J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Bi-Directional Hydrogen Isotopes Permeation throughthe First Wall of a Magnetic Fusion Power Reactor
Haishan ZHOU1)*, Yoshi HIROOKA1,2), Naoko ASHIKAWA1,2) and Takeo MUROGA1,2)
Department of Fusion Science, The Graduate University for Advanced Studies, Toki 509-5292, Gifu, Japan1) 2)National Institute for Fusion Science, 322-6 Oroshi, Toki 509-5292, Gifu, Japan*Present address: Institute of Plasma Physics, CAS, P.O. Box 1126, Hefei, 230031, China(Received: 25 September 2014 / Accepted: 19 January 2015)

Plasma-driven permeation (PDP) and gas-driven permeation (GDP) through a first wall candidate material
F82H have been investigated using a laboratory-scale steady state plasma facility. Experiments indicate that GDP
may take place in the opposite direction of PDP, which then results in an unwanted increase in edge plasma density.
A hydrogen flow from the plasma side to the gas side has also been identified. A one-dimensional diffusion
analysis code, DIFFUSE, has been utilized to simulate the experiments. Multiple hydrogen isotopes (D/T)
bi-directional permeation has been studied by DIFFUSE-code modelling as well. The two isotopic (D/T) flows
appear to be independent of each other, driven by their own concentration gradients.
Keywords: plasma-driven permeation, gas-driven permeation, reduced activation ferritic steel, first wall, blanket,
fusion power reactor

effects on hydrogen permeation. For D-T fusion reactor
studies, isotope effects must be taken into accountbecause both D and T are the fuels. Isotope effects onhydrogen transport can be divided into two classes [5],i.e., intrinsic effects and synergistic effects. The formerclass is related to the differences in the transport
properties of each of the individual isotopes. The latterclass is due to the competition of the various isotopes fortraps and the coupling of isotopes through the process of
surface recombination. In the present work, theevaluation using DIFFUSE code has been extensively
performed to deal with the multiple hydrogen isotopes (D,T) permeation cases as well.

1. Introduction

The first wall of a fusion power reactor is defined as
the plasma-facing surfaces of the blanket units, which arerequired to operate at elevated temperatures for an efficientheat exchange. For the blankets employing self-cooled
breeder, the first wall is exposed to the edge plasma,containing energetic D+ and T+ ions on the one side andon the other side it is exposed to T2 gas bred in blankets.
Under these conditions, it is highly possible that thesehydrogen isotopes would penetrate the first wall by aphenomenon called “bi-directional permeation”: (1)deuterium as well as tritium would be transported into theblanket by plasma-driven permeation (PDP), which willprobably necessitate isotope separation; and (2) tritium
would flow in the counter direction to the edge plasma by
gas-driven permeation (GDP), which will affect edge
plasma density [1,2].
For the hydrogen isotopes permeation through thefirst wall, the fundamental materials science questions thatneed to be addressed are: (1) can plasma-driven andgas-driven permeation of hydrogen isotopes in the twocounter directions actually take place under fusion reactor
relevant conditions; and (2) will hydrogen isotopes in these
two flows “recognize” each other? If they do, what would
the resultant flows be like? To answer the first question,
bi-directional hydrogen permeation experiments have been
performed by exposing membrane samples to plasma andgas simultaneously. A one-dimensional diffusion code:
DIFFUSE [3,4] has been utilized to simulate theexperiments. The second question is related to the isotope

2. Experimental

Details of the experimental setup in the steady state
ECR plasma facility: VEHICLE-1 have already beenpresented in our previous papers [1,2]. For completeness,some of the important features of the bi-directional
experiments are described in this section.
Shown in Fig. 1 is the schematic diagram of thebi-directional permeation setup in VEHICLE-1. F82H
membranes are fixed in such a way that the upstream
surface is exposed to hydrogen plasma, while the other side
is exposed to hydrogen gas. At the plasma side, an orificehas been installed so that a quadrupole mass spectrometer(QMS) can measure the H 2 partial pressure in the
VEHICLE-1 main chamber within the operational pressure.
The plasma near the membrane surface is monitored by an
optical spectrometer. For the PDP experiments, a bias ofhaishanzhou@ipp.ac.cn

