HYDROGEN ISOTOPES PERMEATION
THROUGH FUSION ORIENTED MATERIALS
A. Aiello, C. Fazio, I. Ricapito, G. Benamati
ENEA Fusion Division
CR Brasimone
40032 Camugnano Italy
ABSTRACTThe evaluation of hydrogen and its isotopes
permeability and solubility in fusion oriented
materials is a key issue in fusion technology
concerning safety, fuelling and reliability. Different
devices have been designed and constructed at
ENEA Brasimone Centre, with the aim tocharacterize hydrogen isotopes transport andinventory phenomena in structural and breeding
materials, in the temperature range between 473 K– 1073K and in liquid metal and gas phase.
Several materials were examined, martensitic steels,tungsten, Tritium Permeation Barriers (TPB), andthe main results, together with a brief description ofexperimental devices and procedure, are hereinpresented and discussed.

1phase with hydrogen and deuterium drivingpressures in the range 10-105 Pa. Recently a newexperimental device was designed and constructed
to reach temperatures up to 800°C.
Several materials and deposition processes wereselected to act as Tritium Permeation Barriers(TPB). The most promising candidate material
resulted alumina, deposited on steels using
Chemical Vapour Deposition (CVD) and Hot
Dipping (HD) pack aluminisation. The qualificationof TPB has been conducted using different devices.
The latest generation is represented by Vivaldi, anapparatus in which two different specimens can bedirectly compared in terms of permeability, in thetemperature range between 573 K and 723 K, in gas
and liquid metal phase.
The main results, together with a description of theexperimental devices and methodology, are hereinpresented and discussed.

INTRODUCTION

In considering materials for fusion reactors adetailed understanding of the transport andinventory parameters of hydrogen and its isotopes
through these materials is an important issue
concerning safety, fuelling and reliability. A large
R&D effort has been done in the frame of EU
Fusion Technology programme to select anddevelop structural materials for the first wall andinternal components of DEMO, and to reduce
tritium permeation through structures. Numericalcodes have been developed for the calculation ofrecycling, inventory and permeation of deuterium
and tritium in fusion reactor design concepts in non
steady state conditions. Essential input data forthese codes are the permeability, diffusivity andsolubility of hydrogen isotopes in the structural
material involved. Hydrogen related problems forfusion applications are been studied since several
years in the ENEA Brasimone Centre.
Different experimental devices have beendeveloped in ENEA, with the aim to fully
characterise hydrogen and its isotopes transport andsolubility in different materials, in a wide range oftemperatures, in gas and liquid metal phase.
The experimental activities are focused on thedetermination of hydrogen/deuterium transportparameters in the temperature range 473÷723K
using a time dependant permeation technique in gas

2

EXPERIMENTAL
2.1

Hydrogen isotopes permeation athigh temperature in gas phase: PERI
and PERI 2 devices

The PERI (PERmeation of hydrogen) device wasdeveloped in the ENEA Brasimone laboratories, inorder to evaluate hydrogen transport and inventory
parameters. The permeation device “PERI” isdescribed elsewhere[1,2] and a schematic view
appears in fig. 1.
The specimen divides the apparatus in two sides:
the high pressure side and the low pressure side.
High pressure deuterium gas, with a nominal purityof 99.7 %, was taken from a cylinder, althoughhydrogen gas had also been used. The hydrogen
had an impurity content less than 1 vppm of H2O
and O2, and was produced by a hydrogenelectrochemical generator connected to a hydrogen
purifier.

Fig. 1: Schematics of the permeation apparatus
PERI for the gas-phase measurements.
The hydrogen/deuterium gas is admitted to thesample via a control valve which, together with apressure transducer and a Baratron, enables thepressure to be set at any value between 100-1.5⋅105
Pa. The sample is heated by a resistance furnace.
Gas permeates through the sample causing a rise inpressure in the outlet volume. For each run, ananalysis of the high pressure and permeated gas iscarried out by a quadrupole mass spectrometer inorder to check for possible contaminants. A
National Instruments system (Labview) is used fordata acquisition.

2.2

reading of 1.3⋅102 Pa. Analogously to the PERI
device the pressure rise detected in the calibrated
volume was converted in amount of hydrogen molepermeated per unit of area and time, the results
obtained were compared with those of PERI. The
gas – liquid phases condition was realised by filling
the test section with the liquid lead alloy and thehydrogen gas was admitted with a continuous flow.
The experimental procedure was the same underboth the conditions and a detailed description of theexperimental procedures is given in [3]. The maindifference between Corelli I and II[4] is represented
by the test section design. Since in CORELLI I thesample was welded to the holders with theconsequence of a higher permeation area, in
CORELLI II the sample was designed to makepossible a direct welding to the test section and theholders were eliminated. In order to confirm theresults obtained with CORELLI II and to evaluate
the TPB effectiveness directly by testing coated anda not coated specimens in the same operating
conditions the VIVALDI apparatus was designed
and constructed (fig. 2) [5].

Liquid metal/gas phase method:
Corelli (I and II) and Vivaldi devices

The experimental installation CORELLI I wasconstructed with Ultra High Vacuum (UHV)
components [3]. The test section was a cylindricalcontainer, which hosted a cylindrical hollow
specimen. The geometry of the specimen waschosen in response to the WCLL design
requirement. The container was joined to a feedtank for its filling up with the lead alloy. Further on
the test section a UHV line with a calibrated
volume for the measurement of the hydrogenpressure inside the specimen was foreseen as well
as a UHV line for measuring hydrogen in the test
section and a UHV line to supply hydrogen to thetest section. The hydrogen gas was admitted in thetest section using a pressure controlled valve with afull scale reading of 1.3⋅105 Pa. All the UHV lines
were connected with a turbo-molecular pumping
group.
The measurements performed on the CORELLI I
device were both in a gas – gas phases and a liquid– gas phases conditions. In the first case after bakeout and evacuation the cylindrical test section was
filled with hydrogen gas. The hydrogen thatpermeated through the specimen was released
inside, where it caused a pressure rise in thecalibrated volume. This pressure rise was measured
with a capacitive vacuum gauge with a full scaleFig.2: schematic of Vivaldi apparatus
In VIVALDI it is possible to perform comparativeevaluation of permeation fluxes, determining
directly the TBP. The gas lines of the VIVALDI
installation were the same as those of CORELLI.
Two hollow cylinder specimens were sealed with astandard CF 16 flange, realised with the samematerial of the specimen. The coupling between thesamples and the upper and lower counter flanges
was realised with standard Cu O-ring. As for
CORELLI, in VIVALDI experimental tests were
performed in gas – gas and in liquid – gas phases.
The measurement lines and the pressure transducersof the two specimens were perfectly symmetric toguarantee the same measurement accuracy.
With regard to the experimental procedure, it was
the same applied in CORELLI in gas phase, whilein liquid metal phase a small continuous flow ofhydrogen gas was bubbled through the liquid
maintaining constant the pressure in the section.

THEORY

It is possible to obtain an expression for thepermeation flux J (moles of gas m-2 s-1) of hydrogenatoms through a membrane of uniform thickness d(m), in the case where the driving hydrogenpressure is increased instantaneously from zero to p
(Pa) (always very much higher than the pressure on
the low pressure side), and where the initialconcentration throughout the solid is zero (i.e. theexperimental conditions used for the present work).
Here J(t) is given by the one-dimensional solutionof Fick's law and can be expressed as:∞⎡
D ⋅ Ks ⋅ p 2 ⎡n 2π 2 ⎤⎤n⎢1 + 2 ∑ ( −1) exp⎢ − D 2 t ⎥⎥ (1)
d
d
n =1
⎣
⎦⎦
⎣
1

