Sweep gas chemistry effect on vaporization of LiAlO2

ELSEVIER

Journal of Nuclear Materials 23.3-237 (1996) 1452-1456

journalnf nuclear materials

Sweep gas chemistry effect on vaporization of LiA10 2 A. S u z u k i a.~, M.

Yamawaki

'~' * ,

M. Y a s u m o t o b, K. Y a m a g u c h i a

a Nuclear Engineering Research Laboratory. Faculty of Engineering. The University of ToL3'o. 2-22. Shirakata-Shirane. Tokai-nlura. Ibaraki-ken 319-1 l. Japan b Research Centerjor Nuclear Science and Technology. The University q['ToLyo. 7-3-I. Hongo. Bun~3,o-ku. Tokyo 113. Japan

Abstract The equilibrium vapor pressures in the D 2 - D 2 0 - L i A I O 2 system were measured by means of an atmosphere-controllable high temperature mass spectrometer in the temperature range 1573-1773 K. Based on the reaction, 5LiAIO2(s) + 2D20(g) = 4 L i O D ( g ) + LiAI5Os(s), the heat of formation of LiAlsO 8 was evaluated to be - 4 5 5 3 . 9 k J / m o l , which is in good agreement with the data in previous studies based on the reaction, 5LiAIO2(s) = 4Li(g) + O2(g) + LiAlsOs(s). The results of this study enabled estimation of the maximum allowable temperature with respect to lithium transport by the sweep gas in fusion reactor blanket.

1. Introduction In most blanket designs of the nuclear fusion reactors which employ lithium-containing ceramics (Li 2O, Li 4SIO4 , LiAIO 2 etc.) as tritium breeder materials [l], mixing of hydrogen to inert sweep gas has been proposed in order to enhance the release of bred tritium from the surface of ceramic breeder materials. In some designs, the temperature in the ceramic can be more than 1000 K, and hence it is considered that hydrogen in the sweep gas may cause Li loss. Tetenbaum and Johnson studied the effect of water vapor addition on the vaporization behavior of Li20 by means of vapor pressure measurement, and reported that water vapor enhances the volatility of Li20 in the form of LiOH(g) [2]. Recently, it was observed that D 2 and D20, resulting from D 2 oxidation, enhance the volatility of Li4SiO 4 as Li and LiOD, respectively, by means of Knudsen effusion mass spectrometry [3-6].

* Corresponding author. Nuclear Engineering Research Laboratory, University of Tokyo, 7-3-I. Hongo, Bunkyo-ku, Tokyo 113, Japan. Tel.: + 81-3-3812-2111, ext. 7422; lax: + 81-3-5684-0229; e-mail: yamawaki@ utnhgen.u-tokyo.ac.jp. Tel.: +81-29-287-8434; fax: +81-29-284-0442; e-mail: [email protected].

In order to investigate the sweep gas effect by mass spectrometry, we attached a gas inlet system to the mass spectrometer to allow the introduction of gas into a Knudsen cell as reported in a previous study [6]. Sweep gas for fusion reactor blankets was sinmlated by the inlet gas, while the temperature range to be controlled was restricted to less than 1350 K. In this study, the gas inlet system was modified so as to control the pressure of the gas in the cell at high temperatures, i.e., up to 1773 K. By means of the "atmosphere-controllable high temperature mass spectrometer', g a s / s o l i d equilibria in the D 2 - D 2 0 - L i A I O 2 system were studied, and the maximum allowable temperature with respect to lithium transport in the sweep gas was evaluated.

2. Experimental A Knudsen effusion mass spectrometer described in earlier works [7,8] has been modified to allow the introduction of sweep gases [6]. However, when the temperature of the Knudsen cell was raised, the top portion of the gas inlet tube was heated so high that the apparent conductance of the tube became ve D' small. Therefore, it was not possible to introduce a sufficient amount of gas into the cell in the temperature range higher than ~ 1350 K. In this work, first of all, the gas inlet system was modified as

0022-3115/96/$15.00 Copyright (.c) 1996 Elsevier Science B.V. All rights reserved. PH S 0 0 2 2 - 3 1 1 5 ( 9 6 ) 0 0 2 5 2 - 8

1453

A. Suzuki et al./Journal of Nuclear Materials 233-237 (1996) 1452-1456

The system was calibration method. from Mann's table summation of those atomic molecules.

