Tritium depth profiles and accelerator mass spectroscopy for JET tiles(graphite/CFC tiles)
N.Bekris1, M. Glugla1, U. Berndt1, R.-D.Penzhorn1, M. Friedrich2 W. Pilz2,
1

Forschungszentrum Karlsruhe, Tritium Labor (TLK) P.O.Box 3640, 76021 Karlsruhe, Germany
2
Forschungszentrum Rossendorf, P.O.Box 510119, 01314 Dresden, Germany

1. Introduction
Tritium retention in carbon first walltiles is an important issue not only from aradiological point of view but also forinventory purposes. As the divertor verticaltargets of the ITER machine areconstituted from carbon tiles, it is ofparamount importance to evaluate theamount of tritium that may be retained bythese tiles. For that purpose a total of 24
tiles were removed from the divertor (10),the inner guard limiter (6) and the poloidal
limiter (8) from the JET fusion machine
after
the
first
Deuterium-Tritium
Experiment (DTE1) and sent to the Tritium
Laboratory in Karlsruhe (TLK) for asurface and bulk tritium analysis.
Due to plasma wall interactions a
fraction of the fuelled tritium becomesincorporated into the first wall materials of
fusion machines, so it is expected that they
will be tritium-contaminated.
In addition, during plasma discharges
hydrogen isotopes are implanted into thegraphite tiles as ions or as energetic neutral
atoms having lost their charges after one orseveral electron captures (charge-exchangemechanism). Complex processes, different
to some extent from those known inmetals, govern the transport of hydrogen ingraphite [1]. Plasma particles interact not
only with the outer surfaces of the firstwall but also after diffusion with the innersurfaces of the interconnected pores.
Hydrogen atoms may also combine withthe eroded carbon to form a hydrocarbon
film which is co-deposited on the surfaceof the tiles. In other areas of the tiles theimplanted hydrogen combines with thecarbon to form a hydrogenated carbon
layer whose thickness depends on theenergy of the more energetic particles.
Retention
via
co-deposition
andimplantation of energetic ions into the nearsurface layers as well as bulk effects
including migration through the network ofinterconnected pores and diffusion acrossthe grains are dependent on the plasma
operation conditions (electron temperature,electron density) and on the degree ofneutron and alpha irradiation damage [2].
It is known that hydrogen implanted atroom temperature is retained at the nearsurface until a saturation is reached [3, 4].
The saturation concentration has beenreported to be in the H/C ratio range of
0.3 - 0.6 [5] and depends on the prevailingtemperature. The films found in therelatively cool (< 200°C), shadowed area(not subjected to plasma bombardment) arevery rich in hydrogen content. The H/Cratio measured by Ion Beam Analysis(IBA), showed a value between 0.7 – 0.8[6] It depends also on the implantationenergy (incident ion energy at JET< 50 eV) but seems to be relatively
insensitive to the microstructure of the
carbon [7].
2. Experimental
2.1. Full combustion
The tritium determination wasachieved using the full combustion
technique proposed by Vance et al. [8].
The glass experimental arrangement usedat the TLK is shown schematically infigure 1. A stream of pure air from apressurised cylinder was used with a flowrate of approx. 15 cm3/min. Passing the air
through a bubbler containing water ensured
the carrier gas was humidified.
Cylinders retrieved from selected
zones of the various tiles having a diameterof 7.8 mm were sliced into small disks
having a thickness of 1 mm employing a
diamond-grinding disk. The grinding disk
was rotated with only 100 rpm to avoid
unnecessary heating of the cylinder. To
minimisecross-contaminationthediamond-grinding disk was cleaned after
each cutting.

