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A new vision of plasma facing components

Fusion Engineering and Design

Nygren, Richard E.; Youchison, Dennis L.; Wirth, Brian D.; Snead, Lance L.

This paper advances a vision for plasma facing components (PFCs) that includes the following points. The solution for plasma facing materials likely consists of engineered structures in which the layer of plasma facing material (PFM) is integrated with an engineered structure that cools the PFM and may also transition with graded composition. The key to achieving this PFC architecture will likely lie in advanced manufacturing methods, e.g., additive manufacturing, that can produce layers with controlled porosity and features such as micro-fibers and/or nano-particles that can collect He and transmutation products, limit tritium retention, and do all this in a way that maintains adequate robustness for a satisfactory lifetime. This vision has significant implications for how we structure a development program.

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Flow instabilities in refractory metal, porous media, helium-cooled plasma facing components

Proceedings - Symposium on Fusion Engineering

Youchison, Dennis L.; Nygren, Richard E.

Past numerical investigations of the performance of porous media to enhance heat transfer in helium-cooled devices neglected the susceptibility of multi-channel heat sinks to parallel flow instabilities even though experimental evidence suggests it may be a problem for narrow channel devices. In previous work, our simulations have shown that helium micro-jets do not experience changes in flow distribution due to non-uniform heating. However, jets are difficult to fabricate for large area refractory metal components. The same is not true for narrow channel devices filled with porous media. Although these refractory devices are easier to fabricate, the effects of downstream hot gas expansion can influence the incoming flow distribution in multi-channel configurations.

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Flow instabilities in non-uniformly heated helium jet arrays used for divertor PFCs

Fusion Science and Technology

Youchison, Dennis L.

Due to a lack of prototypical experimental data, little is known about the off-normal behavior of recently proposed divertor jet cooling concepts. This article describes a computational fluid dynamics (CFD) study on two jet array designs to investigate their susceptibility to parallel flow instabilities induced by non-uniform heating and large increases in the helium outlet temperature. The study compared a single 25-jet helium-cooled modular divertor (HEMJ) thimbleand a micro-jet array with 116 jets. Both havepure tungsten armor and atotal mass flow rate of 10 g/s at a 600°C inlet temperature. We investigated flow perturbations caused by a 30 MW/m2 off-normal heat flux applied over a 25 mm2 area in addition to the nominal 5 MW/m2 applied over a 75 mm2 portion of the face. The micro-jet array exhibited lower temperatures and a more uniform surface temperature distribution than the HEMJ thimble. We also investigated the response of a manifolded nine-finger HEMJ assemblyusing the nominal heat flux and a 274 mm2 heated area. For the 30 MW/m2 case, the micro-jet array absorbed 750 W in the helium with a maximum armor surface temperature of 1280°C and a fluid/solid interface temperature of 801°C. The HEMJ absorbed 750 W with a maximum armor surface temperature of 1411°C and a fluid/solid interface temperature of 844°C.For comparison, both the single HEMJ finger and the micro-jet array used 5-mm-thick tungsten armor. The ratio of maximum to average temperature and variations in the local heat transfer coefficient were lower for the micro-jet array compared to the HEMJ device. Although high heat flux testing is required to validate the results obtained in these simulations, the results provide important guidance in jet design and manifolding to increase heat removal while providing more even temperature distribution and minimizing non-uniformity in the gas flowand thermal stresses at the armor joint.

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Flow instabilities in non-uniformly heated helium jet arrays used for divertor PFCs

Fusion Science and Technology

Youchison, Dennis L.