J (t ) =where D (m2 s-1) is the diffusion coefficient of themembrane and KS (mol m-3 Pa-1/2) its Sieverts'
constant. It is also assumed that the diffusion
coefficient D is independent of concentration within
the experimental range and no surface effects arepresent, i.e. the rate limiting process is diffusion
through the material rather than surface reactions.
At steady-state (i.e., t→∞) eq. (1) becomes: J=P/d
p1/2, where P=DKS (mol m-1 s-1 Pa-1/2) is defined as
the permeability of the material. To calculate thepermeated flux through a cylindrical hollow sample,
at steady state condition, the formula for hollow
cylinder is used:temperature. As described in paragraph 2 theamount of permeated hydrogen versus time allows
evaluating the permeation flux, the permeability,
the diffusivity and the solubility of the tested
material. With the PERI device austenitic andmartensitic steels [6], also with an interlayer ofcopper[7], W and W-Re [8] were qualified in terms ofhydrogen permeability and diffusivity. Moreover, at
low temperature the onset of hydrogen trapping
effects on martensitic steels was evaluated too[9,10].
The hydrogen trapping behaviour in steels is ofgreat interest because of its relation with theresistance to hydrogen embrittlement.
As example the hydrogen/deuterium permeabilities
through and diffusivities in Eurofer 97 steel
samples are reported in the Arrhenius plot of figure
3 and 4.
T(°C)
450

400

350

300

250

200

1e-10

Φ(mol m-1s-1Pa-1/2)

3

1e-11

F82H D2 gas
1e-12
EUROFER97 H2 gas
EUROFER97 D2 gas

1e-13
1.5

2.0

1000/T (1/K)

J=

P p
R
R out ln out
R in

(2)
Fig. 3: Permeability of hydrogen/deuterium throughEurofer 97 compared to F82H

The total amount of gas, Q(t) moles of gas, whichhas permeated after time t, in the one-dimensionalcase, is given by:
1

0

1

1

⎛
P p 2 d 2 P p 2 d ∞ ( −1) nP p2n 2π 2 ⎞t−−exp⎜ − D 2 t ⎟
∑
2
2
d
6D
d
π D n =1 n⎝⎠

In the Pressure Rise Method (PRM), Q(t) is
obtained experimentally from the pressure rise data,using the ideal gas law. The values of P and D canbe obtained by fitting the experimental data foreach permeation run. Then, having obtained thepermeability P and the diffusivity D, the Sieverts'
constant is determined directly from therelationship Ks=P/D.
4

RESULTS AND DISCUSSION
4.1

Permeation tests in gas phase

The data that could be obtained with the PERI
device are the amounts of hydrogen gas thatpermeates through a disc sample at a given

400

350

300

250

200

150

1e-8

2 -1

t

D (m s )Q(t ) = ∫ J (t ')dt ' =

T(°C)
450

1e-9

F82H D2 gas
EUROFER97 H2 gas
EUROFER97 D2 gas
1e-10
1.5

2.0

2.5

1000/T (1/K)

Fig. 4 Diffusivity of hydrogen/deuterium in Eurofer 97
compared to F82H

In figure 4 is clearly evident the effect of trapping,with a marked decrease of diffusivity at low
temperatures. The measurements were performed

with the PRM technique and more details are givenin [10].
In the case of W and W-Re, the results obtained interms of permeability with Peri installation in thetemperature range 573÷773K, compared withliterature data, are reported in fig. 5.

700 650

600

550

500

400 (oC)

450

10-9
10-10

P (mol m-1 s-1 Pa-1/2)

10-11
10-12
10-13
10-14

4.2

Permeation tests in liquid metalphase
The qualification of TPB was performed using
different permeation devices, as mentioned in theprevious paragraphs. The first apparatus used wasCorelli I, in which a CVD coated sample wastested[3]. Experimental tests were performed in gas
phase and in liquid metal phase, using a gas drivingpressure of 105 Pa. When operating in liquid metal
the gas was bubbled through the liquid. The
hydrogen permeated fluxes were measured in thetemperature range between 523 and 773K
increasing and decreasing the temperature with astep of 50K. Experimental results in terms of fluxes
are reported in fig. 7, in comparison with barematerials.

