. In the studied area, the classification was determined as follows: moderately sorted medium sand in shore and soil samples and poorly sorted coarse sand in bottom samples (Al Rashdi et al., 2017). The gamma spectroscopic data from this study do not show ¹³⁷Cs peaks in the runs of all samples. This result is in good agreement with the insignificant ¹³⁷Cs activity in the UAE soils according to the global distribution of ¹³⁷Cs atmospheric nuclear tests fallout (UNSCEAR, 1993).:

The estimated average values of radium equivalent (Ra_eq) and absorbed dose rates are shown in Table 1 and Fig. 4. The calculated average value of radium equivalent activities in shore, soil and bottom sediment samples are 15.01 ± 5.19, 54.50 ± 18.31 and 15.44 ± 14.58 Bq/kg, respectively. All the values are below the permissible maximum value of 370 Bq/kg reported by the UNSCEAR (2000). As shown in Table 1, the average values of the estimated absorbed dose rates in the shore and soil samples are 7.49 ± 2.68 and 26.85 ± 8.84 nGy/hr, respectively. They are found to be much less than the world average of 57 nGy/hr, as set by the UNSCEAR (2000). The ⁴⁰K is the major natural radioactivity contributor to the absorbed dose rates.

Close

Fig. 4

Correlations between ²³⁸U (²²⁶Ra), ²³²Th and ⁴⁰K activities in different environments (shore, soil and bottom).

Export to PPT

3.2 ²³⁴U/²³⁸U ratio

Table 2 shows the ²³⁴U/²³⁸U ratio for ten soil samples, which were selected based on having the highest gamma activity concentrations. The ²³⁵U peak was neglected as it was relatively weak compared to the peaks of ²³⁴U and ²³⁸U. This is because both ²³⁴U and ²³⁸U emit distinct alpha particles at specific energy levels (4.75 and 4.2 Mev, respectively) while ²³⁵U emits mixed energy particles. In addition, the crustal uranium distribution, based on the half-lives of the isotopes, is 48.7% for ²³⁴U, 2.27% for ²³⁵U and 49.0% for ²³⁸U (NNDC, 2011). The small radioactive percentage of ²³⁵U resulted in very small peaks, which were not detected by the alpha spectrometer. The ²³⁴U/²³⁸U activity ratios range from 0.59 to 2.24. In closed systems older than 10⁶ years, ²³⁸U decay chain should be at equilibrium where ²³⁴U/²³⁸U is approximately equal 1 in activity ratio (Holden, 1990; Cheng et al., 2000). The current study is for open systems where the daughter to parent (²³⁴U/²³⁸U) activity ratio is out of the secular radioactive equilibrium. The depletion of ²³⁸U in natural objects is a well-known phenomenon (Rosholt, 1959; Thurber, 1962). Two main factors affect the disequilibrium: (1) the direct recoil of ²³⁴Th and its fast decay to ²³⁴U near mineral grain boundaries which lead to higher (²³⁴U/²³⁸U) activity ratios, and (2) the leaching processes of ²³⁴U from crystal lattices that are damaged by energetic alpha decay which lead to lower (²³⁴U/²³⁸U) activity ratios (Andersen et al., 2009; Tokarev et al., 2005). Thus, the observed disequilibrium in the current data can be attributed to chemical and physical geological processes in the area (Peate and Hawkesworth, 2005). The presence of evaporates and carbonates can also cause high ²³⁴U/²³⁸U ratio disequilibrium due to fractionation from water-rock interactions (Riotte and Chabaux, 1999). Faure and Mensing (Faure and Mensing, 2005) illustrated how uranyl ions (UO₂²⁺) tend to form carbonate complexes, thus observable concentrations of ²³⁴U would be found in Ca carbonate minerals.

The correlations between the activities of ²³⁸U (²²⁶Ra) and ²³²Th, between ²³⁸U (²²⁶Ra) and ⁴⁰K and between ²³²Th and ⁴⁰K are depicted in Fig. 4. The degree of correlation between the different radionuclides varies: it is practically non-existent in the soil samples (R² for the correlation between ²³⁸U (²²⁶Ra) and ⁴⁰K is as small as 0.02), there is better correlation in the shore samples (R² for the correlation between ²³⁸U (²²⁶Ra) and ²³²Th is 0.52) and it is best (R² is close to unity, 0.97, for the correlation between ²³²Th and ⁴⁰K) in the bottom sediment samples. These correlations suggest different sources of the radionuclides in the studied samples. Generally, K-bearing minerals such as feldspars and K-salts can be the main supplier of ⁴⁰K whereas much of ²³⁸U is related to carbonate minerals (Faure and Mensing, 2005). The relatively weak or non-existent correlation in the soil samples could be due to the fact that soil particles do not accumulates from a single source (because of the windblowing and diagenetic formation) so their composition is not particularly consistently homogeneous (Speight, 2012). Feldspars are common components of dust particles (sand dunes) (Magill, 2000) while K-salts accumulate in the soil as a result of evaporation (Gornitz, 2008). The rather good correlation between ²³⁸U (²²⁶Ra) and ²³²Th in the shore samples is in good agreement with correlations reported on similar deposits in Egypt (Eissa et al., 2010). The relatively good radionuclides correlation values in the shore and bottom samples may be a result of possibly consistent homogeneity of the sources that compose the material, i.e. carbonate from shells and K-salts from seawater evaporation. The coexistence of comparable marine sources for the radionuclides is better illustrated by the relatively strong correlation in the sea bottom sediments.

The values reported in this study are compared with those reported in other studies as shown in Table 3 and Fig. 5. The estimated activity concentrations in this study are close to the values reported by Alali on shore sediments in Abu Dhabi (Alali, 2003). The activity concentrations of radionuclides in shores of UAE (in the Barakah area), China, Oman, Jordan and the Gulf of Aqaba are very low, in fact they are below the world average. However, in Iran and Turkey, the activity concentrations for all radionuclides are elevated. In Iran, the distribution of activity concentrations along the southern coast of the Caspian Sea area exceeded the international limits (Abdi et al., 2009). The high-activity concentrations in Turkey is due to the presence of zircon, allanite, monazite, thorite, uranothorite and apatite (Orgun et al., 2007). The values reported in India are higher only in ²³⁴R. All radionuclides show higher activity concentrations in Yemen due to the geologic structure of the Juban area which is located near granite and gneiss rocks. The radionuclide activity concentrations of ²³⁸U of the Red Sea are higher than the world average.

Close

Fig. 5

International and regional values of radionuclide activity concentrations (in Bq/kg). The UAE* label indicates average values from the shore, soil and bottom sediment samples collected in this study.

Export to PPT

Fig. 6 depicts spatial distribution radiological maps of the measured radiological activities and radium equivalent in the studied area. The maps were plotted using Arcmap 10 with the kernel smoothing interpolation method. With the exception of the uranium map, higher activity concentrations are shown in the east and the south parts. The activity concentrations decrease from the south to the north. The highest activity concentrations of radionuclides are observed in the south towards the sampling area. The north and west areas of the maps show the lowest activity concentrations of both shore and bottom sediment samples. Tidal fluctuations and wave currents effectively lower the activity concentrations of radionuclides in shore sediments (Buesseler et al., 2011). That is indicated in the low measured values in the current study.

Close

Fig. 6

Spatial distribution maps of the radionuclide activities of ²³⁸U (²²⁶Ra), ²³²Th and ⁴⁰K (in Bq/kg) and the Ra_eq (in Bq/kg) around the Barakah NPP.

Export to PPT