radiological control in a mine with a naturally occurring radioactive

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3Aluna de graduação no Bacharelado em Ciência e Tecnologia,. Universidade Federal de Alfenas, Campus Poços de Caldas. Estrada José Aurélio Vilela, ...
2013 International Nuclear Atlantic Conference - INAC 2013 Recife, PE, Brazil, November 24-29, 2013 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-05-2

RADIOLOGICAL CONTROL IN A MINE WITH A NATURALLY OCCURRING RADIOACTIVE MATERIAL - NORM: I ASSESSMENT OF ACTIVITY CONCENTRATION OF ALPHA EMITTER WITH LONG HALF LIVE IN THE AIR IN A PILOT PLANT. W. S. Pereira1, 2, A. Kelecom2, J. R. S. Pereira3, D. A. Py Júnior1, A. C. A. Silva1 and O. Garcia Filho1; 1

Grupo Multidisciplinar de Radioproteção, Serviço de Radioproteção, Unidade de Tratamento de Minério Indústrias Nucleares do Brasil, CP 961, CEP 37.701-000, Poços de Caldas - MG – Brasil; e-mail: [email protected] 2

Laboratório de Radiobiologia e Radiometria Pedro Lopes dos Santos (LARARA-PLS), Grupo de Estudos em Temas Ambientais (GETA) Universidade Federal Fluminense – UFF, C.P. 100436, CEP 24.001-970, Niterói, RJ, Brazil. e-mail: [email protected] and [email protected] 3

Aluna de graduação no Bacharelado em Ciência e Tecnologia, Universidade Federal de Alfenas, Campus Poços de Caldas Estrada José Aurélio Vilela, 11.999 - Poços de Caldas - MG, CEP 37715-400 e-mail: [email protected]

ABSTRACT The Ore Treatment Unit (OTU) supports a laboratory process responsible for the development of new chemical processes for uranium extraction from ore elements associated with uranium. In 2009, a pilot plant for extraction of uranium from a phosphate ore mine with uranium associated was implanted, which is a case of Naturally Occurring Radioactive Material (NORM) in a mine at Santa Quitéria, CE, Brazil. This pilot plant was supervised by the radiological protection service, aiming the Occupational Exposed Individual’ safety (OEI). During the pilot plant operation the monitoring of radionuclides concentration in air was carried out. During the functioning of the pilot plant 63 high-vol air monitoring and posterior gross alpha counts were made in order to evaluate the alpha emitters. One sampling was made before the beginning of operations in order to evaluate the background which was estimated in 0.003 Bq m-3. Monitoring results varied between 0.001 Bq m-3 and 0.162 Bq m-3 with the average equal to 0.041 Bq m-3. 100 % of the results were below the derived limit for OEI which is equal to 0.360 Bq m-3. Thirty results were below the derived limit for public exposure. By using this criterion the area must be classified as Supervised Area. In order to correctly classify the area, the internal exposure must also be measured. The small values of air concentration of long lived alpha emitters can be explained by the process of uranium extraction that is made by solvent in a wet way that creates few aerosol particles in air that can be monitored by this method.

1. INTRODUCTION The Santa Quitéria Unit (USQ) is a plant of industrial mining and beneficiation of phosphate with uranium as a by-product, situated in the State of Ceará, NE of Brazil. It is now in process of commissioning. This unit is divided into two areas: the phosphate mining and production (conventional mining facility, CMF-USQ), and the production of uranium diuranate (nuclear facility, NF-USQ), that must obey the Brazilian legislation for nuclear installation (NF) [1-3]. This way of classifying the plant is new in Brazil, since until then facilities have always been classified in a unique way as conventional or as nuclear installations. The form of classification adopted here creates a situation where there is a need for dual licensing: one is

