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Radioactivity Assessment of Pavement Blocks Made with Itabirite Ore Waste

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Radioactivity Assessment of Pavement Blocks Made with Itabirite Ore Waste 2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-04-5 RADIOACTIVITY ASSESSMENT OF PAVEMENT BLOCKS MADE WITH ITABIRITE ORE WASTE Carolina Braccini Freire1,2, Valeria Cuccia1, Talita Oliveira Santos3, Clédola Cássia Oliveira de Tello1, Fernando Soares Lameiras1 e Zildete Rocha1 1 Centro de Desenvolvimento da Tecnologia Nuclear (CDTN / CNEN – MG) Avenida Presidente Antônio Carlos, 6.627 Campus da UFMG - Pampulha - CEP 31270-901 Caixa Postal 941 - CEP 30161-970 Belo Horizonte – MG [email protected] [email protected] [email protected] [email protected] 2 Rede Temática de Engenharia de Materiais (REDEMAT/UFOP/CETEC) Praça Tiradentes, 20 Centro – CEP 35400-000 Ouro Preto – MGl [email protected] 3 Departamento de Engenharia Nuclear (UFMG) Av. Antônio Carlos, 6627 Campus UFMG PCA 1 – Anexo Engenharia – Pampulha -CEP: 31.270-90 Belo Horizonte – MG [email protected] ABSTRACT Banded Iron Formation, also known as itabirite is a rock composed of silica and hematite layers. The iron ore exploitation from itabirite is carried out by separating the silica by chemical processes, generating high amount of sandy waste that are stocked in dams. In contrast, the construction of dwellings and other buildings consumes about 210 million tons of aggregates (sand and gravel) annually in Brazil and the excessive extraction of river sand causes many environmental impacts. Studies have indicated the technical feasibility of using these waste ore like aggregates in cement artifacts. However, natural occurring radionuclides are present in many natural resources. Human activities may enhance concentrations of radionuclides and/or enhance potential of exposure to naturally occurring radioactive material (NORM/TENORM). The industrial residues containing radionuclides have been receiving a considerable global attention, because of the large amounts of NORM containing wastes and the potential long term risks of long-lived radionuclides. Although a significant amount of primordial radionuclides by primary geological process is not expected in itabirite, it may occur due to late geologic process. In this concern, this work presents an assessment of natural radionuclides contents in this sand waste, in cement and in regular sand. Also the radon exhalation rate of pavement blocks made with varied contents of the itabirite mining residue was measured by using a continuous radon measuring equipment in a closed circuit. A preliminary estimation of the potential radiological risk by using this ore waste in place of regular sand is also presented. 1. INTRODUCTION Brazil has the second worldwide largest producer of iron ore, in Minas Gerais. Itabirite (banded iron formation) is a rock constituted of quartz and iron oxide layers. The ore iron exploitation from itabirite generates high amount of sandy waste, about thousands tons daily. The Quadrilátero Ferrífero - Iron Quadrangle - in the state of Minas Gerais, Brazil, is the most important region of itabirite exploitation. This waste presents homogeneous characteristics and it is available in high amounts. Therefore, it is an interesting raw material for many industrial segments, among them the manufacture of cement artifacts [1]. The segment of cement artifacts is one of the many industries that are included in the macrosector construction. An important aspect for the sector’s sustainability is the rational use of natural resources and durability increase of its products [2]. The replacement of natural sand by sandy waste in the manufacture of cement artifacts reaches the sustainability’s principles. It decreases the impacts generated by the accumulated waste in the environment and by the extraction of natural resources (sand). Previous works executed by the researchers of Waste Management Service from CDTN, showed this sand waste can be used like aggregate in cement mortars, resulting products with great properties to construction industry. However, natural occurring radionuclides are present in many natural resources. Human activities may enhance concentrations of radionuclides and/or enhance potential of exposure to naturally occurring radioactive material (NORM/TENORM). The industrial residues containing radionuclides have been receiving a considerable global attention, because of the large amounts of NORM containing wastes and the potential long term risks of long-lived radionuclides [3]. The most important factors to consider in evaluating radiological impact of using a new material in construction of dwellings and other buildings are the external exposure to gamma rays and the internal exposure to radon due to the naturally occurring radioactive material of terrestrial origin (also called primordial radionuclides) present in the construction material. Thus, two ways of radiation hazards risks evaluation were made: first, the external exposure by gamma radiation from the 238U and 232Th series and from 40K, and second, the internal exposure due to the inhalation of radon and its daughters. This paper presents an assessment of natural radionuclides contents in this sand waste, in cement and in regular sand. Also the radon exhalation rate of mortar specimens made with itabirite sand waste were measured by using a continuous radon measuring equipment in a closed circuit. A preliminary estimation of the potential radiological risk by using this ore waste in place of regular sand is also presented. 