Tritium Transport, Permeation, and Control · tritium from gases (Example: a 60g U bed can hold 2g...
Transcript of Tritium Transport, Permeation, and Control · tritium from gases (Example: a 60g U bed can hold 2g...
Tritium Transport, Permeation, and Control
Paul Humrickhouse
Idaho National Laboratory
Fusion Safety Program
4th IAEA DEMO Programme Workshop
Karlsruhe Institute of Technology
November 15, 2016
Overview
• Tritium in DEMO: some magnitudes
• Fundamental transport phenomena
– Permeation, regimes
– Isotope effects
– Trapping
• Mitigating tritium losses
– Extraction systems
– Permeation barriers
• Are they necessary?
• Perspectives from the FNSF study
– Parameter uncertainties
Tritium in DEMO
• In DEMO it will be necessary to breed tritium at the same rate it is consumed (55.6 kg/GWf-year)
• The tritium production rate in DEMO will need to be ~104-105 times that of ITER or fission reactors:
• Losses from the blanket and heat transfer systems must be small (~1 g/yr) in order to meet environmental and safety requirements- dose to public from normal operations2 < 0.1 mSv/yr (10 mrem/yr)
• More efficient power generation requires higher temperatures, but these facilitate tritium permeation, which must be mitigated:
• DEMO losses must be <10-5 of the produced tritium
PWR1 CANDU1 Gas-cooled
reactor1
Molten salt
reactor1
ITER DEMO
(2-3 GWfus)
T generated (kg/y) 0.000075 0.1 0.002 0.09 ~0.004 110 - 170
1H. Schmutz, INL/EXT-12-26758, 2012 2DOE-STD-6002-96, “Safety of Magnetic Fusion Facilities: Requirements”
Fundamental Transport Phenomena
• Implantation (FW)
• Diffusion
• Trapping
• Recombination
• Adsorption/dissociation
• Solubility in solids and liquids also plays a key role
CT2,Bulk
2
2 2T,FWC KΓ rT
Wall diffusion
x
C -DΓ m
m
2
rT2 FW1T,C K
Molecular
Recombination Flux
22 TdT p KΓ
Implantation Flux
~ 1x1020 m-2-s-1
Plasma
t
Ct
Bulk Trapping
Gas Driven Permeation • Three things happening:*
• At each surface:
– Molecules dissociating:
– Atoms recombining:
• In the bulk solid:
– Diffusion:
• If surface dissociation/recombination are fast relative to diffusion (diffusion limited):
• Say C2 is small…
• If diffusion is fast relative to surface dissociation/recombination (surface limited):
1 2
C1
Jd,1
Jr,1
Jr,2
Jd,2
C2
JD
2Tdd PKJ
2CKJ rr
x
CCDJ 12
irid JJ ,, isi PKC
r
ds K
KK
Sieverts’ Law,
where is the solubility
0CCi 0DJ 1,1,2,2
1drr JJJ
Permeation Regimes
• When diffusion is rate limiting (W’ >> 1):
• Define permeability:
• When surface effects are rate limiting (W’ << 1):
• A dimensionless number* tells us where the transition is:
• Diffusion-limited (~√P) transport is usually assumed if parameters unknown
• Note that it only makes sense to talk about permeability in this particular regime
• When surface-limited, fluxes are lower than expected from diffusion-limited theory
*Ali-Kahn et al JNM 76/77 (1978) 337-343.
x
PDKJ s
PKJ d
sDK
PxKW d
x
PJ
Perkins and Noda JNM 71 (1978) 349-364.
Isotope effects
• Hydrogen (or deuterium) can recombine with tritium, and their presence alters tritium permeation rates
• Define
• “Surface-limited” and “Diffusion-limited” apply to the sum of all isotopes:
HT
Ts
PPx
PDKJ
Td PKJ2
1
Surface Limited Diffusion Limited
1W 1W
T
H
s PPx
DKJ
Ts P
x
DKJ
“Isotope-limited”
DK
PxKW
s
dDK
PPxKW
s
HTd
Flux unchanged
but inventory in
solid suppressed!