Fig.1. Schematic view of the
combustion apparatus used at the TLK.
Graphite or CFC disks wereintroduced via a joint into a small
receptacle placed on top of the combustion
zone and isolated from this zone by aglass-to-glass seal. To introduce thegraphite disk into the combustion zone theseal was lifted from the outside with two
appropriate magnets. The combustion zone
itself consists of a vertically arranged
quartz tube heated from the outside with acylindrical oven designed for temperaturesof up to 1500 °C. The actual sample
temperatures were determined with athermocouple placed inside of the quartz
tube. The combustion tube itself contained
approx. 30 g of a wire packing consistingof a Cu2O core with CuO on the surface.

The packing is secured between two pieces
of quartz wool packing, on top of which
the disk to be analysed drops by gravity
when the seal is opened. The copper oxide
contributes to ensure complete oxidation of
all molecular hydrogen isotopes and allhydrocarbons
releasedduringthecombustion of the disks. The effluent gases
are then passed through a bubbler
containing water (about 200 cm3) via theshortest route. By this procedure the quartz
tube, extending from the combustion zone
down to the water level in the bubbler,
remains at all times at comparatively hightemperature. The experimental evidence
indicates that most of the tritiated water(> 99%) is retained in this first bubbler. To
assure quantitative trapping of the tritiatedwater, a second bubbler, also filled withwater is placed downstream.
After each combustion 50 µl wateraliquots were taken from the first bubbler
and 1 ml from the second employing
calibrated syringes. These aliquots weremixed with 15 cm3 of a scintillation
cocktail and analysed for total tritium witha liquid scintillation counter (LSC). The
LSC instrument employed at the TLK is a
Tri-Carb 2300TR Liquid Scintillation
Analyser from Packard. Upon completionof an analysis the surface contamination
remaining on the tubing inserted into thewater of the first bubbler was removed byrinsing thoroughly several times with fresh
water. Frequent blank tests without sample
or with virgin graphite disks were carriedout to verify that no cross contamination
had taken place.
2.2. AMS
In order to have more accurate
concentration profiles in the micro-meter
range the accelerator mass spectrometry(AMS) was used. The method is acombination of secondary negative ion
mass spectrometry coupled to nuclear
physics instruments and devices as are thetandem accelerator or the isotope
separation and detection instruments [9].

The principle of tandem accelerator
mass spectrometry is described below anda schematic view of the experimental setup used at the Forschungszentrum
Rossendorf is shown in Fig. 2.

Fig. 2. Schematic illustration of the
AMS experimental set-up in Rossendorf.
The following is a brief description ofeach element of the AMS system.
The sputter ion source produces abeam of caesium ions. The caesium gas
flows into the ion source after the caesium
reservoir is heated. After being surface
ionised with a hot spherical tungsten
ionizer the Cs+ beam is focused on theinvestigated sample, which is set on anegative voltage of -3 kV giving a cathodecurrent of about 1 mA. Using a mechanicalwobbling mechanism the sample crater
was changed from a bell-like shape tocylindrical shape with a surface area of aparallelogram of about 2.5 x 2.5 mm2. The
Cs+ beam spot has a FWHM of about
400 µm. Due to the uniform erosion inside
the cylindrical sputter crater theconcentration depth profile is converted

