ENVIRONMENTAL RADIOLOGICAL SURVEILLANCE AROUND NUCLEAR POWER PLANTS IN ARGENTINA

Transcription

1 ENVIRONMENTAL RADIOLOGICAL SURVEILLANCE AROUND NUCLEAR POWER PLANTS IN ARGENTINA CIALLELLA, H.E.; FERNANDEZ, J.A.; GAVINI, R.M.; LEWIS, E.C.; QUINTANA, E.E. AUTORIDAD REGULATORIA NUCLEAR AV. DEL LIBERTADOR BUENOS AIRES - ARGENTINA ABSTRACT During the last five years of operation of Atucha I Nuclear Power Plant (CNA I) and Embalse Nuclear Power Plant (CNE), a field team from the Environmental Radioactivity Division of Nuclear Regulatory Authority (NRA) has carried out surveillance programs in the surroundings of these facilities, in order to estimate the radiological impact. The argentine nuclear power plants use natural uranium as fuel and heavy water as refrigerant and moderator. During normal operation, radionuclides are released to the environment through liquid and gaseous discharges; mainly tritium which is generated from the neutron activation of heavy water deuterium. One of the environmental radioprotection requirements for a nuclear facility licensing is the compliance of the dose limit (1 msv/year) to the most exposured members of the public (critic group). The authorised annual dose limits established for NRA, to control the release of radioactive effluents are 50 and 100 µsv/year for CNA I and CNE respectively. The surveillance programs carried out around the mentioned nuclear facilities included the radionuclide analysis of H-3, Cs-137, Co-60. I-131 and Sr-90. The matrixes analysed were milk, grass, diet, fish and groundwater, surface and tap water. The results, including the dose assessment, are presented. The measured results of the radionuclides mentioned above were, in general, below the detection limit (except for tritium). The doses estimated for both, liquid and gaseous discharges, varied from 4 to 5 µsv/year for each nuclear power plant, since 1998 to This implies that the population living in the vicinity of these areas receives no significant exposure. Key words: environmental surveillance, nuclear power plants, H-3, Cs-137, Co-60, I-131, Sr-90 I. INTRODUCTION Argentine has in operation Atucha I (CNA I) and Embalse (CNE) nuclear power plants. CNA I is in operation since june 1974, it is located nearby Paraná de las Palmas river, in Buenos Aires province, 100 km north-east from Buenos Aires city. CNE, whose operation started in January 1984, is located in the south coast of Embalse de Río Tercero, in Córdoba province, 110 km south-east from Córdoba city. CNA I has a heavy water pressure reactor (HWPR) from Germany, of 357 electric net power megawatts (MWe) and uses natural uranium as fuel. The reactor core has 253 fuel elements, which contain uranium dioxide (UO 2 ) pellets. This arrangement is immersed in 500 t of heavy water, which acts as moderator and refrigerant. CNE has a pressure tubes reactor, CANDU model from Canada, of 648 electric net power megawatts (MWe) and also uses natural uranium as fuel and heavy water as refrigerant and moderator. The core is horizontally penetrated by 380 fuel elements. In normal operation conditions, this type of reactor has a tritium production rate of 7, Bq/MWe. This is due to the neutron activation of heavy water deuterium. A fraction of this tritium is released to the environment as gaseous discharge through the power plant stack due to the degassed of the primary circuit [1].

