Critical Gaps between Tokamak Physics and Nuclear Science (Step 1: Identifying critical gaps) (Step 2: Options to fill the critical gaps initiated) (Step 3: Success not yet) Clement P.C. Wong General Atomics Fusion Nuclear Science and Technology Annual Meeting August 2, 2010 Rice Conference Room, UCLA
The beautiful picture of fusion development Inexhaustible clean energy for the world
The beautiful picture of fusion development: Inexhaustible clean energy from MFE fusion power reactor Fusion Power Reactor DEMO DIII-D, CMOD NSTX, JET, EAST JT-60, KSTAR FNSF
For MFE we will need ITER and FNSF+IFMIF Fusion Power Reactor DEMO DIII-D, CMOD NSTX, JET, EAST JT-60, KSTAR ITER FNSF IFMIF
We can see the obvious critical issue from dpa effects Under DT reaction, PFM and structural material damages from neutron dpa+he generation and DT He + will become major challenges 150 80-100 ~0 dpa DD neutrons 3 40-60
We need to provide technical connections between major devices 150 80-100 ~0 dpa DD neutrons 3 40-60
ITER has helped to focus world fusion resources It has also identified critical issues when extended to DEMO?
ReNeW Workshop was the First Step for Systems Interface activities
18 Thrusts were Identified
5 Critical areas were selected Critical areas that will have major impacts to the feasibility of fusion power Tritium supply, burnup fraction, tritium processing and TBR Divertor configuration and peak heat flux reduction Transient events: disruptions, ELMs and runaway electrons First wall heat flux and design Plasma surface material Fast pace review on the first 3 areas
Tritium supply, burnup fraction, tritium processing and TBR Critical areas that will have major impacts to the feasibility of fusion power Tritium supply, burnup fraction, tritium processing and TBR Divertor configuration and peak heat flux reduction Transient events: disruptions, ELMs and runaway electrons First wall heat flux and design Plasma surface material
Tritium Breeding Blankets must be developed in the near term to solve the serious issue of external tritium supply Projected Ontario (OPG) Tritium Inventory (kg) Tritium Consumption in Fusion is HUGE! Unprecedented! 55.8 kg per 1000 MW fusion power per year Production & Cost: CANDU Reactors: 27 kg from over 40 years, $30M/kg (current) Fission reactors: 2 3 kg/year/reactor, $84M-$130M/kg (per DOE Inspector General*) *www.ig.energy.gov/documents/calendaryear2003/ig-0632.pdf A Successful ITER will exhaust most of the world supply of tritium, but 5-10 kg will be needed to start DEMO Any future long pulse burning plasma device (including ITER Extended Phase) will need tritium breeding technology The availability and cost of external tritium supply is a serious issue for FNSF development Engineering development and reliability growth stages must be done in a small fusion power device; only fusion break-in stage can be done in ITER M. Abdou, UCLA, 2007 12 30 25 20 15 10 5 1000 MW Fusion 10% Avail, TBR 0.0 0 1995 2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 Year CANDU Supply w/o Fusion ITER-FEAT (2004 start) We cannot wait very long for blanket development
ITER is designed with a burn-up fraction of 0.3% Low tritium burn-up fraction will need large tritium startup inventory The key is to optimize burn-up fraction and tritium processing, t tp Impact of T burn-up fraction in plasma on start-up T inventory for new power plant Implications of tritium burn-up fraction for ITER ~ 0.3% A power reactor consumes ~ 0.5 kg per day, and if t tp is ~ 24 hours like TSTA, then the tritium inventory in the fuel storage will be > 160 kg!! Totally unacceptable. If t tp is reduced to 4 hours, it will be ~ 27 kg. Still too high!! A power reactor with the same as ITER would be unacceptable! L. El-Guebaly, UW M. Abdou, UCLA, 2007
Divertor configuration and peak heat flux reduction Critical areas that will have major impacts to the feasibility of fusion power Tritium supply, burnup fraction, tritium processing and TBR Divertor configuration and peak heat flux reduction Transient events: disruptions, ELMs and runaway electrons First wall heat flux and design Plasma surface material
Most divertors are designed to a peak heat flux of 10 MW/m 2
Peak heat flux for DEMO is very uncertain New divertor configurations should be considered D. Ryutov, ARIES Workshop, San Diego, CA, May 20-21 2010 M. Kotschenreuther, ARIES Workshop, San Diego, CA, May 20-21 2010
Transient events: disruptions, ELMs and runaway electrons Critical areas that will have major impacts to the feasibility of fusion power Tritium supply, burnup fraction, tritium processing and TBR Divertor configuration and peak heat flux reduction Transient events: disruptions, ELMs and runaway electrons First wall heat flux and design Plasma surface material
However even with prefect disruption mitigation, a power reactor will need to be designed to withstand a few unforeseeable events
First wall heat flux and design Critical areas that will have major impacts to the feasibility of fusion power Tritium supply, burnup fraction, tritium processing and TBR Divertor configuration and peak heat flux reduction Transient events: disruptions, ELMs and runaway electrons First wall heat flux and design Plasma surface material
ReNeW PFC panel generated unexpected surprises ITER First wall panels are designed to 1 MW/m 2 and 5 MW/m 2 heat flux while steady state radiation is only 0.5 MW/m 2 Why???
