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Reactivity balance in the IPR-R1 TRIGA reactor
Progress in Nuclear Energy 53 (2011) 1132e1135
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Reactivity balance in the IPR-R1 TRIGA reactor Rose Mary Gomes do Prado Souza*, Amir Zacarias Mesquita Centro de Desenvolvimento da Tecnologia Nuclear (CDTN/CNEN-MG), Av. Presidente Antônio Carlos, 6627, Campus UFMG, Pampulha, 31.270-901 Belo Horizonte, MG, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 May 2010 Received in revised form 14 June 2011 Accepted 18 June 2011
Considering that the power of the IPR-R1 TRIGA reactor, located at the Nuclear Technology Development Center, Brazil, will be increased from 100 kW to 250 kW, some experiments were done in order to evaluate the magnitude of the reactivity effects associated with the reactor operation. The core excess of reactivity obtained was 1.99 $, and the shutdown margin was 1.33 $. The reactivity needed to operate the IPR-R1 reactor at 100 kW was 0.72 $, mainly due to the prompt negative temperature coefficient. A significant amount of reactivity is needed to overcome temperature and allow the reactor to operate at the higher power levels. The loss of reactivity due to xenon poisoning after 8 h of operation at 100 kW was around 0.20 $, and the highest reactivity loss value caused by a void inserted in the central thimble was 0.22 $. From the results obtained, it was possible to balance all the determined reactivity losses with the reactivity excess available in the reactor, considering the present and the future reactor power operation. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: TRIGA research reactor Control rods worth Reactivity Xenon poisoning Void
1. Introduction The thermal power operation of the IPR-R1 TRIGA reactor, at the Nuclear Technology Development Center e CDTN, Belo Horizonte, Brazil, is 100 kW. Its power will be upgraded to 250 kW, and it only depends on the CNEN license to operate the reactor at this power. The current 250 kW core configuration has 63 fuel elements composed of 59 aluminum clad elements and 4 stainless-steel clad fuel elements. The reactor reactivity is controlled by three control rods. Nuclear reactors must have sufficient excess reactivity to compensate the negative reactivity feedback effects such as those caused by the fuel temperature and power defect of reactivity, fuel burnup, void, poisoning production, and also to allow full power operation for predetermined period of time. This paper reports the results of a set of experiments that were accomplished to supply data for the reactivity balance calculation. The experiments which were performed were to: calibrate the control rods; determine the reactivity excess and the shutdown margin; evaluate the losses of reactivity due to simulated voids inserted in the core; determine the reactivity needed to operate the IPR-R1 reactor at 100 kW; and investigate the effect of the xenon poisoning during eight hours of operation at a steady-state power level of 100 kW. From the analysis of these results, it was possible to conclude that the reactivity excess obtained of 1.99 should be increased by * Corresponding author. Tel.: þ55 31 3069 3232; fax: þ55 31 3069 3380. E-mail address: [email protected] (R.M.G.P. Souza). 0149-1970/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pnucene.2011.06.011
loading fuel elements in the core to operate the reactor at the new power. This will be necessary because it is planned to go to 2.5 times the power licensed at present, i.e. to 250 kW. 2. Reactor description The IPR-R1 TRIGA reactor is a pool light water reactor cooled by natural convection. The fuel is a solid, homogeneous mixture of uranium-zirconium hydride alloy containing about 8.5% and 8% by weight of uranium enriched to 20% in 235U, for stainless-steel and aluminum clad elements, respectively (GA-9864, 1970; GA-2025, 1961; GA-471, 1958). The core has cylindrical configuration with an annular graphite reflector. There are 91 locations in the core altogether, which can be filled by fuel elements, graphite elements or other components. The elements are arranged in five concentric rings. The 250 kW core configuration has 63 fuel elements composed of 59 original Al-clad fuel elements and 4 fresh SS-clad fuel elements (Souza and Resende, 2004). Fig. 1 shows the geometrical configuration of the fuel elements, the graphite dummy elements and the control rods loaded in the core. 3. Reactivity measurements 3.1. Control rod worth The power level of the reactor is controlled by three control rods: Regulating, Shim and Safety. The Shim and the Safety control
R.M.G.P. Souza, A.Z. Mesquita / Progress in Nuclear Energy 53 (2011) 1132e1135
1133
Fig. 3. Integral curve of the shim control rod.
