DESIGN OPTIMIZATION OF HIGH-PERFORMANCE HELIUM-COOLED DIVERTOR PLATE CONCEPT

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1 DESIGN OPTIMIZATION OF HIGH-PERFORMANCE HELIUM-COOLED DIVERTOR PLATE CONCEPT X.R. Wang a, S. Malang b, A.R. Raffray a and the ARIES Team a Center for Energy Research, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA , USA, xrwang@ucsd.edu b Fusion Nuclear Technology Consulting, Fliederweg 3, Linkenheim, Germany A helium-cooled plate-type divertor design concept has being proposed within the framework of the ARIES power plant study. W tiles are used as sacrificial armor, W-alloy as main structural material and advanced ODS (oxide dispersion-strengthened) steel as the coolant manifold. An impinging jet cooling scheme is employed to enhance the heat transfer characteristics of the concept in the high heat flux zone. This paper describes the design optimization of the helium-cooled plate divertor through parametric studies including thermo-fluid and thermomechanical analyses. I. Introduction A number of different He-cooled divertor concepts have been considered in the past years for fusion power plant. Designs include the EU FZK He-cooled modular divertor with jet cooling (HEMJ) 1, 2 and the ARIES-CS T- tube configuration with slot-jet cooling. 3, 4 The HEMJ concept is based on the use of multiple-jet cooling; it employs small hexagonal W tiles (18 mm wide, 5 mm thick) as armor, brazed to a W-alloy thimble ( 15 mm x 1 mm) forming a cooling finger. Each finger is cooled with the helium at 10 MPa and 600/700 ºC (inlet/outlet temperatures). The HEMJ concept is designed to accommodate an incident heat flux of at least 10 MW/m 2 and perhaps higher. The ARIES-CS T-tube concept is developed to provide a mid size unit capable of accommodating at least 10 MW/m 2, and the design is suited for both stellarator and tokamak configurations. 3, 4 The T-tube is ~15 mm in diameter and ~0.1 m long and is made up of the W-alloy inner cartridge and outer tube on top of which a W castellated armor layer is attached. The slot jet cooling scheme is used to obtain high velocity and high wall heat transfer coefficient for heat flux accommodation. A crucial issue with the small-scale unit designs (the HEMJ and T-Tube) is reliability since large numbers of elements are needed for a power plant. Use of a larger scale divertor concept would reduce the number of units and the design complexity, providing an important advantage. An early design of a large-scale divertor concept was performed in 2002 by Hermsmeyer and Malang. 5 However, thermal stress was a major concern for this concept. This paper presents an evolution of the helium-cooled plate-type divertor concept with features enhancing the heat transfer and reducing the thermal stresses in order to accommodate a high heat flux of about 10 MW/m 2. Design details are addressed in this paper, and results from the supporting analyses are summarized, including the computational fluid dynamic (CFD) thermo-fluid and the finite element (FE) thermo-mechanical analyses. A comparison with results obtained for the other He-cooled divertor concepts mentioned above is given in Ref. 6. II. General Design Requirements The helium-cooled plate-type divertor design concept was proposed to take advantage of larger scale in order to reduce the number of units, fabrication complexity and possibly cost of the divertor. The goal in designing the plate-type divertor concept for a fusion power plant is to accommodate a high heat flux (of the order of 10 MW/m 2 ) while meeting the main design requirements, namely: (a) the thermal power of the divertor plates has to be used efficiently in the power conversion system; (b) the lifetime of the target plates should not be shorter than the lifetime of the blanket modules in order to avoid additional down time for replacing them; (c) the operating temperature of the plates must be higher than 800 ºC in order to avoid embrittlement under neutron irradiation; (d) the maximum structure temperature must be maintained below the re-crystallization temperature of the W-alloy (~1300 ºC); (e) the thermal stresses must be acceptable; (f) the required pumping power for the divertor coolant should be less than about 10% of the heat extracted from the plates; and (g) it must be possible to align the target plates precisely relative to the plasma with a tolerance < 1 mm. FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG

2 III. Divertor Plate Design III.A. Design Description The allowable heat flux for the plate-type concept is limited by the temperature of the front structure (underlying the armor) and total stress levels, mainly thermal stresses due to the high heat flux on the armor causing large temperature difference between the front structure and the side and back wall of the plate. Possible ways of reducing the thermal stresses include: (1) decreasing the temperature at the front plate by reducing its thickness and enhancing the heat transfer coefficient, (2) raising the temperature of the side and back plates to keep temperature differences small (100~200 ºC) to reduce the plate bending stresses caused by the thermal expansion, and (3) using small castellated tiles to avoid inducing additional stresses in the front plate. Thus, the engineering effort was mainly focused on developing and optimizing the plate geometry and cooling scheme to improve cooling performance with acceptable structure temperature ( 1300 ºC), stresses (3Sm 450 MPa) and pumping power (<10% of the thermal power). Fig.1 and Fig.2 show the configuration of the target plate. The plate unit includes (a) the W armor layer with 5 mm (tor.) x 5 mm (pol.) castellation, (b) the W-alloy front plate with grooves (underlying the W armor), (c) the W- alloy side plates, (d) the W-alloy back plate with grooves for brazing in the front plate and the side plates together into one unit, (e) the ODS-steel inlet and outlet manifolds, and (f) transition pieces from the W plate to the ODSsteel manifolds at the ends of the plate where the heat flux is low. The front plate with the castellation and grooves is fabricated as a single piece and brazed together with the side and back plates. Each divertor plate consists of ~8 parallel slot channels with ~1 m poloidal length and 2 cm toroidal pitch, as illustrated in Fig. 1 (a typical toroidal dimension would be 19.2 cm) and Fig. 2. A thicker back plate (4-10 mm), side wall (2-3 mm each side of the filled He at 10 MPa) between the W-alloy side wall and the ODS manifold are employed to raise the side wall and the back plate temperatures. In this way, the temperature differences between the W front plate (underneath the W armor) and the side/back plates are reduced, and so are the thermal stresses. The thickness of the helium insulator gap and the back plate and the side wall thicknesses can be adjusted according to the level of the volumetric heating rates and the surface heat flux on the W armor. An advantage of the larger-scale plate design is the reduction in number of units, and the associated reduction in complexity and possibly costs. Only 750 plate units are needed for a tokamak power plant with an assumed divertor area of 150 m 2 compared to ~535,000 HEMJ elements and 110,000 T-tubes, respectively. Fig. 1. Helium-cooled divertor target plate (half of the plate length) Fig. 2. Cross-section of the flat-plate divertor target III.B. Cooling Scheme Flow configurations were investigated and optimized to improve cooling performance with acceptable structure temperature, stresses and pumping power. Three impinging jet cooling schemes were explored and compared based on their cooling performance, including (a) a slot-jet cooling scheme similar to the T-tube, (b) a micro-channel (~0.25 mm) cooling, and (c) multiple impinging jet arrays similar to the HEMJ. The goal in designing the flow configuration is to optimize the flow velocity (in the range of 200~300 m/s) and the heat transfer coefficient (in the range of 30~50 kw/m 2 K) to cool the front plate effectively, while maintaining an acceptable pressure drop. Concurrently, the channel structure temperature is increased in order to minimize the temperature difference between the front plate (underlying the W armor) and back to 100~200 ºC range. Schematics of the plate and the helium flow configuration 1024 FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG. 2009

3 are illustrated in Fig. 2 and Fig. 3. The inlet and outlet manifolds are inserted into the slot channel (20 mm wide) extending all the way along the longitudinal (poloidal) direction, and they are tapered in order to balance the flow velocity (~ a few cm/s) in the longitudinal direction as schematically illustrated in Fig.3. The helium coolant is routed through the inlet manifold with low velocity and then flows though thin slot-jets (~0.5 mm) with high jet velocity to cool the front plate which is subjected the heat flux of 10 MW/m 2. After the flow impinges on the front plate surface, the coolant flows as a highly turbulent wall jet along the large inside surface and then returns to