10©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

H. Zhou et al., Bi-Directional Hydrogen Isotopes Permeation through the First Wall of a Magnetic Fusion Power Reactor

100 V or 50 V is applied and the electron density is of theorder of 1016 /m3. The electron temperature is raised up to~10 eV for the improved sensitivity of Hα spectroscopy.
Under such experimental conditions, the variation insurface recycling condition can be detected. At the gas side,
the hydrogen gas pressure is measured by an absolutepressure gauge.
The F82H samples are prepared in the same
dimensions as those commercially available conflat flanges
with an outer diameter of 0.07 m, except that a circulararea of ~0.035 m in diameter inside the knife-edge ismachined down to thicknesses of 5×10-4 to 5×10-3 m to use
as permeation membranes. Plasma-facing surfaces of themembranes are mechanically polished and then cleaned inan ultrasonic bath. During the experiments, the sample
membranes can be heated up to >773 K by plasma
bombardment and heat from a resistive heater.

3. Bi-directional hydrogendemonstration and modelling

(H)

Fig.1

permeation

3.1. Hydrogen flow from the gas side to the plasmaside (GDP flow)
For this experiment, a 6×10-4 m thick F82H
membrane is used. The temperatures on the gas-facing andplasma-facing sides are measured to be ~853 and ~823 K,
respectively. The ion bombarding energy is set at 50 V.
Taking into account the ion species mix in the lowtemperature hydrogen plasma and the particle reflection at
the plasma-facing surface, the net implantation flux isestimated to be ~8.5×1019 H /m2/s.
Gas-driven permeation of hydrogen has been
identified in the upstream hydrogen plasma, as shown in
Fig. 2. The plasma is produced throughout the experiments
and the downstream side is kept in high vacuum during thefirst ~100 s. From t = ~100 s, H2 gas is introduced into thedownstream to a pressure of 9.3×104 Pa to induce GDP (asshown in Fig. 1). Note that both the PH2 and Hα increase.
Also seen here is the initial transient kick-up, which is

Fig.2

A schematic diagram of the bi-directional
permeation setup in VEHICLE-1.

exhibited by both PH2 and Hα, presumably due toion-induced desorption or thermal desorption, although thedetail is unclear at this point. At t = ~350 s, the H2 gas is
pumped out again and the PH2 and Hα decrease to the initiallevel. The steady state GDP hydrogen flow rate in this casehas been evaluated to be about ~9.9×1019 H-atoms/m2/s.
Figure 3 shows the hydrogen release flux from theplasma-facing surface predicted by the DIFFUSE code.
For the DIFFUSE calculation, the input data are exactly thesame as the experiment (e.g., membrane thickness,
temperature, plasma flux, etc.), except that α-Fe is used asa surrogate of F82H because unfortunately F82H is notincluded in this database. The boundary conditions for theplasma side and the gas side are set to be recombination
release and Sieverts’ law, respectively. In the first 100 s,
only plasma is produced. The hydrogen flux reaches~8.5×1019 H-atoms/m2/s (i.e., hydrogen recycling rate

Fig.3

Hydrogen flux and Hα signals detected in the
upstream hydrogen plasma in VEHICLE-1.
Data rearranged from those published in [1].

11

Hydrogen release flux from the plasma
facing surface of the 6×10-4 m thickmembrane predicted by the DIFFUSE code.

