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Preliminary fluid channel design and thermal-hydraulic analysis of glow discharge cleaning permanent electrode
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Preliminary fluid channel design and thermal-hydraulic analysis of glow discharge cleaning permanent electrode Lijun Cai a,∗ , Tao Lin a , Yingqiao Wang a , Mingxu Wang a , So Maruyama b , Yu Yang b , Gabor Kiss b a b
Southwestern Institute of Physics, Chengdu, China ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St. Paul Lez Durance Cedex, France
h i g h l i g h t s • • • •
The plasma facing closure cap has to survive after 30,000 thermal heat load cycles. 0.35 MW/m2 radiation heat load plus nuclear heat load are very challenging for stainless steel. Multilayer structure has been designed by using advanced welding and drilling technology to solve the neutron heating problem. Accurate volumetric load application in analysis model by CFX has been mastered.
a r t i c l e
i n f o
Article history: Received 12 August 2015 Received in revised form 31 December 2015 Accepted 25 January 2016 Available online xxx Keywords: Glow discharge cleaning Permanent electrode Fluid channel Thermal-hydraulic analysis Optimization
a b s t r a c t Glow discharge cleaning (GDC) shall be used on ITER device to reduce and control impurity and hydrogenic fuel out-gassing from in-vessel plasma facing components. After first plasma, permanent electrode (PE) will be used to replace Temporary Electrode (TE) for subsequent operation. Two fundamental scenarios i.e., GDC and Plasma Operation State (POS) should be considered for electrode design, which requires the heat load caused by plasma radiation and neutron heating must be taken away by cooling water flowing inside the electrode. In this paper, multilayer cooling channels inside PE are preliminarily designed, and snakelike route in each layer is adopted to improve the heat exchange. Detailed thermal-hydraulic analyses have been done to validate the design feasibility or rationality. The analysis results show that during GDC the cooling water inlet and outlet temperature difference is far less than the allowable temperature rise under water flow rate 0.15 kg/s compromised by many factors. For POS, the temperature rise and pressure drop are within the design goals, but high thermal stress occurs on the front surface of closure cap of electrode. After several iterations of optimization of the closure cap, the equivalent strain range after 30,000 loading cycles for POS is well below 0.3% design goals. © 2016 Elsevier B.V. All rights reserved.
1. Introduction ITER (a nuclear facility INB-174) needs a reliable wall conditioning method to obtain a clean first wall for plasma breakdown after a venting. Direct Current (DC) Glow Discharge Cleaning (GDC) is chosen as a baseline wall conditioning technique to reduce and control impurity and hydrogenic fuel out-gassing from plasma-facing components [1]. Based on the staged installation strategy of ITER, for the first plasma operation, if no PE is available totally ten TEs [2] will be used and fixed on the Vacuum Vessel (VV) near those ports allocated for GDC, including the upper ports 3, 8, 14, the equatorial
∗ Corresponding author. Fax: +86 28 82850300. E-mail address: [email protected] (L. Cai).
ports 3, 8, 12, 17, and the lower ports 3, 9, 15. During conceptual design, the heat load of GDC electrode during different phases and operation states have been evaluated [3]. The heat load on TE during POS can be neglected due to low plasma discharge parameters and the 0.4 kW surface heat load during GDC is not a big problem for TE design. After first plasma, TEs will be removed and PEs installed. For maintainability reasons each PE will be integrated into port plugs at the upper or equatorial port level and the electrode front surface will be flush with the plasma facing surface of Diagnostic First Wall (DFW). Having taken into account all available locations, GDC layout is decided to provide toroidal uniform coverage to the maximum extent possible which results three PEs in the upper port level 3, 8 and 14, as shown in the left side of Fig. 1, and four PEs in the equatorial port level 3, 8, 12 and 17 as shown in the right side of Fig. 1. Because PEs will be used for all the later plasma opera-
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Please cite this article in press as: L. Cai, et al., Preliminary fluid channel design and thermal-hydraulic analysis of glow discharge cleaning permanent electrode, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.01.060
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Fig. 1. PE locations in the upper and equatorial ports.
tion including the D-T plasma operation, a high heat load totally about 72 kW will be deposited on the electrodes due to the plasma radiation, charge exchange particles and neutron heating. The heat load on PE during GDC or POS can be treated as steady state, a powerful cooling structure shall be designed to take the heat out of electrode and control the temperature rise and resultant thermal stress. Since each PE whether in upper port or equatorial port has similar main components and similar loading condition, so the fluid channel design and thermal hydraulic analysis in terms of one PE will be the reference for other PEs. The detailed design an analysis results will be discussed in the following sections.