R. Nygren SNL, 2010
Correct question: Can physicists manage without such a requirements? R. Nygren SNL, 2010
Main chamber ELM loads Clearly present in higher triangularity configurations JET DIII-D IR TV DIII-D #138219 Before ELM Secondary strike During ELM 68193, 57 s R. A. Pitts et al., APS 2007 See J. G. Watkins, Poster P2-66, Tuesday R.A. Pitts, 19 PSI conference May 2010, San Diego
ITER Thermal Load Specifications: resumé Steady state: q ~ 8 MWm -2, l q > 4.0 cm q ~ 24 MWm -2, l q > 2.5 cm (ELMs) Disruptions q ~ 45-120 MJm -2, l q > 20 cm t = 3.0-6.0 ms VDE (up): q ~ 70-270 MJm -2, l q > 3.0 cm t = 1.5-3.0 ms Start-up: q ~ 25 MWm -2, l q ~ 5.0 cm Several seconds Confinement transients q ~ 250 MWm -2, ~2-3 secs Very high parallel heat fluxes Radiation: SS: 0.5 MWm -2 (photon+cx) Disruptions TQ: ~0.5 MJm -2 t ~ 1 ms (mitigated) CQ: ~0.9 MJm -2 t ~ 10 ms Start-up and rampdown: q ~ 40 MWm -2, l q > 1.2 cm Several seconds R.A. Pitts, 19 PSI conference May 2010, San Diego VDE (down): q ~ 90-300 MJm -2, l q > 3.0 cm
ELMs Mitigation
Extension to FNSF How can we accommodate necessary RMP coils while satisfying TBR, thermal performance and components lifetime requirements? Note: Other ELMs control methods are being investigated, e.g. Low field side (LFS) pellet injection High performance ELMs free plasma (e.g. QH mode) may be the right approach but open issues remain
Plasma surface material Critical areas that will have major impacts to the feasibility of fusion power Tritium supply, burnup fraction, tritium processing and TBR Divertor configuration and peak heat flux reduction Transient events: disruptions, ELMs and runaway electrons First wall heat flux and design Plasma surface material
Surface Material is a Key Item for Fusion Development Surface material is critically important to next generation tokamak devices: Plasma performance is affected by transport of impurities Surface heat removal, tritium co-deposition and inventory will have impacts on material selection for devices beyond ITER Radiation effects from neutrons and edge alphas, material design limits and component lifetimes will have to be taken into consideration DIII-D C-Mod AUG JET-ILW EAST ITER FNSF DEMO C Mo W Be/W/C C/W Be/W/C?? (High neutron and edge alpha fluence) Surface material options C and Be will not be suitable for the next generation devices and DEMO due to surface erosion and radiation damage. Presently W is the preferred choice, but feasibility issues have been identified
Plasma Surface Materials and Effects from Helium 1. Transmuted helium* 2. He ions, generated from DT reaction * Radiation impacts from high energy neutrons in addition to dpa effects
Damages to W First Wall have also been Projected from He + Low energy He + irradiation in plasma simulator NAGDIS H bubble and hole formation on W surface @ > 10 ev D. Nishijima, Journal of Nuclear Materials, 329 (2007) 1029-1033
Significant Issues Projected for W-surface Operation Independent of Alloy Development ITER disruption loading: 10-30 MJ/m 2 for 0.1 to 3 ms When exposed to He at high temperature, W surface showed growth of W nano-structure from the bottom; the thickness increases with plasma exposure time Baldwin and Doerner, Nuclear Fusion 48 (2008) 1-5 Irreversible surface material damage W fuzz most likely will not be observed in Tokamaks, but it can still have major impacts as a form of surface erosion M. Rödig, Int. HHFC workshop, UCSD Dec. 2009 We cannot eliminate un-predicted disruptions even if disruption detection and mitigation work perfectly
Plasma Facing Material Design and Selection Requirements for Next Generation Devices 1. Withstand damage from DT generated He 2. Withstand transient events like ELMs and disruptions Additional critical requirements: Physics performance: 3. Material suitable for high performance plasma operation 4. Suitable for edge radiation to reduce maximum heat flux at the divertor 5. Low physical and chemical erosion rate Engineering performance: 6. Transmit high heat flux for high thermal efficiency conversion 7. Minimum tritium inventory 8. Minimum negative effect to tritium breeding performance 9. Low activation materials 10. Replenish damaged surface material suitable for steady state operation and long lifetime 11. Match materials temperature design requirements 12. Withstand high neutron fluence at high temperature
