Download Carbon as a flow-through, consumable PFC material

Carbon as a flow-through, consumable PFC
material
PERSISTENT SURVEILLANCE FOR
PIPELINE PROTECTION AND THREAT INTERDICTION
Peter Stangeby
University of Toronto
Presented at the ReNew Theme III workshop; Taming the
Plasma Material Interface
UCLA, March 4-6, 2009
UNIVERSITY OF TORONTO
Institute for Aerospace Studies
Tungsten is an attractive PFC material for reactors,
however…
• Tungsten PFCs may not be compatible with AT
operation since AT may involve high plasma
temperature at the edge, resulting in high sputtering
rates from walls and targets.
• Concentrations of high-Z impurities in the confined
plasma must be kept to very low levels.
• Much higher concentrations are permissible for the
low-Z refractory carbon.
• Graphite, however, may not be compatible with the
high neutron fluence of devices like FDF, DEMO and
reactors due to swelling damage.
Tungsten is an attractive PFC material for reactors,
however…
Tungsten experiences a surface roughening effect due to He
bombardment at energies below the threshold for physical
sputtering (simultaneous D + T + 5-10% He, as will occur in
burning plasmas), creating a surface “foam” (nanoscopic
morphology) [Nishijima, Baldwin, Doerner]. “Such structures are
potentially large sources of high-Z dust, could harbour
significant levels of retained hydrogen isotopes in the plethora
of helium nano-bubbles/voids within nano-structures and may
have a dramatic influence on surface-thermal properties of W
plasma facing components.” [Baldwin and Doerner].
The effect becomes significant over ~1000K and could be the
most serious issue for W in DEMO [Konishi and Ueda].
A potential solution to the W ‘foam’ problem:
low-Z coatings
- A solution to the problem has been proposed [Baldwin,
Doerner, Nishijima, Tokunaga, Ueda ]: coating the W with
a low-Z material such as Be, B or C.
- Coating W with a low-Z material could also solve the
problem of unacceptably high concentrations of W in the
confined plasma.
- The low-Z coatings would be consumed and replenished
continuously and so neutron damage would not be an
issue.
- Continuous refurbishment of the plasma-facing surfaces
with coatings would also solve the basic problem of loss of
structure in high duty cycle devices.
Very little tritium retention in low-Z coatings at
high temperature
• Reactors will operate with hot walls, ~500C-1000C, for
reasons of thermal efficiency.
• At high temperatures there is little tritium retention in any
materials, including C, where H/C < 0.003 for T > 1000C [J
Davis].
• Low-Z coatings in hot wall devices will result in little tritium
retention by co-deposition, which is a serious issue for
cold (200-300C) wall devices like ITER.
• In hot wall devices the tritium control problem is less likely
to be retention than permeation into the cooling system.
• Low-Z coatings may also act as a permeation barrier,
although if they reduce the surface recombination rate,
that could increase T retention and permeation; R&D is
required, including into finding the best substrate material.
Unique advantage of C as low-Z coating:
unwanted deposits easily removed by O2-baking
• The continual replenishing of the low-Z coatings will
inevitably result in thick deposits accumulating at some
locations, even if they contain little tritium, will cause other
problems, including dust formation and UFOs causing
disruptions.
• It will therefore be as important to be able to remove, in
situ, the low-Z material as to be able to create, in situ, the
coating in the first place.
• Carbon appears to be unique in this regard since it is
possible both to introduce it into the vessel as a gas e.g.
vapor deposition and to remove it as a gas, as CO and
CO2 using oxygen baking.
• Since little tritium would be contained in the carbon
deposits, there would be little creation of tritiated water,
which is problematic to handle and process.
Estimates of carbon gross, net erosion rates and
tritium codeposition rates
• For a reactor having Pheat = 400 MW, and assuming
Prad = 75% Pheat, thus 0.25Pheat = γkTsφs , where γ =
sheath heat transmission coefficient = 7; Ts = plasma
average temperature in contact with surfaces = 10 eV
assumed here; φs = total D/T-ion flux to all surfaces
[ions/s], targets and walls.
• C physical sputtering due to D/T-ions and selfsputtering, Yeff = 0.005 (Eckstein 2002 yields for
maxwellian ions plus a 3kT-sheath). Chemical
sputtering and RES assumed to be not significant at
assumed surface temperature of 1000C. 80% duty
cycle. Then gross erosion rate = 23 tons/yr.
No chemical sputtering or radiation enhanced
sputtering (RES) of carbon at 1000C
• Chemical sputtering → 0
for T > ~ 700C.
[Toronto, JNM 255 (1998) 153]
• RES ~ 0 until T > 1100C
[Tore Supra, PPCF 40 (1998) 1335].
vs
RES ~ 0 until T > 1900C
[TEXTOR, JNM 220-222 (1995) 467].
Estimates of carbon gross, net erosion rates
and tritium codeposition rates, cont’d
• Net erosion is far more difficult to calculate
reliably. Here, for concreteness, we assume
net/gross = 0.1. Thus net erosion rate = carbon
throughput rate ~2 tons C/yr.
• O2-baking at >700C is extremely rapid, such that
even a graphite substrate, if present, would be
removed quickly.
• At 1000C, T/C < ~ 0.0015 in carbon codeposits,
thus T-throughput via codeposition ~ 1.5 kg T/yr.
Reactivity of carbon with O2
is very high for T > 700C.
• 2 tonsC/yr net deposits
2 gmC/m2/s removed in 1 atm air
• Assume C deposited on
100 m2.
• Thus time to remove:
once/yr:
3 hrs
once/mo: 15 min
once/wk:
3 min
once/day: 30 sec
• W substrate, if exposed,
releases volatile oxides
for T > 1250C.
• SiC substrate, if exposed,
degrades for T >1400C.
pure graphite
codeposits
pure graphite
Conclusion
• Like Li, C may be usable as a flow-through
PFC material
• Experiments are required to assess this
possibility:
- studies of very high temperature O2-baking
of codeposits
- assessment of collateral damage to very
high temperature baking
- identification of best substrate material