Plasma and Fusion Research: Regular Articles

Volume 14, 1405125 (2019)

Eﬀect of Re-Deposition Layers in Plasma-Facing Wall on
Tritium Retention and Tritium Depth Profile
Masao MATSUYAMA, Hideki ZUSHI1) , Kazutoshi TOKUNAGA1) ,Arseniy KUZMIN2) and Kazuaki HANADA1)
Hydrogen Isotope Research Center, Organization for Promotion of Research, University of Toyama, 3190 Gofuku, Toyama
930-8555, Japan1)
Rearch Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan
2)
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan(Received 25 October 2018 / Accepted 4 June 2019)
Eﬀect of the re-deposition layers formed on plasma-exposed stainless steel type 316L (SS316L) in QUEST
on the retention and depth profile of tritium has been studied by both methods of tritium exposure experiments andnumerical analyses of X-ray spectra observed by the β-ray-induced X-ray spectrometry (BIXS). Both samplesof plasma-exposed and non-exposed SS316L were exposed to tritium gas under given temperature, time andpressure conditions. Surface of the former sample was covered with re-deposition layers after exposing to theplasma experiments. After tritium exposure, X-ray spectra induced by β-rays emitted from tritium atoms retainedin the surface layers and/or dissolved into the bulk were measured using an ultra-low energy X-ray detector
consisting of pure Ge semiconductor, and numerical analysis of the observed spectrum was conducted to estimate
a tritium depth profile in the sample. As a result, it was found that the amount of tritium in surface layers of theplasma-exposed sample was about five times larger than that of the non-exposed sample, and the tritium depth
profile for the plasma-exposed sample was about half depth in comparison with that for the non-exposed sample
although the degassing temperature and tritium exposure conditions were the same for both samples. It wassuggested, therefore, that the re-deposition layers played a role of diﬀusion barrier of tritium atoms formed on
the sample surface.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: tritium retention, tritium depth profile, re-deposition layers, β-ray-induced X-ray spectrum, numerical analysis
DOI: 10.1585/pfr.14.1405125

1. Introduction
Reduction of fuel particles in the plasma-facing materials (PFMs) of the future fusion plants is one of the crucial
issues from viewpoint of steady-state operation. In particular, tritium retention in PFMs is one of important issues
from viewpoints of safety and economy as well as a site
limitation of tritium. It is well known that original surface
characteristics of PFMs change by bombardment of highenergy particles such as neutron and fuel particles, by surface erosion due to physical and chemical sputtering, andby formation of re-deposition layers during a long operation [1]. Therefore, tritium retention in the PFMs should
be strongly aﬀected by various phenomena. Especially,
the re-deposition layers formed on the surface of PFMs
directly influence to adsorption/desorption behavior of tritium as was reported by previous papers [2, 3]. Namely,
physical and chemical properties of original surfaces of
PFMs are changed by the re-deposition layers, and new
surface characteristics will appear. It is foreseen, therefore,that tritium retention in surface layers changes depending
on a state of plasma and vacuum conditions of the fusion
author’s e-mail: toyama3h1949@gmail.com

devices. The diﬀusion behavior of tritium in the PFMs will
be similarly aﬀected by the formation of re-deposition layers.
Matsuyama et al. reported the eﬀects of plasma exposure on tritium retention in a stainless steel type 316L
(SS316L) [4, 5]. The samples used in this study were exposed to plasmas of the Large Helical Device (LHD) in
National Institute for Fusion Research and of the Q-shu
University Experiment with Steady-State Spherical Tokamak (QUEST) [5]. The device of QUEST was establishedin the Research Institute for Applied Mechanics (RIAM) of
Kyushu University. Non-exposed SS316L (described hereafter as “bare SS316L”) samples were also used in thesestudies to examine tritium retention behavior in detail. The
amount of tritium retained in the plasma-exposed SS316L
samples was rather large in comparison with that in thebare SS316L samples, when the samples were exposed totritium gas at 393 K after degassing at the same temperature. This may be due to the eﬀect of re-deposition layers
formed by plasma exposure.
On the other hand, the tritium retention in surface layers of specimens increased with increasing both tempera-

