2015 International Nuclear Atlantic Conference - INAC 2015 São Paulo, SP, Brazil, October 4-9, 2015 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-06-9
ACTIVITY CONCENTRATIONS OF 238U AND 226Ra IN SCALES FORMED ON PIPES OF INDUSTRIAL BOILERS IN THE STATE OF PERNAMBUCO, BRAZIL
Claudia M. B. Poggi1, 2, Elvis J. França2,Crescêncio A. Silva Filho, Maria H. P. Gazineu3 Emerson E. G. de Farias1, 2, Clovis A. Hazin2
1Universidade Federal de Pernambuco – UFPE - Departamento de Energia Nuclear Av. Professor Luiz Freire, 1000, 50740-540 Recife, PE [email protected], [email protected]
2Centro Regional de Ciências Nucleares do Nordeste - CRCN-NE/CNEN-PE Av. Professor Luiz Freire, 200, 50740-540 -Recife, PE [email protected]
3Universidade Católica de Pernambuco – UNICAP - Centro de Ciências e Tecnologia Rua do Príncipe, 526, sala D603, 50050-500 Recife, PE [email protected]
ABSTRACT
The procedures employed in the industry can generate significant amounts of solid, liquid and gaseous wastes that usually contain toxic or materials of difficulty degradation. One of the facts that contribute to the formation of such wastes is the generation of steam used in operating processes and industry segments. Currently, steam supplied by boilers is the most economical and practical mode of heat transfer in industrial processes. Due to the high temperature of water used in these processes, compounds which were previously soluble become insoluble, generating residues called scales. This material, which contains stable ions, can also present naturally occurring radionuclides such as 238U and 226Ra, which concentrate over time in piping and equipment surfaces. If not disposed correctly, this material also can contaminate the environment. The main origin of these radionuclides is the use of groundwater in industrial processes. Thus, in regions of naturally enriched in radionuclides such as the Region of Pernambuco, including the municipalities of Paulista and Goiana, there is a greater possibility of radioactive scale formation. Therefore, this study aimed to determine the activity concentrations of 238U and 226Ra present in the solid wastes generated by industries situated in Paulista and Goiana, in order to assess radionuclide disequilibrium. For the sake of comparison, scale samples collected from industries located in the municipality of Caruaru, far from the previous municipalities, were also analyzed. The determination of the activity concentrations for 238U and 226Ra was performed by High Resolution Gamma-Ray Spectrometry. Samples were collected, prepared, packed in plastic containers and set aside for a minimum time of 21 days, for the secular equilibrium to occur between 226Ra and its short lived descendants. The counting time was 80,000 seconds. Gamma energies used for determination of activity concentrations were 1001 keV for 238U, and 352 keV, 609 keV and 1120 keV, for 226Ra. The concentrations ranged from 53 to 1,300 Bq.kg-1 for 226Ra and from < 237 to 410 Bq.kg-1 for 238U. According to the results one can conclude that, for the samples analyzed, statistically different values for 238U and 226Ra indicate different transport and deposition dynamics for radionuclides of the same decay series.
1. INTRODUCTION
Radionuclides can be introduced in various compartments of the ecosystem in different ways. Knowing the behavior and mobility of these radionuclides in the various environmental compartments may facilitate the understanding and interpretation of data. In this context, the matrix water is important due to its constant presence throughout the ecosystem and the large volume used in human activities, mainly in industry. In some of these facilities in order to obtain the final product, it is necessary either to generate heat or to cool down, the water being essential in these processes.
In general, it is essential to take into account not only the surface water but also the groundwater. The latter, in most cases, is more convenient for cooling, due to advantages such as being naturally filtered and purified by percolation. However, groundwater is often harder than surface water and can cause formation of scale, which interfere with heat transfer. The content and nature of impurities generating the waste varies from one place to another and, in general, contain significant quantities of calcium and magnesium salts [1,2] and traces of barium, which can co-precipitate with natural radioactive elements present in the water, as is the case of 226Ra, radionuclide belonging to the 228 U series [1,2].
