Transcript ARIES: Fusion Power Core and Power Cycle Engineering

Accommodation of Prompt
Alpha-Particle Loss in Compact
Stellarators
A. R. Raffray, T. K. Mau, F. Najmabadi,
and the ARIES Team
University of California, San Diego
PSI-17
Hefei, China
May 21-26, 2006
May21-26, 2006/ARR
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Alpha Loss is an Important Issue in MFE,
Governed by the Magnetic Field Ripple
•  loss can impact tokamaks, where size and aspect ratio constraints can
result in coil placement inducing significant ripple, and more
importantly stellarators, where the inherent non-axisymmetry of the
magnetic geometry causes an appreciable magnetic field ripple along the
flux surfaces inside the plasma.
• In this case, a non-negligible fraction of the alpha particles will either be
born or “kicked” into orbits that are trapped in these ripples.
• Depending on the magnetic topology, a fraction of these particles are
promptly lost from the plasma and hit the PFC’s at energies equal to or
close to their born value of 3.5 MeV.
• The resultant footprints on the LCMS show that the bulk of these
particles are lost in a region below the mid-plane on the outboard edge of
the plasma cross-section. On the other hand, the more thermalized
particlesMay21-26,
tend2006/ARR
to diffuse from the plasma uniformly on the LCMS.2
Several Classes of Stellarators Were
Considered As Part of the ARIES-CS Study,
Including NCSX-Based Configurations
Coil Configuration and Plasma
Shape for Example NCSX-Like 3Field Period Concept
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Example In-Core ARIES-CS
Configuration with Port-Based
Maintenance
3
Example Spectrum of Particle Loss from
ARIES-CS Compact Stellarator
•  loss not only
represents a loss
of heating power
in the core, but
impacts the
PFC design
requirement and
lifetime.
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Accommodating Alpha Particle Flux on PFC
• High heat flux could be accommodated by designing
special divertor-like modules.
• e.g. for typical ARIES-CS power plant parameters:
-
Pfusion = 2350 MW
Max. neutron wall load = 5 MW/m2
FW Surface Area = 572 m2
Assumed  loss fraction = 0.1
Assumed  footprint = 0.05
Ave. q’’ on alpha modules = 2.2 MW/m2
Max. q’’ <10 MW/m2 for peaking factor < 4.5
Divertor itself must be designed to accommodate both the
divertor heat load and any incident  loss power.
• Impact of -particle flux on armor lifetime (erosion) is
an additional concern.
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Divertor-Like Modules Can Accommodate
Heat Flux of Up to 10 MW/m2
• T-tube divertor concept developed for ARIES-CS, able
to accommodate ~10 MW/m2, utilizing a He-cooled jet
configuration with W-alloy as structural material and a
thin W armor.
- Inlet and outlet He temperatures ~600°C and ~700°C
- Maximum W alloy temperature < 1300°C
- Total stress intensity < 370 MPa
• Issues requiring further R&D effort include fabrication
methods, W alloy development and thermo-fluid
performance verification.
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ARIES-CS Divertor T-Tube Concept
~15 mm
~100 mm
W armor
W alloy
inner
cartridge
Graded transition
between W and ODS FS
W alloy
outer tube
(~ 1 mm thick)
He coolant
inlet and
outlet
T-tubes
(W alloy)
• T-tubes can be assembled to form
target plate of required size.
Module Body (FS)
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Alpha Particle Flux Impact on Armor
Lifetime
• For these high  energies,
sputtering is less of a concern
and armor lifetime would be
governed by some other
mechanism, such as exfolation,
resulting from accumulation of
He atoms in the armor.
• The implantation depth for ~ 1
MeV He in W is ~1.5 m.
-
Pfusion = 2350 MW
FW Surface Area = 572 m2
Assumed  loss fraction = 0.1
Assumed  footprint = 0.05
 flux on PFC is ~3 x 1018 ions/m2-s
Eff. Vol. generat. rate ~2 x 1024 ions/m3-s
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Sputtering yield of W as a function of
He energy at different incidence angles
(from W. Eckstein, “Sputtering, reflection and
range values for plasma edge codes,” IPP-Report,
Max-Planck-Institut fûr Plasmaphysik, Garching
bei Munchen, Germany, IPP 9/117, March 1998.)