A Possible Si filled W-surface concept could Satisfy most Requirements The concept: Si-filled W-surface (#3,#4,#9) Thin Si surface could protect the W surface from He damage (#1) Exposed W will have a low erosion rate (#5) W-buttons ~2mm thick W with indents could transmit high heat flux, thus retaining high effective kth of W layer, necessary for DEMO (#6,#8,#11) W-buttons filled with Si Enough Si is provided to withstand ELMs and a few disruptions (modeling showed vaporized Si ~10 μm/disruption including vapor shielding effect) W-T melt @ 3410 C, Si-T melt @1412 C, Si-T boil @ 2480 C (#2) Should be able to control tritium inventory at temperature ~1000 C (#7) Suitable real time siliconization could be used to replenish Si when and where needed (#10) (Satisfying requirements #12 TBD)
Initial Results of Transient Tolerant Si-filled W-buttons Si filled W-buttons Loaded DiMES sample 2 Si-W, 3 graphite, 2 W buttons W-buttons with 1 mm dia. indents Shot 14261-14264 Shot 142706 Sample exposed To 4 LSN discharges Exposed in DIII-D lower divertor After one additional disruption, but not fully thermally loaded
Si-W Buttons Exposure Observations As expected Si on the W button surface got removed during normal discharges easily via sputtering or vaporization Favorable result shows much of the Si is retained in the indents even under relatively high heat and particle flux Retained Si could demonstrate the vapor shielding effect to protect the W-button surface from melting Initial results show expected results in the performance of Si-filled W- surface to fulfill its function, much more development and testing will be needed
Conclusions From ITER to FNSF critical gaps have been identified between Tokamak Physics and Nuclear Science and they can only be resolved with close interactions between physics, material, technology and design communities Examples are: Tritium supply, burnup fraction, tritium processing and TBR Divertor configuration and peak heat flux reduction Transient events: disruptions, ELMs and runaway electrons First wall heat flux and design Plasma surface material
Conclusions-Solutions Tritium supply, burnup fraction, tritium processing and TBR Higher T burnup fraction >>0.3% is needed, furthermore a net tritium producing device like FNSF is needed before the tritium supply runs out Divertor configuration and peak heat flux reduction New divertor configuration and with radiation to maintain acceptable peak heat flux is needed for a robust FNSF design with design margin Transient events: disruptions, ELMs and runaway electrons Different schemes of radiation are needed to mitigate damaging peak power flux impacts to surface material First wall heat flux and design Radiation to spread the peak heat flux and ELM-free operation, like QH mode are needed Plasma surface material Si-filled W-surface, which uses radiation to mitigate surface material damage is a possible transient tolerance approach and should be developed
Conclusions-Radiation is the key Tritium supply, burnup fraction, tritium processing and TBR Higher T burnup fraction >>0.3% is needed, furthermore a net tritium producing device like FNSF is needed before the tritium supply runs out Divertor configuration and peak heat flux reduction New divertor configuration and with radiation to maintain acceptable peak heat flux is needed for a robust FNSF design with design margin Transient events: disruptions, ELMs and runaway electrons Different schemes of radiation are needed to mitigate damaging peak power flux impacts to surface material First wall heat flux and design Radiation to spread the peak heat flux and ELM-free operation, like QH mode are needed Plasma surface material Si-filled W-surface, which uses radiation to mitigate surface material damage is a possible transient tolerance approach and should be developed We will need FNSF soon, and radiation is the key to control the damaging transient events. Si-filled W-surface design is proposed as a possible PFM for steady state operation of FNSF and should be organized for more systematic studies.