Assessment of NORM from oil refineries and fields northwest of Mosul

The uranium concentration and radioactivity of radon gas were measured in Al-Kasik refinery and Ain Zala field using the CR-39 detector. Soil and water samples associated with the production stages of oil, sludge and crude oil were collected. The levels of uranium concentration in soil ranged from 0.703 to 1.480ppm, in water samples from 0.681 to 0.716 ppm, in sludge samples from 0.849 to 1.014 ppm, and in crude oil from 0.785 to 0.933 ppm. As for the radioactivity of radon gas, when comparing the radon rate in the samples we obtained with the global values, it was found that it falls within the internationally permissible limit, where the radon rate in the soil was 12.81 Bq/kg and when compared with the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) which has a value of 32 Bq/kg, and in the produced water it was 8.66 Bq/kg compared to (UNSCEAR)which has a value of 50 Bq/kg. In sludge samples 11.81 Bq/kg and when compared with the International Atomic Energy Agency (IAEA) whose value is(8 − 5 × 10) Bq/kg, and in crude oil samples 10.56 Bq/kg and when compared with the International Federation of Oil and Gas Producer (IOGP) whose value is (800 − 4 × 10) Bq/kg. As for the alpha ray hazard index, the results showed that it is within the permissible limits internationally, where the results were less than 1 and therefore does not pose a threat to the health of workers and environment.


Introduction
Human beings are exposed to natural radiation that comes from two main source is cosmic rays that come from outer space and are produced by the glow of the sun and from external galaxies, as well as the natural terrestrial radiation, and the second source is the natural radioactive elements where these elements are widely spread in the earth's crust [1] . The term (NORM) used in the oil and gas industry stands for natural radioactive materials such as uranium and thorium. The radionuclides that are measured in the oil and gas industry are the result of the dissolution of two natural chains, uranium U238 and thorium Th232, where these elements have a long half-life and their presence in the soil depends on the geological structure [2].Natural radionuclides are present in oil and gas reservoirs in varying concentrations. These nuclides are released when oil and gas is extracted from the ground, to accompany oil and water to the surface of the earth, and move to production equipment such as pipelines and oil tanks, and settle inside them within the slag and crusts materials [3]. Workers in the oil and gas industry are exposed to radioactive materials during oil extraction, as well as exposure to mud, and water while drilling oil wells, in addition to that during equipment maintenance and tank cleaning. There are several studies and reports, related to NORM in the oil industry, where the International Atomic Energy Agency (IAEA) [4] , has published safety and security reports on dealing with NORM . Additionally ,the International Association of Oil and Gas Producers (OGP) [5] , has published clarifications about NORM in the oil and gas industry, and the United Nations Scientific Committee concerned with the effects of atomic radiation (UNSCEAR) [6] has published several reports on NORM and provided detailed sexplanation . In Egypt, researchers were able to study the level of NORM in the slag resulting from the oil industry and determine the level of its danger [7]. In Turkey, radium isotopes have been studied in waste and crude oil produced from the oil industry [8]. Our current study aims to measure the concentration of uranium and the radioactivity of radon gas, determine the danger of alpha that affects workers in the oil and gas industry, and compare the obtained results with the internationally permissible limit, including the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the International Atomic Energy Agency (IAEA), and the International Association of Oil and Gas Producers (OGP).

Study Area
The area of this study, as shown in Figure -

Samples Collection
Twenty two different samples were collected from Ain Zala field and Al-Kasak refinery. The samples included soil, production water, sludge, and crude oil samples. Fifteen soil samples were collected from two previously mentioned sites, whereas soil samples were collected from around the oil tanks and some of the site. In the Ain Zala field, samples were taken from around the oil wells. Soil samples were taken at a depth of 0-15 cm and stored in airtight plastic bags. As for slag, two samples were taken from the previously mentioned study sites, and sludge samples were taken from oil tanks and stored in plastic bags as well. Two samples of production water were also taken from a wet gas treatment unit and then placed into one liter (1 liter) plastic containers, and crude oil samples were taken from one of the tanks and stored in (1 liter) plastic containers with a label. The type of sample, the location, and the date the sample was taken is written on each sample .

Sample Preparation
Soil and sludge samples were exposed to a heat source for several hours in order to remove moisture from them. Impurities and gravel were removed from samples using a sieve of 2 mm diameter. Samples were grinded and a fine powder was obtained. Samples were weighed using a sensitive balance ,as for the production water and Crude Oil samples, they remained as they are. After that, all samples were placed inside plastic cups which were sealed to prevent the occurrence of interaction between the air inside the samples and their external surroundings, where the samples were left for 30 days to achieve the ideal balance between radium and radon, reaching 98% of the ideal equilibrium state. This time is calculated through the equilibrium relationship of radioactivity [9] [10]: Where Rn effective means radon and and represents the effectiveness of radium and represents the radon decay constant 0.1814 d −1 and is the time required to reach equilibrium .

