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

Recovery of Hydrogen Isotopes by Liquid-Gas Contactor from Li17Pb83
BlanketSatoshi FUKADA1, Taiki MUNEOKA1, Ryosuke YOSHIMURA1, Kazunari KATAYAMA1,
Yuki EDAO2 and Takumi HAYASHI2
Dept. Advanced Energy Engineering Science, Kyushu University:
Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan. Email: sfukada@nucl.kyushu-u.ac.jp
1

Tritium Process Laboratory, Japan Atomic Energy Agency: Tokai, Ibaraki 319-1184, Japan

2(Received: 11 September 2014 / Accepted: 19 January 2015)

Rates of H2 transfer between Ar+H2 gas bubbles and fluidized Li17Pb83 (Li-Pb) are experimentally and analyticallydetermined. Ar+H2 gas bubbles directly contact with liquid Li-Pb in a SS-304 vessel. The diameter of bubbles andterminal rising velocity are estimated from balance between several forces including surface tension, gravity, inertia
force and drag ones. Then, variations of the H2 concentration in a bubble contacting with Li-Pb are estimated in termsof the H2 capacity in the bubble and diffusion rate through Li-Pb interface. It is found that H2 transfer between bubbles
and liquid is limited by H diffusion in the Li-Pb boundary layer. Variations of the H2 concentration with time aredetermined experimentally, and the results are extended to the design of a liquid-gas contactor for tritium recovery
from a Li-Pb blanket flow in a commercial fusion reactor.
Keywords: Li-Pb eutectic alloy, tritium, breeder, liquid-gas contactor, hydrogen diffusion, recovery

especial. This may be because differences in chemical
conditions of Li-Pb and simultaneous absorption of othergaseous species affect the solubility and diffusivity in
Li-Pb [13-15]. There are also several researches onrecovery of tritium from fluidized breeder underconditions resembling to an actual blanket system [16-22].
These methods are classified into two. The first candidate
method to recover tritium continuously from a fluidized
Li-Pb loop will be a permeation window system. Somebasic experiments related with the H permeation window
were presented previously [16-20]. As another way, adispersed liquid contact or a liquid-bubble contactor
tower was also investigated experimentally [21,22],
where droplets and liquid Li-Pb flow counter-currently ina packed tower. However there is less experimental
information on H isotope recovery from fluidized Li-Pb
by a bubble column or liquid-gas direct contactor.
In order to obtain basic mass-transfer parameters fortritium recovery from the Li-Pb blanket, a bubbling tower
for direct contact between Ar+H2 and Li-Pb or simulant
fluid is set up in our laboratory, and experiment of H2
absorption/desorption is performed. The tritium recovery
system is closely connected with not only the physicalproperties of solubility and diffusivity but also fluidized
conditions of Li-Pb such as the bubble rising velocity,
surface tension, viscous force and so on. All thoseparameters are related with the tritium recovery rate, andthe total design for the blanket system is presented.
Li-Pb of 3.3 L in volume is made up in a SS-304
vessel after its surfaces are cleaned up in the experiment.

1. Introduction

A eutectic alloy of Li17Pb83 (Li-Pb) is a promisingmaterial for a liquid blanket of a fusion reactor, whichmakes it possible to constitute a high-performanceblanket system producing heat for electric power
generation and tritium for the next generation fuel at thesame time effectively [1,2]. Fluidized eutectic Li-Pb alloy
is expected so as to act as a dual coolant along with He
flow and tritium breeder to produce the next-cycle D-T
fuel. In addition, Li-Pb works as a protect shield fromstrong neutron beam and gamma ray along with anenergy convertor from nuclear energy through heat
finally to electricity. The inlet and outlet temperatures aredesigned within allowable ones with use of reduced
activation steel such as F82H or EUROFER. It is
expected that material durability is kept in good conditioneven under the harsh operating conditions of temperatureand neutron/gamma irradiation. Efficient recovery of heatand tritium outside of the reactor chamber is the mostimportant issue. The tritium recovery issue has been
investigated experimentally and analytically in ourprevious works [3-6].
One of the most important physical properties todesign a liquid blanket system of a fusion reactor isdiffusivity and solubility of tritium in breeders. Althoughseveral researches were presented for the solubility,
diffusivity and isotopic exchange of H isotopes in static
Li-Pb [7-12], there seems to be scattering in solubility inauthor’s e-mail: sfukada@nucl.kyushu-u.ac.jp

