- Email: [email protected]
First thermal-hydraulic analysis of a CO2 cooled pebble bed blanket for the EU DEMO
Fusion Engineering and Design 146 (2019) 2218–2221
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
First thermal-hydraulic analysis of a CO2 cooled pebble bed blanket for the EU DEMO
T
⁎
Shuai Wanga, Francisco A. Hernándezb, Guangming Zhoub, , Hongli Chena a b
University of Science and Technology of China (USTC), School of Physical Sciences, Hefei, 230026, Anhui, China Karlsruhe Institute of Technology (KIT), Institute for Neutron Physics and Reactor Technology (INR), 76344, Eggenstein-Leopoldshafen, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: DEMO CO2-cooled pebble bed blanket Thermal-hydraulic analysis
Over the years, the Helium Cooled Pebble Bed (HCPB) Breeding Blanket (BB) has been intensively studied for the EU DEMO. However, several feasibility issues remain open for a HCPB-type DEMO reactor. Some of these issues are linked with the use of He as coolant and are related to the large size of the Primary Heat Transfer System pipework, the resulting large coolant inventory and therefore large expansion volumes required in an ex-vessel loss of coolant accident, the limited operational experience with relevant size He-turbomachinery, the large circulating power and possible considerable pipe leakage, among others. Due to the larger density of CO2, the use of this gas as primary coolant for DEMO can lead to key advantageous features, mitigating most of the issues present for He-cooling and resulting in a higher net efficiency than that of HCPB, as reported in a previous study. Therefore, a CO2-Cooled Pebble Bed (CCPB) has been proposed as an alternative coolant to He for the EU DEMO. After identifying that CO2 have a negligible influence on the neutronic performance, making the CCPB’s TBR almost equal to the HCPB’s one (TBR ≈ 1.15), a first set of thermal-hydraulic analyses with the commercial code of ANSYS CFX are reported here. The analyses are based on the newly proposed design of breeding zone (BZ) in the enhanced HCPB fuel-breeder pin concept for the EU DEMO. Despite the lower heat transfer capability of CO2 compared to He, the fuel-breeder pin design BZ improves the thermo-hydraulic performance to a point that this coolant is able to meet the materials’ temperature requirements. The results show that the CCPB can satisfy the basic thermal hydraulic requirements and that CO2 is also a realistic option as primary coolant for gas-cooled fusion reactors, with the key advantage of the existing > 60 years operational experience in fission industry with this coolant.
1. Introduction The EU Demonstration Fusion Power Plant (DEMO) is considered to be the last step before building a commercial fusion power plant. Among the main objectives of EU DEMO are: (i) demonstrating electricity production of several hundred MW; (ii) achieving tritium selfsufficiency [1,2]. The breeding blanket (BB) plays an important role in achieving the two main objectives. Over the past years, extensive studies for the Helium Cooled Pebble Bed (HCPB) BB has been performed [3–11]. However, the very low density of He easily leads to high circulating power with increasing pressure drops (Δp), thus affecting the net plant efficiency. The current EU DEMO development strategy of prioritizing the use of existing/ mature technologies is a major constraint for the He-turbomachinery [2]. As He-turbomachinery in fission industry is currently proven up to 6 MW with limited experience, introducing a strict limit on the plant
⁎
circulating power. Moreover, chronic releases due to systematic He leakages, which are typical for He-cooled reactors and the large coolant inventory, resulting in safety concerns in the event of a Loss of Coolant Accident (LOCA), are also significant drawbacks on the use of He as a coolant for a BB in EU DEMO. On the other side, CO2 has been used as primary coolant since the 50′s in the nuclear fission industry for gas-cooled reactors (MAGNOX, UNGG and AGR) [12] and it is drawing increasing attention as a secondary coolant to replace the conventional Rankine cycle in power plants Power Conversion System (PCS) [13,14]. Despite of the lower specific heat capacity and thermal conductivity of CO2 (about 22% and 17% of those of He, respectively), its larger density (11 times larger molecular weight of CO2 than that of He) can lead to an advantageous heat dissipation rate per unit of circulating power. These features, together with its relative transparency to neutrons, the more compact and less prone to leakage pipework, the large industrial operating
Corresponding author. E-mail addresses: [email protected], [email protected] (G. Zhou).
https://doi.org/10.1016/j.fusengdes.2019.03.156 Received 5 October 2018; Received in revised form 11 March 2019; Accepted 22 March 2019 Available online 28 March 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 146 (2019) 2218–2221
S. Wang, et al.
Fig. 1. CCPB BB CAD design.