Due to the high temperature of the boiler water, certain compounds, which are soluble in water supply / return, tend to a condition of super saturation, adhering to the heat exchange surface of the boiler. The scales formed in this situation consists of the aggregation and deposition of solids near the surface of the boiler and pipes, on the side of the water, due to the presence of impurities, such as sulfates, carbonates (calcium and / or magnesium), silicates and complexes containing iron, aluminum, calcium and sodium [3].
The precipitates resulting from inadequate treatment of boiler water may originate very rigid and difficult to remove scales [4,5]. These scales can cause problems in boiler performance during vapor formation, since they accumulate at the walls of the ducts which carry water, causing reduction in process efficiency, since the diameter of the ducts decreases significantly over time, and with it, the water flow decreases. Therefore, there is a need to do periodic maintenance on the pipes, thus generating a large amount of solid wastes, causing an increase in environmental problems resulting from these procedures [3].
The material removed from the boiler pipes can contain naturally occurring radioactive elements which, over time, can contaminate the environment. The naturally occurring radionuclides are referenced in the literature by the acronym NORM (Naturally Occurring Radioactive Material). When NORM-containing raw materials go through industrial processes that concentrate the radionuclides derived from soil, water and rocks, the formation of TENORM (Technologically Enhanced Naturally Occurring Radioactive Materials) occurs [6].
The formation of TENORM can occur in many types of processes and was detected in the mining industry, energy production (in scale and sludge formed in the oil production), and in wastes from water and effluent treatment [7,8]. However, no data has been found in the literature on the presence of natural radionuclide disequilibrium in scales from industrial boilers in general. As there is a need to study this type of waste, this study aimed to evaluate the natural radionuclides, especially 238U and 226Ra, in scale samples generated in industrial boilers in the State of Pernambuco.
2. MATERIAL AND METHODS
Scale samples were collected from boiler tubes from different industries in the state of Pernambuco. Samples one and two were collected from industrial laundries in the cities of
INAC 2015, São Paulo, SP, Brazil.
Caruaru and Paulista, respectively, and sample three was collected from a sugar cane plant located in the city of Goiana.
Caruaru is an industrial city 134 km far from Recife, capital of the state of Pernambuco, northeastern Brazil. Paulista and Goiana are both located in a region containing uranium phosphorite and which presents higher than normal uranium concentrations in the soil. This -1 region has high concentration of U3O8 (30 to 500 mg.kg ), and then high concentration of 226Ra, depending on the site analyzed [9].
The scale samples were collected directly from the surface of the tubes inside the boilers. Approximately 400 grams of each sample were collected and then packed in plastic bags, labeled and sent to the laboratory of the Northeast Regional Centre for Nuclear Sciences (CRCN / NE). Figure 1 illustrates one of the samples collected for analysis of radionuclides.
Figure 1: Scale removed from a water pipe sectioned from a boiler
The samples were oven-dried for two hours at 105 °C, cooled in a desiccator for 40 minutes and then ground in a mill jar (LICITTM), at 250 rpm to ensure homogeneity of samples. After dried, analytical portions of 100 g of each sample was carefully weighed and placed in a plastic cylinder 50 mm diameter and 60 mm height. Each container was sealed with silicone sealant and adhesive tape to prevent the escape of radon gas, produced from the decay of 226Ra.
The samples were stored for 21 days, enough time for equilibrium between 226Ra and its descendants with short half-lives from the 238U series. The containers with the samples were placed directly on the detector cap, due to their low activity. After equilibrium, concentration of natural radionuclides in the scale samples was determined by gamma spectrometry [10].
For the quality of the analytical procedure, the reference material IAEA-375 Radionuclides and Trace Elements in Soil produced by International Atomic Energy Agency – IAEA was analyzed together with samples. As the uranium concentration was unexpected it was not possible to use suitable material to this region of the spectrum.
