J. Plasma Fusion Res. SERIES, Vol. 10 (2013)

Tritium distribution on the tungsten surface exposed to deuterium plasma
and then to tritium gas
Kanetsugu ISOBE1)*, Vladimir Kh. ALIMOV1.2), Akira TAGUCHI2), Makiko SAITO1,3),
Yuji TORIKAI2), Yuji HATANO2), Toshihiko YAMANISHI1)

1) Tritium technology group, Japan Atomic Energy Agency, shirakatashirane 2-4, Tokai-mura, Ibaraki 319-1195, Japan.
2) Hydrogen Isotope Research Center, University of Toyama, Gofuku 3190, Toyama-shi, Toyama 930-8555, Japan.
3)ITER project Management group, Japan Atomic Energy Agency, mukooyama801-1, Naka-shi, Ibaraki 319-0193, Japan.
(Received: 12 May 2012 / Accepted: 5 October 2012)

Tungsten (W) samples were exposed to Low-energy (38 eV/D), high flux (1022 D+/m2/s) deuterium (D) plasma
at 495, 545 and 550 K to a fluence of 1026 D/m2. After the plasma exposure, tritium was introduced into the samples
by exposure to deuterium-tritium gas mixture at 473K. Tritium distribution on the W surface was examined by thetechniques of imaging plate and autoradiography. The results of the imaging plate revealed that tritium wasconcentrated mainly within the area exposed to the D plasma, and the concentration of tritium is slightly varied
within the area. Autoradiography of the W surface showed that tritium was concentrated on the grain boundary andblisters.
Keywords: Tungsten, Plasma-surface interaction, Tritium, Imaging plate, Tritium autoradiography.

10 u 10 u 2 mm3 and polishing. The grain size was
20-200 ȝm.
The linear plasma generator used for delivering aplasma beam is described elsewhere [4]. A plasma beam
highly enriched with a species of D2+ to over 80% wasobtained. The bias voltage of -80 V was applied to the W
sample resulting in a incident of 38 eV/D, taking into
account the plasma potential of about -4 V measured by aLangmuir probe. The incident deuterium ion flux was
fixed at 1022 D+/m2/s, and the samples were exposed toion fluence of 1026 D/m2. The sample was passively
heated by the plasma itself and exposure temperature wasset by the thermal contact between the sample and thecooled holder. D plasma exposure to recrystallized W was
carried out at around 495, 545 and 550K. At the fluenceof 1026 D/m2, the deuterium inventory of tungsten
exposed at temperature over the range of 500 - 550 K
become higher than that at other temperature range [15].
The surface area of the W samples exposed to the plasma
was 9 mm in diameter. For a comparison, as-received
recrystallized W sample was also used for tritium loading.
Note that the as-received sample was not exposed to theD plasma.
4 months after the D plasma exposure, the samples
were loaded with tritium from gaseous phase. Fig. 1
shows a schematic view of a tritium exposure apparatus.
The tritium exposure apparatus consisted of a vacuum
pumping system, a quartz tube for gas exposure equippedwith an external heater, quadrupole mass spectrometer

1. Introduction

Since tungsten (W) is one of the candidate materials
for plasma-facing components of fusion reactor, many
investigations relating to the plasma-W interaction havebeen carried out. From a view point of safety, the tritium
inventory in plasma-facing materials is a one of key
issues due to limitation of tritium inventory in thevacuum vessel. Most of the results on hydrogen isotope
retention and recycling in W materials have beenreviewed by Causey and Venhaus [1,2], and Skinner et al[3]. It has been reported that during exposure to D
plasmas, blisters with various shapes and sizes dependingon the exposure temperature are formed on the W surface[4-14]. Blister formation is accompanied by deformationof the near-surface layers [12, 13], and this deformation isthought to create additional traps for hydrogen isotopes.
There is also a possibility that amount of hydrogenisotope which could be solved or trapped in the deformedW near-surface layer increases. In this study, W samples
with D-plasma-induced blisters on the surface wereexposed to D2-T2 gas mixture, and the distribution oftritium on the W surface was examined by the techniquesof imaging plate and autoradiography.

2. Experimental

The W specimens exposed to D plasma wereprepared from polycrystalline tungsten (A.L.M.T. Corp.,Japan) with a purity of 99.99 wt%. The samples were
fully recrystallized at 2070 K for 1 hour after cutting into

author’s e-mail: isobe.kanetsugu@jaea.go.jp

81©2013 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchK. Isobe et al., Tritium Distribution on the Tungsten Surface Exposed to Deuterium Plasma and then to Tritium Gas

to expose the emulsion to the tritium beta electrons. Then,specimens were dipped in a developer solution and thenin a fixer solution. Under exposure to beta electrons, theAgBr grains were transformed into Ag grains. The
distribution of Ag grains was observed by the scanningelectron microscope (SEM).

(QMS), and a tritium source. The W samples were placedin the quartz tube, in doing so pieces of glass wool wereinserted between samples to avoid a contact between
them. The quartz tube with samples was heated invacuum at 473K for degassing until termination of gas
release that was controlled by QMS. At this temperature,
we have already confirmed that deuterium in tungsten is
not released to the vacuum by thermal desorption
spectroscopy [15]. When high vacuum was obtained,
tritium diluted with deuterium was introduced into thequartz tube from the tritium source. W samples wereexposed at 473 K to D (92.2%)-T (7.8%) gas mixture at atotal gas pressure of 1 kPa for 5 hours.

QMSIGCM

3. Results and Discussion

Surface morphology of the recrystallized W after D
plasma exposure at 495K is shown in Fig. 2. Grainboundary could be clearly observed in Fig. 2. Largeblisters with sizes of a few tens of microns and small
blisters with size of less than a few microns are observed.
The large blisters demonstrate a multi-layered structure
like steps. The small blisters are formed on the flat
surface of crystal grains.

Chamber495K
TMPSIPRP
Tritium source SpecimenFig. 1 Schematic view of tritium exposure apparatus.
QMS:Quadropole mass spectrometer,
IG:Ionization gauge, CM:Capacitance manometer
SIP:Suptter ion pump, RP:Rotary pump
TMP:Turbo molecular pump

20μmFig. 2. SEM image of recrystallized W exposed at 495 K
to low-energy high flux D plasma with fluence of 1026
D/m2. The surface was tilted at an angle of 45ºto theelectron beam.

The amount of tritium retained in the W near-surface
layers was evaluated by the imaging plate technique.
Imaging plate (IP) is a 2-dimensional radiation detector
with high sensitivity. IP used in our experiments wasBAS-TR2025 (Fuji Photo Film Co. Ltd.) which consistsof 50 ȝm thickness photostimulable phosphor mounted
on a plastic support. The IP surface was contacted to theW surface and exposed to tritium beta electrons in 1 week.
After the exposure to tritium beta electrons, the IP wasprocessed by an IP reader FUJIFILM BAS2500 (FujiPhoto Film Co. Ltd.) in order to obtain the digital image.
Tritium activity is expressed as the photo-stimulate
luminescence (PSL) intensity, which is proportional to theabsorbed radiation energy.
Tritium autoradiography technique was applied totwo specimens; recrystallized W exposed to the D plasma
at 495K and as-received, non-exposed recrystallized W.
At first, to avoid direct contact of the W samples withAgBr grains in radiographic emulsion, thin collodion film
was deposited on the W surface by dipping the samples incollodion diluted by ethanol. The monolayer of theradiographic emulsion (ILFORD L4) was placed onto thecollodion film by a wire-loop method. The specimens
were kept in a light-tight box at around 273 K for 12 days

Fig. 3 shows the results of IP measurements. IP
measurement can easily observe tritium distribution of
the whole of specimen as shown in Fig. 3. A level ofdarkness corresponds to a concentration of tritium innear-surface layer. As seen Fig. 3 (a, b, c), tritium isconcentrated on the surface area exposed to the D plasma.
It is known that under exposure to high flux D plasmas,
vacancy-type defects, voids and cracks are generates in W,
and these defects are responsible for accumulation
deuterium [11-14, 16]. Obviously, subsequent exposure totritium leads to isotope exchange in the near-surface layer[17] and accumulation of tritium at the defects createdunder previous D plasma exposure.
The average tritium concentration of the W surface
exposed at 550 K is significantly higher than that for theW surface exposed at 545 K, although the difference inthe exposure temperature is only 5 K. Obviously, a small
variation in the exposure temperature within thistemperature range has a significant influence on a82

K. Isobe et al., Tritium Distribution on the Tungsten Surface Exposed to Deuterium Plasma and then to Tritium Gas

concentration of the D-plasma-induced traps generated innear-surface layer. This suggestion is supported by sharp
temperature dependence of the D retention in therecrystallized W exposed to low-energy, high-flux D
plasma [16].
(a)

(b)

(1)

(b)