Fig. 2. Temperature distribution of typical CCPB unit slice (with graphite rods). Fig. 3. Velocity of FW and BZ coolant of typical CCPB unit slice.
experience with this coolant, its abundance and low price makes CO2 an attractive option and a practical alternative to He as primary coolant for DEMO. Thanks to the above-mentioned advantages, it has been shown that a CO2 cooled EU DEMO BB has a higher efficiency than that of Hecooled EU DEMO BB [15]. Using CO2 as a coolant for EU DEMO BB is therefore worth investigating. In this paper, a CO2 Cooled Pebble Bed (CCPB) BB based on the fuelbreeder pin configuration is presented. This configuration has been originally developed for the enhanced EU DEMO HCPB BB [16]. First thermal-hydraulic analyses have been performed, showing the potential of CO2 as a coolant for the BB.
2. The CCPB BB design 2.1. Material selection With the aim to develop a near-term alternative BB for EU DEMO, the materials that have mature industrial experience or worldwide development efforts are therefore preferred. Reduced Activation Ferritic-Martensitic (RAFM) steel EUROFER97 is selected as structural material. A 2 mm layer tungsten is used as the armor material facing the plasma to protect the first wall (FW). Advanced ceramic breeder (mixture of Li4SiO4 and Li2TiO3) is used as tritium breeder. Be12Ti is chosen as neutron multiplier, improving the tritium release and reducing the swelling and water/steam-reactivity. Both functional materials are in form of pebble beds. The purge gas is He at 0.2 MPa with 0.1% 2219
Fusion Engineering and Design 146 (2019) 2218–2221
S. Wang, et al.
2.2. Basic structure Like in the enhanced HCPB BB [16], the blanket segment is based on so-called single module segment. The BZ is formed by rows of fuelbreeder pins. The CCPB BZ has 12 pin. and the pin diameter is 86 mm, nevertheless with a squared matrix configuration, which is the main difference compared to the HCPB fuel-breeder pin design (hexagonal matrix configuration). The squared pin configuration has the advantage of having less pin density than the hexagonal bundle for a same pitch and therefore reducing the number of pins in the BZ. However, it has the disadvantage of resulting in a larger power density in the Be12Ti and therefore causing a local hot spot in the EUROFER97, which would be too large (> 550 °C) at the front. In order to locally limit this power density, graphite rods of Ø70 mm and 150 mm length are introduced between pins. In the future, an optimization of the coolant flow at the front of the pin, behind the FW, could make the use of these rods not necessary. The blanket basic configuration is shown in Fig. 1. 2.3. Gas flow scheme and coolant parameters The coolant flow scheme is the same with that of [17]. The coolants with an inlet temperature of 300 °C firstly flow in parallel through the FW channels and then mixed at the FW manifold to have a mixed temperature of about 370 °C. Thereafter the coolant goes into the BZ inlet channel and exits BZ channel at a temperature of 500 °C. This coolant pressure is 8 MPa, which is a trade-off between: (i) the low primary stresses; (ii) low pressure drops; (iii) piping dimensions; (iv) the expansion tank size with the possible event of in-vessel LOCA. 3. Performance analysis 3.1. Thermal-hydraulic analysis 3.1.1. Geometry and mesh In order to reduce the complexity of the computational model, a simplification is achieved using a basic unit cell in poloidal direction. The 3D-slice model includes a FW with 8 channels, a BZ with 12 pin. and the corresponding back supporting structure (BSS). CFD code ANSYS CFX has been used to perform this analysis. Mesh sensitivity analysis has been studied in a previous study. For a best compromise between mesh quality and size, a detailed mesh of the FW channels cross section has been created and then sweep along the channel length. Finally, the sweep size of 2 mm and the channel cross section sizes of 0.4 mm have been selected [18]. The total mesh number includes about 5 million nodes and 6 million elements. As a heat transfer enhancement, the wetted side of the tubes is roughened. An equivalent sand-grain wall roughness εs of 400 μm has been considered for the cooling channels of FW and BZ which is corresponds to an average surface roughness Ra of about 68 μm.
Fig. 4. Multiplier temperature of typical CCPB unit slice.
Fig. 5. Ceramic breeder temperature of typical CCPB unit slice.
