isotope exchange reaction in li2zro3 packed bed

Fusion Engineering and Design 39–40 (1998) 713–721

Isotope exchange reaction in Li2ZrO3 packed bed

Yoshinori Kawamura *, Mikio Enoeda, Kenji OkunoTritium Engineering Laboratory, Japan Atomic Energy Research Institute, Shirakata Shirane 2-4, Tokai-mura, Naka-gun,

Ibaraki-ken 319-1195, Japan

Abstract

To understand the release behavior of bred tritium in a solid breeder blanket, the tritium transfer rate and tritiuminventory for various mass transfer processes should be investigated. The contribution of the surface reactions(adsorption, desorption and two kinds of isotope exchange reactions) to the release process cannot be ignored. It isbelieved that two kinds of isotope exchange reactions (gaseous hydrogen-tritiated water and water vapor-tritiatedwater) occur on the surface of the solid breeder materials when hydrogen is added to the sweep gas to enhance thetritium release rate. The isotope exchange reaction study in H–D systems was carried out using a Li2ZrO3 packedbed. The exchange reaction between gaseous hydrogen and water was the rate controlling step among the two kindsof exchange reactions. The reaction rate constants were quantified, and experimental equations were proposed. Theequilibrium constant of the isotope exchange reaction in the H–D system was obtained from experimental data andwas found to be 1.17. © 1998 Elsevier Science S.A. All rights reserved.

1. Introduction

The release of tritium bred in a solid breederblanket consists, in order, of a diffusion processof tritium in crystal grains, a release of tritium inthe form of tritiated water from the surface of thegrains to the pores, a migration process of triti-ated water in the pores, and a release of tritiumthrough the boundary layer to the sweep gasstream. To understand the release behavior oftritium bred in the solid breeder blanket, thetritium transfer rate and tritium inventory at eachprocess should be investigated. Many in-situ ex-periments have been carried out to investigate thetritium release behavior, and it has been con-cluded that the diffusion process of tritium in the

crystal grains is the rate controlling step of thebred tritium release process [1–5]. However, it hasbeen reported that the tritium release rate is en-hanced when hydrogen is added to the heliumsweep gas [3,5,6]. It has been pointed out that thetritium inventory due to sorption is larger thanthat due to diffusion by the present authors [7].Therefore, effects of the surface reactions (adsorp-tion, desorption and two kinds of isotope ex-change reaction) cannot be ignored.

It is believed that two kinds of isotope ex-change reactions (gaseous hydrogen-tritiated wa-ter and water vapor-tritiated water) occur on thesurface of the solid breeder materials when hydro-gen is added to the sweep gas. Munakata andNishikawa have studied the isotope exchange re-action in the Li2O packed bed [8]. However, theisotope exchange reaction on the surface of other* Corresponding author.

0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.

PII S0920-3796(97)00189-0

Y. Kawamura et al. / Fusion Engineering and Design 39–40 (1998) 713–721714

Table 1Sample specification

Sample name Li2ZrO3

4.15 g cm−3Theoretical density3.57 g cm−3 (86% T.D.)Density

Grain size 13 mm (diameter)Pellet size 1.0 mm (diameter)

0.06 m2 g−1BET surface area

Power Industries. The sample specification islisted in Table 1.

Fig. 1 shows a schematic diagram of the exper-imental apparatus. The sample of 15–50 g wascharged in a reaction tube. The reaction tube wasa quartz tube of 450 mm length and 22 mm innerdiameter in which a filter plate with several 1-mmholes was mounted 200 mm from the bottom end.The temperature of the packed bed was measuredwith a Type K thermocouple that was inserted ina quartz tube of 1 mm inner diameter to protect itfrom corrosion. The apparatus has two flow chan-nels. The D2/He mixed gas was passed throughchannel 1 (Ch.1) and H2/He mixed gas was passedthrough channel 2 (Ch.2). A gas dryer bedcharged with MS5A was attached to each flowchannel to remove residual water in the gas cylin-der, and a CuO bed heated up to 300°C was alsoattached to each flow channel to oxidize H2 or D2

to H2O or D2O. Helium was used as a carrier gas.The H2/He mixed gas (H2: 1.01%) and D2/Hemixed gas (D2: 0.998%), were used as the samplegases. The flow rate and concentration of thesample gas were adjusted with four mass flowcontrollers.

candidate materials is still not clear. An isotopeexchange capacity involving –OH bases or watertrapped in the lithium ceramic matrix in thecourse of the pellet manufacturing process hasbeen proposed and quantified by Baba et al. [9].They have concluded that Li2ZrO3 has no isotopeexchange capacity. Because of this, the presentauthors chose Li2ZrO3 as the sample, and investi-gated the isotope exchange reactions in a Li2ZrO3

packed bed in this study.

