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Effect of hydrogen loop fluctuation on cold neutron performance with hybrid control system in JSNS
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 600 (2009) 81–83
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Effect of hydrogen loop fluctuation on cold neutron performance with hybrid control system in JSNS Shoichi Hasegawa , Tomokazu Aso, Hideki Tatsumoto, Kiichi Ohtsu, Toshiaki Uehara, Fujio Maekawa, Takashi Kato, Yujiro Ikeda Japan Atomic Energy Agency, Toukai-mura, Naka-gun, Ibaraki 319-1195, Japan
a r t i c l e in fo
abstract
Available online 30 November 2008
Supercritical hydrogen is selected as a moderator material in Japan Spallation Neutron Source Facility (JSNS). Total nuclear heating in the moderator is estimated to be 3.75 kW at 1 MW proton beam power, which provides a hydrogen cryogenic system as a heat load. The heat load changes significantly due to pulse proton beam operation. A control system to mitigate such heat load changes is required for the hydrogen cryogenic system in order to maintain stable operation and steady cold neutron generation. A hybrid control system, which consists of a heater and an accumulator, is developed and installed in the system. In order to build the optimal control method, the behaviors of hydrogen loop are analyzed with an original computer code. The analysis suggests that a stability of hydrogen system and a cold neutron beam performance by the hybrid fluctuation control system are compatible. & 2008 Elsevier B.V. All rights reserved.
Keywords: Hydrogen system Control system
1. Introduction The Japan Spallation Neutron Source Facility (JSNS) has been constructed under the Japan Proton Accelerator Research Complex (J-PARC) project promoted by a joint collaboration between the Japan Atomic Energy Agency (JAEA) and the High Energy Accelerator Research Organization (KEK) [1]. The JSNS aims to produce the world highest intensity of pulsed cold neutron beams and high quality pulse structure for the fundamental researches in materials and life science [2]. Because of neutron quality, cryogenic hydrogen is selected as the moderator material [3]. A cryogenic hydrogen system in JSNS provides cryogenic hydrogen for moderators and removes nuclear heating when the proton beam is injected to the JSNS. There is a close relation between the physical properties of hydrogen and the characteristic of the generated cold neutron beam [4]. The nuclear heating generated in the moderators is estimated to be 3.75 kW at the proton beam power of 1 MW [5–7]. The heat load of nuclear heating changes significantly due to proton beam operation. Such a change of heat load induces a large fluctuation in the cryogenic hydrogen loop, but a hybrid control system in the JSNS works well and is able to mitigate the fluctuation of pressure [8]. This change also affects the neutron beam characteristic. In this work, an optimal control method to maintain the stability of
Corresponding author. Tel.: +81 29 284 3742; fax: +81 29 282 6496.
E-mail address: [email protected] (S. Hasegawa). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.11.088
hydrogen loop is discussed, and the effect of hydrogen loop on the neutron beam performance is estimated.
2. Fluctuation control system for hydrogen loop A schematic configuration of the cryogenic hydrogen system is shown in Fig. 1 [3]. The hydrogen circulation system forms a closed loop filled with cryogenic hydrogen. In the usual closed loop, the quantity and density of the coolant in the system are constant. In such a system, the pressure change is subjected by temperature change. In the case of 1 MW proton beam injection, hydrogen temperature in moderators will rise up a few Kelvin, bringing severe pressure rising. Therefore, more attention should be paid to such a pressure change during the control of the cryogenic fluid in closed loop. The JSNS cryogenic hydrogen system adopted a hybrid control system that consists of a heater and an accumulator [3]. The heater will compensate heat load change caused by the proton beam-on and beam-off and the accumulator will mitigate the density changes in the hydrogen loop due to volume change. Based on former analysis, the heater capacity is designed to be 6.5 kW and the accumulator consists of a bellow that has a capacity change of 15.7 L [8]. The advantage of the hybrid system is that the function of heat compensation and the pressure mitigation can be controlled individually, so that each role becomes clear and a control method is simplified. In the JSNS, supercritical hydrogen is selected as a cryogenic hydrogen moderator, in which a high density of hydrogen is
ARTICLE IN PRESS 82
S. Hasegawa et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 81–83
Fig. 1. Schematic drawing of a hydrogen cryogenic system. Fig. 2. A schematic view of the analysis model.