etc., At present, the main components of PE except for insulation and fasteners will be made of stainless steel. As mentioned previously, the electrode body is the plasma facing and neutron shielding component, and it shall have the capability to survive under the high thermal load during plasma operation. The detailed design conditions, design schemes and thermal hydraulic analysis will be introduced in the following sections. 3. Fluid channels design and analysis inside electrode body 3.1. Design conditions
2. Preliminary design of GDC PE A typical PE as shown in Fig. 2 is mainly composed of four assemblies, i.e., electrode body, cooling tubes, vacuum penetration and ex-vessel components. The electrode body includes head and rod which are embedded in the diagnostic port plug first wall and Diagnostic Shielding Module (DSM) but electrically insulated from them. Fig. 3 shows the current design scheme. Ceramic support blocks are used as the insulator. During incidental and accidental conditions there will be high mechanical and thermal loads applied on the ceramics. The ceramic blocks will be bolted to the electrode body with slightly moveable capability relative to each other to minimize the mechanical and thermal stress. At the end of the rod, the electrode body is insulated and fixed onto the DSM. The elastic cooling pipes are used to feed the power to the electrode body, and to absorb the thermal expansion when these pipes are heated from 20 ◦ C to 240 ◦ C. The relative displacement between electrode body and vacuum feed through are also accommodated by these pipes. The cooling pipes with high potential must be insulated from the port plug closure plate and other components when they are being routed out of port plug. At present, ceramic coating will be adopted to provide the insulation. After feeding out, the cooling pipes are connected to the water manifolds outside the torus vacuum. The vacuum feed through is a double vacuum structure as it penetrating the first confinement barrier, and insulation with closure plate has to be considered. The second vacuum chamber shall be connected to the service vacuum system to allow monitoring of the vacuum state. Inherited from the conceptual design, the front surface of each PE is proposed to be approximately 0.07 m2 . The shape and dimension of the electrode body interrelate strongly with the diagnostic systems around. The design of PE needs to compromise among the weight, the volume for cooling channel, the structural integrity, the insulation gap and the remote handling compatibility
Two main loading conditions as shown in Table 1 including GDC/baking and POS are considered for the design of fluid channels inside electrode. The maximum surface heat load is 0.35 MW/m2 , and the maximum volumetric heat load is 10 W/cm3 at the front and the decay length is 12 cm [3]. The application of this load will be done according to the following formula with P0 = 10 W/cm3 and = 12 cm: P (x) = P0 e
−x
The following parameters are used to develop the coolant circuits. Some of these parameters are system requirements while others are design guidance or goals established from experiences of other ITER in-vessel components or design experience [5,6]. • Provide sufficient mass flow to limit temperature difference less than 50 ◦ C, then reduce the thermal stress below the allowable value • Limit average coolant velocities to∼5 m/s, the target value 1.5–3.5 m/s • Provide circuit with minimum flow velocity of 0.5 m/s • Limit peak stainless steel temperature <400 ◦ C, considering the thermal stresses, creep and thermal fatigue limits • No heat sink to surrounding components. 3.2. Cooling channels in electrode bulk The bulk portion of the electrode body is made from a single forging of 316SS. Within this bulk portion an arrangement of coolant passages are formed by drilling through various directions to form coolant circuits Fig. 4. The coolant channel ends are sealed with welded plugs. The basic arrangement is designed to have a series of layers each having a number of parallel passages forming “ladders”. The layers are positioned radially (along thickness direc-
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Table 1 Thermal and cooling conditions for PE [3,4].
Thermal load Inlet water temperature Maximum outlet temperature Inlet pressure Maximum pressure drop
POS
GDC/baking
About 72 kW for Rad & CX (21 kW) plus neutron heating (51 kW) 70 (±5) ◦ C (Burn period), 100 ◦ C (Dwell period) ≤126 (+10) ◦ C 4.0 (+0.6/−0.2) MPa 1.35 MPa
300 W 240 (+0/−5) ◦ C – 4.4 (+/−0.4) MPa ∼0.01 MPa (estimated, not a requirement)
Fig. 2. Main components of typical GDC PE.