In this study, due to a lack of prototypical experimental data, little is known about the off-normal behavior of recently proposed divertor jet cooling concepts. This article describes a computational fluid dynamics (CFD) study on two jet array designs to investigate their susceptibility to parallel flow instabilities induced by non-uniform heating and large increases in the helium outlet temperature. The study compared a single 25-jet helium-cooled modular divertor (HEMJ) thimble and a micro-jet array with 116 jets. Both have pure tungsten armor and a total mass flow rate of 10 g/s at a 600 °C inlet temperature. We investigated flow perturbations caused by a 30 MW/m2 off-normal heat flux applied over a 25 mm2 area in addition to the nominal 5 MW/m2 applied over a 75 mm2 portion of the face. The micro-jet array exhibited lower temperatures and a more uniform surface temperature distribution than the HEMJ thimble. We also investigated the response of a manifolded nine-finger HEMJ assembly using the nominal heat flux and a 274 mm2 heated area. For the 30 MW/m2 case, the micro-jet array absorbed 750 W in the helium with a maximum armor surface temperature of 1280 °C and a fluid/solid interface temperature of 801 °C. The HEMJ absorbed 750 W with a maximum armor surface temperature of 1411 °C and a fluid/solid interface temperature of 844 °C. For comparison, both the single HEMJ finger and the micro-jet array used 5-mm-thick tungsten armor. The ratio of maximum to average temperature and variations in the local heat transfer coefficient were lower for the micro-jet array compared to the HEMJ device. Although high heat flux testing is required to validate the results obtained in these simulations, the results provide important guidance in jet design and manifolding to increase heat removal while providing more even temperature distribution and minimizing non-uniformity in the gas flow and thermal stresses at the armor joint.

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Ultrahigh heat flux plasma-facing components for magnetic fusion energy

Youchison, Dennis L.

Sandia and Ultramet partnered to design and test refractory metal plasma-facing components and heat exchangers for advanced, high-temperature power conversion systems. These devices consisted of high-temperature helium-to-helium and lithium-to-helium heat exchangers that operate with high efficiency due to the porous foam inserts used in the gas stream, which promote turbulence and provide extended surface area for enhanced convection. Single- and multi-channel helium panels and the Li-He heat exchanger were fabricated from either pure molybdenum, TZM, or tungsten. The design was carried out through an Ultramet subcontractor. The flow path was carefully tailored to minimize the pressure drop while maximizing the heat transfer. The single- and multi-channel helium panels were tested at Sandia's PMTF using an electron beam system and the closed helium flow loop. In 2006, a single-channel tungsten tube was successfully tested to an average heat flux of 14 MW/m{sup 2} with a localized peak of 22 MW/m{sup 2} along the axial centerline at the outer radius. Under this CRADA, multiple square-channel molybdenum components were successfully tested to heat flux levels approaching 8.5 MW/m{sup 2}. The three multi-channel prototypes experienced mechanical failure due to issues related to the design of the large unsupported span of the heated faceplates in combination with prototype material and braze selection. The Li-He heat exchanger was both designed and partially tested at the PMTF for helium and lithium flow.

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Prediction of Critical Heat Flux in Water-Cooled Plasma Facing Components Using Computational Fluid Dynamics

Fusion Science and Technology

Youchison, Dennis L.; Ulrickson, M.A.

Several commercial computational fluid dynamics (CFD) codes now have the capability to analyze Eulerian two-phase flow using the Rohsenow nucleate boiling model. Analysis of boiling due to one-sided heating in plasma facing components (pfcs) is now receiving attention during the design of water-cooled first wall panels for ITER that may encounter heat fluxes as high as 5 MW/m2. Empirical thermalhydraulic design correlations developed for long fission reactor channels are not reliable when applied to pfcs because fully developed flow conditions seldom exist. Star-CCM+ is one of the commercial CFD codes that can model two-phase flows. Like others, it implements the RPI model for nucleate boiling, but it also seamlessly transitions to a volume-of-fluid model for film boiling. By benchmarking the results of our 3d models against recent experiments on critical heat flux for both smooth rectangular channels and hypervapotrons, we determined the six unique input parameters that accurately characterize the boiling physics for ITER flow conditions under a wide range of absorbed heat flux. We can now exploit this capability to predict the onset of critical heat flux in these components. In addition, the results clearly illustrate the production and transport of vapor and its effect on heat transfer in pfcs from nucleate boiling through transition to film boiling. This article describes the boiling physics implemented in CCM+ and compares the computational results to the benchmark experiments carried out independently in the United States and Russia. Temperature distributions agreed to within 10 °C for a wide range of heat fluxes from 3 MW/m2 to 10 MW/m2 and flow velocities from 1 m/s to 10 m/s in these devices. Although the analysis is incapable of capturing the stochastic nature of critical heat flux (i.e., time and location may depend on a local materials defect or turbulence phenomenon), it is highly reliable in determining the heat flux where boiling instabilities begin to dominate. Beyond this threshold, higher heat fluxes lead to the boiling crisis and eventual burnout. This predictive capability is essential in determining the critical heat flux margin for the design of complex 3d components.