Orifice

K n H : ~ t n e 2 ~

calibrated by the conventional silver Ionization cross-sections were taken [9] for atoms, and obtained from a of constituent atoms for non-mono-

~

Z

3. Results a n d discussion

Pt tube (0.6# x 2201, ),

l~sulation tube

Capillary tube (0.20 x 600L)

Fig. I. Schematic drawing of the Knudsen cell with modified gas inlet system.

It can be assumed that the equilibrium in the LiA10 2D 2 - D 2 0 system is established according to the following reactions; 5LiAlO2(s ) = 4Li(g) + 0 2 ( 8 ) + LiAlsOs(s )

(A)

5LiAlOz(s ) + 2 D 2 0 ( g ) = 4LiOD(g) + LiAlsOs(s )

(B)

D2(g)+"O2(g)=D20(g)

(C)

From these reactions, it is considered that the introduction of D z and D 2 0 enhances the vapor pressure of Li and LiOD, respectively. The vapor species observed in run 0 were Li and O 2. Their partial pressures are shown in Fig. 2. The dam were fitted by the method of least squares, which gave the following partial pressure equations (given in Pa). The stated errors (95% confidence level) were derived from an analysis of variance. 1000

logPu=(-21.30+0.66) shown in Fig. 1. This system mainly consists of a gas inlet Pt tube (0.6 mm diameter × 220 mm length) and a capillary tube (0.2 mm diameter × 600 mm length). The conductance of the gas inlet tube was smaller than that of the orifice of the Knudsen cell so as not to destroy the equilibrium in the cell, while larger than that of the capillary tube so that the total conductance of f!,e gas inlet system was to be determir ".d by that of the capillary tube, whose temperature was little affected by heating the Knudsen cell. Consequently, this modification allowed the pressure of the gas in the cell to be controlled at higher temperatures up to 1773 K, where the pressures of vapor species over LiAIO 2 are expected to be large enough to be measured with sufficient accuracy. The powdered LiAIO 2 sample held in a small Pt crucible was installed in a Pt Knudsen cell. Deuterium (D 2) and deuterium oxide ( D 2 0 ) were used instead of hydrogen (H 2) and hydrogen oxide (H=O) because of the large background at mass 18. We performed three runs (run 0, run 1 and run 2) of measurement in the temperature range of 1573-1773 K. No sweep gas was introduced in run 0. D 2 at 4.0 × l0 2 Pa and D 2 0 at 2.0 × 10 .2 Pa, each on average, were introduced in runs 1 and 2, respectively. Prior to each measurement, the sample was kept under vacuum for 5 h at 773 K in order to remove LiOH which had been produced during the exposure to atmosphere.

X--+ T

11.75_+0.38

(1)

1000 log Po2 = ( - 2 3 " 5 8 _+ 1.60) × - - 7 - + 12.13 _+ 0.92

(2)

These data are in good agreement with those of Ikeda et al. [ 10] which are shown in Fig. 2 with dashed lines.

i

I

I

~Li

--

~ 0.1

I

I

I

Yhis work

----Ikeda et al

"~

'

~0.01 0.001

i

t

i

t

i

i

i

5.6 5.7 5.8 5.9 6 6.1 6.2 6.3 6.4 10000/T[K]

Fig. 2. Partial pressures of vapor species over LiAIO 2. (Run 0).

454

A. Suzuki et al./Journal of Nuclear Materials 233-237 (I 996) 1452 1456 •

I

I

I

t

I

I

[

0

e-

I

I

}

I

I

I

I

0.1

D2

O-

N

0.01

0.001

~_

I

I

~

I

r

5.6 5.7 5.8 5.9 6 6.1 10000/T[K]

i

0.01 -

0.001

6.2 6.3 6.4

J i [ i i 5.6 5.7 5.8 5.9 6 6.1 10000/TIK ]

i [ 6.2 6.3 6.4

Fig. 3. Partial pressures in the system of D 2 -LiAIO 2. (Run 1).

