EXPERIMENTAL DETERMINATION OF NEUTRONIC PARAMETERS IN THE IPR-R1 TRIGA REACTOR CORE

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1 2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: EXPERIMENTAL DETERMINATION OF NEUTRONIC PARAMETERS IN THE IPR-R1 TRIGA REACTOR CORE Rose Mary Gomes do Prado Souza, Amir Zacarias Mesquita Centro de Desenvolvimento da Tecnologia Nuclear (CDTN) Comissão Nacional de Energia Nuclear (CNEN) Campus da UFMG - Pampulha Caixa Postal , Belo Horizonte, MG, Brazil. souzarm@cdtn.br, amir@cdtn.b ABSTRACT This paper presents the results of neutronic tests performed in August 2010 in the IPR-R1 TRIGA reactor, located at the Nuclear Technology Development Center - CDTN. These experiments follow the determination of the IPR-R1 Safety Analysis Report, which stated the necessity to perform them annually to ensure the safety of the reactor. The IPR-R1 TRIGA reactor has a nominal power of 100 kw in a steady-state operation, and its power will be upgraded to 250 kw. The license to operate at 250 kw is under final review by the regulatory body of Brazil. So the tests described here will be repeated and new experiments will be conducted in the new power. The control rods were calibrated and the worth of Regulation, Shim and Safety rods were 0.48 $, 3.21 $ and 2.84 $, respectively. The value of the excess reactivity obtained to compensate the negative reactivity due to negative temperature coefficient, the xenon poisoning, etc, was 2.0 $. The shutdown margin obtained was 1.32 $, and the power defect was 0.76 $. 1. INTRODUCTION The aim of this paper is to present the results of recent neutronic tests conducted in the IPR- R1 TRIGA reactor, as determined by the IPR-R1 Safety Analysis Report, which stated the need to perform them annually to ensure the safety of the reactor [1]. The tests performed were: the calibration of the control rods, and the excess of reactivity, the shutdown margin, and the power defect determination. Finally, it was measured the reactivity loss of the core due to one operation at 100 kw, during a period of time of eight hours. It was confirmed the necessity of new fuel elements in order to operate the reactor at the new power. 2. BRIEF DESCRIPTION OF THE IPR-R1 REACTOR The IPR-R1 TRIGA reactor is a pool type research reactor moderated and cooled by light water. The fuel is a solid, homogeneous mixture of U-ZrH alloy containing between 8.5% and 8% by weight of uranium enriched to 20% in 235 U, for stainless-steel and aluminum clad elements, respectively [2, 3]. The core has 63 fuel elements composed of 59 original Al-clad fuel elements and 4 fresh SS-clad elements. The power level of the reactor is controlled by three control rods: Regulating, Shim and Safety. The Shim and the Safety control rods are positioned at symmetrical locations of C-ring, and the Regulating rod at F-ring.

2 Figure 1. IPR-R1 TRIGA core configuration. 3. REGULATING, SHIM AND SAFETY CONTROL RODS WORTH The control rods were calibrated by the positive period method. The reactor is made critical at 20 W, so the temperature increase during the experiment was negligible, with the test rod in its fully inserted position. The test rod is withdrawn a small distance so that the reactor is slightly supercritical and the power starts to increase. After two minutes for the transients to die out, the reactor period is determined from the doubling time. Other rod is inserted into the reactor to bring it back to critical. The previous procedure is repeated until the rod test has been calibrated along its whole length. From the observed periods the corresponding reactivities are computed utilizing the inhour equation. Once the control rods are calibrated, it is possible to evaluate the magnitude of other reactivity changes by comparing the critical rod positions. Figure 2 shows the differential curve and the corresponding integral reactivity worth curve of the Regulating control rod, where the reactivity values are plotted as a function of the rod positions. The maximum in the differential curve occurs approximately in the center, and is small near the ends because of the smaller flux density. The worth of the Regulating rod was 0.48 $ [4].

3 Reactivity [cents] y = -2E-07x x x R 2 = (dr/dz) *0.01 [cents/ step] Regulating Rod Position 0.0 Figure 2. Differential and integral curves of the Regulating control rod. The Shim and Safety rods were intercalibrated, which was to measure one control rod in presence of the other rod, which is used for compensating the reactivity introduced by step withdrawal of the measure rod. Figure 3 shows the integral calibration curves of the Shim and Safety rods. Since it is impossible to calibrate the whole Shim and Safety rods, their total worth were calculated by considering the neutron flux asymmetry. From the Regulating rod calibration curve in Fig. 2, it was possible to see this asymmetry. Taking the ration between the reactivity at the bottom and at the top of this curve, and considering that the neutron flux has the same shape at the places where the Shim and Safety rods are placed, the total worth of these rods were calculated using the curves shown in Figures 3. The Shim and Safety rods worth were 3.21 $ and 2.84 $, respectively [4]. Both rods have sufficient reactivity worth to shut down the reactor independently.