3.1.2. Loads and boundary conditions A total heat flux of 0.37 MW/m2 for the equatorial FW, including the radiation heat flux of 0.31 MW/m2 and a particle heat flux of 0.06 MW/m2 [19]. The power density of different materials calculated by neutronic analysis, have been applied on the corresponding parts. Taking into account the specific heat capacity of CO2 and ΔT through the blanket, a total plant mass flow rate of 10,199.5 kg/s is needed, about 72% to the OB and 28% to the IB. The mass flow rate for one cooling channel of FW is 0.12 kg/s and the inlet mass flow for one pin of BZ is 0.08 kg/s.
Fig. 6. Pin-tube temperature of typical CCPB unit slice.
3.1.3. Results The temperature design limits for different materials have been summarized here:
Vol. H2 as additive. Contrarily to the HCPB pin design, graphite rods are here introduced to limit the power density in Be12Ti, due to the squared pin bundle configuration selected for the breeding zone (BZ), as shown later.
• The maximum design temperature of EUROFER97 is 550 °C, which 2220
Fusion Engineering and Design 146 (2019) 2218–2221
S. Wang, et al.
4. Conclusions An EU DEMO CCPB BB has been presented in this paper. CO2 is selected as the coolant for this blanket, as a low risk alternative to helium. Through a comprehensive CFD analysis of a unit slice in the equatorial outboard region, it has been demonstrated that CO2 is able to efficiently dissipate the heat of the blanket and keep the material temperatures globally under the design limits. Future investigations will be oriented to consolidate a CCPB design. Using CO2 as blanket coolant in an indirect cycle coupled with a standard Rankine power conversion system offers the lowest risk for a high temperature gas cooled heat source. However, such blanket could pave the way for future investigations with advanced power cycles based on direct coupling with a supercritical CO2 power conversion system, provided that the tritium permeation to the coolant could be kept to an enough low value that the inventory to be circulated through the turbine hall does not go beyond the allowable limits.
Fig. 7. FW temperature of typical CCPB unit slice.
Acknowledgments This work is financially supported by China Scholarship Council (Grant No.: CSC-201606340011). Under this Fellowship the author Shuai Wang has joined the KIT. References [1] A.J.H. Donne, The European roadmap towards fusion electricity, Phil. Trans. R. Soc. A 377 (2019), https://doi.org/10.1098/rsta.2017.0432 20170432. [2] G. Federici, et al., DEMO design activity in Europe: progress and updates, Fusion Eng. Des. 136 (2018) 729–741, https://doi.org/10.1016/j.fusengdes.2018.04.001. [3] F.A. Hernandez, et al., Overview of the HCPB research activities in EUROfusion, IEEE Trans. Plasma Sci. 46 (2018) 2247–2261, https://doi.org/10.1109/TPS.2018.2830813. [4] F. Hernandez, et al., A new HCPB breeding blanket for the EU DEMO: evolution, rationale and preliminary performances, Fusion Eng. Des. 124 (2017) 882–886, https://doi.org/10. 1016/j.fusengdes.2017.02.008. [5] P. Pereslavtsev, et al., Neutronic analyses for the optimization of the advanced HCPB breeder blanket design for DEMO, Fusion Eng. Des. 124 (2017) 910–914, https://doi.org/ 10.1016/j.fusengdes.2017.01.028. [6] G. Zhou, et al., Transient thermal analysis and structural assessment of an ex-vessel LOCA event on the EU DEMO HCPB breeding blanket and the attachment system, Fusion Eng. Des. 136 (2018) 34–41, https://doi.org/10.1016/j.fusengdes.2017.12.017. [7] C. Zeile, et al., Structural assessment of the HCPB breeding blanket segments in the EU DEMO reactor under normal operation and a central plasma disruption, Fusion Eng. Des. 136 (2018) 335–339, https://doi.org/10.1016/j.fusengdes.2018.02.024. [8] G. Zhou, et al., A methodology for thermo-mechanical assessment of in-box LOCA events on fusion blankets and its application to EU DEMO HCPB breeding blanket, Kerntechnik 83 (2018) 256–260, https://doi.org/10.3139/124.110868. [9] G. Zhou, et al., Design study on the new EU DEMO HCPB breeding blanket: thermal analysis, Prog. Nucl. Energy 98 (2017) 167–176, https://doi.org/10.1016/j.pnucene. 