H. Zhou et al., Bi-Directional Hydrogen Isotopes Permeation through the First Wall of a Magnetic Fusion Power Reactor

previous experiments indicate that hydrogen PDP flux ismuch higher when the bias is applied [6]. Here the PDP
flow is assumed to be negligible when the bias is off, i.e.,
only GDP. If a PDP flow into the gas side actually exists inthe bi-directional permeation process, the decreasing rateof hydrogen gas pressure at the downstream side should besmaller than that of the GDP-only case.
Hydrogen gas is introduced into a closed volume(PDP downstream) with an initial hydrogen gas pressure of~6.93×104 Pa. Figure 4 shows the time evolution ofhydrogen gas pressures for the two cases (i.e., w/ and w/oPDP). It can be seen that after the transient phases, both of
the hydrogen pressures decrease, indicating hydrogen
outflows to the plasma side. The difference in the two
curves reflects the effect of PDP.
Using Eq. (3) and the steady state portion of thecurves shown in Fig.4, the PDP flux is estimated to be~2.3×1018 H/m2/s, which is about 30% higher thantheoretical prediction for steady state PDP flux in therecombination-diffusion limited regime [7]:reaches ~100%) within one second. From 100 s to 300 s,hydrogen gas is introduced to the plasma downstream side.
An extra steady-state release flux of 1.3×1020 H/m2/s has
been found, suggesting GDP takes place in the opposite
direction of PDP. The hydrogen gas for driving GDP ispumped out from t = 300 s and the steady-state release flux
is found to decrease to the initial value in the first 100 s.
The estimated steady-state hydrogen GDP flux isrelatively close to the experimental data.
3.2. Hydrogen flow from the plasma side to the gas
side (PDP flow)
Hydrogen PDP flow from the plasma side to the gas
side has been investigated as well. The results shown in theprevious section indicate a net hydrogen flow from the gas
side to the plasma side in the bi-directional permeation
experiments. That means the hydrogen gas pressure at the
PDP downstream will keep decreasing due to the loss ofhydrogen. Then the PDP signal may be picked up from thedecreasing rate of hydrogen pressure at the downstreamside of the membrane. The total remaining hydrogenparticle numbers in a closed volume at time t are given as
N1(t) and N2(t) for the cases with and without PDP as:

N0 − Nt − J GDPts + J PDPts = N1 (t )N0 − Nt − J GDPts = N 2 (t ) ,J PDP =

(1) and⎛ −0.14 eV ⎞D = 7.5 ×10−4 exp ⎜⎟kT⎝⎠⎛ 0.48 eV ⎞ .
K r = 4.8 ×10−21 exp ⎜⎟⎝ kT ⎠

(3)

(5)
(6)

4. Modelling on reactor operation with hydrogenisotopes (D,T) bi-directional permeation throughthe first wall

Based on the above idea, bi-directional hydrogen
permeation experiments have been performed with andwithout a bias of -100 V onto the sample membrane. Our

Fig.4

(4),

where J0 is the net implantation flux; L is the membranethickness; D and Kr are the diffusion coefficient andsurface recombination coefficient for F82H [8]:(2)where N0 is the initial particle number, Nt is the numberof particles trapped in the membrane, s is the surface area,
JGDP and JPDP are the steady state GDP and PDP flux,
respectively. The PDP flux can be evaluated by:

1 dN (t ) dN (t )J PDP = ( 1 − 2 ) .
s dt
dt

D J0
L Kr

4.1. Re-evaluation of tritium pressure in a FLiBe
blanketThe T2 equilibrium pressure in FLiBe is available [9].
Assuming a tritium concentration of 0.1 ppm, the T2
pressure is estimated to be ~104 Pa at 800 K. However,fusion reactor studies indicate that the T2 gas pressure in ablanket is related to a number of factors, e.g., fusion power,
tritium breeding ratio, tritium recovery rate, structurematerial, temperature, flow rate of the breeder, etc.
Evaluation of the T2 gas pressure in the tritium loop
was reported in another paper [10], assuming no T leakage
from the first wall. Under these conditions, the pressure
was found to be 103 -104 Pa, depending on the integration
method of the tritium recovery system. Preliminary tritium
analysis for the blanket employing FLiBe was performed
as well [11]. In the presence of tritium permeation barrier,
the T2 pressure varied from 5×103 to 4.5×104 Pa as adecrease of tritium recovery efficiency. To analyze the

Variation of hydrogen gas pressures
measured by an absolute pressure gauge at
the PDP downstream side.

