1 R. Doerner, ARIES HHF Workshop, Dec.11, 2008 PMI issues beyond ITER Presented by R. Doerner...
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Transcript of 1 R. Doerner, ARIES HHF Workshop, Dec.11, 2008 PMI issues beyond ITER Presented by R. Doerner...
U C S DU niversity o f C alifo rn ia S a n D iego 1R. Doerner, ARIES HHF Workshop, Dec.11, 2008
PMI issues beyond ITER
Presented by R. Doerner
University of California in San Diego
Special thanks to J. Roth (IPP-Garching) and
M. Baldwin (UCSD) for their advice
U C S DU niversity o f C alifo rn ia S a n D iego 2R. Doerner, ARIES HHF Workshop, Dec.11, 2008
The missions of ITER & DEMO will force a sea change in emphasis.
• The ITER mission is power generation [Q = 10]– Requires the core plasma to function
– Wall is secondary, its only purpose is to allow core to operate successfully
• The mission of DEMO/reactor is power conversion– Wall/blanket system must function
– Core plasma will become secondary, its purpose will be to supply power to the wall
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego 3R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Even ITER barely begins to address DEMO relevant PMI issues.
• PFC armor materials, Be and C, are not reactor relevant
• Elevated wall temperature necessary for efficient power conversion
• Tritium fuel cycle
• Neutron effects will alter material performance
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego 4R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Tritium loss terms in reactors must be understood and carefully controlled.
• Too much in-vessel inventory and the Tritium Breeding Ratio may drop below unity
• Smaller than expected in-vessel T inventories may result in unexpected on-site tritium surplus– Best possible result for ITER, but perhaps not for DEMO
• Trapped tritium inventories must be understood– Bulk retention in PFCs– Erosion of plasma-facing materials can lead to tritium
trapping due to codeposition (as well as a reduction in PFC lifetime)
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego 5R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Tritium losses include retention of implanted energetic particle flux.
• Different materials exhibit different retention behavior with increasing fluence
• Identical elements can behave differently depending on their structure
• Measurements above a fluence of 1026 m-2 are sparse
• Plenty of low temperature data on W existFrom J. Roth et al., PPCF 50(2008)103001.
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego 6R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Data base for retention in W:e.g. fluence dependence at 500 K
Experimental scatter emphasizes lack of understanding of underlying retention principles.
PISCES
Possibleexplanations:
- Material differences - Sample pretreatment - Exposure conditions - Measurement techniques
U C S DU niversity o f C alifo rn ia S a n D iego 7R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Fortunately, retention in W at high surface temperature is consistently reported to be low.
• Low temperature peak in retention correlates with surface blister formation
• More high temperature (>900K), large fluence retention data needed
• For 1000 m2, 1019 m-2 T retention is ~1.5gm/min, or ~0.1 kg/hour
• T permeation becomes an issue
PISCES
ConservativeITER estimate
U C S DU niversity o f C alifo rn ia S a n D iego 8R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Particle flux in DEMO will contain both T and He.
• Recent results show strong interaction of low energy He with W
• At low temperature, a small He concentration (a few percent) results in nanobubble formation in the near surface region, suppressing blisters and deuterium retention
SRWM-3b D2
Time
500 1000 1500 2000 2500 3000
Par
tial P
ress
ure
(T
orr)
10
-11
10
-10
10
-91
0-8
Temperature (C)
500 1000 1500
He (Torr) D2 (Torr)
SRWM-4b D2-He(20%)
Time
500 1000 1500 2000 2500 3000
Temperature (C)
500 1000 1500
He (Torr) D2 (Torr)
D2 plasma D2/He plasma
TEM imageof W sampleexposed at ~300°C in P-Bto D/He plasma (Eion ~ 60eV)from M. Miyamoto
PISCES
Desorptionspectra fromW exposed at300°C (Eion~60eV)Fluence ~5e25 m-2
U C S DU niversity o f C alifo rn ia S a n D iego 9R. Doerner, ARIES HHF Workshop, Dec.11, 2008
At high temperature He in the incident plasma also exhibits strong interactions with W surfaces.
PISCES
Simultaneous D/He plasma exposure produces identical W nano-structured surfaces [Consistent He plasma exposures: Ts = 1120 K, He+= 4–6×1022 m–2s–1, Eion ~ 40 eV]
Cross section view of fractured W targets after He plasma exposure
U C S DU niversity o f C alifo rn ia S a n D iego 10R. Doerner, ARIES HHF Workshop, Dec.11, 2008
At 1120 K, nano-structured layer thickness does not saturate with He plasma exposure time.