19©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

S. Fukada et al., Recovery of Hydrogen Isotopes by Liquid-Gas Contactor from Li17Pb83 Blanket

After Li-Pb homogeneity is confirmed, bubbles of an
Ar-H2 mixture are supplied into a fresh Li-Pb orconversely bubbles of Ar are done to Li-Pb pre-saturated
with H isotopes. These experimental results are analyzed
using material balance equations between gas bubbles and
Li-Pb. Results are compared among different conditions
and are extended to a design of a tritium recovery systemof a liquid blanket. The present collaboration research is
performed as one of the ITER-BA activities.

Ar or
H +Ar
2

Gas
chromatograph

2. Analysis

2.1. Formation and movement of bubbles in liquid
Transfer of hydrogen isotope from gas bubbles tosurrounding liquid or vice versa is analyzed based on not
only material balance and mass-transfer rate equations but
also a momentum equation to determine the movement of agas bubble. The present analysis is deeply related with thereview study on bubble formation [23,24] and Kondo’s
analysis for Li-Pb [21], where a similar momentum
transfer and mass-transfer analysis is carried out.
Description below outlines the above previous researches.
When gas bubbles are blown through an I-type nozzle
into liquid bulk with a constant (low) flow rate, Q [m3/s], a
relation between the I-type nozzle diameter, D [m], and thediameter of bubble, d [m], is written as follows:     
for low Q

L

Fig. 1 Schematic figure of LiPb-gas bubble contactor
where CD [-] is a drag coefficient, and the buoyancy force
is balanced with the viscous drag force on the left-handside. The CD value is correlated in terms of the kinematic
viscosity of liquid, νL [m2/s], or the dimensionless Re
number, which is defined as Re=ud/νL. When Re<10 in the
Stokes region, CD is equal to CD=24/Re, where fluid
convection inside gas bubbles or liquid is ignored. Whenthe convection is taken into consideration, the following
Hadamard-Rybczynski equation [23] for CD holds:   
(4).
 
   

(1),

where the buoyancy force on the left-hand side is balancedwith the surface tension on the right-hand one, and it isassumed that the direction of gas bubbles blown throughthe nozzle is right angle to the nozzle surface. In addition,
σ [N/m] is a surface tension, ρL and ρG [kg/m3] are thedensities of liquid and gas and g [m/s2] is gravity.
When the bubble flow rate becomes high, on thecontrary, the effect of inertial force should be taken into
consideration. Then the bubble diameter, d [m], and thetotal flow rate Q of gas blown into liquid is related by thefollowing equation:       

 




for high Q

On the contrary when Re>103 in the Newton region, theconstant relation of CD=0.44 holds. In the middle Re region,
experimental data are summarized to an empiricalcorrelation such as CD=(0.55+4.8/Re0.5)2. The CD value isapproximated by 24/Re in the present experimental
condition of D=4.35mm. Consequently, the bubble
terminal rising velocity is determined from Eqs. (1) or (2)and (3) as a function of ρL, ρg, νL, σ and D.
Relation between the total flow rate of gas bubbles
blown into liquid, Q [m3/s], and the number of bubbles
present in the Li-Pb vessel, N [-], are estimated by thefollowing equation when the Li-Pb depth is L [m]:(2),where the buoyancy force is balanced with the inertial
force on the left-hand side. Bubbles formed in Li-Pb bulk
volume rise in the upward direction, and α is defined as adimensionless constant to express the effect of virtual massof surrounding liquid which is estimated as α=11/16 [23].
A gas bubble ejected from a nozzle is accelerated by
the buoyancy force initially, and then it approaches to aterminal velocity due to resistance of viscous force. The
distance approaching to the terminal velocity is estimated
3.9 mm in the present condition. Its value is much smaller
than the distance between the nozzle and liquid surface, L[m]. Therefore, it can be assumed here that the bubble rises
always at the terminal velocity, u [m/s]. The terminal
velocity u is estimated from the following equation:      

 




  









(5).

Since the total surface of the spherical gas bubbles, S [m2],is described as S=πd2N, the total gas-liquid surface present
in the Li-Pb vessel is expressed by the equation:





(6).

The expression of Eq. (6) means that the total surface areaof bubbles in the Li-Pb vessel, S [m2], is determined by thedepth of the liquid Li-Pb vessel, L, and the total gas flow
rate, Q. If Q is low, S becomes a function of the nozzle
diameter, D, straightforwardly. The relating physicalproperties are viscosity of liquid, μL [Pas], and the surface
tension between liquid and gas, σ [N/m].

(3),

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S. Fukada et al., Recovery of Hydrogen Isotopes by Liquid-Gas Contactor from Li17Pb83 Blanket

2.2. H transfer between bubbles and liquid
As seen schematically in Fig. 1, gas bubbles ejectedfrom a nozzle may move upward independently when N issmall. H absorption into Li-Pb or conversely H desorption
from it to bubbles proceeds in the vessel independently. In
order to constitute a rate equation for H transfer from Li-Pb
in vessel to bubbles or vice versa is deduced under thefollowing assumptions:(i) Since Li-Pb is well mixed by bubble movement, the Hconcentration dissolved in the Li-Pb bulk (its volume VLiPb[m3]) is assumed uniform except for a concentration
boundary layer formed near interface between Li-Pb andspherical bubbles.
(ii) Association or decomposition of bubbles ejected in
Li-Pb are neglected. The number N is unchanged during
gas absorption/desorption operation.
(iii) H2 concentration in bubbles is uniform due to insidegas convection.
(iv) Variations of the H2 concentration in the top open
space (volume VS) with time are taken into consideration.
(v) The movement of gas bubbles is taken into
consideration in the rate of H2 transfer from Li-Pb into theopen space but direct desorption of H2 from the Li-Pb
surfaces is ignored.
When the effect of flow inside the liquid or gas phase
is ignored, the one-dimensional transient diffusion
equation for the H isotope concentration, cL [mol/m3], inthe liquid boundary layer is described as follows:





 

  



for r>a region

Fig. 2 Preliminary experiment on number densityof bubbles, N, using simulant liquid, where τ is a
residence time.

(7).

The initial and boundary conditions for the H absorption
operation in the liquid are given by the equations:(8),  
  
(9),     (10).
The solution to Eq. (7) under the initial and boundary
conditions of Eqs. (8)-(10) is obtained as follows: 

 



 
 

      



 
 



Fig. 3 Comparison between measurement and estimationof bubble diametervelocity, u. The second term on the right-hand side
becomes small when the contact time is long. Since thetotal H transfer rate between Li-Pb and bubbles iscalculated as jHS, the H balance in the Li-Pb bulk layer is
described as follows:(11).

The H mass flux from liquid to gas, jH [mol-H/m2s], perunit surface area of one bubble is obtained from the Hconcentration profile of Eq. (11) as follows:  

    











 






     (14).

It is not simple extension to derive Eq. (14) from Eq. (13),This is because the bulk concentration cL,oo is assumed tobe constant before the derivation of Eq. (12). The overall
H2 transfer rate between bubbles and Li-Pb is described interms of the mass-transfer coefficient, km. Its driving force
is expressed by the difference in the H concentration
between surface and bulk changing with time.
The H2 balance equation in the top open space is
described as follows:(12).

In the case of H desorption from Li-Pb to bubbles, theorder in the driving-force term on the right-hand side of Eq.
(12) is interchanged. When the effect of convective flow inthe liquid or gas phase is taken into consideration, the firstterm on the right-hand side of Eq. (12) can be replaced by
the following equation:    



(13)

  where km [m/s] is a mass-transfer coefficient and is afunction of not only temperature but also the terminal









  

(15).

The equilibrium relation for H2 is held between gas in21

S. Fukada et al., Recovery of Hydrogen Isotopes by Liquid-Gas Contactor from Li17Pb83 Blanket

bubbles and Li-Pb interface:    


(16),

where KL [mol-H/m3Pa0.5] is the Sieverts’ constant foratomic H solution in Li-Pb.

