J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Exchange Reaction of Hydrogen Isotopes on Proton Conductor Ceramic ofHydrogen Pump for Blanket Tritium Recovery System
Yoshinori KAWAMURA, Kanetsugu ISOBE and Toshihiko YAMANISHI
Tritium Technology Group, Fusion Research and Development Directorate, Japan Atomic Energy Agency(Received: 24 October 2009 / Accepted: 17 April 2010)

Electrochemical hydrogen pump using ceramic proton conductor has been investigated to discuss its
application for the blanket tritium recovery system of the nuclear fusion reactor. As the series of those work, thetransportation experiments of H2-D2 mixture via ceramic proton conductor membrane have been carried out. Then,the phenomenon that was caused by the exchange reaction between the deuterium in the ceramic and the hydrogenin the gas phase has been observed. So, the ceramic proton conductor which doped deuterium was exposed tohydrogen under the control of zero current, and the effluent gas was analyzed. It is considered that the hydrogen in
the gas phase is taken as proton to the ceramic by isotope exchange reaction, and penetrates to the ceramic bydiffusion with replacement of deuteron.
Keywords: fusion reactor, blanket, tritium, hydrogen pump, hydrogen sensor, isotope exchangepressure system and from the hydrogen compound via
decomposition. These advantages are suitable for BTR,
and it may be possible to establish smart BTR by usinghydrogen pump. So, the authors have investigated the ionictransportation properties of hydrogen isotopes[8-13].
When the author investigated the transportation
properties of H2-D2 mixture as one of a series of those
work, the phenomenon that seems to be the contribution ofexchange reaction between H2 in gas phase and D2 inceramic has been observed[9]. So, in this work, the masstransfer model including the isotope exchange reaction wasassumed, and the author has compared the calculation withthe experimental observation to evaluate the reaction rate
constant.

1. Introduction
Proton conductor ceramic is one of the functional
materials applicable to a nuclear fusion reactor system.
One of the use is a hydrogen sensor. When one side of theproton conductor membrane comes in contact with thereference gas and the other side comes in contact withtarget material (for example, sweep gas or liquid breeder),the electromotive force (EMF) is observed between
membrane. From this EMF, hydrogen concentration in thetarget material can be known[1-5]. Kondo et al. havediscussed about application of ceramic hydrogen sensor tomeasure hydrogen concentration in the liquid breeder
material[6]. The other use is a hydrogen pump. In case of asolid breeder blanket of a nuclear fusion reactor, bred
tritium is purged out by passing helium (He) based sweepgas through breeder zone of the blanket. To enhancerelease rate of tritium from the blanket and to decrease
tritium inventory in the blanket, the dominant chemicalform at the released of tritium is controlled to be HT by useof hydrogen (H2) addition He sweep gas. As the result ofusing the H2 addition sweep gas, the chemical contents ofthe sweep gas are expected to be H2, HT, H2O and HTO.
So, a blanket tritium recovery system (BTR) has toseparate hydrogen isotopes (HT, H2, HTO and H2O) from
He, and send them to main fuel cycle system. The presentauthors have proposed to apply electrochemical hydrogen
pump to BTR[7]. It can selectively extract hydrogenisotopes from the sweep gas via the proton conductor
membrane. Its driving force of hydrogen transportation is
not only the pressure difference, but also the electric
potential difference. Therefore, it may be possible toextract hydrogen isotopes from the low hydrogen partial

2. Experimental
Fig.1 shows a schematic diagram of an experimental
apparatus. The electrochemical hydrogen pump is a double

Fig.1 A schematic diagram of the experimental apparatus.

author’s e-mail: kawamura.yoshinori@jaea.go.jp358©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Y. Kawamura et al., Exchange Reaction of Hydrogen Isotopes on Proton Conductor Ceramic of Hydrogen Pump for Blanket Tritium Recovery System

Table 1 Experimental conditions.