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Prediction of critical heat flux in water-cooled plasma facing components using computational fluid dynamics

Youchison, Dennis L.; Ulrickson, M.A.

Several commercial computational fluid dynamics (CFD) codes now have the capability to analyze Eulerian two-phase flow using the Rohsenow nucleate boiling model. Analysis of boiling due to one-sided heating in plasma facing components (pfcs) is now receiving attention during the design of water-cooled first wall panels for ITER that may encounter heat fluxes as high as 5 MW/m2. Empirical thermalhydraulic design correlations developed for long fission reactor channels are not reliable when applied to pfcs because fully developed flow conditions seldom exist. Star-CCM+ is one of the commercial CFD codes that can model two-phase flows. Like others, it implements the RPI model for nucleate boiling, but it also seamlessly transitions to a volume-of-fluid model for film boiling. By benchmarking the results of our 3d models against recent experiments on critical heat flux for both smooth rectangular channels and hypervapotrons, we determined the six unique input parameters that accurately characterize the boiling physics for ITER flow conditions under a wide range of absorbed heat flux. We can now exploit this capability to predict the onset of critical heat flux in these components. In addition, the results clearly illustrate the production and transport of vapor and its effect on heat transfer in pfcs from nucleate boiling through transition to film boiling. This article describes the boiling physics implemented in CCM+ and compares the computational results to the benchmark experiments carried out independently in the United States and Russia. Temperature distributions agreed to within 10 C for a wide range of heat fluxes from 3 MW/m2 to 10 MW/m2 and flow velocities from 1 m/s to 10 m/s in these devices. Although the analysis is incapable of capturing the stochastic nature of critical heat flux (i.e., time and location may depend on a local materials defect or turbulence phenomenon), it is highly reliable in determining the heat flux where boiling instabilities begin to dominate. Beyond this threshold, higher heat fluxes lead to the boiling crisis and eventual burnout. This predictive capability is essential in determining the critical heat flux margin for the design of complex 3d components.

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Silicon carbide tritium permeation barrier for steel structural components

Buchenauer, D.A.; Kolasinski, Robert K.; Youchison, Dennis L.; Garde, J.; Holschuh, Thomas V.

Chemical vapor deposited (CVD) silicon carbide (SiC) has superior resistance to tritium permeation even after irradiation. Prior work has shown Ultrametfoam to be forgiving when bonded to substrates with large CTE differences. The technical objectives are: (1) Evaluate foams of vanadium, niobium and molybdenum metals and SiC for CTE mitigation between a dense SiC barrier and steel structure; (2) Thermostructural modeling of SiC TPB/Ultramet foam/ferritic steel architecture; (3) Evaluate deuterium permeation of chemical vapor deposited (CVD) SiC; (4) D testing involved construction of a new higher temperature (> 1000 C) permeation testing system and development of improved sealing techniques; (5) Fabricate prototype tube similar to that shown with dimensions of 7cm {theta} and 35cm long; and (6) Tritium and hermeticity testing of prototype tube.

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High heat flux testing of a helium-cooled tungsten tube with porous foam

Fusion Engineering and Design

Youchison, Dennis L.; Lutz, Thomas J.; Williams, B.; Nygren, Richard E.

Utramet, Inc. fabricated one-piece heat exchanger tubes of chemical vapor deposited (CVD) tungsten (W), each with an internal porous mesh fused along either 51 or 38 mm of the axial length of a tube 15 mm in outer diameter. The open porous mesh has a structure of joined ligaments that combines relatively low resistance to flow and a large area for heat transfer. In tests at the Electron Beam Test Stand (EBTS) at Sandia National Laboratories, the maximum absorbed heat load was 22.4 MW/m2 with helium at 4 MPa, flowing at 27 g/s and with inlet and outlet temperatures of 40 and 91 °C and a pressure drop of ∼0.07 MPa. The preparation and testing of the samples was funded through a Phase I grant by the US Department of Energy's Small Business Innovation Research Program. The paper reports the surface temperature distribution indicated by an infrared camera, test conditions, a post-test examination in a scanning electron microscope and other details. © 2007 Elsevier B.V. All rights reserved.

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Features of plasma sprayed beryllium armor for the ITER first wall

Journal of Nuclear Materials

Nygren, Richard E.; Youchison, Dennis L.; Hollis, K.J.