3. Experimental

3.1 Observation of bubble diameter
Bubble diameter d is a key parameter to predict rates of
H transfer between bubbles and liquid. However, when theexperimental apparatus has been set up in our laboratory,
the inside cannot be observed from the outside. Therefore,
another experimental apparatus made of glass vessel filledwith water or ethanol is set up before the Li-Pb experiment.
The reason why water and ethanol are selected here is thatthe bubble diameter is deeply related with the σ/ρL value
judging from Eq. (1). Its value calculated from Eq. (1) for
the Li-Pb and Ar system is d=3.9 mm and is between
water-Ar and ethanol-Ar as seen Fig. 3. The SS-304 tubeof D=2.175mm in inner diameter is inserted in the liquidand a constant flow rate of Ar gas of Q is supplied throughthe tube to liquid. The number of gas bubbles, N, formed iscounted by visual observation using a high-speed
video-camera. Fig. 2 shows an example of the number of
bubbles ejected from a nozzle as a function of Q. The N
value increases almost linearly under a small Q condition.
With the increase of Q, the N value deviates from the linear
relation. The average diameter of gas bubbles formed inliquid bulk d is calculated from Q and N. The results areshown in Fig. 3. The estimated d value is almost coincident
with Eq. (1) in case of small Q condition.

Fig. 4 Cooling curve of Li-Pb including solidifying

Fig. 5 H2 concentration history for H absorption experiment

3.2 Make-up of Li-Pb eutectic alloy
At first, specified amounts of plate-type solid Li rods
(218.07g) of 99.9 % and Pb plates (27.752 kg) of 99.9 %were put in order separately in a SS-304 vessel of 3.3 L involume, and the whole was heated up to around 500oC
with evacuating the vessel by a turbo-molecular pump.
After confirming the melted condition of Li and Pb, thesystem was slowly cooled down below the expected
melting temperature. Variations of temperature duringcooling with time are shown in Fig. 4. Heat radiation rate isbalanced with solidification heat at the melting temperature
at 235oC, where a steady-state temperature condition
comes out. Cycles of heating (melting) and cooling(solidifying) were repeated to confirm whole homogeneityof the liquid metal alloy. Temperature is maintained atconstant during the H absorption or desorption operation,
where the whole Li-Pb layer is in liquid state.

Fig. 6 H2 concentration history for H desorption experimentmixture into the liquid Li-Pb vessel, the Li-Pb vessel madeof SS-304 with a volume of 3.3 L, electric heaters,
temperature controllers and gas chromatography todetermine the H2 concentration in gaseous phase. Afterconfirming the condition where the whole Li-Pb in thevessel is maintained at constant temperature between
400-600oC, an Ar-H2 mixture is introduced under aconstant flow rate, which ranges between 10 – 100

3.3 Experiment of H absorption in or desorption from
Li-Pb
An experimental apparatus was set up, which iscomposed of a gas supply section to feed an Ar or Ar+H2

22

S. Fukada et al., Recovery of Hydrogen Isotopes by Liquid-Gas Contactor from Li17Pb83 Blanket

cm3(NTP)/min. The H2 partial pressure in gas introduced
into Li-Pb is 5.0 kPa or 50.0 kPa in the H absorption
experiment. After supplying the H2-Ar gas mixture to the
Li-Pb vessel sufficiently, the gas line is changed to pure Ar
line. The outlet H2 concentration is determined in a similar
way to the H absorption experiment.