2017.03.013. [10] G. Zhou, et al., Preliminary steady state and transient thermal analysis of the new HCPB blanket for EU DEMO reactor, Int. J. Hydrogen Energy 41 (2016) 7047–7052, https://doi. org/10.1016/j.ijhydene.2016.01.149. [11] G. Zhou, et al., Preliminary structural analysis of the new HCPB blanket for EU DEMO reactor, Int. J. Hydrogen Energy 41 (2016) 7053–7058, https://doi.org/10.1016/j. ijhydene.2016.01.064. [12] M. Grimston, et al., The siting of UK nuclear reactors, J. Radiol. Prot. 34 (2014) R1–R24, https://doi.org/10.1088/0952-4746/34/2/R1. [13] J.I. Linares, et al., Supercritical CO2 Brayton power cycles for DEMO fusion reactor based on helium cooled Lithium Lead blanket, Appl. Therm. Eng. 76 (2015) 123–133, https:// doi.org/10.1016/j.applthermaleng.2014.10.093. [14] J.I. Linares, et al., Sizing of a recuperative supercritical CO2 Brayton cycle as power conversion system for DEMO fusion reactor based on Dual Coolant Lithium Lead blanket, Fusion Eng. Des. 134 (2018) 79–91, https://doi.org/10.1016/j.fusengdes.2018.06.026. [15] S. Wang, et al., Comparative analysis of the efficiency of a CO2-cooled and a He-cooled pebble bed breeding blanket for the EU DEMO fusion reactor, Fusion Eng. Des. 138 (2019) 32–40, https://doi.org/10.1016/j.fusengdes.2018.10.026. [16] F.A. Hernandez, et al., An enhanced, near-term HCPB design as driver blanket for the EU DEMO, Fusion Eng. Des. (2019), https://doi.org/10.1016/j.fusengdes.2019.02.037. [17] G. Zhou, et al., Progress on the helium cooled Molten Lead Ceramic Breeder concept, as a near-term alternative blanket for EU DEMO, Fusion Eng. Des. (2019), https://doi.org/10. 1016/j.fusengdes.2019.01.151 in press. [18] S. Wang, et al., Thermal-hydraulic analysis of the First Wall of a CO2 cooled pebble bed breeding blanket for the EU-DEMO, Fusion Eng. Des. 138 (2019) 379–394, https://doi. org/10.1016/j.fusengdes.2018.11.057. [19] F. Maviglia, et al., Wall protection strategies for DEMO plasma transients, Fusion Eng. Des. (2018), https://doi.org/10.1016/j.fusengdes.2018.02.064 in press.
Fig. 8. BSS temperature of typical CCPB unit slice.
• •
is set by the creep strength of steel. This is yet only a design limit: local hot spots > 550 °C could be tolerated if the stress criteria in those locations are fulfilled following the available codes and standards (e.g. RCC-MRx), as discussed in [17]. Although there is no strict upper temperature limit for Be12Ti, a precaution temperature of about 950 °C has been assumed for this material. Limitation design temperature of 920 °C for the advanced ceramic breeder is assumed, which is set by thermo-mechanical behavior, tritium release and thermal expansion reasons.
The temperature distribution of typical CCPB unit slice is shown in Fig. 2. A maximum temperature of 917.6 °C appears at the ceramic pebble beds close to FW. The minimum temperature is 300 °C which is reached at the FW inlet area of back supporting structure, in a low dpa region. Fig. 3 shows the velocity distribution of FW coolant and BZ coolant. For the high heat flux coming from plasma that directly loads on FW, the required velocity of FW coolant is higher than that of BZ coolant. Thus, the pressure drop for FW and BZ are 0.33 bar and 0.04 bar, separately. Fig. 4 (a) shows the temperature result of Be12Ti (multiplier) and advanced ceramic breeder pebble beds without graphite rods, it is obviously higher than the material limitation. Fig. 4(b) and Fig. 5 shows that the maximum temperatures in the Be12Ti and Li4SiO4 pebble beds are respectively 895.9 °C and 917.6 °C, which are within the design limitations when graphite rods are implemented. The maximum structural temperature is 574.6 °C which appears on Pin-tube. It is higher than the design limit 550 °C, but it is only located in the small area which is shown in Fig. 6. Fig. 7 shows the maximum FW temperature is 512.5 °C which is lower than the design limit. The BSS part is far from the plasma, thus the temperature is lower than the design limit, which is shown in Fig. 8.