25 W, so the temperature increase during the experiment was negligible. Figs. 2e4 show the integral calibration curves of the Regulating, Shim and Safety rods, respectively, where the reactivity values were plotted as a function of the rod positions. 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, the total worth of these rods were calculated using the curves shown in Figs. 3 and 4, respectively. The experimental worth of the control rods for the present core are presented in Table 1. Fig. 1. IPR-R1 TRIGA reactor core.
3.2. Excess of reactivity and shutdown margin rods are positioned at symmetrical locations of C-ring, and the Regulating rod at F-ring. The control rods were calibrated by the positive period method. The period was measured from the doubling time, and the reactivity was obtained from the inhour equation. The Shim and Safety rods were intercalibrated. The idea 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. The reactivity measurements were performed at a power of less than
The reactivity excess (rexc) of the core was experimentally determined from control rod worth critical positions, at low power, and from their calibration curves. The average value obtained was 1.99 $, or 1572 pcm (Souza, 2009). The total reactivity worth of the control system is 6.37 $. With a core excess reactivity of 1.99 $, the shutdown margin with all rods down is 4.38 $ and with the most reactive rod stuck out is 1.33 $ (1051 pcm). This value of the shutdown margin assures that the
Fig. 2. Integral curve of the regulating control rod.
Fig. 4. Integral curve of the safety control rod.
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R.M.G.P. Souza, A.Z. Mesquita / Progress in Nuclear Energy 53 (2011) 1132e1135
Table 1 Results of reactivity (beff of the IPR-R1 reactor is 0.0079).
Regulating worth Shim worth Safety worth Reactivity excess Shutdown margin e shim rod out
r ($)
r (pcm)
0.52 3.05 2.80 1.99 1.33
411 2410 2212 1572 1051
reactor can be shutdown from any operating condition even with the assumption that the highest worth control rod remains fully withdrawn. Table 1 shows these measured values for the current IPR-R1 core configuration. The shutdown margin of 1051 pcm satisfies entirely since the minimum safety limit required for the IPR-R1 TRIGA research reactor is 200 pcm (0.20% dk/k) (GA-9864, 1970; CDTN/CNEN, 2007). 3.3. Loss of reactivity with power increase 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 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. Fig. 5 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. The reactivity needed to operate the IPR-R1 reactor at 100 kW, or the power defect, is 72 cents (569 pcm) (Souza, 2009). 3.4. Loss of reactivity due to xenon poisoning Captures of neutrons in 135Xe result in a negative reactivity effect in thermal reactors, because of the large capture cross section of this fission product (Duderstadt and Hamilton, 1976). In addition to the power raising experiments, the effect of the xenon poisoning during 8 h of operation at a steady-state power level of 100 kW was investigated. The reactivity loss versus the running time is plotted in Fig. 6. After eight hours, the estimated value of the negative reactivity introduced in the reactor was around 20 cents (158 pcm) (Souza, 2009).
Fig. 6. Reactivity loss due to the
135
Xe buildup versus time at 100 kW.
3.5. Loss of reactivity due to void The presence of voids in a reactor has a significant effect on the reactivity, thus being of primary importance for the reactor safety and controllability. The voids can be produced either inherently, such as in cases of subcooled boiling, or they may be introduced on purpose, as for material irradiation. For carrying out void measurements, the void was created by inserting in the core a closed aluminum tube which contained atmospheric air. The reactor was first made critical at low power to have no fuel temperature effect, and this way the reactivity variation was only due to the void. Then, the simulated void was inserted at the desired place, inside the reactor core, which resulted in a change in reactivity. All these positions are shown in Fig. 1. The system was brought back to the critical state with the help of a calibrated control rod. Thus, the change in the reactivity due to the inserted simulated void was determined by the change in the position of the calibrated control rod. Due to an increase in flux from the core edge to the central region, the insertion of a void into the central tube is expected to induce a much larger reactivity than when the same volume of void is inserted into the periphery of the core. This was confirmed by the experiment. A 200 cm3 void introduced in the central thimble induces a larger reactivity loss, 22.0 cents (174 pcm) compared to its 17.2 cents (136 pcm) value when inserting the void in the F24 position, replacing a graphite element (Souza, 2009). A small simulated void of 11.4 cm3 was introduced in the numbered holes between some fuel elements, as shown in Fig. 1. The tube was inserted in the whole active core depth, at once. These insertions substituted water, and caused small losses of reactivity. It was calculated the average value of the reactivity loss per ring, and the results are presented in Fig. 7. The farther the core center the smaller the reactivity loss is. 4. Reactivity balance evaluation
Fig. 5. Reactivity loss versus power level.
