Comparison between the estimated and observed source term from the Angra 1 Nuclear Power Plant of Angra dos reis Brazil

Transcription

1 IAEA-CN-114/1p Comparison between the estimated and observed source term from the Angra 1 Nuclear Power Plant of Angra dos reis Brazil N. S. Falcão Martins, S. S. Peres Instituto de Radioproteção e Dosimetria, Comissão Nacional de Energia Nuclear, Rio de Janeiro, Brazil Abstract. Operation of nuclear power plants results on discharges of radioactive effluents to environment. Dealing with the basic objective of radioactive waste management in order to protect human health and environment now and in the future without imposing undue burden on future generation is made a previous analysis of the source terms discharge. These source terms are essential issues for environmental impact analysis. This paper compares the results of calculated source terms discharged from Liquid Waste Processing System and those measured during the operation of Angra 1 Nuclear Power Plant. 1. Introduction ACKNOWLEDGEMENTS Angra 1 is a PWR, manufactured by Westinghouse Electric Corporation, with installed capacity of 1961 MWt. It is located at Itaorna Beach, district of Angra dos Reis, in Rio de Janeiro [3]. That plant was licensed according to requirements of the National Commission of Nuclear Energy (CNEN) [4]. According to the licensing process, all releases of radioactive materials to the environment must be estimated in order to protect human health and the environmental from radiological impact under normal plant operation. The authorization for initial operation was conceived in September of 1981 [5]. The operation of CNAAA 1 is under supervision of CNEN. During normal operation of nuclear power plants there are several radionuclides released into the environment. Those radionuclides are generated in the reactor core or in its vicinity. The valuation of the expected fission and activation product isotopic concentrations in reactor coolant and the corresponding activity discharges into environmental is made through utilization of mathematical models together with pertinent information concerning plant design bases parameters and operating plant data avaiable. Usually the release rates of fission products, as noble gases and iodine, from fuel pellet/cladding to the primary coolant are calculated from diffusion models. For other fission products, whose diffusion mechanisms cannot be appropriately determined, the release rates are derived of typical operational values and adopted as design base. In the same way, the activity concentration of activation products, generated by the reactions of structural materials elements, impurity on the fuel element walls or in the coolant, is also estimated from other plants operational experience [1]. From both primary coolant activity concentration and principal design base parameters associated with reactor coolant and waste processing systems leakage and cleanup rates, the isotopic activity concentrations of the several radionuclides released into the environment is estimated. The main source of radioactive material release into the environment is the Liquid Waste Processing System (LWPS), through discharges from Waste Monitor Tanks (WMT s) [2]. The main parameters used in the source terms calculation are design base values or have been compiled from the operational records obtained from several plants in operation. This work presents a comparison between expected

2 annual isotopic activity discharges from LWPS of Angra 1 NPP, a 1961MWt Westinghouse PWR, and those activities measured after almost 20 years of the plant operation. 2. Source Terms Assessment Activities of noble gases and iodines in the primary reactor coolant during normal operation with small cladding defects are calculated using diffusion models equations[7]. For other fission products, for which diffusion mechanisms cannot be appropriately described, the release rates are obtained from plant operating experience typical values. In the same way, the activity concentrations of the activation products are also based on similar plants operating experience [1]. The fission products inventory in the primary coolant was calculated taking into account the noble gas and iodine isotope activities present in reactor core after a burnup time corresponding to 650 consecutive days of full potency operation[6]; 0.05% average fuel defect level present at the inicial core loading and uniformly distributed throughout the core; and C reactor coolant full power average temperature [2]. The following diffusion equations were used: dn w i = Dν i N Ci (λ i + Rη i + dt B ) N wi B 0 tb for parent nuclides in the coolant and dn w j = Dν j N Cj (λ j + Rη j + dt B ) N w j + λ i N w i B 0 tb for daughter nuclides in the coolant, where: N = radionuclide concentration; D = clad defects, as a fraction of rated core thermal power being generated by rods with clad defects; R = purification flow; B 0 = initial boron concentration; B = boron concentration reduction rate by feed and bleed; h = removal efficiency of purification cycle for nuclide; λ = radioactive decay constant; n = escape rate coefficient for diffusion into coolant; t = time; C, w, i, j = refers to core, coolant, parent nuclide and daughter.

