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Preliminary optimization of proton energy and target for Lead-Bismuth Eutectic target of a demonstration ADS
Progress in Nuclear Energy 71 (2014) 117e121
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Preliminary optimization of proton energy and target for LeadBismuth Eutectic target of a demonstration ADS Zijia Zhao a, *, Zhong Chen b, Hongli Chen a a b
University of Science and Technology of China, Hefei, Anhui 230027, China Southwest Science and Technology University, Mianyang, Sichuan 621000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 June 2013 Received in revised form 21 November 2013 Accepted 23 November 2013
Accelerator Driven subcritical System (ADS) is recognized as an efficient nuclear waste transmutation device. Recently Chinese Academy of Sciences has made a plan to research and develop ADS. As one of the main options, Lead-Bismuth Eutectic (LBE) is chosen to be both of target material and coolant in the target system. In the present work, the generated neutrons of the liquid lead-Bismuth spallation target were studied for an ADS demonstration facility of 1000 MW thermal power. We have investigated the variation of neutron yield, neutron leadage and spatial energy deposition in terms of the different incident proton energy, target height, buffer height and buffer thickness. The information of generated neutrons, including energy spectrum, neutron angle distribution and neutron spatial distribution of the most effective scheme was studied. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Spallation target Neutron performance ADS
1. Introduction Elimination of nuclear waste is currently a pressing issue in view of the global energy crisis and increasing radiotoxic waste inventories. Problems of transmutation long life nuclear waste have motivated many scientists since 1960s. In 1990s the international community began to consider the concept of ADS as an effective device for elimination of trans-uranic (TRU), especially minor actinides (MA) (Rubbia et al., 1995; Wu et al., September 9e13, 2012, Jin et al., September 9e13, 2012, Chen et al., September 9e13, 2012, Maes, 2006; Cinotti et al., 2004). Spallation target provides primary neutrons that are multiplied by the surrounding blanket, in which the transmutation reaction takes place. These primary neutrons are produced by spallation reactions of heavy target nuclei bombarded by high energy protons that generated by accelerator. LBE becomes a promising materials for the spallation target and coolant due to the following properties: 1) heavy materials with enough spallation neutrons. 2) Low neutron capture cross sections. 3) Stable chemical activity. LBE is also chosen as liquid target of MYRRHA (Maes, 2006) and XADS (Cinotti et al., 2004) program. China Lead Alloy Cooled Reactor Project for ADS Transmutation System has been launched since 2011 by Chinese Academy of Sciences. The demonstration reactor,
* Corresponding author. E-mail address: [email protected] (Z. Zhao). 0149-1970/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pnucene.2013.11.014
which is the concept of lead-alloy cooled accelerator driven subcritical reactor, also have chosen LBE as the spallation target. Spallation reaction theory in a heavy material target has been studied in nuclear physics by cascade model (Barry, 1973), preequilibrium model (Gadioli et al., 1976), and evaporation model (Weisskopf, 1937). To gain as many neutrons as possible, three ways are to be considered naturally: 1) increasing proton current, which results in more protons injected into spallation target, so the neutron yield was increased linearly, 2) increasing proton energy, which results in more collision happens in the target nucleus, and more nucleons escapes, 3) improving target geometry, a big target could be good for neutron yield, but could be bad for the neutron energy, since the main objective of ADS is transmutation of TRU. Generally speaking, capture to fission ratio of Np237, Am241, Am243, and Cm244 turns smaller than 1 when neutron energy reaches 1 MeV (http://t2.lanl.gov/data/ndviewer.html). And building accelerator with strong proton current and high energy, which means high power, at the same time is always a difficult problem, it is necessary to find an economic scheme of the target which provides necessary neutrons with lower proton power. A simplified geometry model was built in part 2, parameters including radius of proton beam and current were decided by blanket power and material properties. The neutron yield, neutron leaked and spatial energy deposition of the target were studied to find the most economical scheme was performed in part 3. The neutronic performance of the scheme including neutron spectrum and spatial neutron distribution was studied in part 4. A brief summary was given at last.