2. Experimental

The Li2ZrO3 sample with 86% of theoreticaldensity was prepared by Mitsubishi Atomic

Fig. 1. A schematic diagram of the experimental apparatus.

Y. Kawamura et al. / Fusion Engineering and Design 39–40 (1998) 713–721 715

The measurement was carried out as follows:1. The Li2ZrO3 bed was heated up slowly from

room temperature to the experimental temper-ature (200–800°C) for more than several hoursunder moisture free H2/He mixed gas flow, todesorb remaining water and to release formedwater. The H2/He gas was used to separate theinfluence of the water formation reaction fromthe isotope exchange reaction. This treatmentwas continued until the water concentration inthe outlet gas of the sample bed fell below 1ppm at the experimental temperatures.

2. The D2 and H2O concentrations in D2, H2O/He mixed gas at the inlet of the Li2ZrO3

packed bed was controlled to be in the concen-tration range of 127–5150 ppm (12.0–522 Pa).Helium gas containing D2 and H2O of a cer-tain partial pressure was passed through theLi2ZrO3 bed at a flow rate between 400 and4000 cm3 min−1, and the change of waterconcentration in the outlet gas of the Li2ZrO3

bed was traced with a hygrometer. Helium gascontaining D2 and H2O passed through thesample bed until steady state was attained.

3. Concentrations of hydrogen isotopes in theoutlet of the sample bed at steady state weremeasured with a cryogenic gas chro-matograph, but the water concentration wasnot measured because H2O, HDO and D2Ocould not be separated. A hygrometer wasonly used to confirm whether steady state wasattained or not.

The experiments were mainly carried out in theD2–H2O system for ease of measurement. How-ever, experiments in the H2–D2O system werealso carried out with the same process. The reasonfor using the sample dried at the experimentaltemperature is that drying Li2ZrO3 at 800°C isimpossible after packing it in the blanket.

In this experiment, it is considered that D2Oformation occurs by the reaction between D2 andoxygen which is generated due to the introductionof H2O into the sampled bed. However, thepresent authors considered that the amount ofD2O formed by the oxidation of D2 is too small toaffect the isotope exchange reaction in theLi2ZrO3 bed because the reaction rate of waterformation is much larger than that of the hydro-

gen oxidizing capacity regeneration reaction [10].The experimental conditions are shown in

Table 2.

3. Results and discussions

The concentration of H2, HD and D2 in theoutlet gas of the sample bed were obtained bymeasurement using a cryogenic gas chro-matograph. However, HD was not treated as aindividual element because of the simple approachused in this work. HD was separated to H2 andD2 as follows:

CH2=cH2

+12

cHD (1)

CD2=cD2

+12

cHD (2)

HDO was also treated as with HD:

CH2O=cH2O+12

cHDO (3)

CD2O=cD2O+12

cHDO (4)

where cH2, cHD and cD2

are concentrations of H2,HD and D2 [mol cm−3], respectively, and, cH2O,cHDO and cD2O are concentrations of H2O, HDOand D2O [mol cm−3], respectively. The capitalletters mean the net concentration of H2, D2, H2Oor D2O.

The total amounts of hydrogen isotopes in theoutlet gas of the sample bed were very similar tothe amount of hydrogen isotope in the inlet gas of

Table 2Experimental conditions

Sample Dried at experimental temperaturePacked bed weight 15–51 gCarrier gas He

400–4000 cm3 min−1Flow rate200–800°CTemperature

D2–H2O system200–5000 ppm at atmospheric pres-Concentration of

D2 sureConcentration of 200–1000 ppm at atmospheric pres-

sureH2OD2/H2O ratio 1–20


the sample bed. Therefore, it is possible to con-sider that the contributions of the water forma-tion reaction and hydrogen absorption to theisotope exchange reaction are small in this work,and can be expressed as

CD2,in=CH2+CD2

(5)

Furthermore, the following equations areformed considering mass balance within thesystem

CH2O,in=CH2O+CD2O (6)

CH2=CD2O (7)

where CD2,in and CH2O,in are the concentrations ofD2 and H2O in the inlet gas of the sample bed[mol cm−3], respectively.