Single
Isotope
Two
Isotopes
HT PP
HT PP
HTTT PPP21
2 HTHH PPP
21
2
Surface-limited and
“Isotope-limited” fluxes have same
dependence on PT
but different
proportionality
constants
Humrickhouse JNM (in preparation)
Tritium Trapping
• Tritium inventory in the bulk material can be affected by traps, which:
– Increase the energy required to move from site to site (i.e. alter the effective diffusion
– Potentially increase the tritium inventory of the material
• Trapping sites result from:
– Impurities and structural irregularities arising from cold work
– Precipitation of alternate phases including gas bubbles
– Radiation damage
trmttt CαCfα
t
C
Ct – Trapped concentration (m-3)
αt – Trapping rate coefficient (s-1)
ft – Probability of landing in a trap site (-)
Cm – Mobile concentration (m-3)
αr – Release rate coefficient (s-1)
kT
E
N
Ccf
D tor
t
o
ttt exp;;
2
D – Tritium diffusion coefficient (m2-s-1)
λ – jump distance or lattice constant (m)
cto – Trap site concentration (m-3)
N – Bulk material atom density (m-3)
o – Debye frequency (s-1)
Et – Trap energy (eV)
Tritium Trapping (2)
• Trap properties are being measured for neutron irradiated (HFIR) materials exposed to tritium plasmas (TPE, ion fluence: 1025-1026 m-2)
• Trap density measured by nuclear reaction analysis (NRA) – shows some deuterium trapped in tungsten even at 500 C
• Thermal desorption reveals a distribution of trap energies for irradiated tungsten
* M. Shimada et.al., J. Nuc. Mat., (2011) in press
TMAP simulation of TDS spectrum
for 0 dpa W, 200C
TMAP simulation of TDS spectrum for
0.025 dpa W, 200C
Tritium flows and loss paths
Breeding
Zone
Breedin
g Zone
Breedin
g Zone
Coolan
t
Coolant
Tritium
Extraction
System
Coolant
Purification
System
Sec
on
dar
y
Pri
mar
y
Back to Blanket
Back to Blanket
• Losses:
– From breeding zone to coolant (permeation through structure)
– From breeder and coolant pipes to building (permeation through pipe walls)
– From primary to secondary coolant (permeation through HX walls)
To Tritium Plant
To Tritium Plant
Minimizing tritium permeation losses
• Our two primary strategies for managing tritium permeation losses are:
– Efficient tritium extraction systems
• Efficient extraction keeps circulating inventories low, reducing the concentration gradients that drive permeation
– Application of permeation barriers to structural materials
• Blocks permeation, potentially increases circulating inventory
• Extraction concepts attempt to do the following:
– Provide a medium (purge gas, getter, etc.) where tritium will preferentially accumulate, relative to structural materials
– Maximize the contact area of the breeder/coolant with this medium
– Minimize the transport distance through the breeder/coolant to reach this medium
– Maximize the residence time in the extraction system (i.e. reduce the flow rate)
Tritium extraction concepts
• Immersed getters
• Liquid getter/cold trap
• Release to purge gas or vacuum
– Bubblers, spray/droplets, extraction columns
• Vacuum permeator
Immersed Getters
• Getters such as U, Zr(Co), Pd, Ti, etc. are commonly used to remove tritium from gases (Example: a 60g U bed can hold 2g tritium)
• Immersed getters (V, Nb, Ta) have been proposed in the past to remove tritium from PbLi
• Principle demonstrated at small scale with V in PbLi, though not to saturation1