2 There are liquid discharges also. CNA I releases liquid discharges to the Paraná de las Palmas river and CNE to the Embalse de Río Tercero lake. In this kind of reactor tritium is the main radionuclide released to the environment, and has a great importance from the radiological point of view because is the main contributor to the effective and collective dose to the public. The ARN performs surveillance programs in the surroundings of nuclear power plants, fully independent from the monitoring carried out by the facilities themselves, in order to estimate the radiological impact [2,3]. The environmental monitoring carried out in both facilities has several objectives, one of them is to obtain information of the correlation between discharges and environmental levels (that allows to implement predictions) and additionally, to know about the radionuclide behaviour in the biosphere. II. EXPERIMENTAL METHOD: The surveillance programs carried out around the mentioned nuclear facilities included representative samples that were taken from the different radionuclide transfer compartments. In order to evaluate the environmental impact of the liquid discharges, river, lake water and fish samples were collected and analysed. To assess the environmental impact of the gaseous emissions, samples of locally produced food, such as milk, vegetables and diet, were taken and analysed. Grass was analysed as an indicator of radioactive material deposition. Additionally, tap water (groundwater and surface water) samples were taken. From the radiological point of view, the radionuclides analysed were mainly fission products (Cs-137, Sr-90 and I-131) and neutron activation products (H-3 and Co-60). The population in the surroundings of Atucha I nuclear power plant is low and is conformed principally by farmers. In the case of Embalse nuclear power plant, the population lives in small areas around the facility and is basically dedicated to tourist activities. In Figures 1 and 2 are shown the selected points around CNA I and CNE respectively. Figure No. 1: The map shows the environmental sample points around CNA I

3 Figure No. 2: The map shows the environmental sample points around CNE The samples were processed at the NRA environmental laboratories localised at the Centro Atómico Ezeiza, located in Ezeiza, province of Buenos Aires. The Sr-90 determination was performed over the concentrated samples or the ashes obtained from the burnt samples. After digestion in different acids, a final Y-90 in equilibrium extraction with HDEHP is used. A liquid scintillation measurement is carried out using the Cerenkov emission. Tritium is measured directly over a sample aliquot by liquid scintillation [4]. Cs-137 and Co-60 determination were performed by gamma spectrometry, using GeHp of high efficiency and resolution. Liquid samples were evaporated till 30 cm 3 geometry and were directly measured over the detector: The ashes were measured with a 60 g pellet geometry. I-131 was measured directly by gamma spectrometry over fresh samples [4]. III. RESULTS For both nuclear power plants the experimental results were obtained from nearly 100 quarterly samples of different sample points, between [5]. In Table 1 is presented the activity concentration in the different analysed samples from CNA I.

4 Table 1 Activity concentration in CNA I environmental samples Environmental sample Surface water Freshwater fish (Bq/kg) Grass (Bq/m 2 ) Drinking water (groundwater) cow milk diet (Bq/kg) Radio Nuclide H Cs-137 <0.002 < <0.002 <0.002 <0.002 Co-60 < < <0.002 <0.006 <0.002 Sr < < < < Cs-137 <0.04 <0.03 <0.06 <0.02 <0.02 Co-60 <0.04 <0.02 <0.02 <0.02 <0.02 Sr < I-131 <0.6 <4.5 <3.3 <3.4 <2.5 Cs-137 <1.0 <4.7 <3.8 <3.3 <2.8 Co-60 <0.9 <3.1 <4.4 <2.8 <2.6 H < Cs-137 < < <0.001 < <0.001 Co-60 <0.002 <0.002 < < <0.009 Sr-90 < < < < < I-131 <0.2 <0.2 <0.3 <0.2 <0.2 Cs-137 <0.01 <0.002 <0.01 <0.009 <0.009 Sr-90 <0.08 <0.02 <0.016 <0.017 <0.017 Cs <0.02 <0.01 Co <0.02 <0.02 Sr <0.03 <0.06

