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Subcritical Reactivity Measurements at Angra 1 Nuclear Power Plant

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Subcritical Reactivity Measurements at Angra 1 Nuclear Power Plant 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 SUBCRITICAL REACTIVITY MEASUREMENTS AT ANGRA 1 NUCLEAR POWER PLANT Renato Yoichi Ribeiro Kuramoto 1 and Anselmo Ferreira Miranda 1 1 Eletrobrás Eletronuclear - Central Nuclear Almirante Álvaro Alberto Física de Reatores de Angra 1 BR 101 Sul - Rodovia Governador Mário Covas, km 517 (Rio Santos) CEP: 23 948-000 – Itaorna 4° Distrito de Angra dos Reis – RJ [email protected] [email protected] ABSTRACT In order to speed up the Angra 1 NPP Physics Tests, this work intends to develop a digital reactivity meter combined with a methodology of the modified Neutron Source Multiplication (NSM) method with correction factors for subcriticality measurements at Angra 1 NPP. In the first part of this work, we have applied the Modified Neutron Source Multiplication (MNSM) Method with fundamental mode extraction[1], in order to improve the monitoring of the subcriticality at Angra 1 NPP during the criticality approach. In the second part, we developed a preliminary subcritical reactivity meter algorithm based on the Point-Reactor Inverse Kinetic model with six delayed neutron groups and external neutron source. The source strength was obtained through the Least Squares Inverse Kinetics Method (LSIKM) . 1. INTRODUCTION Reactor physics tests are carried out at the startup after Angra 1 Nuclear Power Plant (NPP) refueling. Before and during the tests, there are two important items related to reactivity measurements. The first item is related to the subcritical measurements during the criticality approach. The monitoring and prediction of the subcritical multiplication conditions is essential to assure that the operation of the control rods withdrawal or the boron dilution process is carried out with safety in order to get the reactor core criticality. Currently, there were different methods to estimate when criticality will accur. One of these methods, called Neutron Source Multiplication (NSM) method [1], is performed by plotting the inverse ratio of the count rate obtained from the source range detector as a function of the change in the condition being used to bring the plant critical, e.g., dilution of boron concentration or withdrawal the control rod banks. This method assumes that neutron flux distribution is the fundamental mode of the neutron diffusion and the shape of the neutron flux distribution is unchanged during the criticality approach. However, in a real situation, the neutron flux distribution is composed by the fundamental and higher harmonics. Furthermore, the neutrons flux shape changes significantly with the subcritical level[2,3]. The second important item in the reactor physics tests is the control rod worth measurement. For the control rod worth measurement, the boron dilution method is conventionally used. However, the Dynamic Rod Worth Measurement (DRWM) has been widely adopted in reactor physics tests at hot zero power condition. Again, in order to reduce the interval of the tests, one of the methods is to evaluate the control rod worth from subcriticality measurement [3]. More precisely, a digital subcritical reactivity meter that uses neutron count rate data obtained during the control rod drop testing, which is carried out before the reactor physics tests at hot zero power condition, can estimate each control rod worth. In such a way, the following reactor physics tests could be performed with the advanced knowledge of each control rod worth and procedures for detailed control rod worth measurement could be simplified or eliminated from the reactor physics tests. If such method is possible, the measurement using the boron dilution method or the DRWM can be eliminated. From these points of view, the subcriticality measurement using a digital reactivity meter has been investigated. A digital reactivity meter can continuously give real time reactivity using the simple one point reactor kinetics equations[4]. However it has not been used directly as a subcriticality monitor for the following reasons. Firstly, the applicability of the inverse point reactor kinetics equations to a system where the neutron flux distribution is dependent on the subcriticality has not been confirmed. Secondly, in the environment where subcriticality is measured, it is normally expected that the neutron flux level is quite low. Therefore the fluctuation of the neutron signal is quite large, which makes the calculation of reactivity using the inverse kinetics equations difficult[5]. Finally, in order to solve the