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tures of degassing and tritium exposure [5]. When a degassing temperature was above 573 K, significant increasein tritium retention appeared even if tritium exposure temperature was low. Similar tendency of tritium retention wasobserved for the bare SS316L samples. Namely, this indicates that the surface state of SS316L plays an importantrole for tritium retention. Analyses by X-ray photoelectron
spectroscopy (XPS) indicated that changes in the chemical states of SS316L surface at high temperatures werestrongly related to tritium retention behavior. It was concluded, therefore, that the tritium retention in PFMs was
significantly aﬀected by history of the evacuation conditions along with re-deposition layers formed by exposing
to plasmas. In addition to such a heating eﬀect, distribution of tritium near surface of a specimen is of an important
issue to discuss about recycling of fuel particles in PFMs.
Many studies on a depth profile of tritium in metallic materials such as stainless steels have been reported so
far [6–10]. Most of these papers showed that tritium atoms
absorbed in the materials are trapped in surface layers ofa few micrometers in thickness and that tritium concentration in this region could be more than two orders of magnitude higher than that in the bulk [7–10]. Therefore, determination of a tritium depth profile in the materials is of
an important factor to consider recycling of tritium in thesurface layers of PFMs even if the total amount of tritium
retained in PFMs is the same. There are no reports about
the eﬀects of re-deposition layers formed by plasma exposure on the retention and depth profile of tritium.
Some techniques are employed for determination ofa depth profile of hydrogen isotope atoms such as H and
D atoms trapped in the metallic materials. For example, nuclear reaction analysis such as H(15 N, αγ)12 C andD(3 He, p)4 He is applicable to date as a non-destructive
technique for measurement of a depth profile of hydrogenisotopes in surface layers of the materials [11–14]. On theother hand, an etching method of surface layers by acidsolution is principally available for the tritium containing
samples although this technique is destructive and generates the radioactive liquid waste [7, 8]. As a quite diﬀerent
method from these techniques, Matsuyama has proposed a
new method for estimation of a tritium depth profile in thetritium retained specimens [15]. Although the details will
be described later, the new method is a technique using an
X-ray spectrum induced by β-rays which are emitted by
the decay of tritium atoms on the surface and in the bulkof metallic materials. This technique is non-destructive
method for analysis of a tritium depth profile, and is called
as BIXS (β-ray induced X-ray spectrometry).
From these viewpoints, eﬀect of the re-deposition layers on the retention and depth profile of tritium has been
examined in this study from both sides of measurements of
X-ray spectra and their reproduction by applying numerical analyses based on a given depth profile of tritium. The
re-deposition layers were formed on the surface of SS316L
specimen by exposing to plasmas in QUEST.

2. Experimental
2.1 Materials
The sample plates used are stainless steel type 316L,
whose nominal atomic composition (at. %) was as follows: Fe(65.3), Cr(19.0), Ni(11.9), Mn(1.89), Mo(1.28)
and Si(0.72). Size of a plate was 15 × 15 × 0.5 mm3 . It
was polished by a buﬀ and then rinsed with acetone. Twokinds of SS316L samples were used in this study: one isa bare SS316L sample and the other is a plasma-exposed
SS316L sample. It was prepared by the following procedures: a bare SS316L sample was fixed by a sample holder
attached at the top of the linear motion feed through, andit was inserted near inner wall of QUEST. A linear motion
feed through was connected with the MH10 port which islocated at mid-plane of QUEST. The sample was mainly
exposed to hydrogen plasmas during a plasma experimentcampaign (ID No. 450-460) of the spring/summer season
in 2016. After the plasma exposure to a bare SS316L
sample, it was transported from the facility of QUEST
in Kyushu University to the tritium experiment facility ofHydrogen Isotope Research Center, University of Toyama.
Tritium gas diluted with hydrogen was used for tritium exposure experiments, and the concentration of tritium used
was ca. 5% tritium. Powdered Zr-Ni alloy which is wellknown as a hydrogen storage material was used for the
supply/recovery process of tritium gas.