Although the reactivity effects associated with the insertion and removal of experiments in the core are difficult to predict, Table 2 provides a guide to the magnitude of the reactivity effects associated with the IPR-R1 TRIGA reactor operation. The last column shows the results that were obtained in the experiments performed at 250 kW (Souza and Resende, 2004). The measurements done at this power were used as a benchmark for reactor calculations done by WIMSD4 and CITATION codes (Dalle, 1999). Considering that the present reactivity excess of the core is 1.99 $, the Table 2 shows that
R.M.G.P. Souza, A.Z. Mesquita / Progress in Nuclear Energy 53 (2011) 1132e1135
1135
5. Conclusions
Fig. 7. Reactivity loss due to void introduced in the core rings versus radial position.
Table 2 Reactivity specifications of the IPR-R1 TRIGA reactor. Reactivity specification
100 kW
250 kW
Reactivity loss ($) Power Xenon Void water (central thimble) Total Reactivity excess in 2009
0.72 0.04 0.20 0.01 0.22 0.02 1.14 0.05 1.99
1.60 0.27 2.09
if the reactor was operated during a period of about 8 h at 250 kW, with a void inserted in the central tube, this excess of reactivity would not be enough because the total negative reactivity that would be inserted in the core would be 2.09 $. At 100 kW, under the same conditions, the total negative reactivity inserted is 1.14 $ and then 0.85 $ is left to compensate other effects of negative reactivity. It was considered the independence of the void reactivity with the reactor power. In addition to considering the losses due to temperature, fuel depletion, voids, and fission product poisoning, some reactivity excess must also be included for operation and for any other effects. A certain amount of reactivity excess, i.e., additional fuel must therefore be included in the reactor. The decrease of reactivity caused by the fuel burnup is very small and occurs in long term, about 15 cents per year (Dalle, 2005). Therefore, in the final balance of reactivity this value would not be considered because the other ones are more significant. In Souza (2008) experimental values of the fuel element reactivity worth inserted in different rings were obtained. Those values were determined from the difference in the critical position of the control rods when the measured element was inserted in and withdrawn from the core. The highest value of 39.0 cents was obtained by substituting a graphite element in the F21 position for fresh Al-clad fuel element. Based on those results a new arrangement of the IPR-R1 core, adding fuel elements, should be done in order to increase its reactivity excess. It is important to emphasize that the maximum excess reactivity in the core, above a cold, clean, critical, and compact core condition, allowed for the TRIGA Mark I reactor is 3 $ (GA-9864, 1970; CDTN/CNEN, 2007).
As the power of the IPR-R1 TRIGA reactor will be upgraded to 250 kW, it was necessary not only to evaluate the magnitude of the reactivity effects associated with the reactor operation due to the power increase, the xenon buildup, and voids inserted in the core, but also to compare them with the current excess of reactivity of the reactor. The reactivity excess obtained for the current core was 1.99 $, and the shutdown margin with the most reactive rod stuck out of the core was 1051 pcm, hence, higher than the minimum safety limit required (200 pcm). The reactivity worth of the control rods is adequate to allow complete control of the reactor during operation from a shutdown condition to full power, and it also satisfies the condition that at least two control rods should independently have sufficient reactivity worth to shutdown the reactor. The reactivity needed to overcome the temperature and allow the IPR-R1 reactor to operate at 100 kW was 72 cents. The negative reactivity introduced by the xenon in the reactor, after eight hours of operation at 100 kW, was approximately 20 cents. The highest negative value of the reactivity introduced by the void was 22 cents in the central thimble. From the analysis of the results obtained in this work, it was concluded that it will be necessary to increase the excess of reactivity of the core by adding fuel elements to it in order to overcome the negative effects of reactivity during operation at the new power. The experimental results and the theoretical calculations will be used to optimize the new 250 kW reactor core configuration.