2221
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
First thermal-hydraulic analysis of a CO2 cooled pebble bed blanket for the EU DEMO
T
⁎
Shuai Wanga, Francisco A. Hernándezb, Guangming Zhoub, , Hongli Chena a b
University of Science and Technology of China (USTC), School of Physical Sciences, Hefei, 230026, Anhui, China Karlsruhe Institute of Technology (KIT), Institute for Neutron Physics and Reactor Technology (INR), 76344, Eggenstein-Leopoldshafen, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: DEMO CO2-cooled pebble bed blanket Thermal-hydraulic analysis
Over the years, the Helium Cooled Pebble Bed (HCPB) Breeding Blanket (BB) has been intensively studied for the EU DEMO. However, several feasibility issues remain open for a HCPB-type DEMO reactor. Some of these issues are linked with the use of He as coolant and are related to the large size of the Primary Heat Transfer System pipework, the resulting large coolant inventory and therefore large expansion volumes required in an ex-vessel loss of coolant accident, the limited operational experience with relevant size He-turbomachinery, the large circulating power and possible considerable pipe leakage, among others. Due to the larger density of CO2, the use of this gas as primary coolant for DEMO can lead to key advantageous features, mitigating most of the issues present for He-cooling and resulting in a higher net efficiency than that of HCPB, as reported in a previous study. Therefore, a CO2-Cooled Pebble Bed (CCPB) has been proposed as an alternative coolant to He for the EU DEMO. After identifying that CO2 have a negligible influence on the neutronic performance, making the CCPB’s TBR almost equal to the HCPB’s one (TBR ≈ 1.15), a first set of thermal-hydraulic analyses with the commercial code of ANSYS CFX are reported here. The analyses are based on the newly proposed design of breeding zone (BZ) in the enhanced HCPB fuel-breeder pin concept for the EU DEMO. Despite the lower heat transfer capability of CO2 compared to He, the fuel-breeder pin design BZ improves the thermo-hydraulic performance to a point that this coolant is able to meet the materials’ temperature requirements. The results show that the CCPB can satisfy the basic thermal hydraulic requirements and that CO2 is also a realistic option as primary coolant for gas-cooled fusion reactors, with the key advantage of the existing > 60 years operational experience in fission industry with this coolant.
1. Introduction The EU Demonstration Fusion Power Plant (DEMO) is considered to be the last step before building a commercial fusion power plant. Among the main objectives of EU DEMO are: (i) demonstrating electricity production of several hundred MW; (ii) achieving tritium selfsufficiency [1,2]. The breeding blanket (BB) plays an important role in achieving the two main objectives. Over the past years, extensive studies for the Helium Cooled Pebble Bed (HCPB) BB has been performed [3–11]. However, the very low density of He easily leads to high circulating power with increasing pressure drops (Δp), thus affecting the net plant efficiency. The current EU DEMO development strategy of prioritizing the use of existing/ mature technologies is a major constraint for the He-turbomachinery [2]. As He-turbomachinery in fission industry is currently proven up to 6 MW with limited experience, introducing a strict limit on the plant
⁎
circulating power. Moreover, chronic releases due to systematic He leakages, which are typical for He-cooled reactors and the large coolant inventory, resulting in safety concerns in the event of a Loss of Coolant Accident (LOCA), are also significant drawbacks on the use of He as a coolant for a BB in EU DEMO. On the other side, CO2 has been used as primary coolant since the 50′s in the nuclear fission industry for gas-cooled reactors (MAGNOX, UNGG and AGR) [12] and it is drawing increasing attention as a secondary coolant to replace the conventional Rankine cycle in power plants Power Conversion System (PCS) [13,14]. Despite of the lower specific heat capacity and thermal conductivity of CO2 (about 22% and 17% of those of He, respectively), its larger density (11 times larger molecular weight of CO2 than that of He) can lead to an advantageous heat dissipation rate per unit of circulating power. These features, together with its relative transparency to neutrons, the more compact and less prone to leakage pipework, the large industrial operating
Corresponding author. E-mail addresses: [email protected], [email protected] (G. Zhou).
https://doi.org/10.1016/j.fusengdes.2019.03.156 Received 5 October 2018; Received in revised form 11 March 2019; Accepted 22 March 2019 Available online 28 March 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 146 (2019) 2218–2221
S. Wang, et al.
Fig. 1. CCPB BB CAD design.