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H. Zhou et al., Bi-Directional Hydrogen Isotopes Permeation through the First Wall of a Magnetic Fusion Power Reactor

hydrogen isotopes permeation through a bare first wall
made by a reduced activation ferritic steel alloy,
re-evaluation of the tritium pressure has been performed in
the present work.
The overall tritium balance in the loop is given by:∂M T= J1 + J 2 − ∑ J 3−i − J 4 − J 5 ,∂t(7)where MT is the tritium inventory in the loop; J1 is thetritium production rate; J2 is the tritium flow towards theblanket by plasma-driven permeation; J3 is tritium leakage
due to gas-driven permeation, including tritium leakage
from the first wall (J3-wall) and the pipes (J3-pipe); J4 is theextracted tritium by the tritium recovery system; J5 is thetritium leakage in the heat exchanger. Tritium decay isneglected in the present analysis.
For the blanket of a 3 GW fusion reactor with atritium breeding ratio (TBR) of 1.3 [12], the tritium
breeding rate J1 is estimated to be 1.38×1021 T-atoms/s(6.89×10-3 g/s T). The hydrogenic ion implantation flux isassumed to be 1×1020 atoms/m2/s (50% T and 50% D),
which will lead to a steady state PDP flux of 1×1017
atoms/m2/s , i.e., J2 = 2.5×10-4 g/s T for a plasma facing
surface area of 1000 m2. For calculating the tritium leakage
from the first wall by GDP, the total surface area of theblankets modules is 3000 m2 for a helical type reactor [12]and the wall thickness L is 5×10-3 m [13]. The other input
data for the calculation are as follows: the FLiBe flow rateis 2.2×106 m3/s; the tritium solubility is 5.3×10-13 wt
fraction/Pa [14]; the tritium recovery efficiency is 0.99; thesurface area and thickness data for the coolant pipes andheat exchanger are the same as those used in literature [11].
Solving Eq. (7) for a steady state condition, thetritium pressure has been found to be ~1.1×103 Pa, which
is only ~10% of the estimated tritium equilibrium pressure

Fig.6

Fig.5

Time evolution of D/T-concentration profiles
calculated by the DIFFUSE-code forbi-directional PDP-D/T and GDP-T2 from the
upstream
anddownstream
surfaces,
respectively. The D concentration is enlarged
by 100 times.

in FLiBe at a T concentration of 0.1 ppm at 800 K [9].
4.2. Re-evaluation of hydrogen isotopes permeation
through the first wall of FLiBe blankets
Using the tritium partial pressure data shown insection 4.1, hydrogen isotopes permeation through the firstwall of FLiBe blankets has been re-evaluated by
DIFFUSE-code as well.
The input data for DIFFUSE calculation are as
follows: the first wall thicknesses are assumed to be 5×10-3
m and the implanted hydrogen fluxes (D/T) at theplasma-facing side are assumed to be 1×1020 atoms/m2/s(5×1019 D/m2/s and 5×1019 T/m2/s) with a bombarding
energy of 100 eV. At the blanket side, the tritium gas
pressure is 1.1×103 Pa. The first wall temperature isassumed to be the average value of the inlet (773 K) andoutlet (873 K) coolant temperatures. A trap density of 1%
atomic fraction and a tapping energy of 0.62 eV are
assumed.
Figure 5 shows the time evolution of the tritium anddeuterium concentration profiles calculated by the
DIFFUSE-code. Note that the T-concentration profilereflects two contributions: one by PDP from the upstream
surface and the other by GDP from the downstream surface,whereas the D-concentration profile is only due to PDP.
Most interestingly, the two isotopic (D/T) flows appear to
be independent of each other, driven by their own
concentration gradients. Here, note that the Dconcentration shown in Fig. 5 is enlarged by 100 times.
The actual tritium concentration at the plasma-facing
surface (depth ≈ 0) is larger than that of deuterium.
Figure 6 shows the D and T release fluxes at theplasma-facing surface calculated by the DIFFUSE-code.