Two kinds of exchange reactions are expectedto occur in the sample bed. They are consideredto be reversible reactions. When HD and HDOare not treated as individual elements, the sim-plest exchange reactions are expressed as

D2+H2O·Li2ZrO3?H2+D2O ·Li2ZrO3 (8)

H2O+D2O ·Li2ZrO3?D2O+H2O ·Li2ZrO3

(9)

and exchange reaction rates are expressed as

r1=k1CD2qH2O,net (10)

r2=k2CH2qD2O,net (11)

r3=k3CH2OqD2O,net (12)

r4=k4CD2OqH2O,net (13)

where k1 is the rate constant of the exchangereaction between D2 in gas phase and H2O ad-sorbed on Li2ZrO3 [g/mol · s], k2 is the rate con-stant of the exchange reaction between H2 in gasphase and D2O adsorbed on Li2ZrO3 [g/mol · s], k3

is the rate constant of the exchange reaction be-tween H2O in gas phase and D2O adsorbed onLi2ZrO3 [g/mol · s], k4 is the rate constant of theexchange reaction between D2O in gas phase andH2O adsorbed on Li2ZrO3 [g/mol · s], and qH2O,net

and qD2O,net are the net amounts of adsorbed H2Oand D2O on Li2ZrO3 [mol g−1], respectively.Then, qH2O,net and qD 2O,net are given as

qH2O,net=qH2O+12

qHDO (14)

qD2O,net=qD2O+12

qHDO (15)

q=qH2O,net+qD2O,net (16)

where qH2O, qHDO and qD2O are the amounts ofadsorbed H2O, HDO and D2O [mol g−1], respec-tively, and q is the total amount of adsorbedwater [mol g−1].

The mass balance equations in the sample bedunder steady state are expressed as

udCD2

dz= −r1+r2 (17)

udCH2

dz=r1−r2 (18)

udCD2O

dz=r3−r4 (19)

udCH2O

dz= −r3+r4 (20)

where u is the superficial gas velocity [cm s−1],and z is the axial distance along the sample bed[cm]. The boundary conditions are expressed as

z=0�ÍÃ

Ã

Á

Ä

CH2=0

CD2=CD2,in

CH2O=CH2O,in

CD2O=0

(21)

Eqs. (17)–(20) are re-written using Eqs. (5), (6)and (10)–(13) as

udCH2

dz=k1CD2

qH2O,net−k2CH2qD2O,net (22)

udCD2O

dz=k3CH2OqD2O,net−k4CD2OqH2O,net (23)

The following equation is formed using Eqs.(5)–(7), (22) and (23)

qH2O,net=k2CH2

+k3(CH2O,in−CH2)

k1(CD2,in−CH2)+k2CH2

+k3(CH2O,in−CH2)+k4CH2

q

(24)


Nishikawa et al. have carried out isotope ex-change experiments using copper oxide, molecularsieves and copper and reported that the rate ofexchange reaction between hydrogen isotopes inthe chemical form of water is much larger thanthat between hydrogen isotopes in the gas phaseand adsorbed water, and the isotope effect in theexchange reaction among hydrogen isotopes inthe chemical form of water is negligible [11].When that is assumed in this experiment, Eq. (24)is expressed as

qH2O,net=�

1−CH2

CH2O,in

�q (25)

Eq. (22) is expressed using Eq. (25) as

udCH2

dz=kexa

:CD2

−

�CH2

K+CD2

�CH2

CH2O,in

;(26)

where kexa means the mass transfer capacity co-efficient of exchange reaction [1/s] and is given as

kexa=k1q (27)

K means the equilibrium constant [-] in thereaction given in Eq. (8) and is given as

K=k1

k2

=C2

H2

(CD2,in−CH2)(CH2O,in−CH2

)(28)

The solution of Eq. (8) is expressed as

CH2=

b [1−exp{a(b−g)t}]

1−b

gexp{a(b−g)t}

(29)

where

a=kexa(K−1)

CH2O,inK

b=(CH2O, in+CD2, in)K

2(K−1)

+(CH2O, in+CD2, in)2K2−4K(K−1)CH2O, inCD2, in

2(K−1)g=(CH2O,in+CD2,in)K

2(K−1)

−(CH2O,in+CD2,in)2K2−4K(K−1)CH2O,inCD2,in

2(K−1)

t is the space time given by dividing the volumeof the sample bed by the volumetric gas velocity[s]. The concentration of hydrogen in the outlet ofthe sample bed was calculated using Eq. (29) withkexa and K as the parameters and was comparedwith experimental data. Fig. 2 shows an exampleof the comparison. The ratio of unreacted H2O isdefined by subtracting the conversion ratio of H2,XH2