Fig. 3. Support and insulation scheme of PE.
Table 2 Influence of flow rates on the cooling performance. Heat load W
Flow rate (kg/s)
Pressure drop (Pa)
Outlet coolant temperature (◦ C)
Max. coolant temperature (◦ C)
Max. temperature in steel (◦ C)
300 300 300 300 300
0.1 0.15 0.16 0.17 0.2
3583 7856 8886 10022 13751
240.63 240.41 240.39 240.36 240.31
242.36 241.74 241.73 241.59 241.46
244.25 243.79 243.68 243.67 243.54
Table 3 Temperature contour under different heat load but same flow rate 0.15 kg/s. Heat load (W)
Flow rate (kg/s)
Pressure drop (Pa)
Outlet coolant temperature (◦ C)
Max. coolant temperature (◦ C)
Max. temperature in steel (◦ C)
300 660 1020
0.15 0.15 0.15
7856 7845 7864
240.41 240.91 241.41
241.74 243.84 245.39
243.79 258.32 262.85
tion) at increasing distances from each other due to the reduction in the volumetric heating rates. The number of parallel passages is also reduced in each ladder due to the same heating reduction noted. Two layers are considered in the electrode for the deposited neutron heat. The cooling water will go directly from the rod to the first layer (closure cap) because of high surface heat load about 0.35 MW/m2 caused by plasma radiation. After circulating in the
first layer the cooling water then goes down to second layer (bulk), then go back along electrode rod and cooling pipes to the outside of vessel. Fig. 5 shows clearly the fluid domain inside the electrode.
3.3. Electrode closure cap The electrode surface which is directly exposed to the plasma requires a higher density of coolant channels which are unreason-
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Fig. 4. coolant circuits in electrode bulk.
Fig. 5. Fluid domain inside PE.
able to be produced with a deep drilling process used for the bulk. In this region a separate part “cap” is milled with coolant channels which can be tailored to provide the high density of coolant as shown in Fig. 6(a). The cap is then brazed onto the bulk main portion and sealed with a perimeter TIG weld. In this layer, the water will flow serially along 12 channels and the cross section for each channel is about 10 mm × 17 mm, and the flow rate should be higher than 0.5 kg/s to provide the velocity about 3 m/s. The influence of flow rate on the cooling performance is discussed in the following sections. For the cap thickness determination, a balance needs to be found considering multiple design factors [5]: coolant pressure stress (effects on nuclear pressurized equipment exemption), erosion requirements (1–2 mm over life time), fabrication costs (depth
Fig. 6. (a) Closure cap design and (b) stress under pressure.
vs width ratio of channels), complexity of channels routing design (greater number of parallel passages produces difficulties). To gain nuclear pressurized equipment exemption a high ratio of component stress verse coolant pressure stress was required (low primary stress from coolant pressure). With a material allowable stress limit (@125 ◦ C) of 147 MPa a ratio of 1/5 produce a design goal of ∼30 MPa coolant pressure stress within the closure cap. Based on the above
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Fig. 8. Temperature contour on stainless steel during GDC.
3.5. Thermal fatigue analysis under POS condition Since the critical heat load occurs during POS conditions, and the permanent electrode has to survive after 30,000 thermal loading cycles. This is very challenging for stainless steel under 0.35 MW/m2 surface heat load plus neutron heat load, so great efforts were spent on the thermal fatigue analysis of closure cap. According to the structural design criteria for in-vessel components [7] the equivalent strain range got from elastic-plastic analysis should be less than 0.3% for 30,000 loading cycles. Fig. 9 shows that the maximum equivalent strain range is 0.257% well below the design goals. 4. Influence of flow rate and heat load during GDC condition Fig. 7. Temperature contour of (a) water and (b) stainless steel.
consideration, the closure cap thickness is chosen as 6 mm, and the equivalent Mises stress is smaller than 28 MPa shown in Fig. 6(b) under maximum absolute pressure 4.9 MPa.