Fig. 2. Temperature distribution of typical CCPB unit slice (with graphite rods). Fig. 3. Velocity of FW and BZ coolant of typical CCPB unit slice.
experience with this coolant, its abundance and low price makes CO2 an attractive option and a practical alternative to He as primary coolant for DEMO. Thanks to the above-mentioned advantages, it has been shown that a CO2 cooled EU DEMO BB has a higher efficiency than that of Hecooled EU DEMO BB [15]. Using CO2 as a coolant for EU DEMO BB is therefore worth investigating. In this paper, a CO2 Cooled Pebble Bed (CCPB) BB based on the fuelbreeder pin configuration is presented. This configuration has been originally developed for the enhanced EU DEMO HCPB BB [16]. First thermal-hydraulic analyses have been performed, showing the potential of CO2 as a coolant for the BB.
2. The CCPB BB design 2.1. Material selection With the aim to develop a near-term alternative BB for EU DEMO, the materials that have mature industrial experience or worldwide development efforts are therefore preferred. Reduced Activation Ferritic-Martensitic (RAFM) steel EUROFER97 is selected as structural material. A 2 mm layer tungsten is used as the armor material facing the plasma to protect the first wall (FW). Advanced ceramic breeder (mixture of Li4SiO4 and Li2TiO3) is used as tritium breeder. Be12Ti is chosen as neutron multiplier, improving the tritium release and reducing the swelling and water/steam-reactivity. Both functional materials are in form of pebble beds. The purge gas is He at 0.2 MPa with 0.1% 2219
Fusion Engineering and Design 146 (2019) 2218–2221
S. Wang, et al.
2.2. Basic structure Like in the enhanced HCPB BB [16], the blanket segment is based on so-called single module segment. The BZ is formed by rows of fuelbreeder pins. The CCPB BZ has 12 pin. and the pin diameter is 86 mm, nevertheless with a squared matrix configuration, which is the main difference compared to the HCPB fuel-breeder pin design (hexagonal matrix configuration). The squared pin configuration has the advantage of having less pin density than the hexagonal bundle for a same pitch and therefore reducing the number of pins in the BZ. However, it has the disadvantage of resulting in a larger power density in the Be12Ti and therefore causing a local hot spot in the EUROFER97, which would be too large (> 550 °C) at the front. In order to locally limit this power density, graphite rods of Ø70 mm and 150 mm length are introduced between pins. In the future, an optimization of the coolant flow at the front of the pin, behind the FW, could make the use of these rods not necessary. The blanket basic configuration is shown in Fig. 1. 2.3. Gas flow scheme and coolant parameters The coolant flow scheme is the same with that of [17]. The coolants with an inlet temperature of 300 °C firstly flow in parallel through the FW channels and then mixed at the FW manifold to have a mixed temperature of about 370 °C. Thereafter the coolant goes into the BZ inlet channel and exits BZ channel at a temperature of 500 °C. This coolant pressure is 8 MPa, which is a trade-off between: (i) the low primary stresses; (ii) low pressure drops; (iii) piping dimensions; (iv) the expansion tank size with the possible event of in-vessel LOCA. 3. Performance analysis 3.1. Thermal-hydraulic analysis 3.1.1. Geometry and mesh In order to reduce the complexity of the computational model, a simplification is achieved using a basic unit cell in poloidal direction. The 3D-slice model includes a FW with 8 channels, a BZ with 12 pin. and the corresponding back supporting structure (BSS). CFD code ANSYS CFX has been used to perform this analysis. Mesh sensitivity analysis has been studied in a previous study. For a best compromise between mesh quality and size, a detailed mesh of the FW channels cross section has been created and then sweep along the channel length. Finally, the sweep size of 2 mm and the channel cross section sizes of 0.4 mm have been selected [18]. The total mesh number includes about 5 million nodes and 6 million elements. As a heat transfer enhancement, the wetted side of the tubes is roughened. An equivalent sand-grain wall roughness εs of 400 μm has been considered for the cooling channels of FW and BZ which is corresponds to an average surface roughness Ra of about 68 μm.
Fig. 4. Multiplier temperature of typical CCPB unit slice.
Fig. 5. Ceramic breeder temperature of typical CCPB unit slice.
