ENVIRONMENTAL IMPACT ASSESSMENT OF A RADIOLOGICAL UNIT OF ORE PROCESSING, AT CALDAS, MG, BRAZIL

Transcription

1 2009 International Nuclear Atlantic Conference - INAC 2009 Rio de Janeiro,RJ, Brazil, September27 to October 2, 2009 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: ENVIRONMENTAL IMPACT ASSESSMENT OF A RADIOLOGICAL UNIT OF ORE PROCESSING, AT CALDAS, MG, BRAZIL Wagner de S. Pereira 1,2, Delcy de A. Py Júnior 1, Marcos M. Antunes 1, Gabriela T. de Oliveira 1,3, Ana C. A. Silva 1, 4, Alphonse Kelecom 2, 5 e Simone Gonçalves 6 1 Coordenação de Proteção Radiológica, Unidade de Tratamento de Minérios UTM. Caixa Postal 961, CEP , Poços de Caldas, MG, BR. Indústrias Nucleares do Brasil INB. wspereira@inb.gov.br, delcy@inb.gov.br, antunes@inb.gov.br or anasilva@inb.gov.br 2 Curso de Pós-Graduação em Biologia Marinha, Instituto de Biologia, - Depto. De Biologia Marinha. Caixa Postal , CEP: , Niterói, RJ, Brasil. Universidade Federal Fluminense UFF. wspereira@inb.gov.br or kelecom@uol.com.br 3 Curso de Engenharia Química, com ênfase em Ciências Ambientais. Largo Engenheiro Paulo de Almeida Sandeville, 15, CEP: , São João da Boa Vista, SP, BR. Centro Universitário de São João da Boa Vista UNIFAE. 4 Curso de Bacharelado em Ciência e Tecnologia. Campus de Poços de Caldas, Rua Corumba, 72, Poços de Caldas, MG, BR. Universidade Federal de Alfenas - UNIFAL. anasilva@inb.gov.br 5 Programa de Pós-Graduação em Ciência Ambiental. Caixa Postal , CEP , Niterói, RJ, BR. Universidade Federal Fluminense UFF. kelecom@uol.com.br 6 Curso de Pós-Graduação em Gestão Ambiental. Av. Padre Francis Cletus Cox Omi, 1661, CEP Poços de Caldas, MG, BR. Pontifícia Universidade Católica PUC. ABSTRACT This study aims to estimate the radiological impact on the environment (RIE) from the release of water treated by the ore processing unit (UTM) at Caldas, MG, during the years 2005 and In 2005, the UMT operated 400 tons of monazite in a process to obtain rare earths; during the following year there was no industrial process. The effluent waters were tested once a week for points 014 and 025, and once a month for point 076. The critical radionuclides are U-238, Ra-226, Pb-210, Th-232 and Ra-228. U-238 and Th-232 were analyzed by spectrophotometry. Ra-226, Ra-228 and Pb-210, in turn, were analyzed by chemical separation and radiometric methods. The dose estimates were based on the model proposed by the National Commission of Nuclear Energy (CNEN) to three critical groups established according to the points of release. The calculations were performed using the annual average concentration of activity. The values of the maximum permitted dose rate allowed by CNEN is equal a 0.3 msv y -1 onto the critical group. In conclusion, we observed that the maximum dose rates, allowed by CNEN, were not reached, being all below 50% of this limit. This indicates that the treatment of water effluents generated by UTM was conducted in an efficient way, ensuring the safety of the population surrounding the venture. 1. INTRODUCTION The Industrial Complex of Poços de Caldas (CIPC) was the first mining and processing uranium plant in Brazil. It is located in the municipality of Caldas, plateau of Poços de

