Determination of Naturally Occurring Radioactive Material in the Egyptian Oil

Transcription

1 S.U.EL-Kameesy, et al. Arab J. Nucl. Sci. Appl, Vol 50, The 4, Egyptian (2017) Arab Journal of Nuclear Sciences and Applications Society of Nuclear Vol 50, 4, (55-62) 2017 Sciences and Applications ISSN Web site: esnsa-eg.com (ESNSA) Determination of Naturally Occurring Radioactive Material in the Egyptian Oil S.U.EL-Kameesy, H.M.Diab 1, A.B.Ramadan 1 and O.R.Megahid 2 Department of Physics, Faculty of Science, Ain-Shams University, Cairo, Egypt (1) Egyptian Nuclear and Radiological Regulatory Authority, Cairo, Egypt (2) Department of Health and Safety, Midor, Ministry of Petroleum, Cairo, Egypt Received: 5/10/2016 Accepted: 25/11/2016 ABSTRACT 1 The technically enhanced concentrations of Naturally Occurring Radioactive Materials (NORMs) were measured in 27 different samples from an oil refinery where crude oil from different wells are processed. The activity concentrations relevant to naturally occurring radionuclides in the samples (waste water, ash and crude oil) were determined by a gamma hyper pure germanium detector. The measured gamma spectrum for each sample were analysed to determine the gamma energy lines emitted from 238 U, 232 Th and 40 K. The measured NORM concentrations were used to calculate the dose rate for external and internal exposure. The obtained data reveal that in ash samples the 210 Pb shows the highest concentration with an average value of Bq/Kg. Furthermore, in waste water 210 Pb shows also a relatively high concentration with an average value of Bq/Kg. INTRODUCTION NORMs are all radioactive elements found in the environment. They stem mainly from the decay of 238 U and 232 Th series. Another major source of NORM is 40 k with long half-life (1.25 billion years) (1, 2). Long-lived radioactive elements such as uranium, thorium and potassium and any of their decay products, such as radium and radon are examples of NORM. These elements have always been present in the earth's crust and atmosphere, and are concentrated in some places, such as uranium ore bodies which may be mined. The term NORM is also used to distinguish natural radioactive material from anthropogenic sources of radioactive material, such as radioisotopes produced in nuclear reactors and used in nuclear medicine, where incidentally the radioactive properties of a material may be useful. However from the perspective of radiation doses to people, such a distinction is completely arbitrary. (3) Analysis of oil and gas extracted from different wells has shown that the long-lived uranium and thorium isotopes are not mobilized from the rock formations that contain them. However, Ra-226, Ra- 224, Ra-228 and Pb-210 are mobilized, and appear mainly in the water co-produced during oil and gas extraction. These isotopes and their radioactive progeny can then precipitate out of solution, along with sulphate and carbonate deposits as scale or sludge in pipes and related equipment (4). The NORM resulting from the 238 U and 232 Th can be concentrated and accumulated in tubing and surface equipment in the form of scale and sludge as a consequence of physical and chemical processes 1 Corresponding author Omima_trj@hotmail.com 55

2 associated with the oil and gas industry (5,6). The main radioactive waste containing NORM during gas and oil industry is produced water, which contains mainly radium isotopes, solid residues and production equipment (7,8). Solid residues consist of sludge and scales from tubing pipes and other production equipment. These radioactive wastes require treatment or disposal by appropriate methods (9, 10). The activity concentrations of 226 Ra, 228 Ra and decay products in deposits and sludge may vary from normal levels in soils and rocks (less than 0.1 Bq/g) to more than 1000 Bq/g. However, the activity concentration is still low in comparison with the specific activity of most man-made radioactive sources, and to emphasize that the concentrations are very low the deposits are often referred to as low specific activity (LSA) scales (11,12). Further, the large amount of material exceeding the recommended clearance levels in oil and gas production represents a considerable waste problem for the industry. The exemption and clearance levels recommended by the International Atomic Energy Agency are based on limiting the annual doses to members of the public to 10 µsv (13, 14). According to the International Basic Safety Standards, the recommended exemption values for the most important naturally occurring radionuclides that occur in NORM waste from oil and gas production are 10 Bq/g for 226 Ra, 228 Ra and 210 Pb and 1 Bq/g for 228 Th. However, the application of exemption values to naturally occurring radionuclides is limited to the incorporation of radionuclides into consumer products or their use as radioactive sources (e.g. 226 Ra and 210 Po) or for their elemental properties (e.g. thorium and uranium). Experimental Technique and Measurements 15 oil samples,6 waste water samples and 6 ash samples were collected from different oil reservoirs. The collected samples were weighed and baked in cans (100 cm 3 ) for more than six weeks to reach secular equilibrium where the rate of decay of the progeny is equal to that of the parent.six waste water samples were acidified with 20 mm of hydrochloride at the rate of 20 ml as soon as possible after sampling to prevent adsorption of radio-nuclides in the bottles. The analysis of water samples was performed after reducing the water volume by evaporation. Five litters were reduced to one litter and the condensed water was contained into 100 ml marinelli beakers. These marinelli beakers were previously washed, rinsed with a dilute sulfuric acid and dried to avoid any contamination. Hereafter, they were firmly sealed, for at least four weeks, to ensure that no loss of radon occurs thereby ensuring a state of secular equilibrium to be reached between radium isotopes and their respective daughters. Three bi-product ash samples were also collected, weighed and baked in cans (100 cm 3 ) for more than six weeks to reach secular equilibrium where the rate of decay of the progeny is equal to that of the parent. The NORM concentration in each of the collected samples was performed by measuring the gamma ray spectrum resulting from the decay of radionuclides existing in the sample. The gamma spectrum was measured by a gamma ray spectrometer based on a HPGe detector. The detector is a closed end-coaxial type HPGe detector of a vertical configuration (model GC 5019). The detector has 40% relative efficiency and 2.0 kev energy resolutions at 1.33 MeV photons. The detector is shielded by 4 mm Pb, 1 mm Cd and 1 mm Cu. The measured pulse amplitude distribution was analysed by means of maestro program. The energy and efficiency calibration of the spectrometer were performed using an analogous calibrated can, 100 cm 3 which contains well known standard sources of 22 Na, 60 Co and 226 Ra along with a standard solution of HCl. Quality control and quality assurance of the measurements were checked using the International Atomic Energy Agency (IAEA) reference materials (soil 6, IAEA-362). The specific activity of 226 Ra was calculated on the basis of the energy transitions of kev (19.2%) and kev (37.1%) of 214 Pb, kev (46.1%), 1120 kev (15.1%) and kev (15.9%) of 214 Bi. The corresponding specific activity of 232 Th was calculated utilizing the energy 56