3. Analysis model Fig. 2 shows a schematic view of the analysis model [8]. In this analysis work, the effect of the volume change by an accumulator is analyzed. In the hydrogen loop at JSNS, the heating region is restricted at the moderators, heater and heat exchanger to simplify the analysis. Namely, heat input is ignored in the pumps, ortho–para converter and pipe, because heat amount is very small compared to such a nuclear heating. Therefore, the volume is divided into three parts, V1, V2 and V3 in the analysis model. Q2 and Q3 are heat loads of moderators and heater, respectively. For the analysis, the following condition is assumed. When the heat load of beam changes, heated or non-heated volume part is appeared out of the moderator and it develops to downstream with the hydrogen flow velocity. The boundary between the heated part and non-heated part exists. In other words, the diffusion of boundary is neglected. This boundary point moves by the flow velocity, e.g. 0.162 Kg/s and the output temperature of helium–hydrogen heat exchanger is constant. The nuclear heating, Q2 is set to be 3.75 kW for 1 MW proton beam. In the numerical analysis, physical properties are calculated by GASPAK library, Cryodata Inc, in each time step, e.g. 0.01 s. The enthalpy of the hydrogen in each lattice is decided according to the heat load. The analysis code search for suitable pressure with which the conservation of mass agrees at each time step. However, the pressure condition affects the volume of the accumulator. Therefore the pressure condition and the volume of the accumulator are synchronized and calculated.
4. Results and discussion The proton beam injection or shutdown gives the maximum change of heat load to the hydrogen loop. Fig. 3 indicates analysis
1.52
Hydrogen Pressure (MPa)
maintained and phase change does not occur. Therefore the hydrogen system is controlled with an operational pressure of 1.5 MPa that is higher than the hydrogen critical pressure of 1.29 MPa. A performance of moderator neutronics depends on the hydrogen density, at which the density should be kept constant. Therefore, the hydrogen system is controlled to keep constant pressure and temperature. Especially, the hybrid control system has an important role when the beam power changes due to beam-on and beam-off.
1.51
1.5
1.49 Pressure [Acc+Heater (3750W)] (MPa) Pressure [Acc+Heater (4800W)] (MPa)
1.48 0
20
40 60 Time (sec)
80
100
Fig. 3. The hydrogen pressure behavior for proton beam injection.
results of pressure fluctuation when 1 MW proton beam injects at 0 s. The analysis was carried out for two cases; one is that the hydrogen pressure is kept constant to the initial pressure after the beam injection. The dashed curve shows the result of pressure behavior for that case. This case is necessary to adjust the heater power to be more than 3.75 kW which is the heat load of moderators, because the heater cannot compensate heat in V2 in Fig. 2. In other words, to maintain the hydrogen pressure constant or low, the required heater power should exceed the heat load by beam. In this case, the hydrogen temperature into heat exchanger has to be changed. Therefore, the cryogenic system requires the cooling power control to maintain the same inlet temperature as the moderators. On the other hand, the solid curve shows the hydrogen pressure behavior with the heater power keeping at 3.75 kW. There is no large difference in pressure change in the curves. The pressure fluctuation is within 70.01 MPa, corresponding to less than 1% for 1.5 MPa operation pressure. From the view point of pressure stability, the second case has an advantage to saturate the hydrogen pressure quickly. Furthermore,
ARTICLE IN PRESS S. Hasegawa et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 81–83
80
Table 1 Effect on cold neutron beam performance. Peak intensity
Hydrogen density (kg/m3)
83
0–20 meV (%)
75 DM PM CM
3.6 4.5 0.9
FWHM (full width at half maximum) 50–1000 meV (%) 2.4 2.4 0.6
0–20 meV (%)
50–1000 meV (%)
0.0 0.0 0.6
1.5 1.5 1.5
70 stable cold neutron beams, it is important to maintain the hydrogen temperature and the flow velocity constant.
5. Summary
65 0
0.2
0.4 0.6 Beam power (W)
0.8
1
Fig. 4. Relation between beam power and density of moderator.