4. Results and Discussion

An example of the experimental results for H
charge into Li-Pb is shown in Fig. 5, which conditions are
500oC and Q=50cm3(NTP)/min. As seen in the figure,close agreement is obtained between experiment andcalculation by selecting a proper km value as a uniqueparameter. In a similar way to the absorption, the Hdesorption process is also well simulated by the same
analysis as seen in Fig. 6. This may be because therate-determining step is H diffusion in liquid Li-Pb,
which is a linear system. Several experiments arerepeated at 500oC. The unique km value was determined
per each H absorption/desorption run.
The values of the mass-transfer coefficient at 500oC
along with other experimental results at 600oC [18,21] aresummarized in Fig. 7. When several experiments ofabsorption and desorption are repeated at 500oC, each runof absorption or desorption was reproduced by a uniquevalue of km in the range from zero to each saturation
value. The km value is 1-3x10-2 m/s for the H absorption
run and 1-4x10-4 m/s for the desorption one. It is noticed
that different km values were obtained between H
absorption and desorption runs.
One reason to explain the different km values
between H absorption and desorption runs is the effect of
surface reaction. The surface reaction process included
for the H absorption is decomposition of a H2 molecule totwo H atoms and that for the H desorption isrecombination of two H atoms to a H2 molecule. The
decomposition proceeds at relatively higher pressure andrecombination does at lower pressure. The two processes
are nonlinear. Therefore the km value may become
different between absorption and desorption, when theprocess of nonlinear reaction affects the overall process.
Another reason is the effect of Ar absorption into
Li-Pb during the H absorption/desorption experiment [15].
In the present study, only the H2 concentration is detectedin the effluent mixture by means of gas chromatography,
which uses Ar carrier and a TCD detector. Therefore theconcentration of Ar or the total flow rate is not detected.
Ar may be present as a single molecular form in liquid
Li-Pb, which is different from atomic H absorption inliquid Li-Pb [4,9,14]. Therefore, the process may proceedindependently. The effect of Ar simultaneous absorptionin Li-Pb may be less than expected. However, further
experiment will be necessary to clarify the different km
values between H absorption and desorption. In addition,
experiment of different temperature conditions is

Fig. 7 Correlation of mass-transfer coefficient of
H absorption or desorption in/out Li-Pb

Fig. 8 LiPb/He counter-current flow contactor for T
recoverynecessary to compare with previous data [18,21] and toclarify the reasons why the different km values appear in
the H absorption/desorption runs.
Now, we continue the experiment for wider different
temperatures or flow conditions after some modificationof the experimental apparatus. In the near future, theseresults will be presented. In addition, we initiated adesign work for LiPb/inert gas direct contactor for T
recovery from a Li-Pb blanket loop as seen in Fig. 8. Fig.
9 shows the McCabe-Thiele diagram for the LiPb/He
extraction tower designed here. The flow rate and column
height are determined based on diagram evaluation.
5. Conclusions
Recovery rates of H from the liquid Li-Pb blanket bymeans of direct contact between Ar-H2 (Ar) and Li-Pb
eutectic alloy determined experimentally were fitted well23

S. Fukada et al., Recovery of Hydrogen Isotopes by Liquid-Gas Contactor from Li17Pb83 Blanket

by analysis, which was deduced based on balances among
surface tension, inertia, buoyancy and drag forces. It was
found that the bubble diameter and the number of bubbles
formed in Li-Pb or simulant fluid are estimated well by
the analysis. In addition, H absorption/desorption analysis
is given, and the results were correlated well in terms of asingle mass-transfer coefficient. The effluent H2concentration from the Li-Pb vessel was well simulatedby the diffusion-limited analytical solution. Difference in
km values between absorption and desorption was
discussed based on effects of reaction rate process oninterface between gas bubbles and Li-Pb and Ar
absorption. The present study is carried out under thecollaboration work of the ITER-BA study between JAEA
and Kyushu University.

Fig. 9 McCabe-Thiele diagram
counter-current extraction tower

References[1]