Fig. 4. Partial pressures in the system of D20 LiAIO e. (Run 2).

In run 1, D 2 at an average of 4.0 × 10 2 Pa was introduced. The observed vapor species were Li, LiOD, D 2, D 2 0 and 02 as s h o w n in Fig. 3, and the following partial pressure equations were obtained by the same method as in run 0;

a m o u n t of D 2 was too small to be detected. Their partial pressures are s h o w n in Fig. 4. The partial pressure equations given in the same m a n n e r as above are as follows: 1000 l o g P L i = ( -- 18.75 + 1 . 6 0 ) × - + 1 0 . 3 8 +- 0 . 9 6 7"

(8)

1000

log PLi = ( - - 13.65 + 0 . 2 0 ) X - 7"

+ 7.554--+ 0.122

(3)

1000 log PISOD = ( --5.159 +- 1.49) × - + 1.918 + 0.894 7'

(9)

1000

log PLiOD = ( -- 10.96 +_ 1.22) ×

7----7 + 5.786 +_ 0.726

1000 log PD.O = ( 1 0 . 2 4 ± 0.82) × - --7.871 + 0.494 T

(4)

--

(io)

1000

log PD: = (7.775 4- 1.53) X - - - 6 . 1 4 6 + _ 0 . 9 1 4 T log P l ) 2 0

1000

(5) log P o, = ( - 26.55 + 6.70) ×

+ 13.65 + 3.90

1000 - 2.176_+0.554 = (2.338 + 0.924) × - -

(11)

(6) 1000

Io~, Po, = ( - 3 8 . 0 2 + 4.62) × - -

+ 19.95 + 2.68

(7)

The enhancement of Li-vaporization is s h o w n from the fact that the partial pressure of 02 is smaller and the partial pressure of Li is larger in run I than in run 0. Moreover, it is s h o w n that the partial pressure of D 2 0 , which results from the oxidation of the introduced D 2, is large ( P D , O - 0. l Pa) and accordingly the partial pressure of LiOD is lare:e ( P 15OI) ~ 0.2 Pa). Thus it is considered that, in the case of LiAIO 2, the introduction of D . causes the enhancement of Li-vaporization as well as the formation of LiOD. In run "~ D~O at an averaoe of 2.0 × 10 2 Pa was introduced. Li, LiOD, D 2 0 and 02 were detected: the

It can be observed that LiOD is produced by the introduction of D 2 0 . The equilibrium constant of reaction (A) ( K a. for run 0, run I and run 2), reaction (B) ( K B, for run I and run 2) and reaction (C) ( K c, for run 1) are given in Table 1 as a

Tablc 1 Equilibrimn constants of reaction (A), (B) and (C) Run Run Run Run Run Run

0 I 2 1 2 I

log log log log log log

Ka 108.78± 3.06 X 1000/7" + 59.15 + 0.40 K A= 9 2 . 6 2 + 4 . 6 8 X I 0 0 0 / T + 5 0 . 1 6 + 2 . 7 4 KA = 101.57+9.24× 1000/T +55.16+5.46 K n =-48.52_+5.20X 1000/T + 27.49+3.10 Ki~ 41.11 + 6 . 2 0 × 1000/T +23.4I +3.69 Kc = 1 3 . 5 7 + 2 . 9 3 × 1 0 0 0 / T 6.00± 1.72

A. Suzuki et al./Journal of Nuclear Materials 233-237 (1996) 1452-1456 Table 2 2nd law A H~)s of reaction (A) and (BXkJ/mol) Run 0 Run 1 Run 2

1455

D20, respectively. Hence AH~ for reactions (A) and (B) are defined as follows:

A H°9s(A)

A H~gs(B)

2114.04_+ 58.55 1804.78 _+89.72 1975.93 _+ 176.76

938.69 _+89.71 796.87_+ 118.50

d H ( T ) = 2 d H ( T ) L i + 0 . 5 d H ( T ) o : + d H(T)LiA%O. -- 5dH(T)LiA,02".

(A)

d H ( T ) = 4dH(T)LiO H + d H(T)LiA%O~,- 2OH(T)H2O - 5dH(T)LimO2 : ( B ) dH(T)LiA4O, = 2dH(T)A12o, s + 7d ' H(T)LizO

function of temperature. Comparison of K c in run 1 with the data given in [4] shows good agreement: 1000 log K c = (13.57 + 2.918) × - -

T

- 6.004_+ 1.718; this work

(17)

(12)

(18) (19)

The second law A H~'gs calculated from the above equations is given in Table 2. When the free energy function of a reaction is known, A H~'98 can be calculated by the third law treatment. Using the free energy function at temperature T (fef(T)), k tt~9 s is given as follows:

1000

log K c = (13.30 + 0.004) X --

- 5.560 + 0.003 [4] T

A H ° ( 2 9 8 ) = T[ - R In K - A f e f ( T ) ]

(20)

--

(13) For reactions (A) and (B), the standard enthalpy of reaction (AH~) s) can be calculated from each equilibrium constant. Using the second law relation, A H~'9s is derived as follows: AH~) s = AH~- - d H ( T )

(14)

d(Iog K ) A H~- = - 2.303 R - d(l/T)

(15)

all(T) = A[H~-

(16)

H~9s]

The values of d H ( T ) were estimated using J A N A F Thermochemical Tables [11]. However, since no thermochemical data are available for LiAlsO s, D 2 and D20, we estimated d H ( T ) for LiAlsO s from the summation of those of L i 2 0 and A l 2 0 3 multiplied by their compositions, and used d H ( T ) of LiOH and H 2 0 as those of LiOD and

where K is the equilibrium constant of the reaction. We calculated A H~'98 for reactions (A) and (B) using the third law treatment. The data of f e f for LiAIO 2, Li and 0 2 were obtained from J A N A F Thermochemical Tables [11]. The f e f of LiOH and H 2 0 were used for those of LiOD and D20, respectively. And the f e f of LiAI50 s was estimated from the summation of those of L i 2 0 and A120 3 multiplied by their compositions. The third law A H~'gs calculated using f e f described above and the equilibrium constants in Table 1 are given in Tables 3 and 4. The differences of A H2°)s in the three runs are smaller than the stated errors. In previous studies on Li4SiO 4 [3-6], it was reported that the equilibrium constants and A H~'gs of reactions taking place in the D 2 - D 2 0 - L i 4 S i O 4 system were depending on the oxygen potential, and the possibility of nonstoichiometry (Li4SiO 4 x) was pointed out. In the case of LiA10 2, we could not recognize significant differences in A H~'98 between the runs.

Table 3 3rd law A H°9s of reaction (A)(kJ/mol) Temp. (K)

1500

1600

1700

Run 0 Run I Run 2

2048.33 _4=_78.58 1997.07 +_ 119.11 2024.60 _+ 236.29

2044.32 _+ 80.95 2010.26 -+ 122.69 2028.22 _+ 243.34

2040.09 _+ 83.36 2023.23 _+ 126.32 2031.62 _+ 250.64

Avg.: 2044.25 -+ 46.68 Avg.: 2010.19 _+ 70.88 Avg.: 2028.15 _+ 140.59

Table 4 3rd law AH~9s of reaction (B)(kJ/mol) Temp. (K)

1500

1600

1700

Run 1 Run 2

951.19 + 133.54 926.53 + 159.28

952.26 + 137.58 935.42 _+ 164.11

953.21 + 141.74 944.17 + 169.09

Avg.: 952.22 -+-79.48 Avg.: 935.37 _+ 94.80

1456

A. Suzuki c t a l . / J o u r n a l of Nuclear Materials 233-237 (1996) 1452 1456

From the enthalpy o f reactions (A) and (B), we calculated the heat o f fomlation kH~'~> s o f LiAlsO s. These values are as follows: From reaction ( A ) : ~ tt,°29s(LiAl 5Os ) = - 4576.6 k J / m o l

be less than 1-10 Pa, the temperature in a fusion reactor blanket is allowed to be up to 1200 K. In the case of Li4SiO 4, if the condition o f PDe and PD20 are the same as above, the temperature is allowed to be up to 1000 K.