it is necessary to control the heater only, not to control the cooling power of helium refrigerator. Therefore the second case is selected as an operation scenario. The change of the neutron beam when the second control is evaluated. In an incompressible fluid, the influence of density change due to pressure fluctuation can be almost ignored, but that of the temperature change should be considered. A temperature rising in the moderators is dominated by hydrogen flow velocity. The flow velocity of 0.162 kg/s, which is the rated flow velocity, brings about the hydrogen temperature rise of about 3 K with the rated heat load condition, corresponding to the hydrogen density degrease of about 4%. A relation between the density change in the moderators and a proton beam is described in Fig. 4, where the hydrogen flow is set to be the rated value. This result shows that the density decreases about 3 kg/m3 at the 1 MW proton beam power. Table 1 lists for the effects of neutron beam profile when such density decreases are occurred. DM, PM and CM indicate three type of moderator, decoupled, poisoned and coupled moderator, respectively. For DM and PM, the beam peak intensity decreases around 4% at the low energy region, that is almost the same as the density decreasing rate [4]. On the other hand, the spectrum width of high energy region increases little bit. Accordingly the hydrogen flow velocity affects not only the moderator temperature but also the neutron beam performance. And the density change is almost proportional to the beam peak intensity change of DM and PM. Therefore, in order to supply the
For the cryogenic hydrogen circulating loop of the JSNS, the stable operation of hydrogen loop and the effect for the cold neutron beam performance are estimated during heat load change by using the high-power pulsed proton beam with analytical code. The heater output for operating hydrogen loop stably made clear analytically that it is the same as the heat load by the pulsed 1 MW proton beam. The hydrogen density, which has a deep influence on the neutron performance, can be kept almost constant with optimal hybrid control independently of beam condition. It became clear that it is important to maintain the flow velocity for homogeneous neutron beam. References [1] S. Nagamiya, J. Nucl. Sci. Technol. (Suppl. 1) (2000) 40. [2] Y. Ikeda, 1 MW Pulse Spallation Neutron Source (JSNS) under the High Intensity Proton Accelerator Project, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 13. [3] T. Kato, et al., Cryogenic System Design for Cryogenic Hydrogen Moderator of the Spallation Neutron Source in J-PARC, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 645. [4] Materials and Life Science Experimental Facility Construction Team, J-PARC Technical Design Report, Material and Life Science Experimental Facility, JAERITech 2004-001. [5] T. Kai, et al., Neutronic Study on Coupled Hydrogen Moderator for Japanese Spallation Neutron Source, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 657. [6] M. Harada, et al., Optimization of poisoned and unpoisoned decoupled moderators in JSNS, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 697. [7] M. Teshigawara, et al., Development of Cd poisoned moderator, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 601. [8] S. Hasegawa, et al., Estimation of pressure change based on hybrid control system in JSNS hydrogen loop, ICANS-XVIII, Dongguan, April 25–29, 2007.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Effect of hydrogen loop fluctuation on cold neutron performance with hybrid control system in JSNS Shoichi Hasegawa , Tomokazu Aso, Hideki Tatsumoto, Kiichi Ohtsu, Toshiaki Uehara, Fujio Maekawa, Takashi Kato, Yujiro Ikeda Japan Atomic Energy Agency, Toukai-mura, Naka-gun, Ibaraki 319-1195, Japan
a r t i c l e in fo
abstract
Available online 30 November 2008
Supercritical hydrogen is selected as a moderator material in Japan Spallation Neutron Source Facility (JSNS). Total nuclear heating in the moderator is estimated to be 3.75 kW at 1 MW proton beam power, which provides a hydrogen cryogenic system as a heat load. The heat load changes significantly due to pulse proton beam operation. A control system to mitigate such heat load changes is required for the hydrogen cryogenic system in order to maintain stable operation and steady cold neutron generation. A hybrid control system, which consists of a heater and an accumulator, is developed and installed in the system. In order to build the optimal control method, the behaviors of hydrogen loop are analyzed with an original computer code. The analysis suggests that a stability of hydrogen system and a cold neutron beam performance by the hybrid fluctuation control system are compatible. & 2008 Elsevier B.V. All rights reserved.
Keywords: Hydrogen system Control system
1. Introduction The Japan Spallation Neutron Source Facility (JSNS) has been constructed under the Japan Proton Accelerator Research Complex (J-PARC) project promoted by a joint collaboration between the Japan Atomic Energy Agency (JAEA) and the High Energy Accelerator Research Organization (KEK) [1]. The JSNS aims to produce the world highest intensity of pulsed cold neutron beams and high quality pulse structure for the fundamental researches in materials and life science [2]. Because of neutron quality, cryogenic hydrogen is selected as the moderator material [3]. A cryogenic hydrogen system in JSNS provides cryogenic hydrogen for moderators and removes nuclear heating when the proton beam is injected to the JSNS. There is a close relation between the physical properties of hydrogen and the characteristic of the generated cold neutron beam [4]. The nuclear heating generated in the moderators is estimated to be 3.75 kW at the proton beam power of 1 MW [5–7]. The heat load of nuclear heating changes significantly due to proton beam operation. Such a change of heat load induces a large fluctuation in the cryogenic hydrogen loop, but a hybrid control system in the JSNS works well and is able to mitigate the fluctuation of pressure [8]. This change also affects the neutron beam characteristic. In this work, an optimal control method to maintain the stability of
Corresponding author. Tel.: +81 29 284 3742; fax: +81 29 282 6496.