Z. Zhao et al. / Progress in Nuclear Energy 71 (2014) 117e121
2. Geometry model
P ¼
N
y
Efission
NzNs
keff 1 keff
where P is the thermal power of the blanket, N is the total number of neutron in the blanket, Efission and y is the average energy and neutron released from each fission reaction, which was taken as 200 MeV and 2.5, Ns is the neutron source, which should be around 4 1018 n/s for P ¼ 1000 MW and keff ¼ 0.95, this was in consistent with (Rubbia et al., 2001)(1017 w 1018 n/s). The option of the proton energy was at the range between several hundreds of MeV to several GeV, about ten to several dozens of neutrons yield by each proton (Gohar, 2002). Then the request proton current should be no greater than 60 mA. Current density is decided by energy deposition peak and structure material, both depends on the proton energy. However, proton current density of up to 50 mA/cm2 is believed possible for virgin materials in the energy range (Rubbia et al., 2001). Combining the current and the density, radius of the
140 4GeV 2.5GeV 1GeV
120 Neutron Yield(n/p)
Fig. 1 is the simplified physical model used to study the neutron behavior in reactor. The spallation reaction takes place in the region of Spallation Target and Buffer, both of these two regions are filled with LBE. To avoid unnecessary influence, material of regions out of Buffer was set to void in the Monte Carlo model. The proton energy, the height of Spallation Target, the height and thickness of buffer were set as variable respective to evaluate the influence of each descriptor to the neutron yield, neutron leakage and energy distribution. Since the structure material of the Spallation Target and Buffer is mainly iron, and little proportion compared with LBE, its influence is not important. We just neglect for convenience. To decide the radius of the Spallation Target, the current should be determined first, which is closely correlated with the thermal power of the blanket, the effective neutron multiplication factor (keff) and the neutron yield by each proton. The intention of this work is to make an optimization of a spallation target for a demonstration reactor with its power at the range of about 1000 MW, and there are some ADS programs with approach power like ATDS in Japan (Takizuka et al., 1998) (820 MW) and ATW in America (Beller et al., 2001) (840 MW), whose keff are 0.97 and 0.95. However, from the perspectives of shut-down and ‘protected’ transient (Schikorr, 2001), keff ¼ 0.95 was preferred. A rough estimate of the neutron source intensity could be made based on thermal power and keff:
160
100 80 60 40 60
80
100
120
140
Height of Spallation Target(cm) Fig. 2. Neutron yield transforms as a function of Spallation Target height from 60 cm to 150 cm for 1 GeV, 2.5 GeV and 4 GeV protons.
proton beam was decided to be 20 cm to ensure each proton energy was under consideration. It was noted that this was a conservative scheme for the current density, however, it is beneficial for cooling. For possible shift (guessed to be 2 cm (Cho et al., 2004)) of the beam spot and necessary distance between beam tube and the beam (0.5 cm (Cho et al., 2004)), the tube radius was chosen to be 22.5 cm. Energy deposition will be studied later in this work to investigate whether the value is under limitation. Fluka (Ferrari et al., 2005) is used to perform the neutron physic study, which is a particle simulation software using Monte Carlo method. Particles that Fluka simulates including neutron, proton, photon, electron, meson and so on. Energy of particles Fluka simulates lays between less than 1eV and several TeV. Physics models were used to describe the nuclear reactions with high energy, and libraries of ENDF/B, JEF, JENDL etc were used to describe reactions of low energy neutrons. 3. Optimization for neutron performance The height of the Spallation Target, height and thickness of the buffer, proton energy is to be optimized in this part by investigate the neutron yield and leakage.
100
110
120
130
140
150
160
150 140
Neutron increased proportion for 1000MeV Neutron increased proportion for 2500MeV Neutron increased proportion for 4000MeV
Neutron yield(n/p)
130 120
0.050 0.045 0.040 0.035
110
0.030
100
0.025
90
0.020
80
0.015
70
Neutron yield for 1000MeV Neutron yield for 2500MeV Neutron yield for 4000MeV
60 50
0.010 0.005 0.000
40
-0.005
30 100
Fig. 1. Neutronics analysis model.
160
Proportion of neutron increased
118
110
120 130 140 Height of buffer(cm)
150
160
Fig. 3. Neutron yield transforms as a function of Buffer height from 100 cm to 160 cm for 1 GeV, 2.5 GeV and 4 GeV protons.
Z. Zhao et al. / Progress in Nuclear Energy 71 (2014) 117e121
160
120 100 5cm 10cm 15cm 20cm 25cm 30cm 35cm 40cm
80 60 40 20 0 0
Average Energy of Neutron Leaked
4.5
140 Neutron Yield(n/p)
119
0.5GeV 1.0GeV 1.5GeV 2.0GeV 2.5GeV 3.0GeV 3.5GeV 4.0GeV
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
500 1000 1500 2000 2500 3000 3500 4000 4500
0
5
10
Proton energy(MeV)
15
20
25
30
35
40
Fig. 4. Neutron yield as a function of proton energy and buffer thickness.
3.1. Height of Spallation Target and buffer
Fig. 6. Average energy of neutron leaked.