based on the standards for radioactive installation [1-3] that includes, among others, the nuclear and conventional parts for the NF-USQ which is under the responsibility of the Comissão Nacional de Energia Nuclear (CNEN), Instituto Brasileiro de Meio Ambiente e de Recursos Naturais Renováveis (IBAMA) and Superintendência de Meio Ambiente do Ceará (SEMACE), the other is a conventional licensing for the CMF-USQ part, under the responsibility of the federal and the state environmental agencies IBAMA and SEMACE. Regarding the licensing of the radioactive facility, one of the concerns is the existence of a radioprotection service. This one is disciplined by CNEN norm NN-3.01 and resolution N.146/13 (norm CNE-NN-7.01) among others [3-4]. In order to analyze the production process of uranium diuranate in the Ore Treatment Unit, a pilot plant production process was started up looking for process parameters and their optimization. In addition, to support the safe production of uranium diuranate in the pilot plant, the Radiation Protection Service developed a program for monitoring the concentration in air of long half-life alpha-emitting radionuclides. This program also allowed the radioprotection service to obtain parameters to be used in the radiological protection program of the NF-USQ. This paper aims to describe the results of air sampling looking for alpha emitters of long halflife in the pilot plant operated in 2009 at the Ore Treatment Unit at Santa Quitéria, CE.

2. METODOLOGY The air sampling was performed as described by us in 2011 [5]. A descriptive statistics and an analysis of frequency distribution were performed and compared with the derived limit values for long half-life alpha emitting radionuclides, based on the dosimetric model described below.

2.1. Dosimetric Model The model proposes that the dose for radionuclide intake is proportional to the activity concentration of the long half-life alpha-emitters present in the air, to the breathing and occupancy rates; the constant of proportionality is the dose conversion factor of the mixture of radionuclides in the air, calculated following equation 1. D = C Tr To e(g)1μm

(1)

where:  D is the committed effective dose rate, in Sv y-1;  C is the activity concentration, in Bq m-3;  Tr is the breathing rate, in m3 h-1;  To is the occupancy rate, calculated as the product of the number of working days per year [Dt (d y-1)] with the number of worked hours per day [Ht (h d-1)], in h y-1;  e(g)1μm is the dose conversion factor of the mixture, in Sv Bq-1.

INAC 2013, Recife, PE, Brazil.

2.2. Record Levels Derived From Concentration In Air The derived level (DL) of concentration of alpha emitters in air was obtained by equation 2 and data contained in item 2.1. Dosimetric Model. The dose rate utilized is equal the dose limit established by CNEN [6 and 7]. The material considered as a supplier of particulate material containing long half-life alpha emitters is uranium diuranate. It has a ratio of 1:1 between U-238 and U-234. Considered radionuclides are shown in Table 1, as well as their proportions in uranium diuranate, according to [4], their solubility and the dose conversion factors, as in PR-003 [6]. The number of working days per year was estimated at 250 days with 8 hours of work per day. Breathing rate considered was 1.2 m-3 per hour [8].

Table 1 - Alpha emitting radionuclides in uranium diuranate, their amounts in the mixture, type of absorption and dose conversion factor (e(g)1μm); F: fast [5]. Radionuclides Uranium diuranate Type of absorption e(g)1μm 238 U 1 F 4.9 10-7 234 U 1 F 4.0 10-5 weighted e(g)1μm 2.03 10-5 0.02 (Sv y-1) = C (Bq m-3) 1.2 (m3 h-1) [250 (d y-1) 8 (h d-1)] 2.03 10-5 (Sv Bq-1)

(2)