2.2 Procedures
Tritium exposure tests of the plasma-exposed and bare
SS316L samples were conducted under given conditions oftemperature and pressure by using a specially designed tritium exposure device [16]. A given pressure of tritium gas
was supplied to a tritium exposure part by heating Zr-Ni alloy powders at a given temperature and it was completely
recovered by using the same Zr-Ni alloy powders after exposure to tritium gas. Temperature and pressure in the tritium exposure tests were set at 623 K and 1.3 kPa, respectively. Tritium gas was exposed a sample for 4 hr in eachrun. After the exposure, the sample was quickly cooled
down at room temperature, and the residual tritium gas in
the exposure part was recovered. Subsequently, evacuation of the exposure part by an ion pump was continued
for one night to avoid the tritium contamination of ambient air when the exposure part was opened to take out thetritium exposure sample.
After the tritium exposure and evacuation, the sample was taken out and the amount of tritium retained ina sample was evaluated by using the technique of BIXS,
where argon was used as a working gas. The X-ray spectra
were measured by an ultra-low energy X-ray detector. In
order to detect low energy X-rays higher than 1 keV, a thinberyllium plate (ca. 8 µm in thickness) was employed as an
entrance window material of the X-ray detector. Electricalsignal of the X-rays was accumulated for a given time bya multi-channel analyzer, and energy width (ΔE) in each1405125-2

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channel was set to ΔE = 32.8 eV.
The amount of tritium retained in surface layers canbe evaluated from the intensity of characteristic X-raysof Ar(Kα,β ), which means the sum of X-ray intensities ofAr(Kα ) and Ar(Kβ ). Thickness of the surface layers was
defined as an average escape depth of β-rays in SS316L. It
was estimated to be 0.22 µm for 5.7 keV which is the average energy of β-rays and 0.07 µm for 3.2 keV which isthe energy of K-shell absorption edge of Ar. On the other
hand, depth profile of tritium in the bulk was estimatedby applying numerical analysis of the X-ray spectrum observed. Shape of bremsstrahlung X-ray spectrum in the observed X-ray spectrum gives important information about
the depth profile of tritium. How to analyze an X-ray spectrum is described in detail elsewhere [15].

3. Numerical Analysis of Tritium
Depth Profiles Based on BIXS
β-rays emitted from nuclei of tritium atoms have continuous energies in the range from 0 to 18.6 keV. Whentritium atoms exist in the bulk of a material, most of kinetic
energy of β-rays is consumed by the interactions with constituent atoms of a material. Consumed energies are partly
converted into characteristic and bremsstrahlung X-rays ina material. The former X-rays show a line spectrum, while
the latter ones do the continuous spectrum. Although penetration power of the X-rays induced by β-rays is muchgreater than that of the β-rays, the produced X-rays are
partly absorbed in a material because these are low energy
X-rays. Therefore, practical intensity of an X-ray spectrum
observed at the surface of a material depends on not only
the number of tritium atoms but also a depth profile of tritium atoms in the material. This is a reason why shape ofa bremsstrahlung spectrum changes with a depth profile oftritium in materials.
As an example, X-ray spectra obtained by a numerical calculation method are shown in Fig. 1, assuming thata given amount of tritium dissolves into a SS316L sample.
Depth profiles of tritium as shown in Fig. 2 were given ineach calculation. In addition, it was assumed that a penetration depth of β-rays in SS316L was 0.22 µm as mentioned above. Appearance of the plural characteristic Xray peaks and a broad and weak peak was confirmed in
the spectra of Fig. 1. The latter peak is bremsstrahlung Xrays and the maximum shifts to higher energy side withextension of a tritium depth profile. Namely, it can be understood that the shape of bremsstrahlung X-ray spectrum
depends on a depth profile of tritium in a SS316L sample.
Appearance of two peaks around 3 keV is due to areason that argon gas (Ar(Kα ) = 2.957 and Ar(Kβ ) =3.190 keV) was used as a working gas of BIXS in
the calculation of an X-ray spectrum. The peak ofAr(Kα ) consists of Ar(Kα1 ) (= 2.9577 keV) and Ar(Kα2 )(= 2.9556 keV) peaks. As the energy diﬀerence between
these peaks is very small as 2.07 eV, it is impossible to sep-

Fig. 1 X-ray spectra calculated by basing on the diﬀerent depth
profiles of tritium in bare SS316L. Tritium depth profiles
from DP1 to DP3 are shown in Fig. 2.