3 The activation products inventory in the reactor coolant is based on operating experience of similar reactors [7]. It was considered that the plant has already operated half of its operational lifetime, i.e., about 20 years, taking the components use into account. All liquid wastes generated during the normal operation of the plant is collected and processed by the LWPS, for later recycle or environment release. The great volume of water treated by this system is originated from equipments drains and can be recycled. However, a small part of this waste water originating from leaks of equipments located inside the containment and auxiliary buildings are collected directly in the floors drains. For their high impurity degree, they could not be recycled, and are liberated for the environment after treatment at LWPS. A much greater volume of water, originating from the wash of clothes, people, areas and/or other components, that can be slightly polluted, is also collected by LWPS and released into the environment through the WMT s. The estimated source terms release is based on reactor coolant radioactive inventory, on the estimated volume of water originated at several sources and on radioactivity removal design parameters of the treatment systems. The assessment of source terms releases of CNAAA 1 is based on all phases of normal operation of the plant, including maintenance stages and refueling. The estimation of potential input sources entering LWPS, in terms of volume, is based in the operational experience obtained from other plants of Westinghouse [1]. In terms of activity, this estimation is based on the previous presented hypothesis, as discussed above. 3. LWPS Effluent Monitoring To control the release of radionuclide activities discharged into environment and evaluate the radiological environmental impact, the operator maintains a monitoring and accountancy program for the radioactive waste of CNAAA 1. For the liquid effluents coming from the LWPS through the WMT s, it is established that, before the discharge of each batch, a representative sample of the effluent should be taken and analyzed for the determination of the main gamma emitting radionuclides present. An aliquot of each sample is then separate and, at the end of every month, are used to set up compound representative samples of the releases that happened in that period, for tritium, 89Sr and 90Sr determination. The release data regarding each batch, like: day, hour and total period of discharge, discharge volume, dilution flow regarding each discharge and report of results of gamma spectrometry analysis are registered and kept filed. At the end of every semester, it is sent to CNEN the Half-yearly Effluent and Waste Report (RER) [8] containing, among other information, the monthly account of each radionuclide activity released into the environment through the WMT s. Parallel to the operator s monitoring program, DEPRA/IRD maintains a program of analysis of the monthly samples for determination of gamma emitters, tritium, 89Sr and 90Sr. These results are compared with the ones on the RER to verify the effectiveness of the release maintained by the operator. 4. Source Terms Comparison The LWPS source terms and the principal design parameters were used on the estimations presented in the Chapter 11 of the Final Report of Safety Analysis of CNAAA 1 (RFAS) [2]. These source terms were estimated assuming an operation period of 20 years, which corresponds to the actual point in time. The monthly total activities of each radionuclide released by the LWPS effluents, used for comparison with the design source terms, are in the RER s released on 1999, 2000 and 2001[9,10,11,12,13,14]. As it can be observed in Fig. 1, the activities of the estimated fission products and the measures in LWPS effluents in the years of 1999, 2000 and 2001 are similar, having the same magnitude. For the tritium, the difference between the measured and estimated activity is of one order of magnitude, maximum.

4 1E+05 1E+14 1E+13 1E+12 1E+11 1E+10 1E+09 1E+08 1E+07 1E+06 Zr95 Nb95 Cs134 Cs137 H3 FSAR 6,29E+06 1,70E+07 1,18E+08 3,70E+08 3,56E ,93E+06 2,45E+07 9,79E+07 3,18E+08 1,28E ,20E+06 5,28E+06 1,38E+08 4,43E+08 1,86E ,72E+06 1,84E+07 5,39E+07 2,98E+08 8,74E+12 Figure 1. Fission products and tritium For the activation products, estimated by operational experience and plant design parameters, one can observe on Fig. 2, that the measured activities can be up to two ordes of magnitude greater than the estimated ones, as in the case of 54Mn and 59Fe. 1E+09 1E+08 1E+07 1E+06 1E+05 1E+04 1E+03 Mn54 Fe59 Co58 Co60 FSAR 7.78E E E E E E E E E E E E E E E E+08 Figure 2. Activation products 5. CONCLUSIONS It is known that the activities of radionuclides released into the environment through the LWPS of a nuclear power plant are highly dependent on the operation of the plant [1]. Nevertheless, it is wanted that the source terms determined for licensing ends be conservative, once they are used for evaluating the potential doses and radiological environmental impact of the installation. The result of this evaluation is used to establish the control of waste release, ascertaining the basic objective of environmental radioprotection that is to protect the public's health and the environment now and for the future generations [15]. From the presented results, one can conclude that the methodology used for the estimative of the annual activities of fission products released from LWPS of CNAAA 1 is similar to the accountancy in the last years of operation of the plant. However, the methodology used in the estimative of the annual activities of activation products underestimates the release expectations, specially for tritium, which is both a fission and activation product. The fact that the differences among estimated and measured tritium activities are in an intermediate band among that found for fission and for activation products, can be seen as an indication of this underestimation.