Two water-cooled mockups with CuCrZr heat sinks and plasma sprayed beryllium (PS Be) armor, 5 and 10 mm thick respectively, were fabricated at Los Alamos National Laboratory and thermally cycled at Sandia at 1 and 2 MW/m2. The castellated surface of the CuCrZr mechanically locked the armor. The resulting PS Be morphology controlled cracking during thermal cycling. Post test examinations showed transverse cracks perpendicular to the surface of the armor that would relieve thermal stresses but not degrade heat transfer. The mockups and two others previously produced for the European Fusion Development Agreement had somewhat porous armor, with a thermal conductivity estimated to be about 1/4 that of fully dense beryllium, due to the low (600-650 °C) substrate temperature during deposition specifically requested by EFDA to avoid subsequent heat treating of CuCrZr. Some melting of the armor was expected and observed in the tests. © 2007 Elsevier B.V. All rights reserved.

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ITER first wall Module 18 - The US effort

Fusion Engineering and Design

Nygren, Richard E.; Ulrickson, M.A.; Tanaka, T.J.; Youchison, Dennis L.; Lutz, Thomas J.; Bullock, J.; Hollis, K.J.

The US will supply outboard Module 18 for the International Thermonuclear Experimental Reactor. This module, radially thinner than other modules with a "nose" that curves radially outward to mate with the divertor, has the potential for high electromagnetic (EM) loads from vertical displacement events and high heat loads. The 316LN-IG shield block and first wall (FW) panels must be slotted to mitigate the EM loads and progress in developing the design is summarized. The FW has beryllium (Be) armor joined to a water-cooled CuCrZr heat sink with embedded 316LN-IG cooling channels. The US Team is considering possible fabrication methods as the design develops. Brief results of high heat flux experiments at Sandia on mockups with plasma-sprayed Be armor prepared at Los Alamos National Laboratory are noted. © 2005 Elsevier B.V. All rights reserved.

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Liquid metal integrated test system (LIMITS)

Fusion Engineering and Design

Tanaka, T.J.; Bauer, F.J.; Lutz, Thomas J.; McDonald, Jimmie M.; Nygren, Richard E.; Troncosa, K.P.; Ulrickson, M.A.; Youchison, Dennis L.

This paper describes the liquid metal integrated test system (LIMITS) at Sandia National Laboratories1. This system was designed to study the flow of molten metals and salts in a vacuum as a preliminary study for flowing liquid surfaces inside of magnetic fusion reactors. The system consists of a heated furnace with attached centrifugal pump, a vacuum chamber, and a transfer chamber for storage and addition of fresh material. Diagnostics include an electromagnetic flow meter, a high temperature pressure transducer, and an electronic level meter. Many ports in the vacuum chamber allow testing the thermal behavior of the flowing liquids heated with an electron beam or study of the effect of a magnetic field on motion of the liquid. Some preliminary tests have been performed to determine the effect of a static magnetic field on stream flow from a nozzle. © 2004 Elsevier B.V. All rights reserved.

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Effects of ion beam assisted deposition, beam sharing and pivoting in EB-PVD processing of graded thermal barrier coatings

Surface and Coatings Technology

Youchison, Dennis L.; Gallis, Michail A.; Nygren, Richard E.; McDonald, Jimmie M.; Lutz, Thomas J.

The development of advanced thermal barrier coatings of yttria stabilized zirconia (YSZ) that exhibit lower thermal conductivity through better control of electron beam-physical vapor deposition (EB-PVD) processing is of prime interest to both the aerospace and power industries. Recently, processing technology was developed to create graded TBCs by coupling ion beam assisted deposition (IBAD) with substrate pivoting in the alumina-YSZ system. The Electron Beam-1200 kW (EB-1200) PVD system was used to deposit a variety of TBC coatings with micron layered microstructures and reduced thermal conductivity of 1.5 W/mK. The use of IBAD produced fully stoichiometric coatings at a reduced substrate temperature of 600 °C and a reduced oxygen background pressure of 0.1 Pa. In addition to the process technology, the results of Direct Simulation Monte Carlo plume modeling and spectroscopic characterization of the PVD plumes are presented. © 2003 Elsevier B.V. All rights reserved.

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FILM-30: A Heat Transfer Properties Code for Water Coolant

Youchison, Dennis L.