3.2. Proton energy and buffer thickness
The relationship between the height of Spallation Target and neutron yield under various proton with energy 1 GeV, 2.5 GeV and 4 GeV was shown in Fig. 2. High energy and high target brings more neutrons. For the 1 GeV proton, when target height increased from 60 cm to 100 cm, the neutron yield increased near 5%, when the target height increased from 100 cm to 150 cm, the neutron yield increased less than 1%. In the same target height regions, for the 4 GeV proton, the neutron yield increased 15% and 3%, for 2.5 GeV proton, the neutron yield increased 12% and 1%. These results indicated that the neutron yield reaches saturation when the target height near 100 cm, so 100 cm was chosen as the height of spallation target in this section. The neutron yield and increased proportion was analyzed as a function of the buffer height for 1 GeV, 2.5 GeV and 4 GeV protons in Fig. 3. The proportion of neutron increased was less than 3.5% for each proton energy when buffer got higher than the Spallation Target (100 cm), so a buffer of 100 cm in height was chosen at last.
The impact of the proton energy to the neutron yield with difference buffer thickness was shown in Fig. 4. Fig. 4 demonstrates that the proton energy and the buffer thickness both play favorable roles in the neutron yield, and the results are just consistent with the analysis in part 1. Also the economical efficiency of each scheme in the Fig. 4 was defined as the average Energy Consumption (Ec) for one generated neutron,
EC ¼ EP =NY where EP was incident proton energy, and NY was neutron yield by each proton. The each scheme’s economical efficiency was shown in the Fig. 5. With the proton energy increased from 0.5 GeV to 4 GeV, EC reduced by 34% (5 cm in buffer thickness) to 37% (40 cm in buffer thickness) respectively. The tendency is particularly obvious when proton energy increase from 0.5 GeV to 1.5 GeV. However, when the proton energy reaches about 1.5 GeV, EC can hardly be affected by the proton energy, but by thickness of buffer. EC decreased by 16% when buffer thickness increased from 5 cm to
42 40 38 36 34 32 30 28 26 24
120 3
5cm 10cm 15cm 20cm 25cm 30cm 35cm 40cm
Energy depostion W/cm
Average Energy Consumption(MeV/n)
46 44
0.5GeV 1.0GeV 1.5GeV 2.0GeV 2.5GeV 3.0GeV 3.5GeV 4.0GeV
100 80 60 40 20 0
0
500
45
Buffer Thickness(cm)
1000 1500 2000 2500 3000 3500 4000 4500 Proton Energy(MeV)
0
20
40
60
80
Depth in LBE (cm) Fig. 5. Average energy consumption of neutrons as a function of proton energy and buffer thickness.
Fig. 7. Bragg peak in LBE for 500 MeVe4000 MeV protons.
100
120
Z. Zhao et al. / Progress in Nuclear Energy 71 (2014) 117e121
1E17 Neutron spectrum
Neutron number
1E16 1E15 1E14 1E13 1E12
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
Fig. 10. Spatial distribution of neutrons flux (n/cm2, normalized to one proton incident).
Neutron Energy(GeV) Fig. 8. Neutron spectrum in the target.
40 cm. The parameters of scheme with the minimum EC were 40 cm buffer thickness and 2000 MeV proton energy (25 MeV/n). But a target with its radius 62.5 cm (40 cm for buffer and 22.5 for target) seems too large, which may reduce the efficiency of the blanket. In order to make balance, 5% sacrifice of the energy consumption could reduce the target radius to 47.5 cm, while 10% sacrifice could reduce the target radius to 37.5 cm. Energy of neutron leaked should also be checked to see if it was higher than 1 MeV for MA transmutation performance. Average energy of neutron leaked from target of each scheme was shown in Fig. 6. The result showed the average neutron energy decreased obviously when the buffer thickness increased, as analyzed in part 2. When buffer reached about 40 cm (35 cm for 500 MeV and 1000 MeV proton energy), average energy of neutron leaked was lower than 1 MeV, so a thicker buffer is not suggested. For the 15 cm buffer thickness, the average neutron energy was between 1.75 MeV and 2.25 MeV for different energy of protons, which met the transmutation request well. Fig. 7 showed the energy deposition of proton in a target with the proton energy range from 0.5 GeV to 4 GeV (normalized to 4 1018 neutron source). The Bragg peak appear for proton energy at 0.5 GeV (about 110 W/cm3) at 20 cm below the LBE surface, which was 2e4 times higher than others (50e30 W/cm3). There
was no obvious Bragg peak when the proton energy was higher than 1000 MeV. Energy deposition turns more gently as proton energy grows higher. So proton with energy 1000 MeV or more was better for power flattening. As a brief summary, scheme with proton energy 1500 MeVe 2000 MeV and buffer thickness near 15 cm satisfied the requirement of the economy, MA transmutation and power flattening. And the energy for the proton was well in the range of linear accelerator, circular accelerator and FFAG (Van Tuyle et al., 2001). 4. Neutron performance analysis for the chosen scheme To get detailed spallation neutron performance for the blanket design, neutron performance including neutron spectrum, neutron direction distribution and neutron spatial distribution of scheme with 1.5 GeV proton and 15 cm buffer thickness (11.9 mA in current and 17.9 MW in proton power to obtain 4 1018/s neutron source) were studied in this part. The results were placed in Figs. 8e11. It was found in Fig. 8 that the most probable energy lays at about 2 MeV, 76.7% of spallation neutron energy was higher than 1 MeV, 18.5% was higher than 10 MeV, and 3.5% was higher than 100 MeV, average neutron energy was 16.2 MeV. In Fig. 9, neutrons with their energy lower than 10 MeV was almost isotropic (which was the most important part in the spallation neutrons), with the energy
0.6 Neutron leaked(n/cm/p)
4.50E+016
Neutron number (n/3 degree)
4.00E+016 3.50E+016 lower than 1MeV 1MeV to 10MeV 10MeV to 100MeV 100MeV to 1500MeV
3.00E+016 2.50E+016 2.00E+016 1.50E+016
0.5 0.4 0.3 0.2 0.1
1.00E+016
0.0
5.00E+015
-100
0.00E+000 0
20
40
60
80
100
120
140
160
180
-50
0
50
100
Height(cm)
Polar angle (degree) Fig. 9. Angle distribution of neutrons yield.
Fig. 11. Neutron leaked distribution along the height (normalized to one proton incident), the horizontal axis was the same with that in Fig. 10.
Z. Zhao et al. / Progress in Nuclear Energy 71 (2014) 117e121
growing, neutrons were more likely to be at the same direction with incident proton. That due to both pre-equilibrium model and cascade model generated forward neutrons. The neutron spatial distribution was shown in Figs. 10 and 11. There was a higher peak under the LBE surface (where the horizontal axis was 0 for Figs. 10 and 11) and a lower peak above. Special care should be taken for axial position and height design for the blanket in order to receive as many neutrons as possible. 5. Conclusion Spallation target was the heart of ADS, economy of the neutron affects the performance of transmutation of the entire system directly. In the present work, a numerical design was performed to get optimized parameters for a spallation target for a 1000 MW ADS. LBE was chosen as the target material. Radius of proton spot was decided by proton energy, material performance, power and keff of the blanket, which was decided to be around 20 cm. Proton energy, target height, buffer height and thickness were set variable see neutron yield, neutron leakage and energy deposition distribution. A scheme with proton energy around 1.5e2 GeV, buffer thickness about 15 cm, target and buffer height near 100 cm was preferred in this work. Detailed neutron performance, including neutron energy spectrum, neutron angle and spatial distribution of this scheme was performed to make source for the blanket.
Acknowledgments This work was supported by the Natural Science Foundation of China under grant No. 91026004 and the "Strategic Priority Research Program" of the Chinese Academy of Sciences under grant No. XDA0304000. The author appreciated helpful discussions with members in Institute of Nuclear Energy Safety Technology of Chinese Academy of Sciences.
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References Barry, G.W., 1973. Nuclear reactions at high energy. Phys. Rev. D 7 (5), 1441. Beller, D.E., Van Tuyle, G.J., Bennett, D., et al., 2001. The US accelerator transmutation of waste program. Nucl. Instrum. Meth. Phys. Res. Sec. A: Acce. Spectr. Detect. Assoc. Equip. 463 (3), 468e486. Chen, Hongli, Chen, Zhao, Zhou, Tao, Jin, Ming, Bai, Yunqing, Wang, Weihua, September 9e13, 2012. Preliminary Thermal-hydraulic Design and Analysis of China Lead Alloy Cooled Research Reactor (CLEAR-I). NUTHOS-9, Kaohsiung, Taiwan. Cho, C.H., Song, T.Y., Tak, N.I., 2004. Numerical design of a 20 MW leadebismuth spallation target for an accelerator-driven system. Nucl. Eng. Des. 229 (2), 317e 327. Cinotti, L., Giraud, B., Abderrahim, H.A., 2004. The experimental accelerator driven system (XADS) designs in the EURATOM 5th framework programme. J. Nucl. Mater. 335 (2), 148e155. Ferrari, A., Sala, P.R., Fassò, A., Ranft, J., CERN-2005-10, 2005. FLUKA: a Multi-particle Transport Code. INFN/TC_05/11, SLAC-R-773. Gadioli, E., Erba, E.G., Tagliaferri, G., 1976. Pre-equilibrium model evaluation of neutron spectra from proton-induced reactions. Phys. Rev. C 14 (2), 573. Gohar, Y., 2002. Target Buffer Assessment for Accelerator-driven Transmuters. Argonne National Lab., IL (US). http://t2.lanl.gov/data/ndviewer.html. Jin, Ming, Chen, Zhao, Chen, Hongli, et al., September 9e13, 2012. Natural Circulation Characteristics of China Lead Alloy Cooled Research Reactor CLEAR-I. NUTHOS-9, Kaohsiung, Taiwan. Maes, D., 2006. Mechanical design of the small-scale experimental ADS: MYRRHA. Energy Conv. Manage. 47 (17), 2710e2723. Rubbia, C., Rubio, J.A., Buono, S., Carminati, F., Fiétier, N., Galvez, J., Gelès, C., Kadi, Y., Klapisch, R., Mandrillon, P., Revol, J.P., Roche, Ch, 1995. Conceptual Design of a Fast Neutron Operated High Power Energy Amplifier. CERN/AT/9544 (ET). Rubbia, C., Aleixandre, J., Andriamonje, S., 2001. A European roadmap for developing accelerator driven systems (ADS) for nuclear waste incineration. ENEA Report, 88e8286. Schikorr, W.M., 2001. Assessments of the kinetic and dynamic transient behavior of sub-critical systems (ADS) in comparison to critical reactor systems. Nucl. Eng. Des. 210 (1), 95e123. Takizuka, T., Sasa, T., Tsujimoto, K., et al., 1998. Studies on Accelerator Driven Transmutation Systems[C]. Fifth International Information Exchange Meeting, p. 383. Van Tuyle, G., Hill, D., Beller, D., et al., 2001. A roadmap for developing ATW technology: system scenarios & integration. Progr. Nucl. Energy 38 (1), 3e23. Weisskopf, V., 1937. Statistics and nuclear reactions. Phys. Rev. 52 (4), 295. Wu, Y., Bai, Y., Wang, W., et al., September 9e13, 2012. Overview of China Lead Alloy cooled Reactor Development and ADS Program in China. NUTHOS-9, Kaohsiung, Taiwan.
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Preliminary optimization of proton energy and target for LeadBismuth Eutectic target of a demonstration ADS Zijia Zhao a, *, Zhong Chen b, Hongli Chen a a b
University of Science and Technology of China, Hefei, Anhui 230027, China Southwest Science and Technology University, Mianyang, Sichuan 621000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 June 2013 Received in revised form 21 November 2013 Accepted 23 November 2013
Accelerator Driven subcritical System (ADS) is recognized as an efficient nuclear waste transmutation device. Recently Chinese Academy of Sciences has made a plan to research and develop ADS. As one of the main options, Lead-Bismuth Eutectic (LBE) is chosen to be both of target material and coolant in the target system. In the present work, the generated neutrons of the liquid lead-Bismuth spallation target were studied for an ADS demonstration facility of 1000 MW thermal power. We have investigated the variation of neutron yield, neutron leadage and spatial energy deposition in terms of the different incident proton energy, target height, buffer height and buffer thickness. The information of generated neutrons, including energy spectrum, neutron angle distribution and neutron spatial distribution of the most effective scheme was studied. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Spallation target Neutron performance ADS
1. Introduction Elimination of nuclear waste is currently a pressing issue in view of the global energy crisis and increasing radiotoxic waste inventories. Problems of transmutation long life nuclear waste have motivated many scientists since 1960s. In 1990s the international community began to consider the concept of ADS as an effective device for elimination of trans-uranic (TRU), especially minor actinides (MA) (Rubbia et al., 1995; Wu et al., September 9e13, 2012, Jin et al., September 9e13, 2012, Chen et al., September 9e13, 2012, Maes, 2006; Cinotti et al., 2004). Spallation target provides primary neutrons that are multiplied by the surrounding blanket, in which the transmutation reaction takes place. These primary neutrons are produced by spallation reactions of heavy target nuclei bombarded by high energy protons that generated by accelerator. LBE becomes a promising materials for the spallation target and coolant due to the following properties: 1) heavy materials with enough spallation neutrons. 2) Low neutron capture cross sections. 3) Stable chemical activity. LBE is also chosen as liquid target of MYRRHA (Maes, 2006) and XADS (Cinotti et al., 2004) program. China Lead Alloy Cooled Reactor Project for ADS Transmutation System has been launched since 2011 by Chinese Academy of Sciences. The demonstration reactor,
* Corresponding author. E-mail address: [email protected] (Z. Zhao). 0149-1970/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pnucene.2013.11.014
which is the concept of lead-alloy cooled accelerator driven subcritical reactor, also have chosen LBE as the spallation target. Spallation reaction theory in a heavy material target has been studied in nuclear physics by cascade model (Barry, 1973), preequilibrium model (Gadioli et al., 1976), and evaporation model (Weisskopf, 1937). To gain as many neutrons as possible, three ways are to be considered naturally: 1) increasing proton current, which results in more protons injected into spallation target, so the neutron yield was increased linearly, 2) increasing proton energy, which results in more collision happens in the target nucleus, and more nucleons escapes, 3) improving target geometry, a big target could be good for neutron yield, but could be bad for the neutron energy, since the main objective of ADS is transmutation of TRU. Generally speaking, capture to fission ratio of Np237, Am241, Am243, and Cm244 turns smaller than 1 when neutron energy reaches 1 MeV (http://t2.lanl.gov/data/ndviewer.html). And building accelerator with strong proton current and high energy, which means high power, at the same time is always a difficult problem, it is necessary to find an economic scheme of the target which provides necessary neutrons with lower proton power. A simplified geometry model was built in part 2, parameters including radius of proton beam and current were decided by blanket power and material properties. The neutron yield, neutron leaked and spatial energy deposition of the target were studied to find the most economical scheme was performed in part 3. The neutronic performance of the scheme including neutron spectrum and spatial neutron distribution was studied in part 4. A brief summary was given at last.