Acknowledgments Thanks to the IPR-R1 TRIGA reactor operators for their cooperation in the experimental works, and to the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) and to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the financial support.
References CDTN/CNEN, 2007. Safety Analysis Reporter of the IPR-R1 TRIGA Reactor. Belo Horizonte, Brazil. RASIN/TRIGA IPR-R1 (in Portuguese). Dalle, H.M., 1999. Neutronic Evaluation of the IPR-R1 TRIGA Reactor to Operate at 250 kW. CDTN/CNEN CDTN. Belo Horizonte, Brazil (NI-CT4-04/99) (in Portuguese). Dalle, H.M., 2005, TRIGA IPR-R1 reactor simulation using monte carlo transport methods. ScD Thesis, Universidade Estadual de Campinas, São Paulo (in Portuguese). Duderstadt, J.J., Hamilton, L.J., 1976. Nuclear Reactor Analysis. Wiley & Sons, New York. GA-9864, 1970. Safeguards Summary Report for the New York University Triga Mark I Reactor. Gulf General Atomic, San Diego, CA. GA-2025, 1961. Hazards Report for the 250 kW TRIGA Mark II Reactor. Gulf General Atomic, San Diego, CA. GA-471, 1958. Technical Foundations of TRIGA. Gulf General Atomic, San Diego, CA. Souza R.M.G.P., Resende M.F.R., 2004. Power upgrading tests of the TRIGA IPR-R1 nuclear reactor to 250 kW. In: Second World TRIGA Users Conference, Atominstitute Vienna, Austria. Souza R.M.G.P., 2008. Fuel element reactivity worth in different rings of the IPR-R1 TRIGA reactor. In: Fourth World TRIGA Users Conference, Lyon, France. Souza, R.M.G.P., 2009. Results of the Neutronic Tests in the IPR-R1 TRIGA Reactor at 100 kW e Core with 63 F.E. CDTN/CNEN. Belo Horizonte, Brazil. NI-SERTA-01/09 (in Portuguese).
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Reactivity balance in the IPR-R1 TRIGA reactor Rose Mary Gomes do Prado Souza*, Amir Zacarias Mesquita Centro de Desenvolvimento da Tecnologia Nuclear (CDTN/CNEN-MG), Av. Presidente Antônio Carlos, 6627, Campus UFMG, Pampulha, 31.270-901 Belo Horizonte, MG, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 May 2010 Received in revised form 14 June 2011 Accepted 18 June 2011
Considering that the power of the IPR-R1 TRIGA reactor, located at the Nuclear Technology Development Center, Brazil, will be increased from 100 kW to 250 kW, some experiments were done in order to evaluate the magnitude of the reactivity effects associated with the reactor operation. The core excess of reactivity obtained was 1.99 $, and the shutdown margin was 1.33 $. The reactivity needed to operate the IPR-R1 reactor at 100 kW was 0.72 $, mainly due to the prompt negative temperature coefficient. A significant amount of reactivity is needed to overcome temperature and allow the reactor to operate at the higher power levels. The loss of reactivity due to xenon poisoning after 8 h of operation at 100 kW was around 0.20 $, and the highest reactivity loss value caused by a void inserted in the central thimble was 0.22 $. From the results obtained, it was possible to balance all the determined reactivity losses with the reactivity excess available in the reactor, considering the present and the future reactor power operation. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: TRIGA research reactor Control rods worth Reactivity Xenon poisoning Void
1. Introduction The thermal power operation of the IPR-R1 TRIGA reactor, at the Nuclear Technology Development Center e CDTN, Belo Horizonte, Brazil, is 100 kW. Its power will be upgraded to 250 kW, and it only depends on the CNEN license to operate the reactor at this power. The current 250 kW core configuration has 63 fuel elements composed of 59 aluminum clad elements and 4 stainless-steel clad fuel elements. The reactor reactivity is controlled by three control rods. Nuclear reactors must have sufficient excess reactivity to compensate the negative reactivity feedback effects such as those caused by the fuel temperature and power defect of reactivity, fuel burnup, void, poisoning production, and also to allow full power operation for predetermined period of time. This paper reports