E-mail address: [email protected] (S. Hasegawa). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.11.088
hydrogen loop is discussed, and the effect of hydrogen loop on the neutron beam performance is estimated.
2. Fluctuation control system for hydrogen loop A schematic configuration of the cryogenic hydrogen system is shown in Fig. 1 [3]. The hydrogen circulation system forms a closed loop filled with cryogenic hydrogen. In the usual closed loop, the quantity and density of the coolant in the system are constant. In such a system, the pressure change is subjected by temperature change. In the case of 1 MW proton beam injection, hydrogen temperature in moderators will rise up a few Kelvin, bringing severe pressure rising. Therefore, more attention should be paid to such a pressure change during the control of the cryogenic fluid in closed loop. The JSNS cryogenic hydrogen system adopted a hybrid control system that consists of a heater and an accumulator [3]. The heater will compensate heat load change caused by the proton beam-on and beam-off and the accumulator will mitigate the density changes in the hydrogen loop due to volume change. Based on former analysis, the heater capacity is designed to be 6.5 kW and the accumulator consists of a bellow that has a capacity change of 15.7 L [8]. The advantage of the hybrid system is that the function of heat compensation and the pressure mitigation can be controlled individually, so that each role becomes clear and a control method is simplified. In the JSNS, supercritical hydrogen is selected as a cryogenic hydrogen moderator, in which a high density of hydrogen is
ARTICLE IN PRESS 82
S. Hasegawa et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 81–83
Fig. 1. Schematic drawing of a hydrogen cryogenic system. Fig. 2. A schematic view of the analysis model.
3. Analysis model Fig. 2 shows a schematic view of the analysis model [8]. In this analysis work, the effect of the volume change by an accumulator is analyzed. In the hydrogen loop at JSNS, the heating region is restricted at the moderators, heater and heat exchanger to simplify the analysis. Namely, heat input is ignored in the pumps, ortho–para converter and pipe, because heat amount is very small compared to such a nuclear heating. Therefore, the volume is divided into three parts, V1, V2 and V3 in the analysis model. Q2 and Q3 are heat loads of moderators and heater, respectively. For the analysis, the following condition is assumed. When the heat load of beam changes, heated or non-heated volume part is appeared out of the moderator and it develops to downstream with the hydrogen flow velocity. The boundary between the heated part and non-heated part exists. In other words, the diffusion of boundary is neglected. This boundary point moves by the flow velocity, e.g. 0.162 Kg/s and the output temperature of helium–hydrogen heat exchanger is constant. The nuclear heating, Q2 is set to be 3.75 kW for 1 MW proton beam. In the numerical analysis, physical properties are calculated by GASPAK library, Cryodata Inc, in each time step, e.g. 0.01 s. The enthalpy of the hydrogen in each lattice is decided according to the heat load. The analysis code search for suitable pressure with which the conservation of mass agrees at each time step. However, the pressure condition affects the volume of the accumulator. Therefore the pressure condition and the volume of the accumulator are synchronized and calculated.
4. Results and discussion The proton beam injection or shutdown gives the maximum change of heat load to the hydrogen loop. Fig. 3 indicates analysis
1.52
Hydrogen Pressure (MPa)
maintained and phase change does not occur. Therefore the hydrogen system is controlled with an operational pressure of 1.5 MPa that is higher than the hydrogen critical pressure of 1.29 MPa. A performance of moderator neutronics depends on the hydrogen density, at which the density should be kept constant. Therefore, the hydrogen system is controlled to keep constant pressure and temperature. Especially, the hybrid control system has an important role when the beam power changes due to beam-on and beam-off.