8
He Implantation and Behavior in W Armor Quite
Complex, Consisting of a Number of Mechanisms
• Due to high heat of solution, inertgas atoms essentially insoluble in
most solids.
Implanted
He atom
• This can then lead to gas-atom
precipitation, bubble formation
and ultimately to destruction of the
material.
• He atoms in a metal may occupy
either substitutional or interstitial
sites. As interstitials, they are very
mobile, but they will be trapped at
lattice vacancies, impurities, and
vacancy-impurity complexes.
1. M. S. Abd El Keriem, D. P. van der Werf and F. Pleiter,
"Helium-vacancy interactions in tungsten," Physical review B,
Vol. 47, No. 22, 14771-14777, June 1993.
2. W. D. Wilson and R. A. Johnson, in Interatomic Potentials and
Simulation of Lattice Defects, edited by P. C. Gehlen, J. R.
Beeler and R. I. Jaffee (Plenum New York, 1972), p375.
3. A. Wagner and May21-26,
D. N. Seidman,
2006/ARRPhys. Rev. Letter 42, 515 (1979)
PORE or
VOID
BULK W
Bulk diffusion
Trapping
Desorption
Detrapping
Trapped He
• The following activation energies were
estimated for different He processes in
tungsten [1,2]:
-
Helium formation energy:
5.47 eV
Helium migration energy:
0.24 eV
He vacancy binding energy: 4.15 eV
He vacancy dissociation energy: 4.39 eV
- From [3], D (m2/s) = D0 exp (-EDif/kT);
D0 = 4.7 x 10-7 m2/s and EDif = 0.28
eV
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Recent Experimental Data on Cyclic He
Implantation and Release in W
(from S. B. Gilliam, S. M. Gidcumb, N. R. Parikh, D. G. Forsythe, B. K. Patnaik, J. D.
Hunn, L. L. Snead and G. P. Lamaze, J. Nucl. Mat. 347 (2005) 289.)
• Results indicate that He retention decreases
drastically when a given He dose is spread
over an increasing number of pulses, each
one followed by W annealing to 2000°C, to
the extent that there would be no He
retention below a certain He dose per pulse.
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Effective Diffusion Analysis of Experimental Results
for He Implantation and Release in W
(from S. B. Gilliam, S. M. Gidcumb, N. R. Parikh, D. G. Forsythe, B. K. Patnaik, J. D. Hunn, L. L.
Snead and G. P. Lamaze, J. Nucl. Mat. 347 (2005) 289.)
Simple effective
diffusion model
Fast desorption:
C=0
Symmetry:
dC/dx=0
Effective
diffusion
• Set effective activation energy to
reproduce final He retention level
in experiments for given number
of cycles, He dose implantation
and temperature anneals
x=
x=0
Cg (x,t)
t
• Activation energy derived from
analysis would not be that of bulk
diffusion but of the rate 2Cg (x,t)  controlling mechanism, much
 D(T)
 probably some form of
2
 t
 trapping/detrapping mechanisms.
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- D0 = 4.7 x 10-7 m2/s
-  = 1.5 m (for ~ 1 MeV He 11in W)
Effective Diffusion Activation Energy (Eeff,diff) as
a Function of Dose per He Implantation from
Modeling of Experimental Results
Effective Diffusion Activation Energy ( eV)
6
Single crystal
Polycrystalline
5
4
3
2
1
(The curve fit has been drawn to suggest a possible variation
of the activation energy with the He dose or concentration)
0
1.0E+15
1.0E+16
1.0E+17
1.0E+18
1.0E+19
1.0E+20
Dose per He implantation (ions/m2)
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Possible Explanation of Effective Activation
Energy Results
• In general, trapping would increase with He irradiation dose,
which creates defects and vacancies (followed by an anneal of the
unoccupied trapped sites during the ensuing temp. transient).
• At very low dose, He migration in W should be governed by bulk
diffusion (Eeff,dif ~0.24-0.28eV)
• As the dose per cycle increases, trapping sites are formed or
activated and Eeff,dif increases. It seems that there is a nearthreshold of He dose at which Eeff,dif increases rapidly to ~3.3-3.6
eV and stays there over a dose range of ~2 orders of magnitude.
• Above this range, Eeff,dif increases rapidly to ~ 4.2-4.8 eV, indicating
an increase in trapping perhaps due to He build up in vacancies
(the vacancy dissociation energy is ~4.4 eV)
• Overall, the dependence of activation energy with dose is about the
same for SC and polycrystalline W except that it is shifted to
lower doses for the latter case.
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Application of Modeling Results to Estimate He
Retention in ARIES-CS W Armor Due to  Loss
•
For ARIES-CS, both He irradiation and temperature are steady-state
- Choice of Eeff,diff is tricky as the experimental results were obtained for
cyclic conditions with a finite He dose per implantation.
•
As a rough guide, the experimental temperature anneals were
integrated and manipulated to estimate the Tuniform which would result
in the same diffusive time scale as the 60 s shown for each cycle.
-
•
Effective Tuniform ~ 1590°C,
Corresponding maximum dose rate ~ 1.6x1017 ions/m2-s.
Increasing Tuniform would result in shorter effective time and larger max.
dose rates (e.g. ~ 3 s and 3 x 1018 ions/m2-s for a ~1800°C temperature),
and reducing it would have the opposite effect.
For ARIES-CS, the steady state  dose rate is ~ 3 x 1018 He ions/m2-s
and the experimental results would be best applied for high temp. cases
with Eeff,diff corresponding to the max. dose, ~ 4.8 eV.
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Estimate of He Retention in W Armor Due to  Loss as a
Function of Operating Temperature and Characteristic
Diffusion Length
• Simple effective diffusion
analysis for Eeff,dif = 4.8 eV
• Not clear what is the max.
He conc. limit in W to avoid
exfolation
- perhaps ~ 0.15-0.2 at.%[4,5]
15% at.
He in W
• High W temp. and shorter
diffusion dimensions help,
perhaps a nano-structured
porous W
- e.g. 50-100 nm at >~1800°C
Alpha particle flux
Porous W
(~10-100 m)
4.
5.
G. Lucas, personal communication.
S. B. Gilliam, S. M. Gidcumb, N. R. Parikh, D. G.
Forsythe, B. K. Patnaik, J. D. Hunn, L. L. Snead and
G. P. Lamaze,
J. Nucl. Mat. 347 (2005) 289.
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Fully dense W
(~ 1 mm)
Coolant
Structure
(W alloy)
15
Effect of Changing the Activation Energy of Governing He
Migration Process on Estimated He Retention
• A reduction in Eeff,diff would have a major effect, e.g. for a
diffusion distance of 100 nm and a temperature of 1800°C, the
steady state He to W inventory ratio, IHe/W:
- IHe/W ~ 0.1 for Eeff,diff =4.8 eV
- IHe/W ~ 4x10-5 for Eeff,diff =3.4 eV
- IHe/W ~ 8x10-13 for Eeff,diff =0.24 eV.
• An interesting question is whether at a such high W operating
temperature some annealing of the defects might further help
helium release.
• Further R&D requiired to understand and better characterize
integrated effect of governing He migration processes under
prototypical conditions
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Plasma Processes Incorporated is Working on
Manufacturing Porous W with Nano Microstructure
Which Could Enhance He Release
• After W precursors are injected into the plasma flame, the vapor phase is
quenched rapidly to solid phase yielding nano-sized W powder
• Nano tungsten powders have been successfully produced by plasma technique
and the product is ultra pure with an ave. particle size of 20-30 nm. Production
rates of > 10 kg/hr are feasible.
• Process applicable to molybdenum, rhenium, tungsten carbide, molybdenum
carbide and other materials.
• The next step is to utilize such a powder in the Vacuum Plasma Spray process to
manufacture porous W (~10-20% porosity) with characteristic microstructure
dimension of ~50 nm .
TEM images of
tungsten
nanopowder,
p/n# S05-15.
(from S. O’Dell,
PPI, personal
communication)
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Conclusions
• Alpha loss is a major issue in stellarators, impacting
the survival of the first wall.
• The armor lifetime under the alpha flux would depend
on a number of parameters:
- -particle energy spectrum
- armor material choice, configuration and temperature
- activation energies of processes governing He behavior in armor
• Use of a nano-sized porous W armor and high
operating temperature would help to enhance the
release of implanted He.
• Further R&D required to make sure that a credible
solution exists for a CS.
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