2 Caldas, Minas Gerais, Brazil. In the middle of the 1980 s, with the end of the economic viability of the uranium exploration, the CIPC was disabled. In 2005, the CIPC disabled installations, at the time already called UTM (Unidade de Tratamento de Minério, ore processing unit in Portuguese), operated 400 tons of monazite in a process to obtain rare earths. In the following year there was no industrial process. The open pit disabled mine, the mine waste piles and tailings dam are term sources of heavy metals and radionuclides, causing environmental impact. Thus, a program of effluent treatment was set up aiming to minimize the environmental impact of the UTM operation and to monitor the impact from the term sources generated by the operation of CIPC. Similarly, an environmental monitoring program was maintained to confirm that the effluent treatment indeed mitigated the impacts and set the effluents into the limits allowed by the Brazilian law for their release to the environment. Several studies were conducted in this region to assess the radiological impact on the environment (RIE) during operation of the plant [1-4]. Other authors focused on the transport of radionuclides in this region during the same phase [5, 6]. Before the operation of CIPC, the background of that region had been established by Amaral and colleagues [7]. This approach is very important, since the Brazilian legislation in force at that time [8] legislated on the additional dose of the venture and the current legislation [9] focuses on the additional dose due to the practice. The management of the sterile pile has been studied by Fernandes [10] and Fernandes and collaborators [11] aiming to the process of decommissioning of the plant. In order to carry out the RIE assessment, a model of environmental dispersion of released radionuclides must be developed. The dispersion of radionuclides released by the practice, the process of incorporation of these radionuclides, their concentration in the environmental compartments of interest (water, food, air, etc...) must be modeled and monitored for confirmation of the model. Furthermore, a dosimetric model should be developed too. In this respect, the CNEN has developed a model of RIE assessment [12] that integrates the environmental and dosimetric models. This model provides derived limits at interface points with the environment. The present study aims to estimate the dose to which individuals of the public (IP) of UTM are exposed, based on derived limits established by the CNEN in 2005 [12], when the plant was in operation, and in 2006 when there was no operation at all. 2. METHODOLOGY 2.1. Establishment of effluents release points The UTM has three points of release of liquid effluents to the environment that are linked to three term sources. The points of release of effluents and their term sources are presented in Table 1.

3 Table 1 - Derived limits established by the National Commission of Nuclear Energy, for the interface points UTM-environment and their sources terms, Bq L -1, seg. [12]. 238 U 232 Th 226 Ra 228 Ra 210 Pb Point Location Source term 014 Ribeirão das Antas Open pit mining Put out Ribeirão Soberbo tailings pond Corrego da Consulta Put out Conceptual Model CNEN has developed a model based on the concept of critical pathways analysis [12]. It predicts a homogeneous group of people most exposed to the release of radionuclides by the practice (critical group). It also provides the establishment of the most important radionuclides in terms of dose contribution (critical radionuclides) and the most important routes of exposure (critical pathways). Another concern of the model is the formulation of a philosophy of conservative calculation, for the actual value of the impact to be significantly lower than the value calculated by the model. The critical exposition route was the release of the liquid effluents by UTM. The critical group was planned as a hypothetical population group for each interface point with the environment. The critical radionuclides were: U-238, Ra-226, Pb-210 and the Th-232 and Ra Radionuclides Analysis Analyzed radionuclides belong to the U-238 and Th-232 series, i.e.: U-238, Ra-226 and Pb- 210, from the U-238 family, and Th-232 and Ra-228, from the Th-232 family. Uranium and thorium were measured by spectrophotometry using arsenazo [13], modified by [14]. Ra-226 activity was determined by radiochemistry and total alpha radiometry, and Ra-228 and Pb- 210 by radiochemistry and total beta radiometry, as described in [15] Derived Limits The derived limits are the activity concentrations of radionuclides in any environmental compartment, which when reached would result in a dose to the surrounding population (critical group) equal to the dose limit (0.3 msv y -1 ) allowed for the IP. Higher values were not allowed by law at the time [8], and neither now [9]. The model developed by the CNEN [12] set limits for critical radionuclides at the interfaces with the environment that appear in Table 1. The maintenance of the values of activity concentration in the effluent below the derived limits ensures that the hypothetical critical groups are receiving a dose lower than the maximum allowed by CNEN (0.3 msv y -1 ), the regulatory instance.