, defined as CH2/CH2O,in, from 1. The solid line

is the calculated value when the backward reac-tion is ignored in the reaction of Eq. (8). kexacontrols the slope of the lines at early space time,and K controls the outlet concentration at theequilibrium state. kexa and K are obtained whenthe calculated values (line) agree with the data(solid circles). It seems that the equilibrium stateis attained below 0.4 l min−1 flow rate, whichgives a K value of about 1.0. When it is assumedthat the amount of adsorbed water on Li2ZrO3 isnot affected by the isotope effect, the totalamount of adsorbed water on Li2ZrO3 is ex-pressed as

q=4.1×10−8ABETP1/2 exp�10700

RT�

(30)

where ABET surface area [m2 g−1], P is the partialpressure of total water vapor [Pa], R is a gasconstant [J/mol ·K] and T is an absolute tempera-ture [K]. Eq. (30) has been proposed by thepresent authors [7]. The forward reaction rate, k1,was obtained using kexa, Eqs. (27) and (30). How-ever, in the case when the experimental tempera-ture is low, for example at 400°C, the reaction didnot reach the equilibrium state under the condi-tions of this work. The shape of the fitting linewhich shows the calculated outlet concentrationwas not affected by the change of K. Therefore,kexa was obtained from curve fitting, but K wasnot obtained at low temperature. Then, the reac-tion rate constant of the backward reaction in theD2–H2O system was estimated from kexa whichwas obtained from the curve fitting of the data inthe H2–D2O system since the backward reactionin the D2–H2O system is equal to the forwardreaction in the H2–D2O system. Finally, the equi-librium constant, K, is calculated as the ratio ofthe reaction rate constant of the forward reaction


Fig. 2. An example of the curve fitting.

in the D2–H2O system to the reaction rate con-stant of the forward reaction in the H2–D2Osystem.

Fig. 3 shows the temperature dependence of theexchange reaction rate constant. k1 was obtainedfrom the experiments in the D2–H2O system, andk2 was obtained from the experiments in theH2–D2O system. The following equations wereobtained from Fig. 3:

k1=1.92×109 exp�

−42900

RT�

(31)

and

k2=1.73×109 exp�

−43200

RT�

(32)

During the experiments in the temperaturerange below 400°C, it was seen that the reactivitygradually becomes smaller. It is believed that thisis the reason for the scattering of the data at400°C. When the sample that was dried at theexperimental temperature was used, the rate con-stant at 500°C was not obtained because of thesmall reactivity. In this work, k1 and k2 are ex-

pressed with Eqs. (31) and (32), respectively.However, it could be considered that the elemen-tary reaction in the temperature range below500°C is different from that above 500°C in theexchange reaction of Eq. (8). The data obtainedusing the sample dried at 800°C are also plottedin Fig. 3. The rate constants obtained using thesample dried at 800°C almost agree with thoseobtained using the sample dried at the experimen-tal temperature. Below 700°C, however, the rateconstants obtained using the sample dried at800°C are smaller than those obtained using thesample dried at the experimental temperature, andthe reaction is not seen below 400°C. Taniguchi etal. have observed –OH bases on the surface ofLi2O using FTIR, and have reported that thesignal corresponding to a certain –OH base dis-appears when Li2O is heated to 973 K and recov-ery of the –OH base on the surface of Li2O is notseen even when Li2O is left in a humidified atmo-sphere at lower temperature [12]. Accordingly, itis probable that the –OH base or residual waterwhich can be released at high temperature con-tribute to the exchange reaction.


Fig. 3. Temperature dependence of the reaction rate constants.

For Li2ZrO3 dried at 800°C, the rate constant,k1, is expressed as

k1=1.36×1013 exp�

−120000

RT�

(33)

The experiment in the H2–D2O system usingthe sample dried at 800°C was not carried out.The rate constant of the exchange reaction in theLi2O bed investigated by Munakata andNishikawa [8] is also shown in Fig. 3. The activa-tion energy of Li2O is 144 kJ, and it is close to thecase when Li2ZrO3 dried at 800°C was used.Therefore, it is probable that the elementary pro-cess of the exchange reaction for Li2O is similar toLi2ZrO3 dried at 800°C. An investigation thatincludes surface analysis is needed to make clearthe mechanism of the exchange reaction.

The equilibrium constant, K, is expressed usingEqs. (31) and (32) as

K=1.11 exp�300

RT�

(34)

The value of K in the temperature range be-tween 250°C and 800°C is about 1.17. This valueis smaller than the value of K for Li2O, 2.2,reported by Munakata and Nishikawa [8].

