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Management of dust in fusion devices

Management of dust in fusion devices. ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009 Charles. H. Skinner Princeton Plasma Physics Laboratory Presented by R. Maingi. Outline: Dust production Dust hazards Dust monitoring

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Management of dust in fusion devices

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  1. Management of dust in fusion devices ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009 Charles. H. Skinner Princeton Plasma Physics Laboratory Presented by R. Maingi • Outline: • Dust production • Dust hazards • Dust monitoring • Dust removal • R&D needs Supported by US DoE DE-AC02-76CH03073 C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  2. Dust production • Dust in fusion devices is produced by : • Thermal overload of plasma facing surfaces. • Brittle destruction of carbon surfaces (‘shrapnel’). • Melt layer loss (aerosol) from metal surfaces. • Disintegration of codeposited layers • Chemical agglomeration of sputtered Cn clusters. • Debris from in-vessel activities. Tungsten droplet tracks in QSPA ELM simulator at Troitsk, 1.6 MJ/m2 first pulse. Zhitlukhin et al., J. Nucl. Mater., 363-365, 301 (2007) Increase in duty cycle and plasma stored energy in next-step long pulse devices will cause huge scale-up in dust produced. Up to 400 µm codeposit expected from 10 days of ITER operations. Thermal and mechanical stability uncertain. 0.1 mm Iron spheres from TEXTOR-94 with the large sphere showing a regular surface texture J Winter, Plasma Phys. Control. Fusion, 40 (1998) 1201 C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  3. Dust hazards: 1. Public safety • Dust particles may be radioactive from tritium or activated metals, toxic and /or chemically reactive with steam or air. • Tritiated dust is respirable • Radiation dose depends on residence time in body. • In-vitro dissolution rate of tritiated dust from TFTR measured in simulated lung fluid. • Only 8% of carbon tritide was dissolved after 110 days ! • Low solubility means tritium will remain for long time increasing radiation dose to lung. TEM microphotograph of tritiated dust particles from TFTR. The count mean diameter is 1.23 µm. Tritium activity Data needed on BeT dust to determine allowable occupational exposure ! Cheng et al., Fus. Technol., 41 (2002) 867 C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  4. Tritiated dust levitation by beta induced static charge • Radioactive decay of tritium via beta emission leaves a positive charge on a dust particle. • Tritiated particles could be uniquely more mobile than other dust. • Movie of tritiated dust from TFTR--> • A release of radioactive dust in an accident would have major consequences. • To assure public safety the mobilisable dust inventory of ITER will be maintained below 670 kg. Fus. Sci. Technol., 45 (2004) 11 C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  5. Dust hazards 2. Hot dust - Vac. Vessel protection • Dust on hot surfaces will produce H2 with air ingress and can react with air. • Hot = > 600°C for C, > 400 C for Be and W. • H limited to 2.5 Kg in ITER. • Need to avoid overpressure events that could rupture the vacuum vessel and jeopardize its primary safety function: confinement of dust and tritium. • Quantities of hot dust that could produce 2.5 Kg of H in steam reaction during ITER accident are 6 kg of Be, C, and W dust, or, if carbon is not present, 11 kg of Be and 77 kg of W dust. • Reliable detection of dust at this level is problematic. • ITER Design Basis Accidents include potential for: • H2 explosion • Dust explosion triggered by H2 ignition. • Pure dust explosion S. Ciattaglia IAEA RCM meeting Vienna Dec 2008. C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  6. Dust hazards 3. Plasma Contamination Simulated profiles of neutral carbon density • A third limit on dust is related to potential transport of tungsten dust to the plasma core. Tungsten is a very efficient radiator and the core tungsten concentration needs to be in the 10-5 range or below to sustain a burning plasma. • Ambipolar effects charge up tokamak dust and ion drag then leads to core contamination. (Pigarov/Krasheninnikov DUSTT code) • However the relation between core tungsten and surface tungsten dust is not known at present. (A. Pigarov et al., Phys. Plasmas 12 (2005) 122508, PSI-17) C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  7. Dust hazards:4. Plasma operations -first mirror survivability • Dust accumulation can ‘blind’ the first diagnostic mirrors • Optical diagnostics necessary for machine operation. • Mirror lifetime not clear • Cleaning techniques unproven ITER divertor Image from disruption simulator Diagnostic mirrors in divertor dome Divertor target plate will be eroded ~1 cm at strike point over 3 year lifetime. Material will go somewhere ! C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  8. Potential dust monitoring techniques: 500 µm ITER In-vessel viewing system head ITER strategy: • Monitor erosion with In-Vessel Viewing System. • Conservative assumption that 100% of erosion products turn into dust (JT60, Tore Supra measure ~ 10%) New systems proposed: • Dedicated laser erosion monitors for divertor • Local dust detectors based on capacitance manometer. • Needs adaption and hardening to tokamak environment and nuclear environment. C. Neri ENEA, Frascati Electrostatic dust detector 100 µm dia. of human hair C.H. Skinner J. Nucl. Mater., 376 (2008) 29–32 Converted capacitance manometer GF Counsell et al, Rev. Sci. Instrum. 