the results of a set of experiments that were accomplished to supply data for the reactivity balance calculation. The experiments which were performed were to: calibrate the control rods; determine the reactivity excess and the shutdown margin; evaluate the losses of reactivity due to simulated voids inserted in the core; determine the reactivity needed to operate the IPR-R1 reactor at 100 kW; and investigate the effect of the xenon poisoning during eight hours of operation at a steady-state power level of 100 kW. From the analysis of these results, it was possible to conclude that the reactivity excess obtained of 1.99 should be increased by * Corresponding author. Tel.: þ55 31 3069 3232; fax: þ55 31 3069 3380. E-mail address: [email protected] (R.M.G.P. Souza). 0149-1970/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pnucene.2011.06.011
loading fuel elements in the core to operate the reactor at the new power. This will be necessary because it is planned to go to 2.5 times the power licensed at present, i.e. to 250 kW. 2. Reactor description The IPR-R1 TRIGA reactor is a pool light water reactor cooled by natural convection. The fuel is a solid, homogeneous mixture of uranium-zirconium hydride alloy containing about 8.5% and 8% by weight of uranium enriched to 20% in 235U, for stainless-steel and aluminum clad elements, respectively (GA-9864, 1970; GA-2025, 1961; GA-471, 1958). The core has cylindrical configuration with an annular graphite reflector. There are 91 locations in the core altogether, which can be filled by fuel elements, graphite elements or other components. The elements are arranged in five concentric rings. The 250 kW core configuration has 63 fuel elements composed of 59 original Al-clad fuel elements and 4 fresh SS-clad fuel elements (Souza and Resende, 2004). Fig. 1 shows the geometrical configuration of the fuel elements, the graphite dummy elements and the control rods loaded in the core. 3. Reactivity measurements 3.1. Control rod worth The power level of the reactor is controlled by three control rods: Regulating, Shim and Safety. The Shim and the Safety control
R.M.G.P. Souza, A.Z. Mesquita / Progress in Nuclear Energy 53 (2011) 1132e1135
1133
Fig. 3. Integral curve of the shim control rod.
25 W, so the temperature increase during the experiment was negligible. Figs. 2e4 show the integral calibration curves of the Regulating, Shim and Safety rods, respectively, where the reactivity values were plotted as a function of the rod positions. 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, the total worth of these rods were calculated using the curves shown in Figs. 3 and 4, respectively. The experimental worth of the control rods for the present core are presented in Table 1. Fig. 1. IPR-R1 TRIGA reactor core.
3.2. Excess of reactivity and shutdown margin rods are positioned at symmetrical locations of C-ring, and the Regulating rod at F-ring. The control rods were calibrated by the positive period method. The period was measured from the doubling time, and the reactivity was obtained from the inhour equation. The Shim and Safety rods were intercalibrated. The idea 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. The reactivity measurements were performed at a power of less than
The reactivity excess (rexc) of the core was experimentally determined from control rod worth critical positions, at low power, and from their calibration curves. The average value obtained was 1.99 $, or 1572 pcm (Souza, 2009). The total reactivity worth of the control system is 6.37 $. With a core excess reactivity of 1.99 $, the shutdown margin with all rods down is 4.38 $ and with the most reactive rod stuck out is 1.33 $ (1051 pcm). This value of the shutdown margin assures that the
Fig. 2. Integral curve of the regulating control rod.
Fig. 4. Integral curve of the safety control rod.
1134
R.M.G.P. Souza, A.Z. Mesquita / Progress in Nuclear Energy 53 (2011) 1132e1135
Table 1 Results of reactivity (beff of the IPR-R1 reactor is 0.0079).