the outlet manifold. Fig. 5. Cross section of the transition region IV. Design Support Analysis Fig. 3. Longitudinal section through the target plate channel (length scaled-down to ¼) III.C. W-Alloy/Steel Transition A concept for the transition from the W-alloy plate to the ODS steel manifolds is proposed to avoid thermocyclic plasticization of the W-alloy/steel joints due to a large mismatch of thermal expansion coefficient of the W (4-6 x 10-6 K -1 ) and the ODS steel (10-14 x 10-6 K -1 ). It consists of placing a Ta-alloy piece between the W-alloy and the ODS steel because the the rmal expansion coefficient of the Ta-alloy is right between those of the W-alloy and the ODS steel. The transition concept is illustrated in Fig. 4 and Fig. 5. Analysis of the transition from the W plate to the ODS steel manifold at both ends of plate is underway at the University of California, Los Angeles. 7 The detailed analysis will include sophisticated thermal cycling calculations to find out the long-term behavior of such transition zones, taking into account plastic deformation of all the materials in this zone (Walloy, Ta-alloy, and ODS steel). Fig. 4. Transition pieces of the plate Detailed 3-D CFD (using CFX 8 ) and 3-D FE thermomechanical simulations (using ANSYS Workbench 9 ) were performed to explore the allowable heat flux at the divertor plate with respect to the temperature, stress, deflection and pumping power limits, and optimize the design configuration. IV.A. CFD Thermo-fluid Simulations Three impinging jet cooling schemes were investigated to assess their performance based on the resulting structure temperature, local heat transfer coefficient and pumping power. The cooling schemes include: (a) a slot-jet cooling scheme similar to the T- tube, (b) a micro-channel cooling similar to Ref. 5, and (c) multiple impinging jet arrays similar to the HEMJ. All the CFD simulations were based on the same thermal loads and mass flow rates. Standard turbulent flow model, k- with wall enhancement was assumed in the CFD simulations. Typical parameters for the slot-jet cooling scheme are summarized in the table I. TABLE I. Divertor Design Parameters He pressure, MPa 10 Peak heat flux, MW/m 2 10 Volumetric heating rate, MW/m 3 53 He inlet temperature, ºC 600 He outlet temperature, ºC 677 Nozzle width D, mm 0.5 Nozzle-to-wall distance H, mm 1.2 Length of the plate, cm 100 Width of the plate, cm 19.2 Height of the plate, cm 6 Width of the channel, cm 2 Number of the parallel channel per plate 8 FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG

4 For the micro-channel cooling scheme, the width of the slot-jet, D, and the radial thickness of the micro-channel, H, were assumed to be 0.5 mm and 0.25 mm, respectively. For the cooling scheme using the multiple impinging jet arrays, the diameter of nozzle and the space of the nozzle-to-wall were assumed to be 0.6 mm and 1.2 mm, respectively. thermal stresses. The ODS insert manifolds were excluded from the thermo-mechanical model. The heat transfer coefficient or the temperature at the interface of the He/W structure were obtained from the CFX thermofluid analyses and used as thermal boundary condition for computing the volumetric temperature in the FE model; a coupled thermo-mechanical analysis was then performed. The following mechanical boundary conditions were applied: (1) no channel bending and in-plane back side, and (2) symmetry condition in the middle of the channel. This resulted in a conservative boundary condition since the entire surface at the back plate was restrained from free bending. Fig. 6. Velocity distribution in the channel illustrating the jet flow to cool the heated zone Fig. 6 shows the velocity distributions for the slot-jet cooling scheme. The maximum velocity is about 258 m/s, and the corresponding heat transfer coefficient is ~4.19x10 4 W/m 2 -K while the pumping power is less than 10% of the thermal power. Both the velocity and the heat transfer coefficient for the micro-channel and multiple jet cooling schemes are higher than those for the slot-jet cooling scheme, but with a correspondingly higher pumping power. Results from experimental investigation of the thermal performance for the gas-cooled divertor plate concept are reported by Gayton. 