77 (2006) 093501 C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  9. Potential dust removal technology 1. • Fusion power reactor cannot have high (~ 90%) availability without remote dust removal. • Plasma operation will not be permitted if dust inventory exceeds safety limits. • Dust monitoring is not enough. Electrostatic dust conveyor a possibility: • 5 g/min (air) 1 g/min (vac) dust transported (even vertically) in travelling electrostatic wave by JAERI/Mitusubishi during ITER EDA. • Potential for large improvement with modern macroelectronics technology. Traveling electrostatic wave: potential gradient is created by three different voltages on the three electrodes. . Y Oda et al., “Development of dust removal system for fusion reactor” J. Fus. Energy 16 (1997) 231 C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  10. Dust removal 2. • Nanotechnology and large area displays are a rapidly evolving area. • Propose to apply advances in macroelectronics to develop electrostatic dust transporter • Mosaic of these devices could cover VV floor • Use low activation substrate e.g. SiO2 Before and after images of a dust shield prototype on a metal plate. Demonstration of a flexible E-Ink display on a transistor backplane made at Princeton University on steel foil. [Y. Chen, K. Denis, P. Kazlas, P. Drzaic, SID 2001 Technical Digest, Paper P-12.2] From “Dust Particle Removal by Electrostatic and Dielectrophoretic Forces with Applications to NASA Exploration Missions”C.I. Calle et al ESA Proc. Annual Meeting on Electrostatics 2008. C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  11. R&D needed: Developing and validating dust inventory diagnostics to nuclear safety standards • Estimation of dust production rates. • Quantification of fraction of erosion that becomes dust. Tokamak + ELM/disruption simulator data needed. • Adaption of local dust diagnostics to tokamak environment • Adaption of local dust diagnostics to nuclear environment. • Validation of erosion measurements in realistic reactor geometry. • Qualification of dust measurement techniques and uncertainties (for nuclear regulators). • Dedicated large-scale mockup facility with tile castellations, hidden areas… dedicated to studying dust behavior, mobilisation factors, measurement and removal. • ….. Acknowledge ITER dust task force lead by S. Ciattaglia C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  12. R&D needed: Management of hot dust • Estimation of hot dust inventories. • R&D on diagnostics on dust on hot surfaces. • Direct measurement seems difficult. • Chuyanov proposal to measure reactivity by small calibrated injection of steam. • Development and validation of 3D models of dust-H explosion • Engineering measures to overpressure of vessel: • Design requirement to limit maximum air entering the VV- needs feasibility analysis. • Injection of an inert gas into the VV in accident situations - needs model validation to establish time scale for mixing and engineering optimization • Crosscheck safety/plasma physics and design codes • (timing of transients could/should be different, e.g. temperature of dust could be below the self-sustaining reaction temperature limit). • ….. C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  13. R&D needed: Managing plasma contamination issues • Benchmarking of models on dust transport with calibrated dust injection experiments. • Benchmarking of melt layer aerosol transport codes in disruption simulators and tokamaks. • Deriving limits on tolerable surface dust in different locations. • Developing countermeasures to mitigate contamination. • … C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  14. R&D needed: Developing countermeasures to clean dusty diagnostic mirrors remotely • Helium puff ? • Vibration ? • Laser cleaning ? • … • related to general issue of survival of first mirror in tokamak environment - potential erosion of and deposition on mirror. “Self Cleaning Sensor UnitA key element of minimizing dust is preventing it from clinging to the front surface of the imaging sensor. To combat against this, the EOS 40D features a Canon-designed Self Cleaning Sensor Unit. The low-pass filter at the front of the sensor shakes off dust automatically with ultrasonic vibrations, removing dust from the sensor assembly.” http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcategoryid=139&modelid=15653#ModelFeaturesAct C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  15. R&D needed: Means to remove dust remotely without impacting tokamak availability Mars Rover Spirit after a dust storm. • Electrostatic dust conveyor ? • …. Will fusion reactor interior end up looking like this ? C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

  16. Final points: • Issues such as dust are not an problem for contemporary tokamaks but are potential show-stoppers for fusion reactors ! • Environment needed that can foster and fundcreative solutions to these issues. • Development path needed from lab experiments, small scale simulators to next-step devices. • A next-step long-pulse, flexible, high power, hot wall machine with relevant first-wall materials and appropriate access and flexibility would be ideal to validate the most promising candidates for dust monitoring and removal. • Work should be coordinated with ITER R&D and ITPA. C. H. Skinner ReNeW Research Needs Workshop Theme III: Taming the Plasma Material Interface, UCLA, March 2-6, 2009

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