5 In Table 2 is presented the activity concentration in the different analysed samples from CNE. Table 2 Activity concentration in CNE environmental samples Environmental sample Surface water Freshwater fish (Bq/kg) Grass (Bq/m 2 ) Drinking water cow milk diet (Bq/kg) Radio Nuclide H Cs-137 < <0.001 <0.001 <0.07 <0.002 Co-60 <0.004 <0.001 <0.001 <0.05 <0.002 Sr-90 <0.003 < < < Cs-137 <0.09 <0.08 <0.06 <0.03 <0.04 Co-60 <0.04 <0.03 <0.03 <0.02 <0.02 Sr < <0.03 I-131 <4.0 <5.0 <5.0 <4.1 <2.7 Cs-137 <0.8 <6.7 <4.7 <4.5 <3.1 Co-60 <0.5 <4.7 <3.5 <3.1 <2.7 H Cs-137 <0.004 <0.002 <0.001 <0.002 <0.002 Co-60 <0.003 <0.001 <0.001 <0.002 < Sr-90 <0.002 < < <0.003 < I-131 <0.2 <0.3 <0.3 <0.2 <0.1 Cs-137 <0.01 <0.01 <0.01 <0.02 <0.01 Sr-90 <0.04 <0.01 <0.02 <0.02 <0.02 Cs <0.02 <0.03 Co <0.01 <0.02 Sr <0.01 <0.03 Moreover, to assess the environmental impact of the emissions, effective doses to the most exposured members of the public (critic group) were calculated. The concentration levels of the radionuclides in the environmental samples were used to calculate the effective doses of the critic group. The doses were calculated taking into account the critical exposition pathways and considering that the population consume local products. In Table 3 are shown the results [6]. The critic group in Atucha I is located 1 km West from the stack of the nuclear power plant. In the case of Embalse nuclear power plant the critic group is 1 km South.

6 Table 3 Effective doses from Nuclear Power Plants Year Doses (msv/year) CNA I CNE 1998 < < < < < < <0.002 < <0.002 < CONCLUSIONS: The measured results of the radionuclides mentioned above were, in general, below the detection limit of used techniques (except for tritium). In the case of CNA, positive H-3 levels were detected in aqueous matrixes, but near environmental concentration. The Sr-90 fish determinations deserve an explanation where positive results were obtained. These low activity levels obtained were basically produced by residual fall out coming from nuclear weapons test carried out in the seventies. The positive values were probably obtained by concentration effect of the mentioned matrix. In the case of CNE, H-3 activity levels in tap and surface water were detected attributable to the nuclear power plant operation. The activity levels presented in drinking and lake water were practically the same because the tap water was coming from the Embalse de Río Tercero lake. The rest of radionuclides gave similar results that the concentration levels founded in the surroundings of CNA I. One of the environmental radioprotection requirements for a nuclear facility licensing is the compliance of the dose limit (1 msv/year) to the most exposured members of the public (critic group). The authorised annual dose limits established for NRA, to control the release of radioactive effluents are 50 and 100 µsv/year for CNA I and CNE respectively. The resulting doses estimated to individuals of the critic group for both, liquid and gaseous discharges, varied from 4 to 5 µsv/year for each nuclear power plant since 1998 to These values are far of the dose constraint and only represent less than 0.5% of the annual dose limit. This implies that the population living in the vicinity of these areas receives no significant exposure. REFERENCES: [1] National Council on Radiation Protection and Measurements. Tritium in the Environment. NCRP, NCRP Report No. 62; [2] Monitoraje ambiental en la zona de emplazamiento de la Central Nuclear de Atucha. Menossi, C. A., Ciallella N. R., Bruno H. A., Escribano T. L.; Comisión Nacional de Energía Atómica. NT 8/78, Argentina (1978). [3] Monitoreo ambiental en los alrededores de las centrales nucleares de la República Argentina realizado en los años 1996 y Canoba, A. C., López, F. O., Bruno, H. A.; 4o Congreso Regional de Seguridad Radiológica y Nuclear (IRPA). La Habana, Cuba. Octubre Memorias, Tomo 4, tema 14, pág. 9-12, [4] Laboratory Techniques Manual (2002). Nuclear Regulatory Authority, Argentina. [5] Annual Reports ( ). Nuclear Regulatory Authority, Argentina. [6] Safety Reports Series No.19, Generic Models for in assessing the impact of discharges of radioactive substances to the environment. IAEA (2001).

7