10 IV.B. FE Thermo-mechanical Analysis A plate divertor concept with the slot-jet cooling scheme was selected as the reference design for the thermo-mechanical analysis because of its superior cooling performance compared to the other two impinging jet cooling methods. The 3-D FE model of the plate included a full length of the channel (20 mm wide and ~ 1 m long in the poloidal direction) with a W armor layer (~ 5 mm thick) on the front plate. A castellated armor was assumed (5 mm (tor.) x 5 mm (pol.)) to reduce Fig. 7. Temperature distribution of the plate The temperature distribution of the plate (including the W armor) is shown in Fig. 7 and the distribution of the combined (primary + thermal) stresses is shown in Fig. 8. The maximum temperature of the W armor is 1834 ºC and the maximum temperature at the interface between the W armor and the front plate (underneath the W armor) is 1296 ºC, which is within the 1300 ºC recrystallization limit assumed for the W-alloy. As shown in Fig. 8, the maximum combined stress is 394 MPa for the 5 mm x 5 mm tile castellation, which is within the assumed 3Sm limit of 450 MPa. The maximum pressure stress is 140 MPa (within the assumed Sm limit of 150 MPa). The maximum total deformation is about 1 mm. A plate design with some slight modifications was also assessed. It included increasing the channel width (from 20 to 22 mm) and the thickness of the side wall (from 2 to 3 mm) under a volumetric neutron heat 1026 FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG. 2009

5 generation similar to that for ARIES-AT (17.5 MW/m 3 for an average wall load of 3.2 MW/m 2 ). CFD thermofluid and FE thermo-mechanical results indicated that the temperature, stress and pumping power all meet the design constraints for a heat flux of 10 MW/m 2. example, the FZK HEMJ or the ARIES T-tube) in the high heat flux region. An example of such a design is described in Ref. 6. ACKNOWLEDGMENTS The work at UCSD was supported under U.S. Department of Energy Grant number DE-FG02-04ER REFERENCES Fig. 8. Combined stress (primary + secondary) distribution of the plate V. CONCLUSIONS AND OUTLOOK A helium-cooled W plate-type divertor design concept has been developed within the framework of the ARIES power plant study. The initial results from the supporting analyses are encouraging. They show that the concept can accommodate a peak heat flux of 10 MW/m 2 while accommodating the temperature, stress and pumping power constraints under uniform heat load conditions. However, concerns exist as to the dynamic stress in case of heat flux transient or during reactor startup/shutdown because of the different thermal time constants in the front and back parts of the plate. These would further limit the maximum allowable heat flux and would need to be addressed as part of the future effort on developing this concept.. The typical variation of surface heat flux over the poloidal length of the divertor target results in a peak to average value in the range of 2-3. This brings up the interesting possibility of a combined plate-type divertor design including an integrated section with smaller units (for 1. T.IHLI, R. KRUESSMANN, I. OVCHINNIKOV, P. NORAJITRA, V. KUZNETSOV, and R. GINIYATULIN, An Advanced He-cooled Divertor Concept: Design, Cooling Technology, and Thermohydraulic Analyses with CFD, Fusion Engineering and Design, 75-79, 371(2005). 2. P. NORAJITRA, R. GINIYATULIN, T. IHLI, G.JANESCHITZ, W. KRAUSS, R. KRUESSMANN, V. KUZNETSOV, I. MAZUL, V. WIDAK, I. OVCHINNIKOV, R. RUPRECHT, AND ZEEP, He-cooled Divertor Development for DEMO, Fusion Engineering and Design, 82, 2740(2007). 3. T. IHLI, A.R. RAFFRAY, S.I. ADBDEL-KHALIK, S. SHIN, and the ARIES-CS Team, Design and Performance Study of the Helium-cooled T-tube Divertor Concept, Fusion Engineering and Design, 82, 249(2007). 4. A.R. RAFFRAY, L. EL-GUEBALY, T. IHLI, S. MALANG, X. WANG, and the ARIES-CS Team, Engineering Design and Analysis of the ARIES-CS Power Plant, Fusion Science & Technology, 54, (2008). 5. S. HERMSMEYER and S. MALANG, Gas-Cooled High Performance Divertor for a Power Plant Fusion Engineering and Design, 61-62, 197(2002). 6. A.R. RAFFRAY, S. MALANG, and X. WANG, Optimizing the Overall Configuration of a He- Cooled W-Alloy Divertor for a Power Plant, accepted for presentation at the 25 th SOFT, September S. SHARAFAT, A. AOYAMA, Thermo- Mechanical Stress Analysis of a Tungsten-tantalum- Steel Transition Joint, Internal Report University of California Los Angeles, August ANSYS Inc., ANSYS CFX Documentation, ANSYS Inc., ANSYS Release 11.0 Documentation, E.GAYTON, L.CROSATTI, D.L. SADOWSKI, S.I. ABDEL-KHALIK, M. YODA AND S. MALANG, Experimental and Numerical Investigation of the Thermal Performance of the Gas-cooled Divertor Plate Concept, this issue. FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG