Use of Nb or Ta Alloys for Permeator and HX Applications in the DCLL TBM

U.S. Department of EnergyPacific Northwest National Laboratory

R. J. Kurtz

Pacific Northwest National Laboratory

ITER-TBM MeetingMarch 2-4, 2005Los Angeles, CA

PbLi Flow Schematic

He inletHe outlet

PbLipump

Cryo-Vacuum pumpVacuum Permeator2000 Nb or Ta Tubes

Ri = 10 mmtw = 0.5 mmPop < 1 MPaPac ~ 8 MPa

BlanketConcentric pipes

Heat ExchangerNb or Ta Tubes

~20,000 m2

Ri = 10 mmtw = 1.0 mm

Pop = 8-10 MPaPac = ?

T2 outlet

Inter-cooler Pre-cooler Recuperator

Pressure boundary (90°C)

Generator

Turbo-compressorPower turbine

Closed Brayton Cycle

700°C PbLi

460°C PbLiPT2 in PbLi ~0.5 Pa (inlet)

PT2 in PbLi <0.03 Pa (outlet)

Hydrogen Permeability of Selected MetalsBuxbaum and Kinney, Ind. Eng. Chem. Res., 1996

Critical Challenges for Use of Nb or Ta AlloysOperational and anticipated accident loading stresses are low.• Tmax = 700°C, T/TM = 0.36 for Nb and 0.30 for Ta• The maximum effective stress is (assuming thin wall tube and pressure loads only):

• < 8.7 MPa under normal operating conditions, 69.3 MPa under accident loading conditions.

Compatibility with the environment is much more challenging.• Compatibility with liquid metals generally not a problem.• Reaction with gaseous impurities such as O2, N2, COX and CHX the main concern.• At 700°C and low Group V metals (V, Nb and Ta) do not form a protective scale.• Refractory metals will tend to reach equilibrium with reactive gases at some time

during the service life of the structural component.• Present day refractory metal alloys contain reactive metal alloying elements that

can profoundly effect the thermodynamic relationships between reactive gases and the metal, the kinetics of gas-metal reactions and the post-exposure mechanical properties.

σ o =12

σ1 −σ 2( )2 + σ 2 −σ 3( )2 + σ 3 −σ1( )2( )1

2

PO2

High Temperature Deformation of Group V Refractory Metals

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Pionke and Davis, 1979

Thermodynamics of Oxidation Reactions

All Group V metals have high affinity for oxygen.Reactive alloy additions (e.g., Ti and Zr) typically have substantially greater negative free energies of formation of carbides, oxides and nitrides than the matrix element. Thus internal oxidation tends to occur resulting in the formation of compounds.Extremely low oxygen partial pressures are required to prevent oxygen pickup.To prevent formation of NbO2:• 500°C - 6.6x10-45 atm• 700°C - 5.6x10-34 atm

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Charlot and Westerman, BNWL-1842, 1974

Solution and Terminal Solubility of Oxygen in NbCharlot and Westerman, BNWL-1842, 1974

The mechanical properties of refractory metals can be strongly affected at impurity concentrations much lower than the terminal solubility.For this reason the equilibrium between impurities in solution in the metal and in the gas phase as a function of pressure and temperature become the critical thermodynamic criteria for compatibility.For oxygen in equilibrium with Nb(Fromm, 1972):

Even at 1200°C the oxygen pressures are below detectable limits.

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logCO =12

logPO2− 4.5 + 20200 /T

Kinetics of Oxygen Pickup in Nb

The observed oxygen concentration can be significantly lower than thermal equilibrium values.• Protective scale formation (generally

does not occur in refractory metals at high temperature and low oxygen partial pressure).

• Application of protective coating (e.g., Pd).

• The oxygen impingement flux is directly proportional to the oxygen partial pressure.

The oxygen pressure limit can be derived from the impingement flux and a limiting oxygen concentration in Nb.

10-1

100

101

102

103

104

105

106

10-12 10-11 10-10 10-9 10-8 10-7 10-6

Oxy

gen

in N

b,

wppm

Oxygen Partial Pressure, torr

1000 h

1 y

10 y

Assumes 3 mm wall thickness and oxygen ingress from one surface only

Γ =P

2πmkT( )1

2

T = 700°C

Effect of Gaseous Impurities on DBTT of Group V Metals

Ghoniem, APEX Study Meeting, 1998

-250

-200

-150

-100

-50

0

50

100

101 102 103 104

DBTT,

°C

Carbon content, wppm

V

Nb

Mo

-250

-200

-150

-100

-50

0

50

100

101 102 103 104

DBTT,

°C

Oxygen content, wppm

V

Nb

Mo

Synergistic Effect of H and O2 on V-4Cr-4Ti Tensile Ductility

H does not substantially change the yield or ultimate tensile strengths of V-4Cr-4Ti.A 20% increase in tensile strength is found for H levels of about 350 wppm.The main effect of H is to reduce tensile ductility.Above 400 wppm H, where hydride formation sets in, the ductility decreases drastically.H is a more potent embrittling element when it acts synergistically with oxygen.

0

5

10

15

20

25

30

35

0 200 400 600 800

1050°C/1h1150°C/1h1200°C/1h850 wppm O2

1050°C/1hPre-Oxidized

Tota

l Elo

ngat

ion,

%

Hydrogen Concentration, wppm

Maximum Estimated Interstitial Levels for Various Refractory Metals

Ghoniem, 1998~200~150~100Cr, Mo, W

Charlot and Westerman, 1974~300Mo-TZM

Charlot and Westerman, 1974<4000Nb-1Zr (Weld)

Charlot and Westerman, 1974~8000Nb-1Zr (Wrought)

Zinkle and Ghoniem, 2000~1500V

Ghoniem, 1998~10,000~4000~2000V, Nb, Ta

Charlot and Westerman, 1974<2100~3000~3000Nb

ReferenceCNOMaterial

Contaminant Levels, wppm

Impurity Pickup in a Vacuum Environment (Permeator Application)

-50

0

50

100

150

200

250

300

350

400 600 800 1000 1200

Hydrogen

Carbon

Nitrogen

Oxygen

Chem

istr

y ch

ange,

ppm

Temperature, °C

Exposure: 1000 h, 2.7x10-7 torr

Aerospace StructuralMetals Handbook, 1990 Ed.

Exposure of Nb-1Zr for 1000 h in a high vacuum furnace resulted in ~ 50 ppmoxygen pickup at 700°C.The oxygen partial pressure in this vacuum was probably considerably lower than the total pressure of 2.7x10-7

torr (~3x10-9 torr).Thus the oxygen partial pressure limit to avoid unacceptable impurity pickup needs to be in the range 10-10 to 10-11

torr.Cryo-pumped vacuum systems are capable of producing ultra-high vacuums (e.g., ~10-10 to 10-11 torr total pressure) but considerable operational care is required (bakeout, high purity purge gases, etc.)

Permeation of Deuterium in Nb

Terai et al, JNM, 1997

Effect of Oxide Film on Mass Transfer Coefficient

10-5

10-4

10-3

10-1 100 101 102 103 104

K,

m/s

Exposure time, min.

45 torr O2 at 600 °C

Terai et al., J. Nuc. Matls., 1997

The overall mass transfer coefficient of deuterium from PbLi to the purge gas through the Nb wall was smaller by 2-5 orders of magnitude than determined by deuterium diffusion in Nb.

Mass transfer limited by the formation of Nb oxides on the surface acting as a permeation barrier.

Permeance of Pd Coated Ta Membrane Run for 31 Days at 420°C With Weekly Backflushes

Buxbaum and Kinney, Ind. Eng. Chem. Res., 1996

Hydrogen embrittlementfound to be a serious problem with Ta and Nbmembranes.

To avoid embrittlementcracking the minimum temperature needed to be:• 350°C for Ta• 420°C for Nb

Permeability of Bulk Ta MembranesRothenberger et al., J. Mem. Sci., 2003

PH2 = 0.1 - 2.9 Pa

Permeability of Pd-Coated Ta MembranesRothenberger et al., J. Mem. Sci., 2003

PH2 = 0.1 - 2.9 Pa

Impurity Pickup in a He Environment(HX Application)

The rate of impurity pickup by refractory alloys in HX applications is largely limited by the impurity levels in the He coolant.The rates of surface reaction and bulk diffusion of impurities does not significantly effect the rate of impurity ingress in the relatively impurity rich He environment. For alloys containing reactive solutes the rate of bulk diffusion may be substantially lower than for the pure metal. For example, oxygen diffusion in V-Ti alloys is ~100 times slower than for pure V.For a closed secondary coolant loop operated at a He pressure of 8-10 MPa the mass of impurities present is limited by:• The initial impurity inventory contained in the He charge and makeup.• Impurities introduced by component outgassing.

Secondary sources impurity sources include:• Adsorbed impurities.• Impurity in-leakage via molecular flow.• Impurity in-leakage via surface diffusion.

For a given impurity concentration in the He, CHe, the maximum impurity level attained in the refractory metal is:

Cmetal =mHe

mmetal

CHe

Typical Impurities in He Coolant - HGTR Example

<<0.1<<0.1O2

1.51.5N2

<0.1<0.1CO2

52CH4

51CO

0.051H2O

5020H2

Gas Turbine CycleSteam Cycle

Range of Partial Pressure, Pa

Impurity

Natesan et al., 2003

For an HGTR system the oxygen partial pressure is limited by the He coolant passing through the graphite core. For a fusion system gettering of the He must be used to control the oxygen partial pressure.

Strategy for He Coolant Impurity Control

The initial charge gas should be purified to the highest extent possible.The system should be heated slowly, with the purification system operating. Adsorbed gases and component outgassingcan be taken up by the purification system without severe contamination of metal components.

Summary - I

Thermodynamics favors impurity pickup by refractory metal permeator or HX tubing.Refractory metals can tolerate certain levels of gaseous impurities before serious mechanical property degradation occurs. Reactivesolute additions such as Ti and Zr may significantly increase this tolerance.Kinetic factors will control behavior for the vacuum permeator and impurity inventory control in the He coolant for HX tubing. For a vacuum permeator oxygen ingress can be limited by controlling the oxygen partial pressure within the range 10-10 to 10-11 torr. Use of a Pd coating may provide additional protection against fouling due to impurity ingress.

Summary - II

For HX applications high tritium permeation is undesirable so surface conditions that provide a permeation barrier would be beneficial. To avoid excessive impurity ingress the He coolant must be highly purified. The level of purification needed will be dictated by the mass of He relative to the mass of refractory metal tubing and component outgassing.Other factors such as fabricability, weldability, fracture toughness, cost and the potential for dissimilar metal corrosion (refractory to ferritic steel transition) should be considered in evaluating the feasibility of using refractory metals in these applications.