pipe. The outer pipe is the quartz tube and the inner pipe is
the proton conductor ceramic. When the voltage is applied
between the outer and the inner surface electrode of theproton conductor ceramic tube, hydrogen transfers throughthe tube in spite of the chemical potential difference, and it
works like a pump. The proton conductor ceramic used inthis work was a strontium cerate that had added theytterbium of 5% (SCO: SrCe0.95Yb0.05O3-), and was madeby TYK Co. Its shape was a test tube type (one end wasclosed). The size was 105mm in length, 12mm in outer
diameter, and 1.0mm in thickness. The porous platinum
electrodes were attached on the inside and outside surface,
and its length in axial direction was 56mm from the closedend (outside).
The experiments were carried out as follows. At first,
as the preparation, the deuterium (hydrogen) transportation
experiment was carried out to exchange the proton(deuteron) in the ceramic to the deuteron (proton). A
detailed procedure of the transportation experiment has
been described in our previous report[9]. The
concentrations of hydrogen isotopes used in thepreparation were about 0.1% and 100%. After that, thecurrent was controlled to be zero by using the potentiostat
to prevent the ionic deuterium (hydrogen) transport. And
then, the hydrogen pump was purged by He until nodeuterium (hydrogen) detected in the effluent gas by gas
chromatograph (GC). In this situation, hydrogen(deuterium) was introduced to the one side of the ceramic
membrane of the hydrogen pump, and the effluent gas
from the hydrogen pump was analyzed by the GC.
Hydrogen (deuterium) concentration in the introduced gas
was about 1000 ppm. Its flow rate was adjusted to become
0.2 or 0.6 l/min (S.T.P.). He gas was introduced to anotherside with 0.2 l/min (S.T.P.) of flow rate, however, nohydrogen isotopes were detected in the effluent gas. The
experimental temperature was 873K. The experimental
conditions were listed in table 1.

Fig.2 Change of the hydrogen isotope concentrations in theeffluent gas of the hydrogen pump. a) ordinary
release curve, and b) release curve of which HD istreated as 1/2H2 and 1/2D2.

was introduced (Electromotive force measurement mode).
D2 and HD has been detected in the outlet gas at first, andtheir concentrations decreased with the elapsed time. In theeffluent gas of the He purge side, hydrogen isotopes were
not detected by the gas chromatograph. Fig.2b is same withFig.2a, but HD has been distributed as 1/2H2 and 1/2D2 to
H2 and D2, respectively. The amount of D2 released fromthe hydrogen pump was calculated from the release curveand it was 2.6 x 10-4 mol-D2, and the amount of H2 taken to
the ceramic was also calculated to 3.0 x 10-4 mol-H2 in thiscase. The release curve of H2 and that of D2 seem to besymmetry. During He purge, hydrogen isotopes have notbeen detected in the effluent gas. So, the present author
considered that the most of D2 has been released by isotope
exchange reaction with H2.
Fig.3 shows the concept of the mass transfer modelthat takes into account the isotope exchange reaction on thesurface of the electrode of the proton conductor ceramic.
Deuteron in the ceramic is replaced by the hydrogen(proton) via isotope exchange reaction on the surface of theelectrode. Then, proton diffuses to the ceramic withreplacing to deuteron. Platinum (Pt) has been used as theelectrode. Generally, Pt has a catalytic function for
dissociation of hydrogen molecule. So, the reaction has
been assumed to occur on the surface of electrodedominantly. At that time, the current between theelectrodes is controlled to become zero. So, in ideal,hydrogen and deuterium are not released from another side

3. Results and Discussion
Fig.2a shows concentration changes of hydrogenisotopes in the outlet gas of the hydrogen pump. H2/Hemixture (H2: 1002 ppm) was introduced to the hydrogen
pump after the D2 transportation experiment. The hydrogen
pump was controlled to be zero in electric current when H2359

Y. Kawamura et al., Exchange Reaction of Hydrogen Isotopes on Proton Conductor Ceramic of Hydrogen Pump for Blanket Tritium Recovery System

The hydrogen concentration change in the membrane
is considered to be caused by the capture of hydrogen by
the surface reaction (isotope exchange) and the diffusion ofhydrogen (proton) in the ceramic. When the surfacereaction is the isotope exchange reaction of the gaseoushydrogen with the deuteron on the surface of electrode, thereaction rate is expressed taking into account the reverse
reaction as;∂q1∂q= k exc1q2 − k'ex c 2q1 = − 2 ,
∂t
∂t

€

Fig.3 A schematic of the mass transfer process.