In order to determine the counting efficiency of the detector for gamma rays as a function of energy, it was necessary to use a certified standard, with the same amount of sample used in the analyzes, equal densities for the standard and the samples, the same geometry and
INAC 2015, São Paulo, SP, Brazil. counting time. The counting efficiency of the detector for gamma rays, as a function of energy, was determined by using an internal soil standard spiked with 5 ml of 152Eu solution (activity: 28.23 ± 1.13 kBq.L-1; reference date: April 10th, 2007). This standard solution was provided by IRD/CNEN (Institute of Radioprotection and Dosimetry, Brazilian Commission of Nuclear Energy). Gamma energies used for determination of activity concentrations were 1001 keV for 238U, and 352 keV (Pb-214), 609 keV(Bi-214) and 1120 keV(Bi-214) for 226Ra.
RESULTS
Table 1 shows the results for the activity concentration of the radionuclides 238U and 226Ra in scale samples collected in boilers from the different industrial plants. The detection limits were 3,0 Bq.kg-1 for 226Ra and 237 Bq.kg-1 for 238U. The concentrations ranged from 53 to 1,300 Bq.kg-1 for 226Ra and from < 237 to 410 Bq.kg-1 for 238U. Analytical uncertainties in this table were expanded at the 95% confidence level.
Table 1: Radionuclides in scales Activity Sample Locality Nuclide concentration (Bq.kg-1)
Ra - 226 53 ± 5 1 Caruaru
U-238 293 ± 5
Ra - 226 1,300 ± 85 2 Paulista
U-238 < 237
Ra - 226 410±240 3 Goiana
U-238 290±57
The 226Ra activity concentration value found for the sample collected in a Caruaru industry was lower than those values obtained for the samples of scale collected from industries located in the cities of Paulista and Goiana. According to Silva et al. [11], the region where the municipalities of Paulista and Goiana are inserted is rich in uranium phosphorite and covers areas cut by aquifers. Thus, the phosphate deposits constitute sources of radionuclides into groundwater. This way, water drawn from wells for industrial use may contain natural radionuclides of the 228U serie, resulting probably in greater activity concentrations in the scale formed in steam boilers of the industries in that region [12,13,14].
According to Table 1, activity concentrations in scale samples were greater for 226Ra than for 238U. This higher deposition rates for 226Ra can be explained by the tendency for deposition of alkaline earth metals, especially calcium and magnesium, but also radium, in encrustation. In water, radium is found in low amounts in relation to other members of the family 2A [15]. However, radium may accumulate over time producing deposits, due to the great influence of variables such as pressure and temperature inside the boilers.
INAC 2015, São Paulo, SP, Brazil.
The activity concentration value of 238U for the scale sample collected from the industry in Goiana was lower than the detection limit. Also, values for activity concentrations of 238U were always lower than for 226Ra, indicating a difference in the dynamics of the transport and deposition of 238U compared to 226Ra. Radium and uranium are distinct elements with different physical and chemical parameters. The decay of radionuclides from the natural radioactive series is usually in secular equilibrium. However, for wastes from industrial processes (NORM / TENORM) in general, the secular equilibrium state is normally altered [16].
The disequilibrium between Ra-226 and its parent radionuclides (U) in oil scales and sludge has been studied by Omar et al. [17] and Jonkers et al. [18]. According to Omar et al. [17] scales in the oil industry presented much higher activity concentration values for 226Ra than for 238U. The daughter/parent ratios 226Ra/238U were much higher than unity, ranging from 41.0 to 55.0 for oil scales produced in an oil/gas production unity in Malaysia.
4. CONCLUSIONS
According to the results one can conclude that, for the samples analyzed, statistically different values for 238U and 226Ra indicate different transport and deposition dynamics for radionuclides of the same decay series.
ACKNOWLEDGMENTS
The authors are grateful to CAPES, for the financial support through the PhD scholarship; to the Division of Environmental Monitoring (SEAMB/CRCN/CNEN), for the technical support necessary for the development of this research; and to UFPE, in especial to the Department of Nuclear Energy, for all academic support.