Z. Zhao et al. / Progress in Nuclear Energy 71 (2014) 117e121
2. Geometry model
P ¼
N
y
Efission
NzNs
keff 1 keff
where P is the thermal power of the blanket, N is the total number of neutron in the blanket, Efission and y is the average energy and neutron released from each fission reaction, which was taken as 200 MeV and 2.5, Ns is the neutron source, which should be around 4 1018 n/s for P ¼ 1000 MW and keff ¼ 0.95, this was in consistent with (Rubbia et al., 2001)(1017 w 1018 n/s). The option of the proton energy was at the range between several hundreds of MeV to several GeV, about ten to several dozens of neutrons yield by each proton (Gohar, 2002). Then the request proton current should be no greater than 60 mA. Current density is decided by energy deposition peak and structure material, both depends on the proton energy. However, proton current density of up to 50 mA/cm2 is believed possible for virgin materials in the energy range (Rubbia et al., 2001). Combining the current and the density, radius of the
140 4GeV 2.5GeV 1GeV
120 Neutron Yield(n/p)
Fig. 1 is the simplified physical model used to study the neutron behavior in reactor. The spallation reaction takes place in the region of Spallation Target and Buffer, both of these two regions are filled with LBE. To avoid unnecessary influence, material of regions out of Buffer was set to void in the Monte Carlo model. The proton energy, the height of Spallation Target, the height and thickness of buffer were set as variable respective to evaluate the influence of each descriptor to the neutron yield, neutron leakage and energy distribution. Since the structure material of the Spallation Target and Buffer is mainly iron, and little proportion compared with LBE, its influence is not important. We just neglect for convenience. To decide the radius of the Spallation Target, the current should be determined first, which is closely correlated with the thermal power of the blanket, the effective neutron multiplication factor (keff) and the neutron yield by each proton. The intention of this work is to make an optimization of a spallation target for a demonstration reactor with its power at the range of about 1000 MW, and there are some ADS programs with approach power like ATDS in Japan (Takizuka et al., 1998) (820 MW) and ATW in America (Beller et al., 2001) (840 MW), whose keff are 0.97 and 0.95. However, from the perspectives of shut-down and ‘protected’ transient (Schikorr, 2001), keff ¼ 0.95 was preferred. A rough estimate of the neutron source intensity could be made based on thermal power and keff:
160
100 80 60 40 60
80
100
120
140
Height of Spallation Target(cm) Fig. 2. Neutron yield transforms as a function of Spallation Target height from 60 cm to 150 cm for 1 GeV, 2.5 GeV and 4 GeV protons.
proton beam was decided to be 20 cm to ensure each proton energy was under consideration. It was noted that this was a conservative scheme for the current density, however, it is beneficial for cooling. For possible shift (guessed to be 2 cm (Cho et al., 2004)) of the beam spot and necessary distance between beam tube and the beam (0.5 cm (Cho et al., 2004)), the tube radius was chosen to be 22.5 cm. Energy deposition will be studied later in this work to investigate whether the value is under limitation. Fluka (Ferrari et al., 2005) is used to perform the neutron physic study, which is a particle simulation software using Monte Carlo method. Particles that Fluka simulates including neutron, proton, photon, electron, meson and so on. Energy of particles Fluka simulates lays between less than 1eV and several TeV. Physics models were used to describe the nuclear reactions with high energy, and libraries of ENDF/B, JEF, JENDL etc were used to describe reactions of low energy neutrons. 3. Optimization for neutron performance The height of the Spallation Target, height and thickness of the buffer, proton energy is to be optimized in this part by investigate the neutron yield and leakage.