L. Sedano, Ciemat report, 2007.
1H. Feuerstein, Fusion Technology
(14th SOFT), 1986, p. 646
• Issues:
– Integrity of material (getter beds for gases are reduced to fines and require ceramic filters)
– Lifetime of beds under cyclic loading (e.g. daily)
– Deleterious effects of impurities including oxygen
Liquid metal getter with cold trap
• Concept investigated at KIT in the 1980s
– Intermediate NaK loop proposed, which acts as a tritium getter
– NaK cooled so as to precipitate solid hydrides
– Tritium removed from solid hydrides by vacuum pumping
• Might also take the form of a thin film between concentric HX tubes
– Processing rate required for fusion may imply large heat loss
– Li is a possible alternative to Na/NaK but subsequent extraction is more difficult
• High saturated concentration
• Less chemically reactive than Na/NaK
Primary
Intermediate/
Secondary
Alkaline metal film
(Na, NaK, or Li)
Concentric
HX Tubes
J. Reimann, Fusion Technology
(14th SOFT), 1986, p. 1579
Droplets in vacuum
• Concept: spray coolant as small droplets into a purge gas or vacuum
• Small droplets provide high surface area and small transport distance
• “Vacuum Disengager” proposed for HYLIFE-II IFE design study1
• Analytical solution and numerical models suggested 99.9% efficiency; no experiments performed
• Now under investigation for PbLi (as Vacuum Sieve Tray)2
• Different analytical solution and numerical models suggest 70% efficiency achievable
• Measured extraction was lower than predicted by 10x, but models depend on (uncertain) solubility and diffusivity
1T. Dolan, Fus. Tech. 21 (1992) 1949-1954
2F. Okino, FED 87 (2012) 1014-1018
Compact Mass Extractor • Gas/Liquid Contactor – planned for HCLL (TBM and
DEMO)
• Structured packing disperses PbLi flow and creates a large interfacial area between PbLi and gas
• 30% efficiency for single column as tested in MELODIE loop1
• HCLL DEMO (14 inventory re-circulations per day) requires at least 80% efficiency2 even with permeation barriers (with ~100x reduction factor)
• Larger scale tests, optimization planned at TRIEX loop (ENEA)
Sulzer Column
20 cm
Structured
packing
2O. Gastaldi et al. FED 83 (2008) 1340–1347
1N. Alpy et al. FED 49-50 (2000) 775-780
1B. Merrill, 2005/06/15 ARIES meeting
Vacuum permeator
• For a DCLL blanket, higher flow rates must be processed than for an HCLL
– Simple scaling from most efficient MELODIE tests indicates that for a DCLL blanket (~470 inventory re-circulations/day), 240,000 extraction columns would be required1 (!)
• DCLL flow rates are much higher (~470 inventory re-circulations/day required)
• Similar performance in a much smaller device is potentially achievable with a vacuum permeator
• Concept: a shell-and-tube mass exchanger with tritium-laden primary, vacuum secondary, and high-permeability, thin-walled tubes
• Required efficiency depends on the reactor design (losses), but:
– What is achievable?
– How does it scale?
Extraction – Vacuum Permeator
• Analytical solution based on axial and radial transport mechanisms reveals rate-limiting phenomena
– Efficiency:
– Dimensionless numbers:
– : Diffusion through membrane is rate-limiting
– : Mass transport to membrane is rate-limiting
– : Tritium swept through before permeating
• Optimization (minimum size) based on required efficiency, other design constraints is straightforward
• Device can be compact with very high permeability membrane materials (e.g. group V metals: Nb, V, Ta)
• Concept not yet demonstrated experimentally but a number of experiments are planned around the word
Tantalum
650 ˚C
η 0.95
Tubes (#) 687
Tube length (m) 12.4
Velocity (m/s) 3.30
Volume (m3) 0.89
ζ 80.0
1exp1
t =2KTL
vri ioilT
s
rrrKK ln
P. W Humrickhouse and Brad J. Merrill, Fusion Science and Technology 68 (2015) p. 295-302.
Example:
Optimized for FNSF (1 of 4)
1
1
t >>1
Group 5 Metals - oxidation
• Group 5 metals have very high tritium permeabilities and from that standpoint are promising tube materials
• They are compatible with PbLi, but the oxygen partial pressure on the vacuum side must be kept below 10-10
Pa to prevent oxidation1
• Application of a Pd coating can prevent this, and commercial hydrogen purifiers based on this concept are available
• They have a very narrow range of operation around 400 ˚C
– At lower temperatures, hydrides form and embrittle the structure
– At higher temperatures, Pd and substrate diffuse together, reducing (irreversibly) the tritium permeability
1R. Kurtz, 2005 ITER TBM meeting
V. Alimov, International
Journal of Hydrogen Energy
36 (2011) 7737-7746
Potential solutions
• Inter-diffusion of Pd and substrate can be prevented by an intermediate layer that separates them
– Such composites are being actively investigated in the hydrogen energy research community
– These are typically ceramics with some porosity so as not to prevent tritium permeation
– Al2O3, Nb2C, HfN, YSZ, etc. mentioned in literature
• Alloys?