300 s 2000 s 4300 s 9000 s 22000 s
Consistent He plasma exposures: Ts = 1120 K, He+= 4–6×1022 m–2s–1, Eion ~ 60 eV
Growth rate follows time0.5 dependence, similar growth in D/He plasma exposures
PISCES
From M. Baldwin and R. Doerner, NF 48(2008)035001
U C S DU niversity o f C alifo rn ia S a n D iego 11R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Nano-structured surface may lead to several difficulties.
• Thermal properties of W fuzz may lead to overheating of surface and increased vaporization of the surface
• W nano-structures have little strength (can be mechanically dislodged from surfaces easily) possibly resulting in an increase in dust production during shocks (mechanical, thermal, etc) to the surface
• Either, or both, of these effects may decrease the lifetime of plasma facing surfaces
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego 12R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Surface lifetime is also affected by erosion, which is often thought to be independent of temperature.
• Radiation enhanced sublimation is well known for carbon-based materials. Interstitials created in the bulk migrate to the surface where they are less strongly bound and sublimate.
PISCES
From V. Phillips et al., JNM 179-181(1991)25. From J. Roth et al., JNM 111&112(1982)775.
U C S DU niversity o f C alifo rn ia S a n D iego 13R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Temperature dependent erosion is also observed from metal surfaces.
• Be (left) and Au (above) data have been compared with possible models
• Lighting manufacturers have used these models to explain short W filament lifetimes (~3000°C)
1015
1016
1017
1018
900 1000 1100 1200 1300
Measured Be erosion rate (50 eV ion energy)Sputtering + surface sublimation Sputtering + (adatom + surface) sublimation model
Sample Temperature (K)
PISCES
From R. Doerner et al., JAP 95(2004)4471.
U C S DU niversity o f C alifo rn ia S a n D iego 14R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Liquid surfaces are not immune to temperature dependent erosion.
• Temperature dependent erosion mechanisms will reduce the operational temperature window for free liquid surfaces
• Temperature dependent erosion also measured for liquid gallium
1016
1017
1018
1019
150 200 250 300 350 400 450
Helium Plasma Bombardment of LithiumIon Energy = 175 eV
Measured erosion (ion flux = 1.6e18 cm -2s-1)
Measured erosion (ion flux = 5e17 cm -2s-1)calculated evaporative loss from surface
Sample Temperature (C)
Lithium meltingtemperature
PISCES
From R. Doerner et al., JNM 313-316(2003)383.
U C S DU niversity o f C alifo rn ia S a n D iego 15R. Doerner, ARIES HHF Workshop, Dec.11, 2008
PMI at elevated temperature can influence surface material loss rate.
• Material loss rates impact reactor operation– Impurity content in core– Lifetime of wall– Changing thickness will alter thermal gradients in armor which
will in turn effect tritium inventory in the armor– Material mixing
• Increases in material loss rates will provide more material available for codeposition with fuel
• Very little PMI data at elevated (DEMO relevant) wall temperature is available
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego 16R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Different mixed-material phases form in different temperature regions (e.g. in ITER Be-W).
From C.R. Watts, International Journal of Powder Metallurgy 4 (3) (1968) 49.
Reaction kinetics can favor different alloy formation as temperature increases
PISCES
From R. Doerner et al., JNM 342(2005)63.
U C S DU niversity o f C alifo rn ia S a n D iego 17R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Codeposits formed at high temperature tend to retain less tritium.
• Empirical scalings (T, E, r) allow predictions of retention despite the large scatter in the codeposition database
• Most codeposition studies do not extend above 300-400°C
• Accumulation in high temperature codeposits likely to be low– How low?– Where will codeposits form
and what is their temperatureFrom J. Roth et al., PPCF 50(2008)103001.
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego 18R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Role of neutron damage on retention in tungsten is just beginning to attract interest.
• Diffusion/permeation rate of T in damaged material needs investigation at more relevant temperature
• Damage may anneal during high temperature, steady-state operation
• Does neutron damage differ from ion induced damage
• Modeling of trap creation and filling deep (beyond micron scale) in material have started
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego 19R. Doerner, ARIES HHF Workshop, Dec.11, 2008
Summary: PMI studies with realistic temperature are needed.
• Power conversion efficiency will push toward increased temperature armor
• Cost of electricity will push toward compact reactors with high power density, which will push armor temperature up
• Steady-state operation will allow sufficient time for elevated temperature PMI to fully develop
• Existing machines (& ITER) operate at lower temperatures, in pulsed fashion
• Almost all aspects of PMI are temperature dependent– Tritium retention, diffusion, recombination, surface material loss
rates, chemical reactions, damage annealing, codeposition, etc.
PISCES