(3)

(2)
(c)

(a)

(4)

100μm

(d)

(6)

Grain
boundary(b)(5)

BlisterFig. 3. IP images for recrystallized W exposed tolow-energy, high flux D plasma at 495 K (a), 545 K (b),550 K (c), and for as-received recrystallized W (d).
Dashed circles (a-c) indicate the surface areas exposed toD plasma. Tritium was loaded into all samples at 473 K,
1 kPa D-T gas mixture for 5 hours.
However, within the exposed surface area the tritium
distribution is not uniform. Intensities of PSL at points((1)-(6)) in Fig. 3 are shown in Table I. The ratio between
the maximum and minimum PSL intensities on the same
exposed sample surface ((2)/(1), (4)/(3), (6)/(5)) varied
from 4:1 to 6:1. It was reported recently that hydrogenisotopes can be retained by tungsten oxide layers [18].
Probably, the surfaces of the W samples werenonuniformly oxidized before tritium gas exposure.

10μm(c)Table I Intensity of photo-stimulate luminescence atsome points on the plasma-exposed W surface
Point
PSL/mm2(1)320(2)
1800(3)500(4)
2100

(5)
560

20μm(6)3200

Figure 4 demonstrates autoradiographic patterns, inwhich white spots correspond to Ag grains tracing thetritium beta electrons. The Ag grains, as white lines, wereobserved along grain boundary (Fig. 4 (a)). The Ag grainsdecorate also blisters and are distributed uniformly on thegrain surface (Fig. 4 (b, c)). However, the Ag grains are notobserved on the surface of as-received recrystallized W(Fig. 4 (d)). As was mentioned above, various types oftrapping sites for hydrogen isotopes are generated in W
under exposure to high-flux D plasma. Under subsequent
exposure to tritium gas at elevated temperature, tritium isthought to be occupied these trapping sites. It should be
noted that techniques of imaging plate and autoradiography
allow detecting tritium retention in a shallow, near-surface
layer because the escape depth of tritium beta electrons in(d)30μmFig. 4. Autoradiographic patterns of recrystallized W
exposed with D plasma at 495K (a, b, c) and as-received
recrystallized W (d). Tritium was loaded into thesesamples at 473 K, 1 kPa D-T gas mixture for 5 hours.

83

K. Isobe et al., Tritium Distribution on the Tungsten Surface Exposed to Deuterium Plasma and then to Tritium Gas

W does not exceed one micrometer [19, 20]. Thus, resultsof this study shows that under exposure to the D plasma atelevated temperatures, traps in the near-surface layer aregenerated mainly along grain boundaries and in layers
subjected to tensile stress due to material migration above
the surface during formation of blisters [12].

report of Hydrogen Isotope Research Center, Toyama
University, 29-30(2010)71V.
[16] V.Kh. Alimov, W.M. Shu, J. Roth et al., J. Nucl. Mater.,
417(2011)572.
[17] Y. Hatano, M. Shimada, V.Kh. Alimov et al., Proc. 20thIntern. Conf. on Plasma Surface Interactions, May 2012,Achen(submitted to J. Nucl. Mater.)[18] V.Kh. Alimov, B. Tyburska, M. Balden et al., J. Nucl.
Mater., 409(2011)27.K. Miyasaka, T. Tanabe, G. Mank andet al., J. Nucl. Mater., 290-293(2011)448[19] T. Hoshihira, T. Otsuka and T. Tanabe, J. Nucl. Mater.,
386-388(2009)776[20] T. Hoshihira, T. Otsuka and T. Tanabe, J. Nucl. Mater.,
390-391(2009)1029

4. Conclusion

Techniquesofimagingplate(IP)andautoradiography were applied to observe a distribution oftritium on the W surface preliminary exposed to
low-energy, high-flux D plasma at elevated temperatures(495-550 K) and then to tritium gas at 473 K. According
to IP images, tritium was highly concentrated within
D-plasma-exposed surface area; however, the tritium
concentration was varied within this area. With the use ofautoradiography, it was found that tritium wasconcentrated on the grain boundary and along/on blisters
formed under D plasma exposure. These results suggestthat D plasma exposure generates traps for hydrogenisotopes in the W near-surface layer.

Acknowledgements

A part of this work is performed with the supportand under the auspices of NIFS (NIFS11KUHR012) anda KAKENHI Grant-in-Aid for Scientific Research (B)(Project No. 21360458, No. 24360380). Authors reallywish to acknowledge for these supports.

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