Fig. 2 Example of various depth profiles of tritium assumed fornumerical calculation of X-ray spectra. DP1: exponentialfunction, DP2: first order function, DP3: uniform distribution.

arate both X-ray peaks by the present X-ray detector. As aworking gas, other gases such as neon, krypton, and xenon
are also applicable to measure tritium retained in the sur-

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Fig. 4 X-ray spectrum observed for the bare SS316L sample exposed to tritium gas. Tritium exposure conditions: degassing temperature was 673 K and tritium exposure temperature was 623 K.

Fig. 3 Change in the intensities of characteristic X-rays and intensity ratios among them with depth of tritium distribution. Figure [A] shows changes in X-ray intensity andFig. [B] shows changes in intensity ratio with the depthof uniform tritium distribution.

face layers, but argon gas is the best among them fromviewpoints of economy and safety. This is a reason why
argon gas was used in this study. By using a working gas, it
is possible to separate tritium retained in the surface layers
from tritium dissolved into the region deeper than 0.2 µm.
One can apply other working gases such as helium, nitrogen and oxygen when major purpose is to measure only abremsstrahlung X-ray spectrum.
The intensities of plural characteristic X-ray peaks obtained from numerical calculation for a SS316L sample
and ratios among them give important information to estimate qualitatively a tritium depth profile. Figure 3[A]shows changes in the intensities of Ar(Kα ), Cr(Kα ), Fe(Kα )and Ni(Kα ) for the uniform depth profile of DP3 in
Fig. 2. And the ratios Cr(Kα )/Ar(Kα ), Fe(Kα )/Ar(Kα ),Cr(Kα )/Fe(Kα ), and Cr(Kα )/Ni(Kα ) were also describedin Fig. 3[B]. It is seen from Fig. 3[A] that intensities ofcharacteristic X-rays steeply increase with an increase indepth but those saturate at about 30 µm. This is also applicable to the ratios among them although increase in theratio Cr(Kα )/Fe(Kα ) is somewhat moderate in comparison
with other ratios. Such a diﬀerent increase behavior may
be mainly due to the large mass absorption coeﬃcient ofFe(Kα ) X-rays for chromium atoms.

4. Results and Discussion
Figure 4 shows an example of X-ray spectrum ob-

Fig. 5 X-ray spectrum observed for the plasma-exposed
SS316L sample. Tritium exposure conditions: degassing
temperature was 673 K and tritium exposure temperature
was 623 K. The inset shows the whole X-ray spectrum
observed in this experiment to compare with the X-ray
spectrum showed in Fig. 4.

served for a bare SS316L sample. This sample was exposed to tritium gas at 623 K for 4 hr after degassing at
673 K in vacuum. As clearly seen from the spectrum, plural characteristic X-ray peaks such as Ar(Kα,β ), Cr(Kα ) andFe(Kα ) and a broad bremsstrahlung X-ray peak of which
maximum was around 5.5 keV were observed, indicatingthat much tritium adsorbed on the sample surface and apart of the adsorbed tritium diﬀused into bulk during exposure to tritium gas.
On the other hand, Fig. 5 shows the X-ray spectrum
observed for a plasma-exposed sample. A similar Xray spectrum was observed, although a peak intensity of1405125-4

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Fig. 6 Analysis of tritium depth profile for the bare SS316L exposed to tritium gas: [A] is an assumed depth profile, [B]is the X-ray spectrum calculated by basing on the depth
profile shown in [A].

the X-rays was diﬀerent from that observed in Fig. 4 asshown in the inset of Fig. 5. Especially, intensity of theAr(Kα,β ) peak which is proportional to the amount of tritium retained in surface layers increased by a factor of ca.
5, whereas the intensities of bremsstrahlung, Cr(Kα ) andFe(Kα ) X-rays did not increase significantly. Such tendency indicates that the amount of tritium retained in thesurface layers largely increased but the bulk concentration
was not so high. This indicates that diﬀusion of tritium
into the re-deposition layers formed on the surface is low.
It is well known that the plasma-exposed sample is covered with re-deposition layers such as carbon and the oxides of constituent elements of plasma-facing materials [1].
Namely, the re-deposition layers bring about increase in
the amount of tritium in surface layers and at the same timethose prevent diﬀusion of tritium into the bulk. It is considered, therefore, that the former eﬀect may give rise toapparent increase in the recycling ratio of fuel particles on
the surface of PFMs.
It is possible to estimate a depth profile of tritiumin the materials by analyzing the observed X-ray spectra as mentioned in the previous section. Namely, one