2. MATERIALS AND METHODS The itabirite sandy waste sample was collected at a mining industry in the region known like Quadrilátero Ferrífero, in the state of Minas Gerais, Brazil. The cement and the regular sand sample were provided by a conventional company of construction materials. Both the samples were homogenized and packaged in bags containing 3 kg of material, approximately. INAC 2011, Belo Horizonte, MG, Brazil. This work presents results of concentration measurements of radionuclides such as 232Th, 226Ra, and 40K by gamma spectrometry in the itabirite sandy waste, in cement and in regular sand and estimate the radiological hazard risk by using the radium equivalent activity as radiation hazard index. It also presents results of exhalation rate of two kind of mortar specimens: one made of itabirite sandy waste in substitution of regular sand and another made of regular sand commercially used as building construction material, for comparison. 2.1. Gamma Spectroscopy The samples were dried for about 48h in an oven at 60°C and transferred to Marinelli beakers (500mL). Each recipient was sealed for at least four weeks to reach secular equilibrium between 226Ra and 224Ra and their respective immediate progeny [4,5]. The measurements of natural radioactivity were performed by gamma-ray spectrometry: a system from CANBERRA, consisting of a hyper-pure germanium detector (HPGe), coaxial geometry, 15% efficiency, with data acquired and treated with Genie 2000 software. All the gamma emitters nuclides from natural series of U and Th were determined by Gamma Spectrometry. 226Ra (238U chain) and 224Ra (232Th chain) were determined by measuring the areas of 214Bi and 212Pb, respectively, because the ageing time of at least thirty days after sealing the Marinelli beakers, which assures the secular equilibrium between these elements and their progeny [5,6]. This period of time is also enough to restore the equilibrium in 232Th chain, between 228Ac and 228Ra. The specific gamma line used and the detection limits are shown in Table 1 [7,8]. Table 1. Radionuclides determined by Gamma Spectroscopy Analysis and detection limits Used gamma lines Detection limit Radionuclide [energy (keV) - emission probability] (Bq.kg-1) 40K 1460.8 (10.7%) 1.0 214Bi (226Ra) a 609.3 (46.3%) 0.4 228Ac 338.3 (11.4%); 911.6 (27.7%); 968.2 (16.6%) 0.4 212Pb (224Ra) a 238.6 (44.6%) 0.4 a The gamma lines represent 214Bi and 212Pb. 226Ra and 224Ra were determined indirectly. The gamma spectrometry laboratories have been participating regularly in intercomparisons programs like the National Intercomparison Program coordinated by IRD/CNEN (Institute of Radiation Protection and Dosimetry), in Brazil, and by the IAEA (International Atomic Energy Agency). The isotopes 60Co, 65Zn, 106Ru, 133Ba, 134Cs, 137Cs, 226Ra, Th and U are analyzed in the intercomparison programs. INAC 2011, Belo Horizonte, MG, Brazil. Calibration Energy calibration was performed in the energy range from using point sources 0.8 to 2 MeV The following photon emissions and radionuclides were used: 241Am, 133Ba, 152Eu, 137Cs and 60Co. The efficiency curve for calibration was determined by standard solution prepared from SRM (Standard Reference Material) or CRM (Certified Reference Material). 2.2. External exposure evaluation When comparing the specific activity of samples containing different amounts of 226Ra, 232Th 40 and K, it is necessary to introduce the term radium equivalent activity (Raeq). It represents a single quantity that combines specific activities of 226Ra, 232Th and 40K. It is developed as a numerical indicator for external dose to public and internal dose due to radon and its daughters. The maximum allowed value for public dose considerations is 370 Bq kg-1 [9]. The equation 1 shows how radium equivalent activity is calculated. Raeq = ARa + 1.43ATh + 0.077AK, (1) 226 232 40 -1 where ARa, ATh and AK are the specific activities of Ra, Th and K in Bq kg respectively [9,10]. The external hazard index (Hex) is a radiation hazard index defined to evaluate the indoor radiation dose rate due to the external exposure to γ-radiation from the natural radionuclides in the construction building materials of dwellings [9]. This index value must be less than unity to keep the radiation hazard insignificant, i.e. the radiation exposure due to the radioactivity from construction materials to be limited to 1.0mSv y-1. For limiting the radiation dose to this value, the following conservative model, based on infinitely thick walls without windows and doors was proposed, showed in equation 2: Hex = (ARa/370) + (ATh/259) + (AK/4810) ≤ 1, (2) 226 232 40 -1 where ARa, ATh and AK are the specific activities of Ra, Th and K in Bq kg respectively.
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