3.1.2. Loads and boundary conditions A total heat flux of 0.37 MW/m2 for the equatorial FW, including the radiation heat flux of 0.31 MW/m2 and a particle heat flux of 0.06 MW/m2 [19]. The power density of different materials calculated by neutronic analysis, have been applied on the corresponding parts. Taking into account the specific heat capacity of CO2 and ΔT through the blanket, a total plant mass flow rate of 10,199.5 kg/s is needed, about 72% to the OB and 28% to the IB. The mass flow rate for one cooling channel of FW is 0.12 kg/s and the inlet mass flow for one pin of BZ is 0.08 kg/s.
Fig. 6. Pin-tube temperature of typical CCPB unit slice.
3.1.3. Results The temperature design limits for different materials have been summarized here:
Vol. H2 as additive. Contrarily to the HCPB pin design, graphite rods are here introduced to limit the power density in Be12Ti, due to the squared pin bundle configuration selected for the breeding zone (BZ), as shown later.
• The maximum design temperature of EUROFER97 is 550 °C, which 2220
Fusion Engineering and Design 146 (2019) 2218–2221
S. Wang, et al.
4. Conclusions An EU DEMO CCPB BB has been presented in this paper. CO2 is selected as the coolant for this blanket, as a low risk alternative to helium. Through a comprehensive CFD analysis of a unit slice in the equatorial outboard region, it has been demonstrated that CO2 is able to efficiently dissipate the heat of the blanket and keep the material temperatures globally under the design limits. Future investigations will be oriented to consolidate a CCPB design. Using CO2 as blanket coolant in an indirect cycle coupled with a standard Rankine power conversion system offers the lowest risk for a high temperature gas cooled heat source. However, such blanket could pave the way for future investigations with advanced power cycles based on direct coupling with a supercritical CO2 power conversion system, provided that the tritium permeation to the coolant could be kept to an enough low value that the inventory to be circulated through the turbine hall does not go beyond the allowable limits.
Fig. 7. FW temperature of typical CCPB unit slice.
Acknowledgments This work is financially supported by China Scholarship Council (Grant No.: CSC-201606340011). Under this Fellowship the author Shuai Wang has joined the KIT. References [1] A.J.H. Donne, The European roadmap towards fusion electricity, Phil. Trans. R. Soc. A 377 (2019), https://doi.org/10.1098/rsta.2017.0432 20170432. [2] G. Federici, et al., DEMO design activity in Europe: progress and updates, Fusion Eng. Des. 136 (2018) 729–741, https://doi.org/10.1016/j.fusengdes.2018.04.001. [3] F.A. Hernandez, et al., Overview of the HCPB research activities in EUROfusion, IEEE Trans. Plasma Sci. 46 (2018) 2247–2261, https://doi.org/10.1109/TPS.2018.2830813. [4] F. Hernandez, et al., A new HCPB breeding blanket for the EU DEMO: evolution, rationale and preliminary performances, Fusion Eng. Des. 124 (2017) 882–886, https://doi.org/10. 1016/j.fusengdes.2017.02.008. [5] P. Pereslavtsev, et al., Neutronic analyses for the optimization of the advanced HCPB breeder blanket design for DEMO, Fusion Eng. Des. 124 (2017) 910–914, https://doi.org/ 10.1016/j.fusengdes.2017.01.028. [6] G. Zhou, et al., Transient thermal analysis and structural assessment of an ex-vessel LOCA event on the EU DEMO HCPB breeding blanket and the attachment system, Fusion Eng. Des. 136 (2018) 34–41, https://doi.org/10.1016/j.fusengdes.2017.12.017. [7] C. Zeile, et al., Structural assessment of the HCPB breeding blanket segments in the EU DEMO reactor under normal operation and a central plasma disruption, Fusion Eng. Des. 136 (2018) 335–339, https://doi.org/10.1016/j.fusengdes.2018.02.024. [8] G. Zhou, et al., A methodology for thermo-mechanical assessment of in-box LOCA events on fusion blankets and its application to EU DEMO HCPB breeding blanket, Kerntechnik 83 (2018) 256–260, https://doi.org/10.3139/124.110868. [9] G. Zhou, et al., Design study on the new EU DEMO HCPB breeding blanket: thermal analysis, Prog. Nucl. Energy 98 (2017) 167–176, https://doi.org/10.1016/j.pnucene. 