of the ceramic membrane of the hydrogen pump. Finally,
the deuteron in the ceramic membrane is replaced by theproton. In this experimental system, HD appears at theoutlet of hydrogen pump. However, HD was treated as
1/2H2 and 1/2D2 for simple calculation as shown in Fig.2b.
So, the following relations were assumed;c 2 = c DD + 12 c HD ,c 0 = c1 + c 2 ,q0 = q1 + q2 = q1 + q2 ,(1)

where c is concentration of gaseous hydrogen isotope€ [mol/cm3]. Subscripts "HH", "HD" and "DD" mean H2,€ HD and D2, respectively. q is concentration of hydrogenisotope in the proton conductor ceramic [mol/cm3]. q is
the average concentration of hydrogen isotope in theminute section of the ceramic in axial direction [mol/cm3].
Subscripts "1" and "2" mean H2 and €D2 after thedistribution of HD, respectively.
The mass balance of hydrogen isotopes in the axial
direction of the hydrogen pump is described as;DL,i€€€where DL is the axial dispersion coefficient of hydrogenisotope [cm2/s], t is time [s], u is the gas velocity [m/s], z is
the axial distance [cm], as is the ratio of the cross section of
the proton conductor ceramic in radial direction to thecross section of the flow path [cm2/cm2]. In this work, thegas velocity is considered much larger than the axial
dispersion. Therefore, the term of the axial dispersion isomitted and the following equation is derived from eq.(2);(i=1,2).

(3)where K is the equilibrium constant [-] and is the ratio of
the reaction rate constant, kex, to that of the reverse reaction,k’ex.
In the ceramic membrane, proton captured via isotope
exchange reaction diffuses and penetrates. However, thecurrent is controlled to become zero. So, proton captured isassumed to diffuse by replacing with deuteron. If thehydrogen ion transportation is described with the diffusion
equation of one dimension, the following equation isderived; ∂ 2q 1 ∂qi ∂qi= Ds 2i + (i=1,2),∂tr ∂r  ∂r∂ 2c i∂c ∂ c∂q= u i + i + as i (i=1,2), (2)2∂z∂z ∂ t∂t

∂c i
∂c
∂q
= −u i − as i
∂t
∂z
∂t

where kex is the reaction rate constant of the isotope
exchange reaction [cm3/mol•s]. q means hydrogen isotope
concentration of the ceramic at the surface [mol/cm3].
Eq.(5) is derived from Eq.(4) as;

∂q1
∂qcq
= k ex (c1 (q0 − q1 ) − 2 1 ) = − 2 , (5)
∂t
K
∂t

€

c1 = c HH + 12 c HD ,

(4)

(6)

where Ds is the diffusion coefficient of hydrogen isotope in
the ceramic [cm2/s], and r is the distance in radial directionof the proton conductor ceramic tube [cm]. As shown in
Fig.3, the author has assumed that proton diffuses in theceramic by replacing with deuteron under the electric field.
Usually, the diffusion coefficient of H2 and that of D2 isdifferent. However, in this model, they have been assumed
to be same.
The initial and the boundary conditions are asfollows;[Flow Path]c1 (z,0) = 0 (z > 0,t = 0),
q1 (z,0) = 0 (z ≥ 0,t = 0),
c1 (0,t) = c in (t ≥ 0),[ceramic]€€

360

q1 (z,r,0) = 0 (z ≥ 0,Ri ≤ r ≤ Ro ,t = 0),∂q1 (z,Ro ,t)= 0 (z ≥ 0,t ≥ 0),∂r

Y. Kawamura et al., Exchange Reaction of Hydrogen Isotopes on Proton Conductor Ceramic of Hydrogen Pump for Blanket Tritium Recovery System

Fig.4 Comparison of the release curve between theobservations and the calculations.

Fig.5 Comparison of hydrogen isotope concentration in theSCO between this work and the previous work.