3 transition kev (12.4%) and kev (25.9%) of 228 Ac, kev (40.4%) and 2614 kev (35.6%) of 208 Tl and kev (43.3%) of 212 Pb. The activity concentrations of 40 K were obtained from the kev (10.67%) gamma line. The activity concentration is based on the following equation: (13) A=Np / (λ η m) (1) where Np is the net count rate, λ is the abundance of the gamma line in radionuclide, η is the detector efficiency of the specific ɣ-ray and m is the mass of the sample(kg). The uncertainty of activity U (A) can be calculated by the following equation: U 2 (A) A 2 = U2 (Np) N 2 + U2 (η) η 2 + U2 (λ) + U2 (m) (2) λ 2 m 2 The reproducibility of the results and the stability of the counting technique were checked by conducting triplicate analysis on the collected samples. Canberra N- Gamma-Ray Radiation Hazard Indices As the activity of 226 Ra or any of its daughters represents about 98.5% of that of 238 U. Therefore, the contribution from 238 U could be replaced by any of them. The gamma radiation hazards due to the specific radionuclides were assessed by four different indices; the radium equivalent (Ra eq), the absorbed dose rate (D) and the annual effective dose rate (AEDE) and gamma index. Ra eq can be calculated according to the following equation (14). Ra eq A 1.43 A A Ra Th Where, A Ra, A Th and A K are the activity concentrations of 226 Ra, 232 Th and 40 K, in Bq/kg respectively. The Ra eq is related to the external gamma dose and internal dose due to radon and its daughters. The maximum value of Ra eq must be less than 370 Bq/kg. Another hazard parameter is the representative level index I γr that can be calculated using the following formula (14) ; ARa ATh AK I r (4) The gamma index ( I γr ) is in the permissible range as long as it is less than unity. The absorbed dose rate D (Gy/h) was calculated using the following equation: (15) D = 0.462A Ra A Th A K (5) The annual effective dose equivalent was calculated using the following equation: (15) AEDE = Dose Rate (ngyh -1 ) x 8760 h x 0.7 Sv Gy -1 x (6) Where the 0.7 Sv /Gy and 0.2 constants are the conversion factors from the absorbed dose in air to the effective dose, and the outdoor occupancy factor respectively (16). RESULTS AND DISCUSSION The calculated values of the measured NORM concentrations in oil samples are presented in table (1) and illustrated schematically in fig (1). The tabulated data show the average values for the radioactivity concentrations of 226 Ra, 232 Th, 40 K and 210 Pb. These values were found to be Bq/L, 8.619Bq/L, Bq/L and Bq/L respectively and appear to be less than their corresponding values given by the IAEA (8). K (3) 57

4 The average concentration values of NORM in waste water that produced from refinery process are presented in table (1) and illustrated schematically in fig (2). These values are Bq/L, Bq/L, Bq/L and Bq/L for 226 Ra, 232 Th, 40 K and 210 Pb respectively. These concentrations represent a hazardous situation that demands special necessary precautions (20, 21). The average values of NORM concentrations in ash samples as by-product from refinery process of oil are presented in table (1) and illustrated schematically in fig (3). The obtained values are Bq/Kg, Bq/Kg, Bq/Kg and Bq/Kg for 226 Ra, 232 Th, 40 K and 210 Pb respectively. Table (1): Activity concentration (Bq/ kg) of the measured radionuclides in the collected oil samples. Code Ra -226 Th-232 Pb-210 K-40 Ash Ash Ash Ash Ash Ash Average of ash water water water water water water Average of water Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Avarage of oil