2017.03.013. [10] G. Zhou, et al., Preliminary steady state and transient thermal analysis of the new HCPB blanket for EU DEMO reactor, Int. J. Hydrogen Energy 41 (2016) 7047–7052, https://doi. org/10.1016/j.ijhydene.2016.01.149. [11] G. Zhou, et al., Preliminary structural analysis of the new HCPB blanket for EU DEMO reactor, Int. J. Hydrogen Energy 41 (2016) 7053–7058, https://doi.org/10.1016/j. ijhydene.2016.01.064. [12] M. Grimston, et al., The siting of UK nuclear reactors, J. Radiol. Prot. 34 (2014) R1–R24, https://doi.org/10.1088/0952-4746/34/2/R1. [13] J.I. Linares, et al., Supercritical CO2 Brayton power cycles for DEMO fusion reactor based on helium cooled Lithium Lead blanket, Appl. Therm. Eng. 76 (2015) 123–133, https:// doi.org/10.1016/j.applthermaleng.2014.10.093. [14] J.I. Linares, et al., Sizing of a recuperative supercritical CO2 Brayton cycle as power conversion system for DEMO fusion reactor based on Dual Coolant Lithium Lead blanket, Fusion Eng. Des. 134 (2018) 79–91, https://doi.org/10.1016/j.fusengdes.2018.06.026. [15] S. Wang, et al., Comparative analysis of the efficiency of a CO2-cooled and a He-cooled pebble bed breeding blanket for the EU DEMO fusion reactor, Fusion Eng. Des. 138 (2019) 32–40, https://doi.org/10.1016/j.fusengdes.2018.10.026. [16] F.A. Hernandez, et al., An enhanced, near-term HCPB design as driver blanket for the EU DEMO, Fusion Eng. Des. (2019), https://doi.org/10.1016/j.fusengdes.2019.02.037. [17] G. Zhou, et al., Progress on the helium cooled Molten Lead Ceramic Breeder concept, as a near-term alternative blanket for EU DEMO, Fusion Eng. Des. (2019), https://doi.org/10. 1016/j.fusengdes.2019.01.151 in press. [18] S. Wang, et al., Thermal-hydraulic analysis of the First Wall of a CO2 cooled pebble bed breeding blanket for the EU-DEMO, Fusion Eng. Des. 138 (2019) 379–394, https://doi. org/10.1016/j.fusengdes.2018.11.057. [19] F. Maviglia, et al., Wall protection strategies for DEMO plasma transients, Fusion Eng. Des. (2018), https://doi.org/10.1016/j.fusengdes.2018.02.064 in press.
Fig. 8. BSS temperature of typical CCPB unit slice.
• •
is set by the creep strength of steel. This is yet only a design limit: local hot spots > 550 °C could be tolerated if the stress criteria in those locations are fulfilled following the available codes and standards (e.g. RCC-MRx), as discussed in [17]. Although there is no strict upper temperature limit for Be12Ti, a precaution temperature of about 950 °C has been assumed for this material. Limitation design temperature of 920 °C for the advanced ceramic breeder is assumed, which is set by thermo-mechanical behavior, tritium release and thermal expansion reasons.
The temperature distribution of typical CCPB unit slice is shown in Fig. 2. A maximum temperature of 917.6 °C appears at the ceramic pebble beds close to FW. The minimum temperature is 300 °C which is reached at the FW inlet area of back supporting structure, in a low dpa region. Fig. 3 shows the velocity distribution of FW coolant and BZ coolant. For the high heat flux coming from plasma that directly loads on FW, the required velocity of FW coolant is higher than that of BZ coolant. Thus, the pressure drop for FW and BZ are 0.33 bar and 0.04 bar, separately. Fig. 4 (a) shows the temperature result of Be12Ti (multiplier) and advanced ceramic breeder pebble beds without graphite rods, it is obviously higher than the material limitation. Fig. 4(b) and Fig. 5 shows that the maximum temperatures in the Be12Ti and Li4SiO4 pebble beds are respectively 895.9 °C and 917.6 °C, which are within the design limitations when graphite rods are implemented. The maximum structural temperature is 574.6 °C which appears on Pin-tube. It is higher than the design limit 550 °C, but it is only located in the small area which is shown in Fig. 6. Fig. 7 shows the maximum FW temperature is 512.5 °C which is lower than the design limit. The BSS part is far from the plasma, thus the temperature is lower than the design limit, which is shown in Fig. 8.
2221