Regulating worth Shim worth Safety worth Reactivity excess Shutdown margin e shim rod out
r ($)
r (pcm)
0.52 3.05 2.80 1.99 1.33
411 2410 2212 1572 1051
reactor can be shutdown from any operating condition even with the assumption that the highest worth control rod remains fully withdrawn. Table 1 shows these measured values for the current IPR-R1 core configuration. The shutdown margin of 1051 pcm satisfies entirely since the minimum safety limit required for the IPR-R1 TRIGA research reactor is 200 pcm (0.20% dk/k) (GA-9864, 1970; CDTN/CNEN, 2007). 3.3. Loss of reactivity with power increase 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 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. Fig. 5 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. The reactivity needed to operate the IPR-R1 reactor at 100 kW, or the power defect, is 72 cents (569 pcm) (Souza, 2009). 3.4. Loss of reactivity due to xenon poisoning Captures of neutrons in 135Xe result in a negative reactivity effect in thermal reactors, because of the large capture cross section of this fission product (Duderstadt and Hamilton, 1976). In addition to the power raising experiments, the effect of the xenon poisoning during 8 h of operation at a steady-state power level of 100 kW was investigated. The reactivity loss versus the running time is plotted in Fig. 6. After eight hours, the estimated value of the negative reactivity introduced in the reactor was around 20 cents (158 pcm) (Souza, 2009).
Fig. 6. Reactivity loss due to the
135
Xe buildup versus time at 100 kW.
3.5. Loss of reactivity due to void The presence of voids in a reactor has a significant effect on the reactivity, thus being of primary importance for the reactor safety and controllability. The voids can be produced either inherently, such as in cases of subcooled boiling, or they may be introduced on purpose, as for material irradiation. For carrying out void measurements, the void was created by inserting in the core a closed aluminum tube which contained atmospheric air. The reactor was first made critical at low power to have no fuel temperature effect, and this way the reactivity variation was only due to the void. Then, the simulated void was inserted at the desired place, inside the reactor core, which resulted in a change in reactivity. All these positions are shown in Fig. 1. The system was brought back to the critical state with the help of a calibrated control rod. Thus, the change in the reactivity due to the inserted simulated void was determined by the change in the position of the calibrated control rod. Due to an increase in flux from the core edge to the central region, the insertion of a void into the central tube is expected to induce a much larger reactivity than when the same volume of void is inserted into the periphery of the core. This was confirmed by the experiment. A 200 cm3 void introduced in the central thimble induces a larger reactivity loss, 22.0 cents (174 pcm) compared to its 17.2 cents (136 pcm) value when inserting the void in the F24 position, replacing a graphite element (Souza, 2009). A small simulated void of 11.4 cm3 was introduced in the numbered holes between some fuel elements, as shown in Fig. 1. The tube was inserted in the whole active core depth, at once. These insertions substituted water, and caused small losses of reactivity. It was calculated the average value of the reactivity loss per ring, and the results are presented in Fig. 7. The farther the core center the smaller the reactivity loss is. 4. Reactivity balance evaluation
Fig. 5. Reactivity loss versus power level.
Although the reactivity effects associated with the insertion and removal of experiments in the core are difficult to predict, Table 2 provides a guide to the magnitude of the reactivity effects associated with the IPR-R1 TRIGA reactor operation. The last column shows the results that were obtained in the experiments performed at 250 kW (Souza and Resende, 2004). The measurements done at this power were used as a benchmark for reactor calculations done by WIMSD4 and CITATION codes (Dalle, 1999). Considering that the present reactivity excess of the core is 1.99 $, the Table 2 shows that
R.M.G.P. Souza, A.Z. Mesquita / Progress in Nuclear Energy 53 (2011) 1132e1135
1135
5. Conclusions
Fig. 7. Reactivity loss due to void introduced in the core rings versus radial position.