100
110
120
130
140
150
160
150 140
Neutron increased proportion for 1000MeV Neutron increased proportion for 2500MeV Neutron increased proportion for 4000MeV
Neutron yield(n/p)
130 120
0.050 0.045 0.040 0.035
110
0.030
100
0.025
90
0.020
80
0.015
70
Neutron yield for 1000MeV Neutron yield for 2500MeV Neutron yield for 4000MeV
60 50
0.010 0.005 0.000
40
-0.005
30 100
Fig. 1. Neutronics analysis model.
160
Proportion of neutron increased
118
110
120 130 140 Height of buffer(cm)
150
160
Fig. 3. Neutron yield transforms as a function of Buffer height from 100 cm to 160 cm for 1 GeV, 2.5 GeV and 4 GeV protons.
Z. Zhao et al. / Progress in Nuclear Energy 71 (2014) 117e121
160
120 100 5cm 10cm 15cm 20cm 25cm 30cm 35cm 40cm
80 60 40 20 0 0
Average Energy of Neutron Leaked
4.5
140 Neutron Yield(n/p)
119
0.5GeV 1.0GeV 1.5GeV 2.0GeV 2.5GeV 3.0GeV 3.5GeV 4.0GeV
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
500 1000 1500 2000 2500 3000 3500 4000 4500
0
5
10
Proton energy(MeV)
15
20
25
30
35
40
Fig. 4. Neutron yield as a function of proton energy and buffer thickness.
3.1. Height of Spallation Target and buffer
Fig. 6. Average energy of neutron leaked.
3.2. Proton energy and buffer thickness
The relationship between the height of Spallation Target and neutron yield under various proton with energy 1 GeV, 2.5 GeV and 4 GeV was shown in Fig. 2. High energy and high target brings more neutrons. For the 1 GeV proton, when target height increased from 60 cm to 100 cm, the neutron yield increased near 5%, when the target height increased from 100 cm to 150 cm, the neutron yield increased less than 1%. In the same target height regions, for the 4 GeV proton, the neutron yield increased 15% and 3%, for 2.5 GeV proton, the neutron yield increased 12% and 1%. These results indicated that the neutron yield reaches saturation when the target height near 100 cm, so 100 cm was chosen as the height of spallation target in this section. The neutron yield and increased proportion was analyzed as a function of the buffer height for 1 GeV, 2.5 GeV and 4 GeV protons in Fig. 3. The proportion of neutron increased was less than 3.5% for each proton energy when buffer got higher than the Spallation Target (100 cm), so a buffer of 100 cm in height was chosen at last.
The impact of the proton energy to the neutron yield with difference buffer thickness was shown in Fig. 4. Fig. 4 demonstrates that the proton energy and the buffer thickness both play favorable roles in the neutron yield, and the results are just consistent with the analysis in part 1. Also the economical efficiency of each scheme in the Fig. 4 was defined as the average Energy Consumption (Ec) for one generated neutron,
EC ¼ EP =NY where EP was incident proton energy, and NY was neutron yield by each proton. The each scheme’s economical efficiency was shown in the Fig. 5. With the proton energy increased from 0.5 GeV to 4 GeV, EC reduced by 34% (5 cm in buffer thickness) to 37% (40 cm in buffer thickness) respectively. The tendency is particularly obvious when proton energy increase from 0.5 GeV to 1.5 GeV. However, when the proton energy reaches about 1.5 GeV, EC can hardly be affected by the proton energy, but by thickness of buffer. EC decreased by 16% when buffer thickness increased from 5 cm to
42 40 38 36 34 32 30 28 26 24
120 3
5cm 10cm 15cm 20cm 25cm 30cm 35cm 40cm
Energy depostion W/cm
Average Energy Consumption(MeV/n)
46 44
0.5GeV 1.0GeV 1.5GeV 2.0GeV 2.5GeV 3.0GeV 3.5GeV 4.0GeV
100 80 60 40 20 0
0
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1000 1500 2000 2500 3000 3500 4000 4500 Proton Energy(MeV)
0
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Depth in LBE (cm) Fig. 5. Average energy consumption of neutrons as a function of proton energy and buffer thickness.
Fig. 7. Bragg peak in LBE for 500 MeVe4000 MeV protons.
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Z. Zhao et al. / Progress in Nuclear Energy 71 (2014) 117e121
1E17 Neutron spectrum
Neutron number
1E16 1E15 1E14 1E13 1E12
1E-8
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1
Fig. 10. Spatial distribution of neutrons flux (n/cm2, normalized to one proton incident).
Neutron Energy(GeV) Fig. 8. Neutron spectrum in the target.