1.51
1.5
1.49 Pressure [Acc+Heater (3750W)] (MPa) Pressure [Acc+Heater (4800W)] (MPa)
1.48 0
20
40 60 Time (sec)
80
100
Fig. 3. The hydrogen pressure behavior for proton beam injection.
results of pressure fluctuation when 1 MW proton beam injects at 0 s. The analysis was carried out for two cases; one is that the hydrogen pressure is kept constant to the initial pressure after the beam injection. The dashed curve shows the result of pressure behavior for that case. This case is necessary to adjust the heater power to be more than 3.75 kW which is the heat load of moderators, because the heater cannot compensate heat in V2 in Fig. 2. In other words, to maintain the hydrogen pressure constant or low, the required heater power should exceed the heat load by beam. In this case, the hydrogen temperature into heat exchanger has to be changed. Therefore, the cryogenic system requires the cooling power control to maintain the same inlet temperature as the moderators. On the other hand, the solid curve shows the hydrogen pressure behavior with the heater power keeping at 3.75 kW. There is no large difference in pressure change in the curves. The pressure fluctuation is within 70.01 MPa, corresponding to less than 1% for 1.5 MPa operation pressure. From the view point of pressure stability, the second case has an advantage to saturate the hydrogen pressure quickly. Furthermore,
ARTICLE IN PRESS S. Hasegawa et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 81–83
80
Table 1 Effect on cold neutron beam performance. Peak intensity
Hydrogen density (kg/m3)
83
0–20 meV (%)
75 DM PM CM
3.6 4.5 0.9
FWHM (full width at half maximum) 50–1000 meV (%) 2.4 2.4 0.6
0–20 meV (%)
50–1000 meV (%)
0.0 0.0 0.6
1.5 1.5 1.5
70 stable cold neutron beams, it is important to maintain the hydrogen temperature and the flow velocity constant.
5. Summary
65 0
0.2
0.4 0.6 Beam power (W)
0.8
1
Fig. 4. Relation between beam power and density of moderator.
it is necessary to control the heater only, not to control the cooling power of helium refrigerator. Therefore the second case is selected as an operation scenario. The change of the neutron beam when the second control is evaluated. In an incompressible fluid, the influence of density change due to pressure fluctuation can be almost ignored, but that of the temperature change should be considered. A temperature rising in the moderators is dominated by hydrogen flow velocity. The flow velocity of 0.162 kg/s, which is the rated flow velocity, brings about the hydrogen temperature rise of about 3 K with the rated heat load condition, corresponding to the hydrogen density degrease of about 4%. A relation between the density change in the moderators and a proton beam is described in Fig. 4, where the hydrogen flow is set to be the rated value. This result shows that the density decreases about 3 kg/m3 at the 1 MW proton beam power. Table 1 lists for the effects of neutron beam profile when such density decreases are occurred. DM, PM and CM indicate three type of moderator, decoupled, poisoned and coupled moderator, respectively. For DM and PM, the beam peak intensity decreases around 4% at the low energy region, that is almost the same as the density decreasing rate [4]. On the other hand, the spectrum width of high energy region increases little bit. Accordingly the hydrogen flow velocity affects not only the moderator temperature but also the neutron beam performance. And the density change is almost proportional to the beam peak intensity change of DM and PM. Therefore, in order to supply the
For the cryogenic hydrogen circulating loop of the JSNS, the stable operation of hydrogen loop and the effect for the cold neutron beam performance are estimated during heat load change by using the high-power pulsed proton beam with analytical code. The heater output for operating hydrogen loop stably made clear analytically that it is the same as the heat load by the pulsed 1 MW proton beam. The hydrogen density, which has a deep influence on the neutron performance, can be kept almost constant with optimal hybrid control independently of beam condition. It became clear that it is important to maintain the flow velocity for homogeneous neutron beam. References [1] S. Nagamiya, J. Nucl. Sci. Technol. (Suppl. 1) (2000) 40. [2] Y. Ikeda, 1 MW Pulse Spallation Neutron Source (JSNS) under the High Intensity Proton Accelerator Project, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 13. [3] T. Kato, et al., Cryogenic System Design for Cryogenic Hydrogen Moderator of the Spallation Neutron Source in J-PARC, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 645. [4] Materials and Life Science Experimental Facility Construction Team, J-PARC Technical Design Report, Material and Life Science Experimental Facility, JAERITech 2004-001. [5] T. Kai, et al., Neutronic Study on Coupled Hydrogen Moderator for Japanese Spallation Neutron Source, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 657. [6] M. Harada, et al., Optimization of poisoned and unpoisoned decoupled moderators in JSNS, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 697. [7] M. Teshigawara, et al., Development of Cd poisoned moderator, ICANS-XVI, Du¨sseldorf-Neuss, May 12–15, 2003, p. 601. [8] S. Hasegawa, et al., Estimation of pressure change based on hybrid control system in JSNS hydrogen loop, ICANS-XVIII, Dongguan, April 25–29, 2007.