Table 2 Reactivity specifications of the IPR-R1 TRIGA reactor. Reactivity specification
100 kW
250 kW
Reactivity loss ($) Power Xenon Void water (central thimble) Total Reactivity excess in 2009
0.72 0.04 0.20 0.01 0.22 0.02 1.14 0.05 1.99
1.60 0.27 2.09
if the reactor was operated during a period of about 8 h at 250 kW, with a void inserted in the central tube, this excess of reactivity would not be enough because the total negative reactivity that would be inserted in the core would be 2.09 $. At 100 kW, under the same conditions, the total negative reactivity inserted is 1.14 $ and then 0.85 $ is left to compensate other effects of negative reactivity. It was considered the independence of the void reactivity with the reactor power. In addition to considering the losses due to temperature, fuel depletion, voids, and fission product poisoning, some reactivity excess must also be included for operation and for any other effects. A certain amount of reactivity excess, i.e., additional fuel must therefore be included in the reactor. The decrease of reactivity caused by the fuel burnup is very small and occurs in long term, about 15 cents per year (Dalle, 2005). Therefore, in the final balance of reactivity this value would not be considered because the other ones are more significant. In Souza (2008) experimental values of the fuel element reactivity worth inserted in different rings were obtained. Those values were determined from the difference in the critical position of the control rods when the measured element was inserted in and withdrawn from the core. The highest value of 39.0 cents was obtained by substituting a graphite element in the F21 position for fresh Al-clad fuel element. Based on those results a new arrangement of the IPR-R1 core, adding fuel elements, should be done in order to increase its reactivity excess. It is important to emphasize that the maximum excess reactivity in the core, above a cold, clean, critical, and compact core condition, allowed for the TRIGA Mark I reactor is 3 $ (GA-9864, 1970; CDTN/CNEN, 2007).
As the power of the IPR-R1 TRIGA reactor will be upgraded to 250 kW, it was necessary not only to evaluate the magnitude of the reactivity effects associated with the reactor operation due to the power increase, the xenon buildup, and voids inserted in the core, but also to compare them with the current excess of reactivity of the reactor. The reactivity excess obtained for the current core was 1.99 $, and the shutdown margin with the most reactive rod stuck out of the core was 1051 pcm, hence, higher than the minimum safety limit required (200 pcm). The reactivity worth of the control rods is adequate to allow complete control of the reactor during operation from a shutdown condition to full power, and it also satisfies the condition that at least two control rods should independently have sufficient reactivity worth to shutdown the reactor. The reactivity needed to overcome the temperature and allow the IPR-R1 reactor to operate at 100 kW was 72 cents. The negative reactivity introduced by the xenon in the reactor, after eight hours of operation at 100 kW, was approximately 20 cents. The highest negative value of the reactivity introduced by the void was 22 cents in the central thimble. From the analysis of the results obtained in this work, it was concluded that it will be necessary to increase the excess of reactivity of the core by adding fuel elements to it in order to overcome the negative effects of reactivity during operation at the new power. The experimental results and the theoretical calculations will be used to optimize the new 250 kW reactor core configuration.
Acknowledgments Thanks to the IPR-R1 TRIGA reactor operators for their cooperation in the experimental works, and to the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) and to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the financial support.
References CDTN/CNEN, 2007. Safety Analysis Reporter of the IPR-R1 TRIGA Reactor. Belo Horizonte, Brazil. RASIN/TRIGA IPR-R1 (in Portuguese). Dalle, H.M., 1999. Neutronic Evaluation of the IPR-R1 TRIGA Reactor to Operate at 250 kW. CDTN/CNEN CDTN. Belo Horizonte, Brazil (NI-CT4-04/99) (in Portuguese). Dalle, H.M., 2005, TRIGA IPR-R1 reactor simulation using monte carlo transport methods. ScD Thesis, Universidade Estadual de Campinas, São Paulo (in Portuguese). Duderstadt, J.J., Hamilton, L.J., 1976. Nuclear Reactor Analysis. Wiley & Sons, New York. GA-9864, 1970. Safeguards Summary Report for the New York University Triga Mark I Reactor. Gulf General Atomic, San Diego, CA. GA-2025, 1961. Hazards Report for the 250 kW TRIGA Mark II Reactor. Gulf General Atomic, San Diego, CA. GA-471, 1958. Technical Foundations of TRIGA. Gulf General Atomic, San Diego, CA. Souza R.M.G.P., Resende M.F.R., 2004. Power upgrading tests of the TRIGA IPR-R1 nuclear reactor to 250 kW. In: Second World TRIGA Users Conference, Atominstitute Vienna, Austria. Souza R.M.G.P., 2008. Fuel element reactivity worth in different rings of the IPR-R1 TRIGA reactor. In: Fourth World TRIGA Users Conference, Lyon, France. Souza, R.M.G.P., 2009. Results of the Neutronic Tests in the IPR-R1 TRIGA Reactor at 100 kW e Core with 63 F.E. CDTN/CNEN. Belo Horizonte, Brazil. NI-SERTA-01/09 (in Portuguese).