40 cm. The parameters of scheme with the minimum EC were 40 cm buffer thickness and 2000 MeV proton energy (25 MeV/n). But a target with its radius 62.5 cm (40 cm for buffer and 22.5 for target) seems too large, which may reduce the efficiency of the blanket. In order to make balance, 5% sacrifice of the energy consumption could reduce the target radius to 47.5 cm, while 10% sacrifice could reduce the target radius to 37.5 cm. Energy of neutron leaked should also be checked to see if it was higher than 1 MeV for MA transmutation performance. Average energy of neutron leaked from target of each scheme was shown in Fig. 6. The result showed the average neutron energy decreased obviously when the buffer thickness increased, as analyzed in part 2. When buffer reached about 40 cm (35 cm for 500 MeV and 1000 MeV proton energy), average energy of neutron leaked was lower than 1 MeV, so a thicker buffer is not suggested. For the 15 cm buffer thickness, the average neutron energy was between 1.75 MeV and 2.25 MeV for different energy of protons, which met the transmutation request well. Fig. 7 showed the energy deposition of proton in a target with the proton energy range from 0.5 GeV to 4 GeV (normalized to 4 1018 neutron source). The Bragg peak appear for proton energy at 0.5 GeV (about 110 W/cm3) at 20 cm below the LBE surface, which was 2e4 times higher than others (50e30 W/cm3). There
was no obvious Bragg peak when the proton energy was higher than 1000 MeV. Energy deposition turns more gently as proton energy grows higher. So proton with energy 1000 MeV or more was better for power flattening. As a brief summary, scheme with proton energy 1500 MeVe 2000 MeV and buffer thickness near 15 cm satisfied the requirement of the economy, MA transmutation and power flattening. And the energy for the proton was well in the range of linear accelerator, circular accelerator and FFAG (Van Tuyle et al., 2001). 4. Neutron performance analysis for the chosen scheme To get detailed spallation neutron performance for the blanket design, neutron performance including neutron spectrum, neutron direction distribution and neutron spatial distribution of scheme with 1.5 GeV proton and 15 cm buffer thickness (11.9 mA in current and 17.9 MW in proton power to obtain 4 1018/s neutron source) were studied in this part. The results were placed in Figs. 8e11. It was found in Fig. 8 that the most probable energy lays at about 2 MeV, 76.7% of spallation neutron energy was higher than 1 MeV, 18.5% was higher than 10 MeV, and 3.5% was higher than 100 MeV, average neutron energy was 16.2 MeV. In Fig. 9, neutrons with their energy lower than 10 MeV was almost isotropic (which was the most important part in the spallation neutrons), with the energy
0.6 Neutron leaked(n/cm/p)
4.50E+016
Neutron number (n/3 degree)
4.00E+016 3.50E+016 lower than 1MeV 1MeV to 10MeV 10MeV to 100MeV 100MeV to 1500MeV
3.00E+016 2.50E+016 2.00E+016 1.50E+016
0.5 0.4 0.3 0.2 0.1
1.00E+016
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5.00E+015
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0.00E+000 0
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100
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160
180
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Height(cm)
Polar angle (degree) Fig. 9. Angle distribution of neutrons yield.
Fig. 11. Neutron leaked distribution along the height (normalized to one proton incident), the horizontal axis was the same with that in Fig. 10.
Z. Zhao et al. / Progress in Nuclear Energy 71 (2014) 117e121
growing, neutrons were more likely to be at the same direction with incident proton. That due to both pre-equilibrium model and cascade model generated forward neutrons. The neutron spatial distribution was shown in Figs. 10 and 11. There was a higher peak under the LBE surface (where the horizontal axis was 0 for Figs. 10 and 11) and a lower peak above. Special care should be taken for axial position and height design for the blanket in order to receive as many neutrons as possible. 5. Conclusion Spallation target was the heart of ADS, economy of the neutron affects the performance of transmutation of the entire system directly. In the present work, a numerical design was performed to get optimized parameters for a spallation target for a 1000 MW ADS. LBE was chosen as the target material. Radius of proton spot was decided by proton energy, material performance, power and keff of the blanket, which was decided to be around 20 cm. Proton energy, target height, buffer height and thickness were set variable see neutron yield, neutron leakage and energy deposition distribution. A scheme with proton energy around 1.5e2 GeV, buffer thickness about 15 cm, target and buffer height near 100 cm was preferred in this work. Detailed neutron performance, including neutron energy spectrum, neutron angle and spatial distribution of this scheme was performed to make source for the blanket.
Acknowledgments This work was supported by the Natural Science Foundation of China under grant No. 91026004 and the "Strategic Priority Research Program" of the Chinese Academy of Sciences under grant No. XDA0304000. The author appreciated helpful discussions with members in Institute of Nuclear Energy Safety Technology of Chinese Academy of Sciences.
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