– V-Ni, Pd-Cu, V-Ti, V-Cu, others mentioned in literature
• Other coatings- Pd is necessary for separation from other gases, but we only need to prevent oxidation
Edlund, Journal of Membrane
Science 107 (1995) 147-153
Permeation Barriers • Such barriers are typically low-permeability metals (e.g. aluminum) or
ceramics such as Al2O3, Cr2O3, Er2O3
• These have achieved permeation reduction factors as high as 10,000 in the laboratory
• They have not performed as well in-pile- “Barriers that can provide a significant permeation reduction in the laboratory must be essentially defect-free1”
• Performance of permeation barriers in a radiation environment, on the necessary time scales, must be demonstrated
Levchuck et al JNM 328 (2004) 103 1R. Causey et al in Comprehensive Nuclear Materials, 2012
Hollenberg et al FED 28 (1995) 190-208
Natural oxide barriers
• Nickel alloys (Incoloy 800H, Inconel 617, Haynes 230) have been investigated for use in high temperature gas-cooled reactors
• These are adopted in the EU DEMO and TBM designs for ex-vessel helium systems
• A stable, protective Cr2O3 layer is expected1, even when hydrogen impurities exceed H2O impurities by a factor of 200 to 1
• This layer forms a very effective natural permeation barrier that makes these materials potentially attractive for fusion systems
• This has been demonstrated with MANET, but with EUROFER only a modest (~30x) PRF was demonstrated at high H2O concentrations
Aiello FED 84 (2009) 385-389
Serra JNM 240 (1997) 215-220
1Wright, INL/EXT-06-11494, 2006
Permeation Barriers Under Irradiation
• Possible reasons for reduced performance:
– Degradation of layer (area defect model1):
– Radiation-enhanced permeation2 (composite diffusion model1):
• Permeation, barrier, and and getter performance under irradiation extensively studied under TPBAR program to generate tritium in LWRs
• Despite considerable advances in understanding, phenomena still not completely understood
M
M
B
B xx
PAQ
Px
AQeff
Md
1R. Causey et al in Comprehensive Nuclear Materials, 2012 2W. Luscher et al., JNM 437 (2013). D. Senor, PNNL-22873 (2013).
Do we need permeation barriers?
• Maybe, and depends on the blanket type
• HCLL DEMO loss estimates with TES efficiency of 80% and PRFs of 5 on RAFM blanket structures and 100 on steam generator tubes:
• Note the influence of PbLi solubility; 1 g/y target not met even under optimistic assumptions
F. Franza et al., Fus. Sci. Tech 64 (2013) 631-635 A. Santucci et al., IEEE Trans. Plas. Sci. 42 (2014) 1053–1057
Insights from FNSF study
• In the FNSF study (which employs a DCLL blanket) a number of design aspects help mitigate tritium permeation losses:
– SiC flow channel inserts (FCI) act as permeation barriers within the blanket
– Relatively high PbLi flow rates characteristic of the DCLL result in less tritium generation per pass and lower tritium partial pressures between the blanket and tritium extraction system
– Efficient tritium extraction from PbLi in the form of a vacuum permeator
– Co-axial piping: tritium permeating out of the hot leg enters the cold leg and is returned to the blanket
• Total losses are influenced dramatically by parameter uncertainty in:
– PbLi solubility
– HX and ex-vessel piping permeability (Incoloy 800 oxidation state)
• Depending on assumptions losses are between ~0.03 g/yr (barriers NOT needed) and ~3 g/yr (barriers needed)
Summary and Conclusions
• DEMO must breed and process an unprecedented amount of tritium, and safety requires that losses be a very small fraction of this
– High temperatures make this difficult
• Tritium transport depends on a variety of physical phenomena including diffusion, recombination, adsorption, and trapping
– Measured properties are often highly uncertain, and so are calculations based on them
• Efficient tritium extraction is essential to limiting losses and inventories
– Concepts exist, experimental validation needed (ongoing)
• Permeation barriers in the form of coatings have not performed well under irradiation to date,
– Reasons for this are not completely understood
– They may not be necessary for some design concepts, but this is still not clear in light of parameter uncertainties
– Application more credible in more accessible and maintainable locations (e.g. on the outside of ex-vessel pipes)