(21) From reaction ( B ) : k 11~29S (LiAI.sO s ) = -- 4553.9 k J / m o l (22) They are in good agreement, and also in agreement with the data o f Ikeda et al., which was calculated from the heat o f reaction (A) to be - 4577.5 k J / m o l [ I 1]. In the blanket designs, the sum o f the partial pressures o f Li-containing species (ptotal) is r e c o m m e n d e d to be Li 0.01 Pa as m a x i m u m pressure with respect to Li transport in the s w e e p gas. We estimated the temperature at which p~oul reaches 0.01 Pa. We defined 71~, as follows:

p~o,~,,(at 7;,,:,,~) = 0.01 Pa

(23)

P~I~tal = PLi + PI.iOD

(24)

PLi and PLiOD can be calculated from the equilibrium constants of reactions (A), (B) and (C). when PD, and P m o are given. Fig. 5 shows T,~:.~ calculated from the data of run 1. The range of PD~ and PD,O are 1-1000 Pa and 0.01-100 Pa, respectively. For example, if PD, iS equal to 100 Pa, which corresponds to a concentration of 0.1% of 101325 Pa (1 atm) sweep gas, and PD20 iS controlled to Tmax [K] 1400 1300 1200 1100 1000

100 1

10 lr~0v " " P(D2) [Pa]

0.1

P(D20) [Pa]

Fig. 5. Temperature at which the sum of the partial pressures of Li-containing species reaches 0.01 Pa.

4. S u m m a r y The vaporization behavior o f LiAIO 2 in simulated s w e e p gas conditions in a fusion reactor blanket was studied by means o f an atmosphere-controllable mass spectrometer, in which the gas inlet system was modified to allow the pressures o f the gases introduced in the Knudsen cell to be controlled at high temperatures (up to 1773 K). From the resulting thermodynamic data, the m a x i m u m allowable temperature with respect to lithium transport in the s w e e p gas was estimated.

References [1] N. Roux, C.E. Johnson and K. Noda, J. Nucl. Mater. 191 194 (1992) 15. [2] M. Tetenbaum and C.E. Johnson, J. Nucl. Mater. 120 (1984) 213. [3] R.-D. Penzhorn, H.R. lhle, S. Huber, P. Schuster and H.J. Ache, J. Nucl. Mater. 191 (1992) 173. [4] H.R. lhle, R.-D. Penzhom and P. Schuster, Proc. 17th SOFT Conf., Rome, Italy, Sept. (1992) 1389. [5] R.-D. Pcnzhorn, Proc. Ceramic Breeder Blanket Interactions2, Paris, Sept. (1993). [6] M. Yamawaki, A. Suzuki, M. Yasumoto and K. Yanlaguchi, J. Nucl. Mater. 223 (1995) 80. [7] M. Yamawaki, T. Oka, M. Yasumoto and H. Sakurai, J. Nucl. Mater. 201 (1993) 257. [8] M. Yamawaki, M. Yasumoto, C. Nakano and M. Kanno, High Temp. High Pressures 14 (1982) 423. [9] J.B. Mann, J. Chem. Phys. 40 (1964) 1632. [10] Y. Ikeda, H. lto and G. Matsumoto, J. Nucl. Mater. 97 (1981) 47. [11] M.W. Chase, Jr., C.A. Davies, J.R. Downey, Jr., D.J. Fruip, R.A. McDonald and A.N. Syvcrud, J. Chem. Rcf. Data, Vol. 14, Suppl. l (1985) 1228.