5 We recommend the reevaluation of the parameters used in the determination of the source terms of effluent release of LWPS of this plant type, for ends of licensing, in the light of monitoring results and other available operational data, as well as the incorporation of the obtained results in safety's analysis of similar plants.

6 REFERENCES [1] Directorate of Regulatory Standards USAEC. Final Environmental Statement Concerning Proposed Rule Making Action: Numerical Guides for Design Objectives and Limiting Conditions for Operation to Meet the Criterion As Low as Practicable for Radioactive Material in LWR Effluents, vol 2: Analytical Models and Calculations, [2] Furnas Centrais Elétricas S.A. Final Safety Analysis Report (RFAS) Central Nuclear Almirante Álvaro Alberto Unit 1, Revisão 24, Capítulo 11, [3] Furnas Centrais Elétricas S. A. FURNAS Geração. Ministério de Minas e Energia, Furnas Centrais Elétricas S. A. Divisão Gráfica, C.S.P. DP dezembro de [4] Comissão Nacional de Energia Nuclear. Norma CNEN-NE-1.04 Licenciamento de Instalações Nucleares. Serviços de Normas e Especificações, julho de Enter first reference here. [5] Furnas Centrais Elétricas S. A. As Usinas Termelétricas e a Questão Ambiental. Ministério de Minas e Energia, Furnas Centrais Elétricas S. A. Divisão Gráfica, C.S.P. DP setembro de [6] Furnas Centrais Elétricas S.A. Final Safety Analysis Report (RFAS) Central Nuclear Almirante Álvaro Alberto Unit 1, Revisão 24, Capítulo 15, [7] WCAP-8253, Source Term Data for Westinghouse Pressurizer Water Reactors. Westinghouse Electric Corporation, Pittsburg, Pa , April [8] Comissão Nacional de Energia Nuclear. Norma CNEN-NE-1.14 Relatórios de Operação de Usinas Núcleoelétricas. Serviços de Normas e Especificações, [9] Eletrobrás Termonuclear S A ELETRONUCLEAR. Usina Nuclear Angra 1 Relatório Semestral de Liberação de Efluentes e Rejeitos - 10 semestre de Gerencia de Monitoração GMO.O, [10] Eletrobrás Termonuclear S A ELETRONUCLEAR. Usina Nuclear Angra 1 Relatório Semestral de Liberação de Efluentes e Rejeitos - 20 semestre de Gerencia de Monitoração GMO.O, [11] Eletrobrás Termonuclear S A ELETRONUCLEAR. Usina Nuclear Angra 1 Relatório Semestral de Liberação de Efluentes e Rejeitos - 10 semestre de Gerencia de Monitoração GMO.O, [12] Eletrobrás Termonuclear S A ELETRONUCLEAR. Usina Nuclear Angra 1 Relatório Semestral de Liberação de Efluentes e Rejeitos - 20 semestre de Gerencia de Monitoração GMO.O, [13] Eletrobrás Termonuclear S A ELETRONUCLEAR. Usina Nuclear Angra 1 Relatório Semestral de Liberação de Efluentes e Rejeitos - 10 semestre de Gerencia de Monitoração GMO.O, [14] Eletrobrás Termonuclear S A ELETRONUCLEAR. Usina Nuclear Angra 1 Relatório Semestral de Liberação de Efluentes e Rejeitos - 20 semestre de Gerencia de Monitoração GMO.O, [15] International Atomic Energy Agency. Principles of Radioactive Waste Management, Safety Series No. 111-F, IAEA, Vienna, [16] US Nuclear Regulatory Commission. Calculation of Release of Radioactive Materials in Gaseous and Liquid Effluents from Light-Water Cooled Power Reactors, Regulatory Guide 1.112, revision 1, Office of Nuclear Regulatory Research, 1977.