A FORTRAN computer code has been written to calculate the heat transfer properties at the wetted perimeter of a coolant channel when provided the bulk water conditions. This computer code is titled FILM-30 and the code calculates its heat transfer properties by using the following correlations: (1) Sieder-Tate: forced convection, (2) Bergles-Rohsenow: onset to nucleate boiling, (3) Bergles-Rohsenow: partially developed nucleate boiling, (4) Araki: fully developed nucleate boiling, (5) Tong-75: critical heat flux (CHF), and (6) Marshall-98: transition boiling. FILM-30 produces output files that provide the heat flux and heat transfer coefficient at the wetted perimeter as a function of temperature. To validate FILM-30, the calculated heat transfer properties were used in finite element analyses to predict internal temperatures for a water-cooled copper mockup under one-sided heating from a rastered electron beam. These predicted temperatures were compared with the measured temperatures from the author's 1994 and 1998 heat transfer experiments. There was excellent agreement between the predicted and experimentally measured temperatures, which confirmed the accuracy of FILM-30 within the experimental range of the tests. FILM-30 can accurately predict the CHF and transition boiling regimes, which is an important advantage over current heat transfer codes. Consequently, FILM-30 is ideal for predicting heat transfer properties for applications that feature high heat fluxes produced by one-sided heating.

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Thermal performance of a dual-channel, helium-cooled, tungsten heat exchanger

Fusion Technology

Youchison, Dennis L.; North, Mark T.

Helium-cooled, refractory heat exchangers are now under consideration for first wall and divertor applications. These refractory devices take advantage of high temperature operation with large delta-Ts to effectively handle high heat fluxes. The high temperature helium can then be used in a gas turbine for high-efficiency power conversion. Over the last five years, heat removal with helium was shown to increase dramatically by using porous metal to provide a very large effective surface area for heat transfer in a small volume. Last year, the thermal performance of a bare-copper, dual-channel, helium-cooled, porous metal divertor mock-up was evaluated on the 30 kW Electron Beam Test System at Sandia National Laboratories. The module survived a maximum absorbed heat flux of 34.6 MW/m2 and reached a maximum surface temperature of 593 °C for uniform power loading of 3 kW absorbed on a 2-cm2 area. An impressive 10 kW of power was absorbed on an area of 24 cm2. Recently, a similar dual-module, helium-cooled heat exchanger made almost entirely of tungsten was designed and fabricated by Thermacore, Inc. and tested at Sandia. A complete flow test of each channel was performed to determine the actual pressure drop characteristics. Each channel was equipped with delta-P transducers and platinum resistance temperature devices (RTDs) for independent calorimetry. One mass flow meter monitored the total flow to the heat exchanger, while a second monitored flow in only one of the channels. The thermal response of each tungsten module was obtained for heat fluxes in excess of 5 MW/m2 using 50 °C helium at 4 MPa. Fatigue cycles were also performed to assess the fracture toughness of the tungsten modules. A description of the module design and new results on flow instabilities are also presented.

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Recent High Heat Flux Tests on W-Rod-Armored Mockups

Nygren, Richard E.; Youchison, Dennis L.; McDonald, Jimmie M.; Lutz, Thomas J.; Miszkiel, Mark E.

In the authors initial high heat flux tests on small mockups armored with W rods, done in the small electron beam facility (EBTS) at Sandia National Laboratories, the mockups exhibited excellent thermal performance. However, to reach high heat fluxes, they reduced the heated area to only a portion ({approximately}25%) of the sample. They have now begun tests in their larger electron beam facility, EB 1200, where the available power (1.2 MW) is more than enough to heat the entire surface area of the small mockups. The initial results indicate that, at a given power, the surface temperatures of rods in the EB 1200 tests is somewhat higher than was observed in the EBTS tests. Also, it appears that one mockup (PW-10) has higher surface temperatures than other mockups with similar height (10mm) W rods, and that the previously reported values of absorbed heat flux on this mockup were too high. In the tests in EB 1200 of a second mockup, PW-4, absorbed heat fluxes of {approximately}22MW/m{sup 2} were reached but the corresponding surface temperatures were somewhat higher than in EBTS. A further conclusion is that the simple 1-D model initially used in evaluating some of the results from the EBTS testing was not adequate, and 3-D thermal modeling will be needed to interpret the results.

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58 Results
58 Results