3.4. Thermal hydraulic analysis ANSYS CFX 14.5 was used to run the thermal hydraulic analysis based on the current cooling circuit design. To guarantee the validity, most of initial efforts were spent on the mesh quality of calculation models and also on the selection of turbulence parameters. For POS condition 0.5 kg/s flow rate is set as the initial value. Fig. 7(a) shows the maximum local water temperature is 151.8 ◦ C induced by the sharp corner in the analyzing model. The inlet and outlet temperature difference is only 17.75 ◦ C well under the design goal value 50 ◦ C. The pressure drop is 0.15 MPa far smaller than 1.35 MPa. Fig. 7(b) shows the stainless steel maximum temperature is 409 ◦ C in sharp corner region, but the vast majority of the electrode bulk is less than 400 ◦ C. A subsequent model without sharp corners has validated that this value can be eliminated by fabrication fillet treatment. For GDC loading condition simulation 0.15 kg/s flow rate is set as the initial value. The inlet and outlet temperature rise is only 0.2 ◦ C, and the pressure drop is only 0.008 MPa. The maximum temperature rise on the stainless steel is about 4 ◦ C as shown in Fig. 8. Based on the results of thermal hydraulic analysis, the design goals are well achieved for the cooling circuit design.
For GDC loading condition, the allowable water temperature rise is about 11.8 ◦ C (boiling point 251.8 ◦ C under 4 MPa pressure), so different flow rates have been considered to evaluate the cooling performance and to reduce the temperature rise to the maximum extent. From the results in Table 2, when increasing the flow rate from 0.1 kg/s to 0.2 kg/s, the temperature both of water and stainless steel will not change a lot but the pressure drop will be exponentially increased. So 0.15 kg/s is chosen to keep the minimum water velocity inside electrode bigger than 0.5 m/s, and the thermal stress is about 57 MPa in this condition which means far less than the allowable stress of stainless steel under this temperature. According to the experiences on some test beds and other tokamak devices, the heat load deposited on the electrode during the glow discharge cleaning is not constant, the heat flux density and distribution will change even for the same feeding of electric current. So based on the design baseline heat load 300 W for GDC condition, the heat load was increased by 2 times or 3 times. The results in Table 3 shows that current coolant circuit design and corresponding cooling water parameters can withstand the fluctuation, which means the temperature rise is still lower than the allowable value 11.8 ◦ C even for 3 times normal heat load. 5. Conclusion and discussion The main components of a typical PE include electrode body, cooling tubes, feed through parts and ex-vessel assemblies. The preliminary design activities are planned according to the schedule, and recent design and analysis progress of fluid channels inside electrode are introduced in this paper. Detailed design principles, design goals, and concrete design parameters for the coolant
Please cite this article in press as: L. Cai, et al., Preliminary fluid channel design and thermal-hydraulic analysis of glow discharge cleaning permanent electrode, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.01.060
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Fig. 9. Equivalent strain range of closure cap surface after 30,000 loading cycles.
circuits have been discussed here. Based on two main working conditions, a great deal of thermal hydraulic analysis and thermal fatigue analysis have been done to investigate the cooling performance of cooling circuits and to validate the design rationality. The results show that the present coolant channel design of electrode can withstand the thermal load during the GDC condition, which means the temperature and thermal stress are within reasonable range. For critical POS condition, the thermal hydraulic requirements such as temperature rise or pressure drop are satisfied. High surface stress is caused by large temperature gradient along the thickness direction of closure cap. After several iterations of optimization of the closure cap structure, the equivalent strain range after 30,000 loading cycles for POS are well below 0.3% design goals. Acknowledgements
information in this paper is governed by the applicable terms of the ITER Joint Implementation Agreement. References [1] Y. Yang, S. Maruyama, et al., Re-design of ITER glow discharge cleaning system based on a fixed electrode concept, Fusion Eng. Des. 89 (2014) 1944–1948. [2] Private communication. SRD-18-GC (Glow Discharge Wall Conditioning) from DOORS. [3] Private communication. GDC heat load. [4] Private communication. System Requirement (SRD) Document. SRD-26-PH, −CV, −DR −DY (TCWS) from DOORS. [5] Private communication. System Design Description Document (DDD) Diagnostic First Wall DFW. [6] M. Merola, R. Matera, F. Sevini, Experimental results of the thermal fatigue tests for I A.E.A. benchmark components, J. Nucl. Mater. 233–237 (1996) 620–625. [7] Private communication. Structural Design Criteria for ITER In-vessel Components (SDC-IC).
The author would like to thank all the colleagues from IO and CNDA for their good comments and suggestions. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization or any member of ITER. Dissemination of the
Please cite this article in press as: L. Cai, et al., Preliminary fluid channel design and thermal-hydraulic analysis of glow discharge cleaning permanent electrode, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.01.060
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Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Preliminary fluid channel design and thermal-hydraulic analysis of glow discharge cleaning permanent electrode Lijun Cai a,∗ , Tao Lin a , Yingqiao Wang a , Mingxu Wang a , So Maruyama b , Yu Yang b , Gabor Kiss b a b
Southwestern Institute of Physics, Chengdu, China ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St. Paul Lez Durance Cedex, France
h i g h l i g h t s • • • •
The plasma facing closure cap has to survive after 30,000 thermal heat load cycles. 0.35 MW/m2 radiation heat load plus nuclear heat load are very challenging for stainless steel. Multilayer structure has been designed by using advanced welding and drilling technology to solve the neutron heating problem. Accurate volumetric load application in analysis model by CFX has been mastered.
a r t i c l e
i n f o
Article history: Received 12 August 2015 Received in revised form 31 December 2015 Accepted 25 January 2016 Available online xxx Keywords: Glow discharge cleaning Permanent electrode Fluid channel Thermal-hydraulic analysis Optimization
a b s t r a c t Glow discharge cleaning (GDC) shall be used on ITER device to reduce and control impurity and hydrogenic fuel out-gassing from in-vessel plasma facing components. After first plasma, permanent electrode (PE) will be used to replace Temporary Electrode (TE) for subsequent operation. Two fundamental scenarios i.e., GDC and Plasma Operation State (POS) should be considered for electrode design, which requires the heat load caused by plasma radiation and neutron heating must be taken away by cooling water flowing inside the electrode. In this paper, multilayer cooling channels inside PE are preliminarily designed, and snakelike route in each layer is adopted to improve the heat exchange. Detailed thermal-hydraulic analyses have been done to validate the design feasibility or rationality. The analysis results show that during GDC the cooling water inlet and outlet temperature difference is far less than the allowable temperature rise under water flow rate 0.15 kg/s compromised by many factors. For POS, the temperature rise and pressure drop are within the design goals, but high thermal stress occurs on the front surface of closure cap of electrode. After several iterations of optimization of the closure cap, the equivalent strain range after 30,000 loading cycles for POS is well below 0.3% design goals. © 2016 Elsevier B.V. All rights reserved.
1. Introduction ITER (a nuclear facility INB-174) needs a reliable wall conditioning method to obtain a clean first wall for plasma breakdown after a venting. Direct Current (DC) Glow Discharge Cleaning (GDC) is chosen as a baseline wall conditioning technique to reduce and control impurity and hydrogenic fuel out-gassing from plasma-facing components [1]. Based on the staged installation strategy of ITER, for the first plasma operation, if no PE is available totally ten TEs [2] will be used and fixed on the Vacuum Vessel (VV) near those ports allocated for GDC, including the upper ports 3, 8, 14, the equatorial
∗ Corresponding author. Fax: +86 28 82850300. E-mail address: [email protected] (L. Cai).
ports 3, 8, 12, 17, and the lower ports 3, 9, 15. During conceptual design, the heat load of GDC electrode during different phases and operation states have been evaluated [3]. The heat load on TE during POS can be neglected due to low plasma discharge parameters and the 0.4 kW surface heat load during GDC is not a big problem for TE design. After first plasma, TEs will be removed and PEs installed. For maintainability reasons each PE will be integrated into port plugs at the upper or equatorial port level and the electrode front surface will be flush with the plasma facing surface of Diagnostic First Wall (DFW). Having taken into account all available locations, GDC layout is decided to provide toroidal uniform coverage to the maximum extent possible which results three PEs in the upper port level 3, 8 and 14, as shown in the left side of Fig. 1, and four PEs in the equatorial port level 3, 8, 12 and 17 as shown in the right side of Fig. 1. Because PEs will be used for all the later plasma opera-
http://dx.doi.org/10.1016/j.fusengdes.2016.01.060 0920-3796/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Cai, et al., Preliminary fluid channel design and thermal-hydraulic analysis of glow discharge cleaning permanent electrode, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.01.060
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Fig. 1. PE locations in the upper and equatorial ports.
tion including the D-T plasma operation, a high heat load totally about 72 kW will be deposited on the electrodes due to the plasma radiation, charge exchange particles and neutron heating. The heat load on PE during GDC or POS can be treated as steady state, a powerful cooling structure shall be designed to take the heat out of electrode and control the temperature rise and resultant thermal stress. Since each PE whether in upper port or equatorial port has similar main components and similar loading condition, so the fluid channel design and thermal hydraulic analysis in terms of one PE will be the reference for other PEs. The detailed design an analysis results will be discussed in the following sections.
etc., At present, the main components of PE except for insulation and fasteners will be made of stainless steel. As mentioned previously, the electrode body is the plasma facing and neutron shielding component, and it shall have the capability to survive under the high thermal load during plasma operation. The detailed design conditions, design schemes and thermal hydraulic analysis will be introduced in the following sections. 3. Fluid channels design and analysis inside electrode body 3.1. Design conditions
2. Preliminary design of GDC PE A typical PE as shown in Fig. 2 is mainly composed of four assemblies, i.e., electrode body, cooling tubes, vacuum penetration and ex-vessel components. The electrode body includes head and rod which are embedded in the diagnostic port plug first wall and Diagnostic Shielding Module (DSM) but electrically insulated from them. Fig. 3 shows the current design scheme. Ceramic support blocks are used as the insulator. During incidental and accidental conditions there will be high mechanical and thermal loads applied on the ceramics. The ceramic blocks will be bolted to the electrode body with slightly moveable capability relative to each other to minimize the mechanical and thermal stress. At the end of the rod, the electrode body is insulated and fixed onto the DSM. The elastic cooling pipes are used to feed the power to the electrode body, and to absorb the thermal expansion when these pipes are heated from 20 ◦ C to 240 ◦ C. The relative displacement between electrode body and vacuum feed through are also accommodated by these pipes. The cooling pipes with high potential must be insulated from the port plug closure plate and other components when they are being routed out of port plug. At present, ceramic coating will be adopted to provide the insulation. After feeding out, the cooling pipes are connected to the water manifolds outside the torus vacuum. The vacuum feed through is a double vacuum structure as it penetrating the first confinement barrier, and insulation with closure plate has to be considered. The second vacuum chamber shall be connected to the service vacuum system to allow monitoring of the vacuum state. Inherited from the conceptual design, the front surface of each PE is proposed to be approximately 0.07 m2 . The shape and dimension of the electrode body interrelate strongly with the diagnostic systems around. The design of PE needs to compromise among the weight, the volume for cooling channel, the structural integrity, the insulation gap and the remote handling compatibility
Two main loading conditions as shown in Table 1 including GDC/baking and POS are considered for the design of fluid channels inside electrode. The maximum surface heat load is 0.35 MW/m2 , and the maximum volumetric heat load is 10 W/cm3 at the front and the decay length is 12 cm [3]. The application of this load will be done according to the following formula with P0 = 10 W/cm3 and = 12 cm: P (x) = P0 e
−x
The following parameters are used to develop the coolant circuits. Some of these parameters are system requirements while others are design guidance or goals established from experiences of other ITER in-vessel components or design experience [5,6]. • Provide sufficient mass flow to limit temperature difference less than 50 ◦ C, then reduce the thermal stress below the allowable value • Limit average coolant velocities to∼5 m/s, the target value 1.5–3.5 m/s • Provide circuit with minimum flow velocity of 0.5 m/s • Limit peak stainless steel temperature <400 ◦ C, considering the thermal stresses, creep and thermal fatigue limits • No heat sink to surrounding components. 3.2. Cooling channels in electrode bulk The bulk portion of the electrode body is made from a single forging of 316SS. Within this bulk portion an arrangement of coolant passages are formed by drilling through various directions to form coolant circuits Fig. 4. The coolant channel ends are sealed with welded plugs. The basic arrangement is designed to have a series of layers each having a number of parallel passages forming “ladders”. The layers are positioned radially (along thickness direc-
Please cite this article in press as: L. Cai, et al., Preliminary fluid channel design and thermal-hydraulic analysis of glow discharge cleaning permanent electrode, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.01.060
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Table 1 Thermal and cooling conditions for PE [3,4].
Thermal load Inlet water temperature Maximum outlet temperature Inlet pressure Maximum pressure drop
POS
GDC/baking
About 72 kW for Rad & CX (21 kW) plus neutron heating (51 kW) 70 (±5) ◦ C (Burn period), 100 ◦ C (Dwell period) ≤126 (+10) ◦ C 4.0 (+0.6/−0.2) MPa 1.35 MPa
300 W 240 (+0/−5) ◦ C – 4.4 (+/−0.4) MPa ∼0.01 MPa (estimated, not a requirement)
Fig. 2. Main components of typical GDC PE.
Fig. 3. Support and insulation scheme of PE.
Table 2 Influence of flow rates on the cooling performance. Heat load W
Flow rate (kg/s)
Pressure drop (Pa)
Outlet coolant temperature (◦ C)
Max. coolant temperature (◦ C)
Max. temperature in steel (◦ C)
300 300 300 300 300
0.1 0.15 0.16 0.17 0.2
3583 7856 8886 10022 13751
240.63 240.41 240.39 240.36 240.31
242.36 241.74 241.73 241.59 241.46
244.25 243.79 243.68 243.67 243.54
Table 3 Temperature contour under different heat load but same flow rate 0.15 kg/s. Heat load (W)
Flow rate (kg/s)
Pressure drop (Pa)
Outlet coolant temperature (◦ C)
Max. coolant temperature (◦ C)
Max. temperature in steel (◦ C)
300 660 1020
0.15 0.15 0.15
7856 7845 7864
240.41 240.91 241.41
241.74 243.84 245.39
243.79 258.32 262.85
tion) at increasing distances from each other due to the reduction in the volumetric heating rates. The number of parallel passages is also reduced in each ladder due to the same heating reduction noted. Two layers are considered in the electrode for the deposited neutron heat. The cooling water will go directly from the rod to the first layer (closure cap) because of high surface heat load about 0.35 MW/m2 caused by plasma radiation. After circulating in the
first layer the cooling water then goes down to second layer (bulk), then go back along electrode rod and cooling pipes to the outside of vessel. Fig. 5 shows clearly the fluid domain inside the electrode.
3.3. Electrode closure cap The electrode surface which is directly exposed to the plasma requires a higher density of coolant channels which are unreason-
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Fig. 4. coolant circuits in electrode bulk.
Fig. 5. Fluid domain inside PE.
able to be produced with a deep drilling process used for the bulk. In this region a separate part “cap” is milled with coolant channels which can be tailored to provide the high density of coolant as shown in Fig. 6(a). The cap is then brazed onto the bulk main portion and sealed with a perimeter TIG weld. In this layer, the water will flow serially along 12 channels and the cross section for each channel is about 10 mm × 17 mm, and the flow rate should be higher than 0.5 kg/s to provide the velocity about 3 m/s. The influence of flow rate on the cooling performance is discussed in the following sections. For the cap thickness determination, a balance needs to be found considering multiple design factors [5]: coolant pressure stress (effects on nuclear pressurized equipment exemption), erosion requirements (1–2 mm over life time), fabrication costs (depth
Fig. 6. (a) Closure cap design and (b) stress under pressure.
vs width ratio of channels), complexity of channels routing design (greater number of parallel passages produces difficulties). To gain nuclear pressurized equipment exemption a high ratio of component stress verse coolant pressure stress was required (low primary stress from coolant pressure). With a material allowable stress limit (@125 ◦ C) of 147 MPa a ratio of 1/5 produce a design goal of ∼30 MPa coolant pressure stress within the closure cap. Based on the above
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Fig. 8. Temperature contour on stainless steel during GDC.
3.5. Thermal fatigue analysis under POS condition Since the critical heat load occurs during POS conditions, and the permanent electrode has to survive after 30,000 thermal loading cycles. This is very challenging for stainless steel under 0.35 MW/m2 surface heat load plus neutron heat load, so great efforts were spent on the thermal fatigue analysis of closure cap. According to the structural design criteria for in-vessel components [7] the equivalent strain range got from elastic-plastic analysis should be less than 0.3% for 30,000 loading cycles. Fig. 9 shows that the maximum equivalent strain range is 0.257% well below the design goals. 4. Influence of flow rate and heat load during GDC condition Fig. 7. Temperature contour of (a) water and (b) stainless steel.
consideration, the closure cap thickness is chosen as 6 mm, and the equivalent Mises stress is smaller than 28 MPa shown in Fig. 6(b) under maximum absolute pressure 4.9 MPa.
3.4. Thermal hydraulic analysis ANSYS CFX 14.5 was used to run the thermal hydraulic analysis based on the current cooling circuit design. To guarantee the validity, most of initial efforts were spent on the mesh quality of calculation models and also on the selection of turbulence parameters. For POS condition 0.5 kg/s flow rate is set as the initial value. Fig. 7(a) shows the maximum local water temperature is 151.8 ◦ C induced by the sharp corner in the analyzing model. The inlet and outlet temperature difference is only 17.75 ◦ C well under the design goal value 50 ◦ C. The pressure drop is 0.15 MPa far smaller than 1.35 MPa. Fig. 7(b) shows the stainless steel maximum temperature is 409 ◦ C in sharp corner region, but the vast majority of the electrode bulk is less than 400 ◦ C. A subsequent model without sharp corners has validated that this value can be eliminated by fabrication fillet treatment. For GDC loading condition simulation 0.15 kg/s flow rate is set as the initial value. The inlet and outlet temperature rise is only 0.2 ◦ C, and the pressure drop is only 0.008 MPa. The maximum temperature rise on the stainless steel is about 4 ◦ C as shown in Fig. 8. Based on the results of thermal hydraulic analysis, the design goals are well achieved for the cooling circuit design.
For GDC loading condition, the allowable water temperature rise is about 11.8 ◦ C (boiling point 251.8 ◦ C under 4 MPa pressure), so different flow rates have been considered to evaluate the cooling performance and to reduce the temperature rise to the maximum extent. From the results in Table 2, when increasing the flow rate from 0.1 kg/s to 0.2 kg/s, the temperature both of water and stainless steel will not change a lot but the pressure drop will be exponentially increased. So 0.15 kg/s is chosen to keep the minimum water velocity inside electrode bigger than 0.5 m/s, and the thermal stress is about 57 MPa in this condition which means far less than the allowable stress of stainless steel under this temperature. According to the experiences on some test beds and other tokamak devices, the heat load deposited on the electrode during the glow discharge cleaning is not constant, the heat flux density and distribution will change even for the same feeding of electric current. So based on the design baseline heat load 300 W for GDC condition, the heat load was increased by 2 times or 3 times. The results in Table 3 shows that current coolant circuit design and corresponding cooling water parameters can withstand the fluctuation, which means the temperature rise is still lower than the allowable value 11.8 ◦ C even for 3 times normal heat load. 5. Conclusion and discussion The main components of a typical PE include electrode body, cooling tubes, feed through parts and ex-vessel assemblies. The preliminary design activities are planned according to the schedule, and recent design and analysis progress of fluid channels inside electrode are introduced in this paper. Detailed design principles, design goals, and concrete design parameters for the coolant
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Fig. 9. Equivalent strain range of closure cap surface after 30,000 loading cycles.
circuits have been discussed here. Based on two main working conditions, a great deal of thermal hydraulic analysis and thermal fatigue analysis have been done to investigate the cooling performance of cooling circuits and to validate the design rationality. The results show that the present coolant channel design of electrode can withstand the thermal load during the GDC condition, which means the temperature and thermal stress are within reasonable range. For critical POS condition, the thermal hydraulic requirements such as temperature rise or pressure drop are satisfied. High surface stress is caused by large temperature gradient along the thickness direction of closure cap. After several iterations of optimization of the closure cap structure, the equivalent strain range after 30,000 loading cycles for POS are well below 0.3% design goals. Acknowledgements
information in this paper is governed by the applicable terms of the ITER Joint Implementation Agreement. References [1] Y. Yang, S. Maruyama, et al., Re-design of ITER glow discharge cleaning system based on a fixed electrode concept, Fusion Eng. Des. 89 (2014) 1944–1948. [2] Private communication. SRD-18-GC (Glow Discharge Wall Conditioning) from DOORS. [3] Private communication. GDC heat load. [4] Private communication. System Requirement (SRD) Document. SRD-26-PH, −CV, −DR −DY (TCWS) from DOORS. [5] Private communication. System Design Description Document (DDD) Diagnostic First Wall DFW. [6] M. Merola, R. Matera, F. Sevini, Experimental results of the thermal fatigue tests for I A.E.A. benchmark components, J. Nucl. Mater. 233–237 (1996) 620–625. [7] Private communication. Structural Design Criteria for ITER In-vessel Components (SDC-IC).
The author would like to thank all the colleagues from IO and CNDA for their good comments and suggestions. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization or any member of ITER. Dissemination of the
Please cite this article in press as: L. Cai, et al., Preliminary fluid channel design and thermal-hydraulic analysis of glow discharge cleaning permanent electrode, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.01.060