Measurements
The concentration of uranium and specific radioactivity of radon gas was measured for all samples using the solid state nuclear trace detector CR-39, where the samples were placed in irradiation chambers consisting of two plastic cups with identical size and shape, placing a filter paper between them and closing the mugs with adhesive tape tightly. The CR-39 detectors were left inside the chambers for 30 days [11] [12]. After that, the chemical 6.25 N NaOH erosive solution was prepared. At the end of the experiment, the reagents were removed and placed inside the chemical erosive solution and placed inside the water bath at a temperature of 70 ° for 3 hours. The radiation background reached 180 . −2 and was subtracted from the intensity of the effects of alpha particles emitted from the samples under study. Microscopic observation was carried out using an optical microscope with a magnification of 400X . Figure 3 shows the irradiation system that was used. The radon concentration in the samples was calculated by the following relationship: Where C s represents the radon activity within the samples in units (Bq. m −3 ), h represents the height of detector from the sample surface in cm units, T represents the irradiation time in days , λ Rn represents the radon decay constant and L represents the thickness of the sample in cm .

5-Finding the number of radon atoms N Rn in the samples :
A Rn = N Rn λ Rn … … … . 6 After determining the number of radon atoms, the number of uranium atoms is found through the law of radiation balance λ U N U = λ Rn N Rn … … … .7 6-Determining the mass of uranium in the samples : Where W U represents the mass of uranium in the samples, A U represents the mass number of uranium, and N av represents the number of Avogadro, N U number of uranium atoms in the samples .

7-Determining the uranium concentration(CU) in samples in ppm units :
Where W S represents the mass of the sample in (gm) . Table 1 shows the location of the sample, its type, the sample weight, number of traces, and the radon concentration in the air and the samples. The results show that the highest radioactivity of radon gas and the concentration of uranium in soil, where the radon value soil samples in Alksak refinery were 18.06 Bq/kg , the uranium concentration was 0.941 ppm, while the lowest values of radioactivity were for radon gas and the concentration of uranium in Alksak refinery; the radon value was of 8.7 Bq/kg and the value of uranium concentration was 0.703 ppm . when comparing the results that were obtained with the permissible limits UNSEARS (32) Bq/kg [6] . As the results were within the permissible global limit, therefore they pose no risk to humans. In sludge samples, the highest radioactivity of radon gas (12.62 Bq/kg) and the concentration of uranium was in Ain Zala field 1.019 ppm , and the lowest radioactivity of radon gas and the concentration of uranium were in sample entry number 19 in table 1 for Ain Zala field with a value of 10.52 Bq/kg and uranium with a concentration of 0.849 ppm. The sludge results obtained with IAEA (8 − 5 × 10 5 Bq/kg [1] show that the results are within the universally permitted limit. As for the crude oil sample, the highest radioactivity of radon gas and the concentration of uranium in Ain Zala field where the radon value was 11.56Bq/kg, uranium with a concentration of 0.785 ppm. The lowest radioactivity value of radon gas and the lowest concentration of uranium was in AlKasik refinery and the lowest concentration of uranium was in AlKasik refinery 9.73 Bq/kg for radon and 0.933 ppm for uranium concentration, compared to the permissible limit. For the OGP (800 − 4 × 10 5 ) Bq/kg [15] , we find that it is within the universally permitted limit. As for the water samples associated with the production of sludge, the highest radioactivity value was for radon gas in Ain Zala field 8.88 Bq/kg, and the highest uranium concentration was in Ain Zala and uranium with a concentration of 0.716 ppm. The lowest radioactivity of radon and the uranium concentration was in Alksak refinery 8.44 Bq/kg and uranium at a concentration of 0.681 ppm When comparing the results we obtained with the permissible limit UNSEAR ( 50) Bq/kg [16] we find that what we obtained is within the permissible limits globally. As for the level of risk of alpha rays, all results were within the global permissible limit 1≤ and thus do not form a risk to the health of workers in the oil industry. Figure  4 shows the rate of radioactivity of radon in the samples, where it was found that the rate of radioactivity of radon in soil is 12.81 Bq/kg , water produced 8.66 Bq/kg and sludge 11.81 Bq/kg , and Crude Oil Bq/kg 10.65 Table 1the location of the sample, its type, the sample weight, number of traces, and the radon concentration in the air and the samples  Figure 3 shows the rate of radioactivity of radon in the samples, where it was found that the rate of radioactivity of radon in soil is 12.81 Bq/kg , water produced 8.66 Bq/kg and sludge 11.81 Bq/kg , and Crude Oil Bq/kg 10.65 .

Results and Discussion
The radon rate obtained in the collected samples is within the permissible limits when compared with global values .

Conclusions
The radioactivity of radon gas and the concentration of uranium were calculated in the samples collected from Al-Kasak refinery and Ain Zala field. The results showed that the average concentration of uranium for soil samples, produced water, slag and crude oil was within the internationally permissible limit. The level of danger of alpha rays was calculated. The results we obtained were less than 1 and therefore do not constitute any danger to the health of workers and the environment.