10-15
10-16
10-17
10-18

773 723

W H (this work) PRM
2

673

623

573

523

473 (K)

10-5

W + 5%Re D (this work) PRM2

p (H2)=105 Pa

W + 5%Re D (this work) CFM
2
MANET D (Serra [1])2
W H (Frauenfelder [2])2W H (Zakharov [3])
2

10-6

1.0

1.2

1.4

1.6

1000/T (K)

Fig. 5: W and W-Re permeabilities compared withliterature data

The maximum temperature reached with PERI was
773 K, too low to obtain a complete set of data on
W and W-Re permeabilities. For that reason the
PERI II apparatus was developed in which thespecimen housing is realised in TZM, amolybdenum alloy, and a different furnace allows
to reach temperatures up to 1073K.
The first results, obtained with a 1 mm of thickness
specimen, are reported in the Arrenhius plot of fig.
6, compared with literature data.
The agreement with previous experimental results
can be considered good.
Temperature [°C]
838
1E-12

727

-1/2

Φ (mol m sec Pa )W-H2 gas

-1

-1

1E-13

1E-14

PERI2
Frauenfelder
Serra
Perujo

0,95

1

1,05

1,1

1000/T (1/K)

Fig. 6: W permeabilities (PERI 2 apparatus)

1,15

J (mol m-2 s-1)

10-19

10-7

10-8

10-9

10-10

10-11
1.2

CVD/MANET with Pb-17Li
CVD/MANET without Pb-17Li
316 L
MANET
1.4

1.6

1.8

2.0

2.2

1000/T (K)

Fig. 7: Hydrogen permeated fluxes through CVD coatedand bare specimens in gas phase and liquid metal phase

It is evident that the Permeation Reduction Factor(PRF), defined as the ratio between flux in the barespecimen and in the coated one, in gas and liquid
metal phase is about 20, very far from the expected
results. For this reason it was supposed the presenceof hydrogen leaks through the structure, with anincrease in the measured fluxes.
In the second generation apparatus, named Corelli
II, the specimen was directly welded to thepermeation chamber, reducing the non coatedsurface in contact with gas. In Corelli II an Hot
Dipping coated sample was tested. The gas pressure
and test temperatures were the same adopted inCorelli I device.
In this case the PRF in gas phase was similar to thatobtained on disk shaped sample[4], while in liquid
metal phase it decreased of more than one order ofmagnitude (fig. 8).

T (K)

Hot-Dipping / MANET II tube

T ( °C)
450

750
350

400

700

650

600

CVD 1 T increase
CVD 2 T increase
CVD 1 T decrease
CVD 2 T decrease
Reference specimen
Disk shaped sample

)
-1/2

1E-6

1E-11

-1 -1

Φ (mol m s Pa

J (mol m

-2

-1

s )

1E-5

1E-7

1E-8
bare MANET II
gas phaseliquid Pb-Li phase

1E-9

1E-10
1,3

550

300

1E-4

1,4

1,5

1E-12

1,6

1,7

1000 / T (K)

1,8

1,3

1,4

1,5

1,6

1,7

1,8

1000/T (1/K)

Fig. 8: HD cotad sample: permeated flux in gas andliquid metal phase compared with bare material

In fig. 9 the SEM micrograph of the HD coatedsample is reported. The presence of surface
microcracks, probably produced by a chemicalinteraction between a not transformed intermetallic
Fe-Al phase and lead lithium, can be observed.

Fig. 10: Hydrogen permeabilities through Eurofer 97 bareand CVD coated specimens

The PRF obtained on the CVD coated samples wasin the range between 3 and 8, demonstrating thepresence of unrecoverable coating defects.
Considering that the minimum acceptable value of
PRF foreseen in the WCLL blanket design is 75, it
was decided not to perform permeation experimentson these samples in liquid metal.
In fig. 11 the SEM micrograph of the transversal
section of the first tested CVD coated specimen,shows the low quality of the coating, with large
detachments.

Fig. 9: Surface of HD sample tested in the Corelli II
deviceVivaldi apparatus was designed to perform
comparative tests between coated and barecylindrical specimen in gas phase and liquid metal
phase, with the aim to directly determine the PRF in
the same operative conditions simply comparing thepermeated fluxes[5].
The apparatus was used to characterise CVD and
HD coated samples, to obtain a complete set of data
to be used in the final coating selection. In fig. 10
are reported the hydrogen permeabilities obtainedon two CVD coated samples, compared in the same
experimental conditions with bare Eurofer 97
specimens.