inverse kinetics equations for a subcritical system, it is necessary to know strength of the neutron source of the system accurately[6,7]. But this is usually quite difficult to do. This work intends to overcome these difficulties and develop a digital reactivity meter combined with a methodology of the Modified Neutron Source Multiplication (MNSM) method[2,3] with correction factors for subcriticality measurements at Angra 1 NPP. In the first part of this work, we have applied the MNSM Method with fundamental mode extraction, in order to improve monitoring of the subcriticality at Angra 1 NPP. In the second part, we developed a preliminary subcritical reactimeter code based on the Point-Reactor Inverse Kinetic model with six delayed neutron groups and external neutron source[8]. The source strength was obtained through the Least Squares Inverse Kinetics Method (LSIKM)[6,7]. 2. ANGRA 1 NUCLEAR POWER PLANT The Angra 1 Nuclear Power Plant (NPP) is a 2 Loop Westinghouse PWR with a rated power of 657 MWe. Owned by the Brazilian company Eletrobrás-Eletronuclear, Angra 1 NPP was synchronized to the grid in April 1982 and went into commercial operation in January 1985. Figure 1 shows the layout of Angra 1 reactor core. The positions labeled with A, B, C and D indicates the control rod banks, DA and DB represents the shutdown banks and S the incore fixed neutron source. The label SR indicates the positions of the ex-core source range detectors N31 and N32. All data analyzed in this work have been collected during Cycle 16 of Angra 1 NPP. INAC 2011, Belo Horizonte, MG, Brazil. A A DA D DA C C DA B DA A S DB DB A SR SR N31 D B C B D N32 A DB DB S A DA B DA C C DA D DA A A Figure. 1. Layout of Angra 1 reactor core . 3. CRITICALITY PREDICTION USING 1/M CURVE Angra 1 NPP startup procedures monitor the approach to criticality through the use of the inverse of the subcritical multiplication factor M, or 1/M curve. In order to correct the effects from the disturbances in the distributions of neutron flux and importance field and improve the criticality approach, we have applied three corrections factors to the neutron counting multiplication factor. This methodology is known as Modified Neutron Source Multiplication (MNSM) Method with fundamental mode extraction[2,3]. According to this method, the 1/M is modified as follow: 1 ext sp im Qref = Cl Cl Cl (1) M Ql where: Qref and Ql are the source range detector count rate of the reference state and l-th subcritical state, respectively. The first factor in Eq. 1 is called extraction correction factor ext Cl which is related to the extraction of the fundamental mode. The second is the spatial sp correction factor Cl and corrects a spatial effect induced by the disturbance of the im fundamental mode. The last one is the importance field correction factor Cl that corrects the effect induced by the disturbance of neutron importance field. Each correction factors depends on the subcritical level l and how the reactivity is added temporally and spatially. In other words, these correction factors changes with the control rods positions. In such way, ext sp im Cl , Cl and Cl where calculated by Yoichiro SHIMAZU, Kennichiro OKAZAKI and Masashi TSUJI and are listed in Table 1 for different control rod configuration[3]. INAC 2011, Belo Horizonte, MG, Brazil. Table 1. Correction factors for different control rod positions[3]. A B C D ext sp im Cl Cl Cl ARI ARI ARI ARI 1 1 1 228 100 0 0 0.590 1.329 1.492 228 228 100 0 1.456 0.789 0.697 228 228 228 100 0.949 0.900 0.922 228 228 228 160 0.816 1.125 1.048 The data analyzed in this work have been collected during Cycle 16 of Angra 1 NPP. Figure 2 shows the count rate of the source range detector N32 and the whole withdrawal sequence of control rod banks during a criticality approach in Angra 1 NPP (Cycle 16). 8000 250 count rate 7000 A B C 200 6000 D 5000 150 4000 count rate (cps) count rate 100 3000 control rod bank (steps) positions rodbank control 2000 50 1000 0 0 10:40:00 10:43:28 10:46:56 10:50:24 10:53:52 10:57:20 11:00:48 11:04:16 11:07:44 11:11:12 11:14:40 11:18:08 11:21:36 11:25:04 11:28:32 11:32:00 11:35:28 11:38:56 11:42:24 11:45:52 11:49:20 11:52:48 11:56:16 11:59:44 12:03:12 12:06:40 12:10:08 12:13:36 12:17:04 12:20:32 12:24:00 12:27:28 12:30:56 12:34:24 12:37:52 12:41:20 12:44:48 12:48:16 Figure. 2. Count Rate of source range detector N32 and the whole withdrawal sequence of control rod banks during a criticality approach in Angra 1 NPP (Cycle 16). The inverse of the subcritical multiplication factor (1/M) curve was calculated using the count rate of the N31 and N32 source range detector recorded during each rest state of the control rod banks (see Fig.
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