∂q (z,Ri ,t)
av Ds 1∂r= −k ex c1 (z,t)(q0 − q1 (z,Ri ,t)) +€(z ≥ 0,t ≥ 0),

sample1 and 2 excluding the lot of raw material. The
present author has reported about the hydrogenconcentration in the SCO under various partial pressure ofH2[13]. The comparison of the results with our previous
work is shown in Fig.5. The results in this work wereplotted against the partial pressures of hydrogen isotope in
the transportation experiments. As mentioned in thesection2, the concentration of hydrogen isotope in thetransportation experiments in the preparation were about
0.1% and 100%. So, the data were plotted against thepartial pressures at about 102 Pa and 105 Pa. The amountsof the released hydrogen isotope from the SCO are larger
than the data in our previous work. However, theirdifferences are within one order of magnitude. In theprevious work, the hydrogen concentration in the SCO has
been estimated from the proton conductivity observed
experimentally and the diffusion coefficient reported by
Schober et al[14]. Taking into account the error range onthe diffusion coefficient, the difference between this workand the previous work may be not so large.
As listed in table 2, it does not seem that there is somerule in the estimated diffusion coefficient. As mentioned
before, Schober et al. have reported that the diffusion
coefficient of hydrogen in SCO, D0, is 2.185x10-3 cm2/s
and its activation energy, ΔH, is 0.46±0.16 eV[14]. Fig.6
shows the comparison between the diffusion coefficientestimated in this work and the reported one. The grey areain Fig.6 is caused by the error range of the activationenergy. The estimated diffusion coefficients are in this area,and so may thought to be comparatively reasonable.

c 2q1 (z,Ri ,t)K(7)where Ri and Ro mean the location that is the surface of€ ceramic tube inside and outside, respectively. av means theratio of the surface area of the inside electrode on theceramic tube to the volume of ceramic tube between theelectrodes [cm2/cm3]. Eqs.(1,3,5-7) were solved
numerically to obtain the change of hydrogenconcentration in the effluent gas of the hydrogen pump.
The solutions were compared with the experimentalobservations. And then, the reaction rate constant, kex, theequilibrium constant, K, and the diffusion coefficient, Ds,
were obtained.
Fig.4 shows a comparison of the observed release
curves with the calculated release curves. Change on theshape of the release curve was hardly observed eventhough the equilibrium constant, K, was changed.
Therefore, the reverse reaction might be able to be ignoredin this estimation. Estimation of rate constants and thediffusion coefficients were listed in table 2. The observedamounts of hydrogen isotope released from the ceramic
were also listed in this table. The left side of the tablemeans that D2 has been used in transportation experimentand H2 has been used in the exchange experiment. The
right side means reverse cases. There is no difference inTable 2 The amount of hydrogen isotope released from the ceramic, the estimation of diffusion coefficient and reaction rate constant.

361

Y. Kawamura et al., Exchange Reaction of Hydrogen Isotopes on Proton Conductor Ceramic of Hydrogen Pump for Blanket Tritium Recovery System(EMF) is measured to know the fuel concentration. This issimilar to this experimental system. So, it is probable thatthe fuel gas permeates through the membrane and is mixed
with the reference gas. Matsumoto et al. have carried out
the experiments of hydrogen isotope sensor[5], but they
have not reported such phenomena. However, they havenot measured the contents of hydrogen isotope mixture gas
during their experiments. It is necessary to confirm
experimentally whether the permeation to the reference
side occur or not.

4. Conclusion
The isotope exchange reaction between the hydrogenisotope in the gas and that in the proton conductor ceramicof hydrogen pump was investigated, and the amount ofhydrogen captured in the proton conductor ceramic, thediffusion coefficient of hydrogen isotope in the ceramic
and the rate constant of the isotope exchange reaction wereestimated using the mass transfer model.

Fig.6 Comparison of the diffusion coefficient of hydrogenisotope in SCO.

There is not so large difference among the estimatedreaction rate constants. Therefore, the equilibrium constantof this exchange reaction is thought to be nearly 1.0
In this work, water vapor has not been taken into
account in the mass transfer model. However, water vapor
is expected to be in the sweep gas of solid breeder blanket
actually. Also, in this experiment, the existing of watervapor in the gas is probable. Therefore, it should beconsidered that the estimated parameters in this work havebeen affected by water vapor. Experiment and estimationof the exchange reaction between gaseous hydrogen andwater vapor is necessary in future work.
From the result of this work, the author thinks that theattention may be necessary when hydrogen sensor with theproton conductor ceramic is applied to the tritium plant offusion reactor. In case of hydrogen sensor, the referencegas, of which hydrogen isotope concentration is known,flows through one side of proton conductor membrane, andthe sample gas, for example DT fuel, flows throughanother side of membrane. And, the electric current is
controlled to become zero, and the electromotive force

5. Acknowledgement
In order to proceed to this work, the authors received
funding support from Ministry of Education, Sports,
Science and Technology of Japan (Grant-in Aid for
Scientific Research for Priority Area (2007-2011), AreaNo.476). Authors really wish to acknowledge for thisfunding support.
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