4 Reactivity (cents) 250 y = -8E-07x x x R 2 = Safety Shim 50 y = -9E-07x x x R 2 = Control Rod Position Figure 3. Integral curve of the Shim and Safety control rods. 4. EXCESS OF REACTIVITY The value of excess reactivity, rexc, must be such as to compensate the effects of negative feedback reactivity due to negative temperature coefficient, the xenon poisoning, burning the fuel (long term) and the introduction of samples for irradiation. The excess reactivity is [5]: r exc k 1 eff k eff To measure the excess reactivity (rexc) of the core, the reactor was left critical at low power with various configurations of the control rods. The rexc values were experimentally determined from the reactivity of each control rod position obtained from the respective calibration curve. Then the average value of the core excess reactivity obtained was (2.0 ± 0.02) $ [4]. The corresponding experimental value of k eff is SHUTDOWN MARGIN The total reactivity worth of the control system is 6.53 $. With a core excess reactivity of 2.0 $, the shutdown margin with the most reactive rod (Shim) stuck out of the core is 1.32 $ [4]. This value of the shutdown margin assures that the reactor can be shutdown from any operating condition, even with the assumption that the highest worth control rod remains fully withdrawn. The shutdown margin of 1043 pcm satisfies entirely since the minimum safety limit required for the IPR-R1 TRIGA research reactor is 200 pcm [3].

5 Reactivity Loss (cents) 6. POWER DEFECT The experiment was performed by increasing the reactor power, and, consequently, the fuel temperature by withdrawing the Shim rod in a number of steps. Initially, the reactor was critical at 20 W. The reactivity was determined from the calibrated curves, considering each critical rod position. Figure 4 shows the relationship between the reactor power level, raised in steps of 10 kw, and the associated reactivity loss to achieve a given power level. Because of the prompt negative temperature coefficient a significant amount of reactivity is needed to overcome temperature and allow the reactor to operate at high power levels. The reactivity needed to operate the IPR-R1 reactor at 100 kw, or the power defect, was 0.76 $ [4]. 80 y = E-03x E-01x E+00 R 2 = 9.972E Power (kw) Figure 4. Core reactivity loss with the power increase. 7. CONCLUSIONS This paper presents the results of neutronic tests which are performed annually as determined by the RAS of the IPR-R1 TRIGA reactor. The control rods were calibrated by positive period method and the worth of Regulating, Shim and Safety rods were 0.48 $, 3.21 $ and 2.84 $, respectively. The value of the excess reactivity obtained to compensate the negative reactivity due to negative temperature coefficient, the xenon poisoning, etc, was 2.0 $ (k eff = ). The shutdown margin obtained was 1.32 $, and the power defect was 0.76 $. After 8 hours of irradiation at 100 kw, with samples inserted in various place in the core, the reactor consumed approximately 16.2 cents of reactivity, mainly due to xenon poisoning. Considering all these reactivity values, the r exc should be increased to the new power operation [6, 7].

6 ACKNOWLEDGMENTS Thanks to the IPR-R1 reactor operators Fausto Maretti Júnior, Dante Marco Zangirolami, Luiz Otávio Sette Câmara and Paulo Fernando Oliveira for their cooperation during the experimental work. This research project is supported by Nuclear Technology Development Center (CDTN), Brazilian Nuclear Energy Commission (CNEN), Research Support Foundation of the State of Minas Gerais (FAPEMIG), and Brazilian Council for Scientific and Technological Development (CNPq). REFERENCES 1 CDTN/CNEN, Safety Analysis Report of the IPR-R1 TRIGA Reactor. (RASIN/TRIGA-IPR-R1/CDTN). CDTN, Belo Horizonte, Brazil (2007). (In Portuguese). 2 General Atomic, Technical foundation of TRIGA. San Diego, California (1958). (GA- 471). 3 General Atomic, Safeguards Summary Report for the New York University TRIGA Mark I Reactor, San Diego, California (1970). (GA-9864). 4 R.M.G.P. Souza, Resultados da Calibração das Barras de Controle, do Excesso de Reatividade, da Margem de Desligamento e do Defeito de Potência do TRIGA IPR-R1 Núcleo com 63 E.C. Belo Horizonte, CDTN (2010). (NI-SERTA-03/10). (In Portuguese). 5 J.J. Duderstadt, L.J. Hamilton, Nuclear Reactor Analysis, J. Wiley & Sons New York, N.Y. (1976). 6 R.M.G.P. Souza, A.Z. Mesquita, Reactivity balance in the IPR-R1 TRIGA reactor. Progress in Nuclear Energy (2011). 7 R.M.G.P. Souza, M.F.R. Resende, Power Upgrading Tests of the TRIGA IPR-R1 Nuclear Reactor to 250 kw". Second World TRIGA Users Conference, Atominstitute Vienna, Austria, September, (2004).