into a time dependent particle beam fromthe ion source.
The injection magnet is double
focusing and provides mass analysis of thesecondary negative ions after being
extracted from the ion source by a voltageof 23 kV. The injection magnet bends thenegative ion beam by 80° to select certain
mass atoms or molecules. Some pumps
provide a vacuum of about 10-6 mbar
inside the beam-line. Before the negativeions reaches the accelerator a retractable
Faraday cup was sometimes used for themeasurement of the high intensity
components such as deuterons and carbon.
The tandem accelerator consists of
two accelerating gaps with a large positive
voltage in the middle. Part of it is a largepressure vessel containing the SF6
insulating gas at a pressure of 8 bar. The
negative ions travelling down the beamtube are accelerated towards the positive
terminal. At the terminal they are passed
through a nitrogen gas electron stripper
and emerge as positive ions. These are
repelled from the positive terminal,
accelerating again to ground potential at
the exit of the accelerator. The nametandem accelerator comes from this dual
acceleration concept. The Rossendorf
Tandetron can be operated at a maximum
voltage of 3 MV but for tritium theoptimised terminal voltage was found to beabout 1.5 MV.
The analysing and switching magnet
are employed for further mass separation.
The negative ions with mass 3 (T-, HD-,H3-) selected by the injection magnet are
accelerated by the Tandetron. The T+ ionsgenerated by stripping are further
accelerated by the high energy acceleration
tube and acquire an energy of up to
3 MeV, while the HD+ or the atomic H3+ions generated by charge exchanges in the
stripper disintegrate immediately into
atomic ions. The more stable charged
atomic ions (H+ and D+) having lower
energies and masses than tritium are easily
suppressed by the 30° switching magnet
having a radius of curvature of about1.4 m. Only a very small fraction of thenegative molecular ions undergoes acharge-exchange and turn into positive
molecular ions (HD+) which could becounted as tritium.
The surface barrier detector is used
to count the ions. The molecular ionspassing the switching magnet together withthe 3 MeV tritium ions are finally stopped
by a 17 µm thick Al foil installed in frontof the implanted silicon detector. The few
HD+ ions in the beam were cracked in the
stopping foil to 1 MeV protons and 2 MeV
deuterons, while H3+ ions are converted to
1 MeV protons. The 1 MeV protons are
stopped inside the foil, while the deuterons
passes the foil with lower energy than thetritium ions and can be separated in theenergy spectrum of the detector.
2.2.1. AMS on high tritium samples
In order to prevent any contaminationof the versatile Tandetron during
measurements of samples with high tritium
inventory a dedicated AMS facility with a
100 kVtandemaccelerator
wasconstructed and put into operation at the
Forschungszentrum
Rossendorf[10].
Instead of a gas stripper this tandem
accelerator is equipped with a long-lived
carbon foil for electron stripping.

H2O(g) +C(s) → H2(g) + CO(g)2H2O(g) + C(s) → 2H2(g) + CO2(g)In this oxidation path the oxygen is
supplied by the water which decomposes
to carbon dioxide (or monoxide) andhydrogen. Nevertheless, water is almost
always observed as a reaction productinstead of molecular hydrogen.
The most probable mechanism
involves the interaction of water withcarbon. In a first step, water is adsorbed on
the carbon surface [14]. This adsorption
starts at relatively low temperatures(400 °C) generating a transient species.
This step is followed by the evolution of
the transient species to give carbon dioxide
and water. The whole process is described
below:
2H2O(g) + C(s) → 2H2-CO2(tr)2H2-CO2(tr) + O2(g) → CO2(g) +2H2O(g)3. Results and discussion
The full combustion technique implies
carbon oxidation. The involved reactions
are:
2C+ O2 → 2CO
C+O2 → CO2
In the same way all hydrogen isotopes
trapped, bonded or even co-deposited in
the carbon surface will also be oxidised
according to the reaction
2Q2(s) + O2(g) → 2Q2O(g)There are experimental indications
that the presence of moisture is necessary
for the full combustion of charcoal [11].
For the quantitative determination oftritium in a solid by full oxidation a moist
carrier gas is also recommended tominimise memory effects [12, 13].
Therefore, it is necessary to consider thereactions of carbon with steam. The overall
reaction can be described by:(Q = H, D, or T)

and condensed as liquid into the bubbler
located downstream.

In the following section, thecombustion results obtained with samples
from a TFTR and a JET tile are discussed
in detail.
3.1. Tritium profile of JET CFC tile IN3s1
The analysis of deuterium retained in
the JET fusion machine revealed a nonuniform distribution of the retention overthe vessel [15, 16]. During DTE1 the torus
retained about 40% of the introduced
tritium [17]. Numerous tritium analyses
have also revealed that the most of thetritium retained by the vessel is located in
the inner divertor region especially in tiles
3 and 4 [18]. A cross section of the JET

Fig. 3. Cross section of the MKIIA
divertor in JET. The illustrated surface
tritium activities are in MBq/cm2.
Starting from the inner divertor region(top tile 1) and going along the poloidal
direction (next divertor tiles) the tritium
surface concentration increases graduallyshowing a peak maximum for the edge ofthe tile 3 and the bottom of the tile 4(3270 MBq/cm2
and860 MBq/cm2
respectively). These positions are actuallyin zones not accessible to the plasma(shadowed zones). It appeared that theseparts of the tiles are coated with a thickfilm of co-deposited material (flakes, dust,)
which in some parts is so thick that it has
flaked off and dropped down into thebottom of the vessel.
From the bottom of tile 4 and acrossthe rest of tile 5 the surface tritium
concentration shows a notable decrease
and reaches its minimum on the top of thedivertor dome (10 MBq/cm2). A similar
pattern is observed starting from the top oftile 6 and going across the outer divertor.
However, surprisingly there is no codeposit or flakes on the edge of tile 8 as it
was found for the symmetric tile number 3.
The peak maximum is located once again
in the strike region of tile 7. The same
pattern was observed in deuterium analysesof the same tiles [19].
Besidesthese
surfacetritium
concentrations, the tritium activity in thebulk of the tiles, particularly for tiles 3, 4
and 7, was also measured and the results
concerning tile 3 will be presented
hereafter.
Using a hollow drill several cylinders
were retrieved from the plasma exposed
surface of tile 3 as well as from its
shadowed edge [figure 1 ref.20]
The cylinders were cut to small disks
using the technique described above andcombusted individually in the Vance’s
apparatus. Tritium was analysed by liquid
scintillation.
As it is illustrated in figure 4 there isclearly a diffusion profile through thewhole tile as it was already found out by
PIN diode measurements [20]. We mustalso notice that the x scale in the figures isin millimetres and the last point in eachfigure represents the back side of the tile.

Tritium Activity [Bq]Mark IIA divertor including the tritium
surface concentration measured by the fullcombustion technique in MBq/cm2 isshown in figure 3.

3 10

6

2 10

6

2 10

6

1 10

6

5 10

5

Cyl_1
Cyl_2
Cyl_4

IN3

s1

0
0

5

10

15

20

25

30

35

40

Depth [mm]Fig 4. Tritium profiles for IN3s1
determined by full combustion of small
disks having 1 mm thickness and 7.6 mm
diameter.
The diffusion profile is clearly
exponential and it was possible to fit itwith an equation of the type
A = A B + A 0 • e (− kD ) ,where A is the tritium activity measured or
calculated (for the part lost during cuts),
AB the activity in the bulk of the tile D thedepth and k is a constant having an inverse
length dimension and being dependent on

The surface activity of each cylinder is
not illustrated in the figure 4. Indeed, a lotof co-deposited material coats the plasma
exposed surface of that tile, so that the firstdisk contains a significant amount oftritium and therefore it is omitted from thefigures as it is always off-scale and did not
fit to the above mentioned equation.
Having such an equation allows us also toestimate the activity lost during the cuts
and consequently better assess the tritiumcontent of the whole cylinder.
Table 1 compares the tritium activity
found in the first mm of the plasma
exposed surface of the tile (first disk) andthe bulk of the tile. The high percentage(72%) of tritium found in the bulk ofcylinder 4 arises more from the low tritium
concentration present in the surface of thesample located in a shadowed part of thetile rather than from the amount of tritium
trapped in the bulk of the tile.
It is also remarkable that for cylinder 4a strong diffusion seems to take place fromthe back side of the tile. Such aphenomenon was already observed forother tiles [19] but is not yet elucidated.
Table. 1. Tritium fraction in the bulkof various cylinders # retrieved from JET
divertor tiles after the DTE1 campaign
Tile

3
3
3
4

Cyl.
N°

1
2
4
1
#
diam.7.8mm

Depth
examined
mm
37.2
31.2
32.4
35.3

Tritiumin the 1st
mm
mCi
1.08
0.79
0.33
2.68

Total
Bulk
tritium fraction
bulk+surf
mCi
%
1.82
41
1.24
36
1.19
72
62
6.98

3.2. AMS depth profiles
The AMS tritium profiles are very
different from the profiles obtained by fullcombustion. The depth resolution is of theorder of 0.3 µm instead of 1 mm.
An advantage of AMS is its ability toanalyse different elements for the same
sample giving a better picture of thesample composition. Since the carbon tiles
in JET are coated beryllium, the profile ofsuch an element is very useful to decide
between an erosion or co-deposition area.
The simultaneous depth profiling of D, T,
and Be is illustrated in figure 5 for a JET
graphite tile from the PTE campaign.
16
Deuterium
Tritium

14

Beryllium

Arbitrary scale

the temperature and the nature of the tile
(graphite or CFC). For the CFC tilenumber 3 the k value is fairly constant andabout 0.2 mm-1. A0 represents the tritium
activity on the surface of the tile (this does
not include the tritium content of any
deposited film on top of the surface).

12
10
8
6
4
2
0

5

10

15

20

25

30

35

Depth [µm]Fig.5. Beryllium, deuterium andtritium depth profiles measured by AMS
for a JET CFC tile.
Figure 6 shows the deuterium andtritium AMS depth profiles for samples
retrieved from tile 3 from the inner JET
divertor. As illustrated the profiles of bothelements are characterised by the presenceof two zones. In the first zone
corresponding to the co-deposited layer
with a thickness of about 5 µm the relative
concentration of both elements decreases
sharply with the depth to reach an almost
constant
value
representing
theconcentration of the second zone. In fact
analyses for samples located deeper in thebulk showed a slight further decrease ofthat concentration which corresponds tothe diffusion curve seen in the mm scale ofthe samples in figure 4.

7

IN3 Cyl12 A1Deuterium
Tritium

Arbitrary units

6
5
4
3
2
1
0
0

1

2

3

4

5

6

7

8

Depth [mm]Fig. 6. AMS deuterium and tritiumprofiles for a plasma exposed disc from tileIN3s1 cylinder 12.
Tritium depth profiles measured by
AMS for other carbon samples retrieved
from JET or other fusion machines havealready been reported by Friedrich et al.
[21, 22].
4. Conclusions
Tritium retention in JET clearly occurs
in the strike points region of the inner andouter divertor.
For CFC tiles a large fraction of thetritium is found in the bulk of the tile (upto 60% for tile 4) firmly fixed in the trapsof the carbon net. The most probablemechanism involves tritium diffusion
between the CFC cloths. This strongly
depends on the tile temperature, tile
location in JET and orientation of thecloths with respect to the scrape off layer.
The coring technique followed by afull combustion and liquid scintillation
counting appears to be the mostappropriate method for the absolute tritium
determination on small discs.
In this respect deuterium and tritium
measurements indicate that the innerdivertor collects substantially higher
amounts of tritiated species than the outer
divertor, and clearly acts as a trap for large
amounts of co-deposited material. The
latter covers the inner and cooler part ofthe divertor in zones not accessible by theplasma.
The inner divertor tiles show a clear
diffusion
profile.
Thismechanism
immobilises in the bulk of thecorresponding tiles (3, 4 and 7) substantial
amounts of tritium (~ 40% for tile 3).
AMS analyses on the corresponding
tiles have confirmed that the diffusion
process is taking place for the deuterium as
well.
The existence of large fractions oftritium in the bulk of these tiles represents
an inventory that may affect the use of insitu techniques for tritium detritiation.
Simple isotopic exchange which treats only
the surface would leave substantial
amounts of tritium remaining in the bulk.
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