4 2.5. Dose Estimation The dose estimation was made based on the linear relationship between the activity concentration of radionuclides present in the effluents and the dose caused by these radionuclides. At each point of interface with the environment, a relationship was established between the average concentration of each radionuclide and its derived limit. Then an average of relations of all critical radionuclides was set up. This average is the percentage of the limit allowed for each point of interface of the installation with the environment Environmental Monitoring Program To support the dose evaluation, an environmental monitoring program (EMP) is performed as follows. At point 014 (located at Ribeirão das Antas) a weekly collection is realized; the same occurs at point 025 (Ribeirão Soberbo). At point 076 (Córrego da Consulta), the collection was performed monthly. These waters are analyzed for the presence and amounts of critical radionuclides in the soluble fraction (i.e. passes through a filter of 0.45 μm) and particulate (i.e. retained by a filter of 0.45 μm). The dose was the sum of both fractions. Currently, proposals for changes have occurred in the monitoring program to satisfy the current Brazilian legislation [9], [16, 17] and the good practices recommended by the regulatory instance [18]. 3. RESULTS The critical radionuclides: U-238, Ra-226, Pb-210, Th-232 and Ra-228, were analyzed following the EMP. The average values of activity concentration at the points of interface with the environment in 2005 and 2006 are shown in Table 2. Among the values of concentration activities reported in Table 2, the only value above the derived limit was the concentration of U-238 at point 014, which is associated to the open pit mine and to the put out 08, in That year, at that point, all other radionuclides were below the derived limits set in [12]. The dose assessment of the critical group associated with this point and place was 0.13 msv y -1 which is less than half the maximum permissible dose. Table 2 - Values of average annual activity of natural radionuclides studied at points 014, 025 and 076 during 2005 and 2006, in Bq L Radionuclides N N N 238 U Th Ra Ra Pb N = number of samples For all the other points, radionuclides and year, the activity and concentration values were below the derived limits. This means that in all these cases, the values of doses were below

5 the authorized limit (0.30 msv y -1 ). In this case the largest evaluated dose was the one at the same point 014, with a value of 0.11 msv y -1. The values of dose to which are exposed the critical groups associated with the release of effluents are shown in Table 3. Table 3 - Estimated dose for the hypothetical critical group in msv y -1 from the derived limits established by CNEN [12] Point Location Ribeirão das Antas Ribeirão Soberbo Córrego da Consulta CONCLUSION The assessment of the radiological environmental impact of UTM showed that the exposure limits were not exceeded. The values of dose to which the critical groups are exposed were less than half of the authorized limit. It should be noted that Brazilian legislation regulates the increase in dose to which the IP are exposed, and in this case it should be necessary to discount the background. Since this procedure has not been done, it can be stated that the impact of the installation was less than estimated in this work. Another important information is the observed gradient in the radiological impact at the interface points. It was observed that point 014, located at Ribeirão das Antas and associated with the open pit mine and with the put out 08, shows higher concentrations than point 025, at Ribeirão Soberbo, which is associated with the tailings pond, and which, in turn, is higher than point 076, at Córrego da Consulta, associated with the put out 04. This gradient allows the lower frequency of sampling at point 076. Indeed, being associated with a lower radiological risk, the latter point may have a less stringent monitoring than the other two ones that have larger impacts, being therefore associated with greater radiological risks. Also relevant is the observation of higher doses at all points during 2005, when UTM was operating. As only two years of analysis are reported here, no statistical treatment of data can be carried out. However, our results indicate a trend that needs a further follow up. REFERENCES 1. Fernandes, H.M., Veiga, L.H.S., Franklin, M.R., Prado, V.C.S. and Taddei, J.F. Environmental impact assessment of uranium mining and milling facilities: a study case at the Poços de Caldas uranium mining and milling site, Brazil. Journal of Geochemical Exploration, 52, pp , (1994). 2. Amaral, E.C.S. Modificação da exposição radiação natural devido a atividades agricolas e industriais numa área de radioatividade natural elevada no Brasil. Ph.D. thesis. Instituto de Biofisica, UFRJ, 130 pp. (in Portuguese). (1992).