It is possible to estimate the concentration ratioof hydrogen, added to the blanket sweep gas tothe tritium in the blanket using Eq. (28). Thefollowing equation is formed to transfer tritiumfrom the surface of the breeding material to thegas phase

Cr=XT2+

KT/HXT2

1−XT2

(35)

where Cr is the concentration ratio of hydrogenadded to the sweep gas to tritium in the blanket[-], XT2

is the ratio of the concentration of theelemental form of tritium in the gas phase totritium concentration in the blanket, KT/H is theequilibrium constant in the H–T system. Theequilibrium constant in the H–D system obtainedin this work, 1.17, is used instead of KT/H, becauseKT/H is still not clear. When XT2

is 0.99, theelemental form of tritium in the gas phase is 99%


Fig. 4. Comparison of the mass transfer resistance.

of the total tritium in the outlet from the blanket,and Cr is 116. If the partial pressure of tritium inthe blanket is 1 Pa, one must set the concentrationof hydrogen in the sweep gas at the inlet of theblanket to be above 1145 ppm.

Fig. 4 shows the comparison of the mass transferresistance of the isotope exchange reaction withthat of water desorption. It is assumed that theamount of adsorbed water is not affected by theisotope effect. The mass transfer resistance is givenas the reciprocal of the mass transfer capacitycoefficient. The mass transfer capacity coefficientof the isotope exchange reaction is estimated usingEq. (27), and the water desorption value used is thevalue reported by one of the present authors [13].Under the condition in which the resistance ofwater desorption is smaller than that of the isotopeexchange reaction, it is probable that the amountof tritium in the form of water is larger than thatin the elemental form at the outlet of the blanket,even if hydrogen is added to the blanket sweep gas.

4. Conclusion

A study of the isotope exchange reaction in the

H–D system was carried out using a Li2ZrO3

packed bed. The rate constants of the isotopeexchange reaction between the elemental form ofhydrogen isotopes in the gas phase and in the waterthat forms on the surface of Li2ZrO3 werequantified, and experimental equations were pro-posed. The temperature dependences of rate con-stants between Li2ZrO3 dried at experimentaltemperatures and Li2ZrO3 dried at 800°C did notagree with each other. Further investigation in-cluding surface analysis of Li2ZrO3 is needed. Theequilibrium constant in the H–D system of 1.17was obtained from experimental data in this work.

References

[1] R.G. Clemmer, et al., The TRIO experiment, J. Nucl.Mater. 133–134 (1985) 171–175.

[2] J.P. Kopasz, S.W. Tam, C.E. Johnson, Modeling of tri-tium behavior in ceramic breeder materials, J. Nucl.Mater. 155–157 (1988) 500–506.

[3] H. Werle, et al., The LISA-2 experiment: In-situ tritiumrelease from lithium orthosilicate (Li4SiO4), J. Nucl.Mater. 155–157 (1988) 538–543.

[4] T. Kurasawa, et al., In-pile tritium release behavior fromlithium aluminate and lithium orthosilicate of the VOM-23 experiment, J. Nucl. Mater. 155–157 (1988) 544–548.


[5] M. Briec, et al., In-pile tritium extraction from samples oflithium aluminate, J. Nucl. Mater. 155–157 (1988) 549–552.

[6] C. Alvani, et al., In situ tritium recovery from LiAlO2

pellets: TEQUILA-I, in: Proceedings of the 15th Sympo-sium on Fusion Technology, SOFT, Utrecht, The Nether-lands, Sept 19–23, 1988.

[7] Y. Kawamura, M. Nishikawa, Adsorption characteristicsof water vapor on Li2ZrO3, J. Nucl. Mater. 218 (1994)57–65.

[8] K. Munakata, M. Nishikawa, Isotope exchange reactionon lithium oxide, J. Nucl. Mater. 170 (1990) 187–192.

[9] A. Baba, et al., Isotope exchange capacity of solid breedermaterials, in: International Workshop on Interfacial Ef-

fects in Quantum Engineering and Systems, IEQES-96,Mito, Japan, Aug. 21–23, Session 1, PA17, 1996.

[10] Y. Kawamura, et al., Formation of water in lithiumceramics bed at hydrogen addition to purge gas, J. Nucl.Mater. 230 (1996) 287–294.

[11] M. Nishikawa, T. Takeishi, Y. Matsumoto, I. Kumabe,Ionization chamber system to eliminate the memory effectof tritium, Nucl. Instrum. Methods A278 (1989) 525.

[12] M. Taniguchi, et al., Study on the surface deutoxyl groupon Li2O, in: Proc. Annu. Mtg. of AESJ, Osaka, Japan,Mar. 27–29, M13, 1996.

[13] Y. Kawamura, M. Nishikawa, Adsorption and desorp-tion rate of water on the various ceramic breeder materi-als, Fusion Technol. 27 (1) (1994) 25–35.

.

isotope exchange reaction in li2zro3 packed bed

Documents