C. P. C. Wong, M. Abdou, Y. Katoh, et al., “Progress of
DCLL blanket concept”, Fusion Sci. Technol., 64, 623-630(2013).
[2] G. Veredas, J. Fradera, I. Fernandez, et al., “Design andqualification of an on-line permeator for the recovery oftritium from lead-lithium eutectic breeding alloy”, FusionEng. Des., 86, 2365-2369 (2011).
[3] S. Fukada, T. Terai, S. Konishi, et al., “Clarification of
Tritium Behavior in Pb-Li Blanket System (Overview)”,
Mater. Trans., 54, 425-429 (2013).
[4] S. Fukada, Y. Edao, “Unresolved issues on tritium masstransfer in Li-Pb liquid blankets”, J. Nucl. Mater., 417,
727-730(2011).
[5] S. Fukada, Y. Edao, A. Sagara, “Effects of simultaneoustransfer of heat and tritium through Li-Pb or Flibeblanket”, Fusion Eng. Des., 85, 1314-1319 (2010).
[6] S. Fukada, Y. Edao, Y. Maeda, T. Norimatsu, “Tritium
recovery system for Li-Pb of Inertial Fusion Reactor”,
Fusion Eng. Des., 83, 747-751 (2008).
[7] Y. Edao, H. Ikitsu, H. Noguchi, S. Fukada, “Permeation of
two-component hydrogen isotopes in lithium-lead eutectic
alloy”, Fusion Sci. Technol., 60, 1163-1166 (2011).
[8] H. Okitsu, Y. Edao, M. Okada, S. Fukada, “Analysis ofdiffusion and dissolution of two-component hydrogen(H+D) in lead lithium”, Fusion Eng. Des., 87,1324-1328 (2012).
[9] Y. Edao, H. Noguchi, S. Fukada, “Experiments ofhydrogen isotope permeation, diffusion and dissolution in
Li-Pb”, J. Nucl. Mater., 417, 723-726 (2011).
[10] Y. Edao, S. Fukada, H. Noguchi, Y. Maeda, K. Katayama,“Isotope effects of hydrogen isotope absorption anddiffusion in Li0.17Pb0.83 eutectic alloy”, Fusion Sci.
Technol., 56, 831-835 (2009).
[11] Y. Maeda, S. Fukada, Y. Edao, “Solubility, diffusivity andisotopic exchange rate of hydrogen isotopes in Li-Pb”,
Fusion Sci. Technol., 54, 131-134 (2008).
[12] Y. Edao, H. Noguchi, S. Fukada, “Isotopic exchange rate
between hydrogen and deuterium in the process of
permeating through Li0.17Pb0.83”, Fusion Eng. Des., 85,1225-1228 (2010).
[13] S. Fukada, M. Okada, S. Yoshimura, Y. E.dao,, “Chemicalfor

LiPb/He

activity of hydrogen isotopes in Pb-Li eutectic alloy
blanket”, J. Appl. Nucl. Sci. Technol., 5, 27-31 (2012).
[14] D. Masuyama, T. Oda, S. Fukada, S. Tanaka, “Chemical
state and diffusion behavior of hydrogen isotopes in liquid
lithium-lead”, Chem. Phys. Let., 483, 214-218 (2009).
[15] T. Sakabe, T. Yokomine, T. Kunugi, Z. Kawara, Y. Ueki, T.
Tanaka, “Gas absorption and discharge behaviors oflead-lithium”, Fusion Eng. Des., 89, 1417-1420 (2014).
[16] S. Yoshimura, R. Yoshimura, M. Okada, S. Fukada, Y.
Edao, “Study on hydrogen isotopes permeation in thefluidized state of liquid lithium-lead”, Fusion Sci.
Technol., 65 (2014) in printing.
[17] S. Fukada, M. Okada, Y. Edao, H. Okitsu. S. Yoshimura,“Overall hydrogen permeation through fluidized PbLi
alloy under natural convection”, Fusion Sci. Technol., 64,
636-642 (2013).
[18] T. Terai, K. Uozumi, Y. Takahashi, “Tritium release
behavior from molten lithium-lead alloy by permeation
through stainless steel type 304”, J. Nucl. Mater., 191-194,
272-276 (1992).
[19] H. Moriyama, S. Tanaka, D. K. Sze, J. Reimann, A. Tarlain,“Tritium recovery from liquid metals”, Fusion Eng. Des., 28,
226-239 (1995).
[20] M. Okada, S. Yoshimura, H. Okitsu, Y. Edao, S. Fukada,“Study on hydrogen isotopes permeation underlithium-lead thermal convection”, J. Appl. Nucl. Sci.
Technol., 5, (2012) 117-121.
[21] M. Kondo, T. Oshima, M. Tanaka, T. Muroga, A. Sagara,“Hydrogen transport through interface between gas
bubbling and liquid breeders”, Fusion Eng. Des., 87,
1788-1793 (2012).
[22] N. Alpy, A. Terlain, V. Lorentz, “Hydrogen extraction
Pb-17Li results with a 800mm high packed column”,
Fusion Eng. Des., 49, 775-780 (2000).
[23] A. A. Kulkarni and J. B. Joshi, “Bubble formation andbubble rise velocity in gas-liquid system. A review”, Ind.
Eng. Chem. Res., 44, 5873-5931 (2005).
[24] S. Ramakrishnan, R. Kumar, N.R. Kuloor, “Studies inbubble formation-I, bubble formation under constant flow
condition”, Chem. Eng. Sci., 24, 731-747 (1969).

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