REFERENCES
1. E. Lenzi, L. Favero, E. Luchese, Introdução á química da água: ciência, vida e sobrevivência, LTC, Rio de Janeiro, Brasil (2009). 2. R. Shereve, J. Brink JR, Indústrias de processos químicos, Guanabara, Rio de Janeiro, Brasil (1997). 3. M. Jamialahmadi, H. Müller-Steinhagen, “Scale formation during nucleate boiling - A review,” Corrosion Reviews, 11, pp.25-54 (1993). 4. D. Santos Filho, Tecnologia de Tratamento de Água, Nobel, São Paulo, Brasil (1985). 5. C. Gouvêia, A. Hurtado, R. Borzio, M. Folletto, “Uso de água tratada por osmose reversa para a geração de vapor em indústria de tabaco,” Revista Produção Online, 12, pp. 522-536 (2012). 6. “UNSCEAR- United Scientific Committee on the Effects of Atomic Radiation Source and effects of ionizing radiation,” www.unscear.org/docs/reports/2008/09- 86753_Report_ 2008_Annex_B.pdf (2010). 7. M.H.P. Gazineu, A.A. Araújo, Y.B. Brandão, C.A. Hazin,” Radioactivity Concentration in liquid and solid phases of scale and sludge generated in the petroleum industry” Journal of Environmental Radioactivity,81, pp.47-54 (2005).
INAC 2015, São Paulo, SP, Brazil.
8. R. Kleinschmidt, R. Akber, “Naturally occurring radionuclides in materials derived from urban water treatment plants in southeast Queensland,” Journal of Environmental Radioactivity, 99, pp. 607-620 (2007). 9. S. Saad, Aspectos econômicos do aproveitamento do urânio associado aos fosfatos do Nordeste, Boletim da Comissão de Energia Nuclear, Brasília, Brasil (1974). 10. Paiva, JDS, Souza, EE, Farias, EEG, Carmo, AM, Silva Filho CA, França EJ, “ Applied tools for determining low-activity radionuclides in large environmental samples,” J Radioanal Nucl Chem,305, pp.1-6 (2015). 11. C.M.Silva, G.H.Sousa, R.S.Amaral, “Determinação da concentração de urânio em poços de abastecimento público da região urano-fosfática de Pernambuco”, Phisicae, 3, pp.149-155 (2002). 12. Lima R. A. Avaliação da dose na população da região urano-fosfática do nordeste que utiliza os recursos hídricos da região, UFPE, Recife, Brasil (1996) [Tese]. 13. C.A.Silva Filho, E.J.França, E.M.Souza, F.CA.Ribeiro, T.O.Santos, E.E.G.Farias, G.N.Arruda, J.A.Souza Neto, E.V.Honorato, C.A.Hazin,”Radioactive risk evaluation of mineral water in the Metropolitan Region of Recife, Northeastern Brazil” J Radioanal Nucl Chem,295, pp.1215-1220. 14. C.M.B. Poggi. 226Ra, 228Ra e 40K em incrustações formadas em caldeiras de instalações industriais do Estado de Pernambuco, UFPE, Recife, Brasil (2011) [Dissertação]. 15. D. Lauria, N. Martins, R. Zenaro, Monitoração radiológica ambiental, IRD/CNEN, Rio de Janeiro, Brasil (2007). 16. B. Michalik, J. Brown, P. Krajewski, “ The fate and behavior of enhanced natural radioactivity with respect to environmental protection”, Environmental Impact Assessment Review, 38, pp. 163-171 (2013). 17. M. Omar, M.S. Hamzah, A.K. Wood, “ Radioactive disequilibrium and total activity concentration of norm waste”, J Nucl.& Rel.Tech,5, pp.47-56 (2008). 18. G.Jonkers, F.A. Hartog, W.A.I. Knaepen, P.F.J. Lancée, Characterisation of NORM in the oil and gas production (E&P) industry, Shell Research & Technology, Amsterdam, Netherlands (1997).
INAC 2015, São Paulo, SP, Brazil.