5 Oil-1 Oil-2 Oil-3 Oil-4 Oil-5 Oil-6 Oil-7 Oil-8 Oil-9 Oil-10 Oil-11 Oil-12 Oil-13 Oil-14 Oil-15 AVERAGE S.U.EL-Kameesy, et al. Arab J. Nucl. Sci. Appl, Vol 50, 4, (2017) Ra-226 Th-238 Pb210 K-40 Fig (1): activity concentration (Bq/kg) in oil samples water 1 water 2 water 3 water 4 water 5 water 6 Average of water Fig (2): activity concentration (Bq/kg) in waste water samples Ra-226 Th-238 Pb210 K Ash-1 Ash-2 Ash-3 Ash-4 Ash-5 Ash-6 Average of ash Fig (3): activity concentration (Bq/kg) in ash samples Ra-226 Th-238 Pb210 K-40 59

6 The experimental results of the absorbed dose, the annual effective dose, the radium equivalent and gamma index for all investigated samples are presented in Table (2). The calculated Ra eq activities of all samples are below the recommended value (370 Bq/kg). Moreover, the average value for gamma index for all the investigated samples presented in Table (2) is less than unity. Table (2): Values of the absorbed dose (D), the annual effective dose (AEDE), the radium Equivalent (Ra eq) and gamma index (I ɣr) for all investigated samples. Code D(nGy/ h) AEDE(mSv /y) Ra eq( Bq/ Kg) I ɣr Ash Ash Ash Ash Ash Ash Average of ash water water water water water water Average of water Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Averge of oil As shown in Table (2), the annual effective dose is very low and far below majority of the previous works (11, 12). 60

7 CONCLUSION AND RECOMMENDATIONS The following conclusions are derived from the results of the measurements performed to assess the concentration of NORM in Egyptian oil. NORM concentration in the Egyptian Oil is below the recommended level stated by the IAEA basic safety standard. Thus, it does not cause any measurable external exposure to oil workers. This assures that the workers annual exposure dose is below the annual dose limit (8). The measured NORM 238 U and 232 Th indicate that these radionuclides, with long half-lives, are not separated from the rock formations existing in oil wells. The produced waste water in oil industry is the main source of dose that should be removed from oil and disposed after treatment. REFERENCES (1) E. Rowan, M. Angle, C. Kirby and T. Kraemer Radium Content of Oil- and Gas Field Produced Waters in the Northern Appalachian Basin (USA): Summary and Discussion of Data, Scientific Investigations Report , USGS Reston, VA, United States, (2) F. A. Hartog, W.A.I. Knaepen, and G. Jonkers, Radioactive lead: An underestimated issue, in Proceedings of the 1995 API and GRI Naturally Occurring Radioactive Material (NORM) Conference, pp (API Publication 7104) Houston, USA, October 16-18, (3) F. A. Hartog, W.A.I. Knaepen, G. Jonkers, A.P. Schmidt, R.D. Schuiling, and P.F.J. Lancee, Origin and Encounter of 210 Pb in E&P facilities, in Proc. 2nd International Symposium on the Treatment of Naturally Occurring Radioactive Material, Krefeld, Germany, pp.53-57, November 10-13,1998. (4) A.P. Schmidt, Naturally Occurring Radioactive Materials in the Gas and Oil Industry Thesis, Geologica Ultraiectina, Mededelingen van de Faculteit Aardwetenschappen, University of Utrecht, the Netherlands, p. 144, (5) F.A.Hartog, G. Jonkers, A.P. Schmidt, and R.D. Schuiling, Lead Deposits in Dutch Natural Gas Systems, Society of Petroleum Engineers paper SPE68316-MS, (6) NRPB, NORM in the Oil and Gas Industries, Radiation at Work Series NRPB, (7) C. Oregano, D. Fenton, Radiological assessment of NORM industries in Ireland- Radiation doses to workers and members of the public, Radiological Protection Institute of Ireland, Dec (8) IAEA, Radiation Protection and the Management of Radioactive in the Oil and Gas Industry, Safety Series No. 34, ISBN , (9) IAEA, Regulatory and management approach for the control of environmental residues containing naturally occurring radioactive material (NORM), IAEA-TECDOC-1484, (10) T. strand, Handling And Disposal Of Norm In The Oil And Gas Industry, Norway, (11) Reed, G., Holland, B., MacAtrhus, A., Evaluating the Real Risks of Radioactive Scale in Oil and Gas Production. First Int. Conf. on Health, Safety and Environment, Hague, The Netherlands, 1014 Nov., Technical Paper SPE (12) Saudi Aramco Engineering Procedure (SAEP-0358) Management of Technologically Enhanced Naturally Occurring Radioactive Material (NORM), (13) K. P. Smith, D. L. Blunt, G. P. Williams, and C. L. Tebes, Radiological Dose Assessment Related to Management of Naturally Occurring Radioactive Materials Generated by the Petroleum Industry Environmental Assessment Division Argonne National Laboratory, 9700 South Cass Avenue, Argonne,

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