6 3. Amaral, E.C.S., Godoy, J.M., Rochedo, E.R.R., Vasconcellos, L.M.H. and Pires do Rio, M.A. The environmental impact of the uranium industry: Is the waste rock a significant contributor. Radiation Protection Dosimetry, 22, pp (1988). 4. Amaral, E.C.S., Rochedo, E.R.R., Paretzke, H.G. and Penna Franca, E. The radiological impact of the agricultural activities in an area of high natural radioactivity. Radiation Protection Dosimetry, 45, pp (1993). 5. Azevedo, H. L. P., Amaral, E. C. S. and Godoy, J. M. Evaluation of the 226 Ra transport by river sediments surrounding the brazilian uranium mining and milling facilities. Environmental Pollution, 51, pp (1988). 6. Barcellos, C., Amaral, E. & Rochedo, E. Radionuclide transport by Pocos de Caldas Plateau rivers, Brazil. Environ. Technol., 11, pp (1990). 7. Amaral, E.C.S., Azevedo, H.L.P. and Mendonca, A.H. Pre-operational environment survey at the uranium mine and mill site, Poços de Caldas, Minas Gerais, Brazil. Science of the Total Environment 42, pp (1985). 8. COMISSÃO NACIONAL DE ENERGIA NUCLEAR, Norma CNEN-NE-3.01, Diretrizes Básicas de Radioproteção, 121 pp. (1988). 9. COMISSÃO NACIONAL DE ENERGIA NUCLEAR, Norma CNEN-NN-3.01, Diretrizes Básicas de Proteção Radiológica, 34 pp. (2005). 10. Fernandes, H.M., Franklin, M.R., Veiga, L.H.S., Freitas, P. and Gomiero, L.A. Management of Uranium Mill Tailings: Geochemical Processes and Radiological Risk Assessment. Journal of Environmental Radioactivity, 30(l), pp (1996). 11. Fernandes, H.M. Subsidios ao Descomissionamento da Primeira Indústria de mineração e beneficiamento de urânio no Brasil. O caso do Complexo Minero Industrial de Pocos de Caldas - Niterói. D.Sc. thesis, Departamento de Geoquímica da Universidade Federal Fluminense. 250 pp (in Portuguese). (1997). 12. COMISSÃO NACIONAL DE ENERGIA NUCLEAR, OFÍCIO CNEN SNU n.º 901/91 de 22/07/1991. (1991). 13. Savvin, S.B. Analytical use of arsenazo III: Determination of thorium, zirconium, uranium and rare earth elements, Talanta, v.8, nº9. pp (1961). 14. Savvin, S.B. Analytical applications of arsenazo III II: Determination of thorium, uranium, protactinium, neptunium, hafnium and scandium, Talanta, v.11, nº1, pp 1-6. (1964). 15. Godoy, J.M., Lauria, D.C., Godoy, M.L.P., Cunha, R. Development of a sequential method for determination of 238U, 234U, 232Th, 230Th, 228Th, 228Ra, 226Ra and 210Pb in environmental samples. J. Radioanal. Nucl. Chem. v. 182, nº 1, pp (1994). 16. COMISSÃO NACIONAL DE ENERGIA NUCLEAR, POSIÇÃO REGULATÓRIA 3.01 / 008. Programa de monitoração radiológica ambiental, 6 pp, (2005). 17. COMISSÃO NACIONAL DE ENERGIA NUCLEAR. POSIÇÃO REGULATÓRIA 3.01 / 009, Modelo para a elaboração de programa de relatórios de monitoração radiológica ambiental, 6 pp, (2005). 18. Lauria, D. C. Martins, N. S. and Zenaro, R., Monitoração Radiologica Ambiental. Comissão Nacional de Energia Nuclear, Instituto de Radioproteção e Dosimetria. Coleção IRD, V. 01, N pp, (2007).