Fig. 7 Analysis of tritium depth profile for the plasma-exposed
sample: [A] is an assumed depth profile, [B] is the X-ray
spectrum calculated by basing on the depth profile shownin [A].

can approximately reproduce the observed X-ray spectrum
when a reasonable depth profile is given. Thus X-ray
spectra were repeatedly calculated by assuming various
depth profiles of tritium. Figure 6[A] shows one of tritium depth profiles assumed for the bare SS316L sample,
and Fig. 6[B] describes the X-ray spectrum which was calculated basing on the depth profile described in Fig. 6[A].
Table 1 shows the summary of peak intensities of the characteristic X-rays in the observed and simulated spectra. As
clearly seen from the intensity ratio of both spectra, the ratios were almost 0.5. In addition, intensity ratios of a major
peak in the simulation spectrum were in accord with thatin the observed spectrum within 10%. Namely, these values mean that the characteristic X-ray peaks observed forbare SS316L sample were perfectly reproduced by numerical calculation. On the other hand, the shape of a simulated bremsstrahlung X-ray peak also agreed with that of
the observed peak except for a low energy region. It isseen, therefore, that the tritium depth profile of Fig. 6[A]was able to reproduce thoroughly the observed X-ray spectrum shown in Fig. 4. That is, it was found from the depth

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Table 1 Summary of the peak intensity and ratio in the observed and simulated spectra for bare SS316L sample.

Table 2 Summary of the peak intensity and ratio in the observed and simulated spectra for plasma-exposed SS316L sample.

profile that tritium atoms diﬀused near the center region in
depth of a bare SS316L sample under the given temperature conditions.
Similar numerical calculations were applied to estimate a tritium depth profile for the plasma-exposed sample. The X-ray spectrum was repeatedly calculated in thesame manner by assuming various depth profiles of tritium.
Figure 7[A] shows one of tritium depth profiles assumed
for numerical calculation. Figure 7[B] is the X-ray spectrum calculated by assuming the depth profile shown in
Fig. 7[A]. Figure 7[B] which is well coincident with theobserved spectrum was obtained by applying the depth
profile of Fig. 7[A]. Table 2 shows the summary of intensities of the characteristic X-ray peaks in the observed andsimulated spectra. As clearly seen from the intensity ratio of both spectra, the ratio was almost constant as 1.1.
In addition, intensity ratio of a major peak in the simulation spectrum was in accord with that in the observed
spectrum within 10%. Namely, this means that the characteristic X-ray peaks observed for plasma-exposed sample
were reproduced by numerical calculation. On the other
hand, the shape of a simulated bremsstrahlung X-ray peakalso agreed with that of the observed peak except for a low
energy region. As clearly seen from the depth profile ofFig. 7[A], a region of tritium distribution became narrow
although the exposure temperature conditions of tritium
were the same as the bare SS316L sample, indicating that
diﬀusion of tritium into bulk of the plasma-exposed sample was rather disturbed. It is likely that the decline ofthe diﬀusivity of tritium is due to the re-deposition layers
formed on the surface of SS316L by exposure to plasmas.

Namely, re-deposition layers play a role as a diﬀusion barrier of tritium although they bring about the increase intritium retention in the surface layers.

5. Conclusions
Eﬀect of plasma exposure on the retention and depth
profile of tritium in stainless steel type 316L (SS316L) exposed to tritium gas has been examined by both methodsof experiment and numerical calculation. The amount oftritium retained in surface layers of the plasma-exposed
sample was about five times larger than that of the bare
SS316L sample even if both samples were exposed to tritium gas under the same conditions. However, it was foundfrom numerical calculation of X-ray spectrum induced by
β-rays of the dissolved tritium that the tritium depth profile estimated from the plasma-exposed SS316L sample
was narrower than that from the bare SS316L sample. It
is likely that this is due to decline of diﬀusion rate of tritium atoms in the re-deposition layers formed on the sample surface by plasma exposure. Namely, it was concluded
that re-deposition layers played a role as a diﬀusion barrier
but those brought about the increase in potential of tritium
retention in surface layers.

Acknowledgments
A part of this work has been performed with the support and under the auspices of the National Institute for
Fusion Science (NIFS) Collaboration Research Program(NIFS12KUTR081).

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