Fig. 11: Cross section of CVD coating (Vivaldi
experiment)

Next, permeation experiments were carried out on
HD coated sample in gas phase and, considering thegood PRF obtained, also in liquid metal phase. The
hydrogen permeabilities in gas phase and liquid
metal phase compared with those of the barespecimen are shown, as Arrenhius plot, in fig. 12.

700

650

600

-1/2

)

1E-10

-1 -1

Φ (mol m s Pa

550
HD T increase
HD T decrease
HD T increase 2
Ref T increase
Ref T decrease
Disk shaped sample
HD in gas phase

1E-11

1E-12

1E-13

1E-14
1,3

1,4

1,5

1,6

1,7

1,8

1000/T(1/K)

Fig. 12: Arrhenius plot of HD coated sample
Permeabilities in gas phase and liquid metal phasecompared with those of the reference specimen

Comparing the permeated flux measured in gas–
phase and in liquid metal phase, it appears clearly
that the permeated fluxes increase in liquid metal.
This phenomenon could be related to a chemicalinteraction between coating and liquid metal as
proposed by literature. Considering that the
specimen was dipped into liquid metal four days
before permeation tests, it could be assumed thatany kind of coating damage was produced during
this period, reaching a steady state condition before
starting the permeation tests.
Also in this case SEM examinations were
performed after extraction the specimen from thetest section. In fig. 13 the micrograph of thetransversal section is shown.

Fig.13: Cross section of HD coating

The morphology and microstructure of the coating
is not comparable to the usual one obtained withthis deposition technique. On the basis of themetallographic analysis performed it could bereasonably supposed that the usual procedure for
producing this type of coating was not adopted.
Further investigations are in progress to explain thephenomenon.

4 CONCLUSIONS
Relevant R&D activities were done in ENEA
Brasimone centre on material permeability
qualification in terms of:• Hydrogenisotopes
permeability
behaviour through steels (PERI)• Hydrogenisotopes
permeability
behaviour through refractory metals(PERI 2).
Testing procedure and experimental devices toperform permeation measurements in gas andliquid metal phase were improved in terms offlexibility and reliability, starting from Corelli I
since Vivaldi, now in use for coating qualification.
The coating qualification procedure elaborated andadopted in Vivaldi can be considered as the‘recommended’ procedure, and Vivaldi is now thereference apparatus for coating qualification as TPB
in Europe.
The devices available in ENEA Brasimone r.c.
make it possible to fully qualify different materials
in terms of gas permeation in a wide temperature
range (473-1073K).
Gas-permeability measurements in liquid metal
phase are also possible using Vivaldi, in which a
direct comparison between bare and coatedmaterials or different materials is possible, in thesame operative conditions.

ACKNOWLEDGEMENTS
The authors wish to thanks CEA Grenoble and FZK
Karlsruhe that have provided coated samples usedin the experimental activities herein presented.

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Nuclear Materials, 245, 108-114 (1997)[2] E. Serra, G. Benamati, Material Science andTechnology, 14, 573-578 (1998)[3] E. Serra, C. Fazio et al. ENEA report ERG
FUS ISP CMAT 004 - 1997
[4] A. Aiello, I. Ricapito, M. Chini et al., Fusion
Engineering and Design, 58-9, 737-742(2001)[5] A. Aiello, I. Ricapito, ENEA Report LB-A-R012 - 2001
[6] E. Serra, G. Benamati et al., Journal of Nuclear
Materials, 255(2-3), 105-115 (1998)[7] E. Serra, E. Rigal et al., Fusion Engineeringand Design, 49, 675-679 (2000)

[8] G. Benamati, E. Serra, C.H. Wu et al., Journalof Nuclear Materials, 283, 1033-1037(2000)[9] M. Beghini, L. Bertini et al., Journal of Nuclear
Materials, 288, (2001)[10] A. Aiello, I. Ricapito, G. Benamati et al.,
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