hence, C = 0.36 Bq m-3

3. RESULTS The values of the descriptive statistics of the 63 samples of long half-life alpha-emitters collected in the pilot plant are shown in Table 2. The limit of the Occupationally Exposed Individual (OEI) was defined in the methodology (item 2.2) as 0.36 Bq m-3, and the limit of Individual from the Public (IP) was established following [3] at 10% of the limit of the IOE. The mean activity concentration of alpha-emitters in air was 0.041 Bq m-3, thus somewhat higher than the limit for the public (0.036 Bq m-3). Analysis of the distribution of frequencies (Table 3) indicated that more than half of the results were below the limit for IP and that none of the results exceeded the limit, with 98% of the data below half the value for IOE and only one result somewhat higher than half the value for IOE. These results are depicted in Figures 1 and 2. Table 2 - Descriptive statistics of activity concentration in air samples of long half-life alpha-emitters in the pilot plant at the Ore Treatment Unit. Parameter Average Variance Kurtosis Asymmetry Interval Minimum Maximum N 0.041 0.002 0.161 0.001 0.162 value 0.87 1.23 63 Bq m-3 Bq m-3 Bq m-3 Bq m-3 Bq m-3 INAC 2013, Recife, PE, Brazil.

Table 3 - Frequency distribution of the results of air sampling for long half-life alpha-emitting radionuclides. Class of frequency Frequency % relative Cumulative % 58.73% 0.036 37 58.73% 15.87% 0.061 10 74.60% 11.11% 0.086 7 85.71% 6.35% 0.111 4 92.06% 3.17% 0.136 2 95.24% 3.17% 0.161 2 98.41% 1.59% 0.186 1 100% 0% higher 0 100%

Figure 1 - Results of activity concentration of long half-life alpha-emitters in air samples and activity concentration limit to the public and for IOE.

Figure 2 - Histogram of frequencies and cumulative data for activity concentration of alpha-emitters of long half-life in of air samples.

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4. CONCLUSIONS The pilot plant operated for approximately five months between March and July 2009. During this period 63 air samples were obtained for verification of activity concentration of long half-life alpha emitters present in the air. All results were below the limit of IOE, more than 58% were below the limit for the public and 42% were between the limit of IP and IOE. Thus, based on the criterion of internal exposure via inhalation the area should be classified as supervised. However, for a correct classification of an area it is necessary to consider also the external exposure and the contamination of surfaces so that, using the full data set of the monitoring (internal, external and surface contaminations), a risk analysis should be performed in order to allow a real classification in terms of radiation protection of area. Other factors to be taken into account are the stored amounts of chemicals and their form of storage. As the physical form is liquid, there is a possibility of contamination. In this case, the norm CNEN-NN-3.01 [3] recommends classifying the area as a controlled area. Thus, the values found for air monitoring confirmed that the execution of the operations occurred within the effective parameters, indicating that the operating procedures carried out in collaboration with the Radiation Protection Service, were effective for the plant operation, resulting in safe operation both from the operational point of view and from the radioprotection one.

REFERENCES 1. CNEN (Comissão Nacional de Energia Nuclear). Norma CNEN-NE-1.04, Licenciamento de instalações nucleares. 25 pp. (2002). 2. CNEN (Comissão Nacional de Energia Nuclear). Norma CNEN-NE-1.13, Licenciamento de minas e usinas de beneficiamento de urânio e tório, 34 pp. (1989). 3. CNEN (Comissão Nacional de Energia Nuclear). Norma CNEN-NN-3.01, Diretrizes Básicas de Proteção Radiológica, 34 pp. (2005). 4. CNEN (Comissão Nacional de Energia Nuclear). Resolução 146/13 Norma CNENNN-7.01, certificação da qualificação de supervisores de proteção radiológica, 7 pp (2013) 5. Pereira, W. S. et al, Area monitoring in a deposit of radioactive material: high flow air sampling for determination of alpha emitters of long half-life Annals of 2011 International Nuclear Atlantic Conference - INAC 2011, 6 pp, 2011. 6. CNEN (Comissão Nacional de Energia Nuclear). Posição regulatória 3.01 / 003 coeficientes de dose para indivíduos ocupacionalmente expostos. 52 pp. (2005). 7. CNEN (Comissão Nacional de Energia Nuclear). Diretrizes básicas de proteção radiológica CNEN-NN-3.01. 27pp. (2005). 8. INB (Indústrias Nucleares do Brasil). Monitoração de área por meio de amostragem de aerossol. Instrução Operacional da UTM, IO-UTM-PO-54. 16 pp. (2010).

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