D and T release fluxes at the plasma-facing
surface calculated by the DIFFUSE-code forbi-directional PDP-D/T and GDP-T2. The
total T flux is composed of the re-emission
flux of implanted T particle and the GDP-T
from the blanket side.
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H. Zhou et al., Bi-Directional Hydrogen Isotopes Permeation through the First Wall of a Magnetic Fusion Power Reactor

The difference in T and D fluxes are mainly due to the
T-GDP from the blanket side. The T-GDP flux is about
1.2×1018 T/m2/s, which is 1.2% of the net implantation flux
from plasma.
Taking hydrogen isotopes permeation into account,the first wall recycling rate R is defined as:R=Γ reflection + Γ re −emission + Γ GDP,
Γ plasma[3][4]

M. Baskes, DIFFUSE83, Sandia Report SAND83-8231.
Y. Hirooka, H. Zhou, N. Ashikawa, T. Muroga and A.
Sagara, Fusion Sci. Technol. 64, 345 (2013).
[5] D. K. Brice, J. Nucl. Mater. 122&123, 1531 (1984).
[6] H. Zhou, Y. Hirooka, N. Ashikawa and T. Muroga,
Hydrogen gas and plasma driven permeation in counter
directions through the first wall of a DEMO reactor, Paper
presented at the 28th JSPF Annual Meeting, Kanazawa,
Japan, Nov. 22nd-25th, 2011.
[7]
B. Doyle, J. Nucl. Mater. 111&112, 628 (1982).
[8]
H. Zhou, Y. Hirooka, N. Ashikawa, T. Muroga and A.
Sagara, J. Nucl. Mater. 455,470 (2014).
[9] S. Fukada, Y. Edao, S. Yamaguti and T. Norimatsu,
Fusion Eng. Des. 83, 747 (2008).
[10] S. Fukada, R. Anderl, A. Sagara and M. Nishikawa, Fusion
Sci. Technol. 48, 666 (2005).
[11] Y. Song, A. Sagara, T. Muroga, Q. Huang, M. Ni and Y.
Wu, Plasma Fusion Res. 7, 245016 (2013).
[12] A. Sagara, T. Goto, J. Miyazawa, N. Yanagi, T. Tanaka,
H. Tamura, R. Sakamoto, M. Tanaka, K. Tsumori, O.
Mitarai, S. Imagawa, T. Muroga and the FFHR designgroup, Fusion Eng. Des. 87, 594 (2012).
[13] A. Sagara, H. Yamanishi, T. Uda, O. Motojima, T. Kunugi,
Y. Matsumoto, Y. Wu, H. Matsui, S. Takahasi, T.
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Fukada, Fusion Technol. 39, 753 (2001).
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5th, 2007.

(8)

where Γplasma is the total incident ion flux from edge
plasma; Γreflection is the particle flux by surface reflection;
Γre-emission is the particle flux by remission and ΓGDP is thesteady state plasma- gas-driven permeation flux. Assuming
a particle reflection coefficient of 0.5, the total incident
flux would be 2×1020 D&T/m2/s and the first wall recycling
rate has been estimated to be R = 1.006.

5. Conclusion

A laboratory-scale plasma device: VEHICLE-1 has
been installed with an experimental setup for bi-directionalhydrogen permeation by plasma-driven permeation (PDP)
and gas-driven permeation (GDP) under conditions to be
seen in the first wall environment.
Bi-directional hydrogen permeation through F82H
has actually been demonstrated in a laboratory-scale steadystate plasma facility. Both PDP flow to the gas side and
GDP flow to the plasma side have been clearly identified.
Re-evaluation of the tritium flows in a FLiBe loop has
been performed, taking into account tritium leakage fromthe first wall. The tritium equilibrium pressure has been
estimated to be ~1.1×103 Pa. DIFFUSE-code modelling
predictions indicate that the two isotopic (D/T) flows
appear to be independent of each other, driven by theirown concentration gradients. Under the bi-directionalhydrogen permeation conditions, the first wall recycling
rate has been estimated to be R > 1 for the blanket
employing FLiBe as a coolant/breeder.
Unfortunately, at present deuterium cannot be used in
our laboratory because of local regulation. Further
validation on the bi-directional phenomena using multiplehydrogen isotopes is anticipated in the future.

Acknowledgement

The author would like to thank Mr. J. Yagyu, Dr. M.
Sato, Dr. T. Nakano and Dr. A. Sakasai of JAEA, Naka for
their material preparation.

References[1][2]

Y. Hirooka and H. Zhou, Fusion Sci. Technol. 66, 63(2014).
H. Zhou, Y. Hirooka, N. Ashikawa, T. Muroga and A.
Sagara, Plasma Fusion Res. 8, 2402065 (2013).

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