Tritium aging effects on hydrogen permeation through Pd8.5Y0.19Ru alloy membrane

Fusion Engineering and Design 86 (2011) 2220–2222

Contents lists available at ScienceDirect

Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes

Tritium aging effects on hydrogen permeation through Pd8.5Y0.19Ru alloy membrane Lu Guangda ∗ , Zhang Guikai, Chen Miao, Wang Xiaoying China Academy of Engineering Physics, P.O. Box: 919, Mianyang, 621900, PR China

a r t i c l e

i n f o

Keywords: Pd–Y alloy membrane Hydrogen Permeation Tritium aging effect

a b s t r a c t Palladium alloy membranes have been widely used as permeator to produce pure tritium from gas mixture or tritiated impurities in tritium technology related to fusion reactor. But long term continuous operation will lead to accumulation of 3 He in alloy, resulting in tritium aging effects. In this work, the changes in protium/deuterium permeation characteristics of Pd8.5Y0.19Ru (at.%) alloy membrane with 3 He concentration of 0.042 (He/M) induced by tritium decay was investigated. In the temperature range 573 K–723 K the permeability of the aged membrane decrease seriously, and the permeation separation factor for H–D mixture increase somewhat. © 2011 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

Tritium process is the basic technology for the fuel cycle in fusion reactor. It has been for long time to use palladium alloy membrane as permeater to separate hydrogen isotopes from gas mixture containing impurities or tritiated gas [1–3]. Palladium alloy membranes are also considered as potential candidates for large and continuous hydrogen isotope separation because they present isotope effects in permeation [4–6]. Besides, by combining catalysis with pervasion, palladium membrane reactor has been used as tritium recovery apparatus from tritiated water and other tritiated gases [7,8]. The operation in the fuel cycle system for fusion reactor is continuing and lasting. Under such condition, tritium decay will certainly lead to 3 He accumulation in the alloy and result in degradation on membrane’s engineering performance. Literature reported about aging effects on Pd alloy membranes has mainly focused on the microstructure, mechanical strength, helium behavior, etc. [9–12], but fewer paper reported about changes of their permeability. In this work, we investigated the tritium aging effects on protium/deuterium permeation through Pd8.5Y0.19Ru alloy membrane.

2.1. Palladium alloy membrane

∗ Corresponding author. Tel.: +86 0816 3626710. E-mail address: [email protected] (L. Guangda). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.04.038

The Pd8.5Y0.19Ru (at.%) membrane sample with thickness of 40 ␮m was sealed and saturated with tritium in a stainless steel vessel. After storing for a certain time, tritium was desorbed while helium was retained even far above tritium desorption temperature. Theoretically calculated concentration of 3 He atom in the obtained sample is 0.042 (He/M). XRD inspection showed that tritium aging made the height of five main diffraction peaks depression so seriously that only one peak at 2 near 68.5◦ could be observed clearly, also, it moved to smaller diffraction angle. Evidently, severe distortion in crystal lattice had taken place, and the crystal lattice became bigger. TEM detection showed that a large quantity of dislocation loops or networks came up in aged membrane, and the He bubble was difficult to recognize. 2.2. Measurements of permeability of hydrogen and deuterium In permeation experiments, the membrane was fixed in a permeation box shown in Fig. 1, and the box was assembled to an experimental system. H2 and D2 which was provided by different LaNi4.7 Al0.3 bed permeated through the membrane at constant temperature into a volumetric container. The temperature of the box can be controlled to ±1 K in accuracy, the gas pressures Pin and Pout were measured and data collected once per second by computer in real time. Gas permeation rate was calculated by the Eq. (1): J=

˚A 1/2 V dPout 1/2 = (Pin − Pout ), RT dt d

(1)

L. Guangda et al. / Fusion Engineering and Design 86 (2011) 2220–2222

2221

Fig. 1. Structure of permeation box.

Here:J – Permeation flux, mol/sV – Volume of standard container (include pipe), m3 T – Temperature of standard container, KR – Gas constantPin – Inlet pressure, PaPout – Outlet pressure, PaA – Area of alloy membrane, m2 d – Thickness of alloy membrane, m˚ – Gas permeability of the alloy membranes, mol/(m s Pa0.5 ). The related data between permeation rate and temperature were obtained by altering permeation temperature, and then permeability expression were determined by fitting the data as Eq. (2): ˚ = ˚0 e−EP /RT

(2)

Here:EP – Permeation activation energy, KJ/mol. 2.3. Measurements of permeation separation factor The permeation separation factor of alloy membrane is defined as the ratio between the abundance of light and heavy isotopes at permeated end and the abundance of light and heavy isotopes in feed end, illustrated with H2 –D2 as: (CH /CD )out ˛= (CH /CD )in

(3)

In Eq. (3), the CH and CD is the abundance of protium and deuterium respectively. During the experimental process, the gas composition on the surface of the membrane in the inlet end must be kept invariable, thus, a thin pipe for feed gas supply was placed near the membrane and the feed flux was kept far greater than permeation flux. The gas samples was analyzed with mass spectrometry. 3. Results and discussion 3.1. Tritium aging effect on H2 and D2 permeability Initial experiments showed that the mechanical strength of virgin membrane decline as temperature increase. At 753 K and 0.1 MPa pressure difference the membrane was broken. Therefore, the permeation temperature range was controlled from 573 K to 723 K and the pressure difference was controlled less than 0.05 MPa in later experiments. The hydrogen and deuterium permeability of virgin membrane was firstly measured, shown in Fig. 2. Because primal experimental conditions were controlled not so ideally, there were bigger discrete between two sets of repeated data points and fitted line for protium permeation. The results show that the relation between permeation flux and the pressure difference accord with the Fick’s first law, indicating that hydrogen atom diffusion was the control step in permeation. The fitted equations of permeability of H2 and D2 for virgin membrane are: ˚H , virgin = 2.43 × 10−1523.49/T −1493.25/T

˚D , virgin = 1.38 × 10

␮mol/(msPa0.5 ) 0.5

␮mol/(msPa

)

(4) (5)

Fig. 2. The H2 and D2 permeability of virgin and aged membrane.

Compared Eq. (4) with (5), it can be found that the preexponential factor of permeability of H2 is much bigger than that of D2 , whereas the H2 permeation activation energy is about 3% larger than D2 . Because the pre-exponential factor is dominative, permeability of D2 is much less than that of H2 . For aged membrane, the permeation box was made smaller than that for virgin membrane so that avoiding the membrane broken. The obtained permeability of H2 and D2 at different temperatures are shown in Fig. 2, too. The fitted equations of permeability are: ˚H ,aged = 0.331 × 10−1267.41/T −1214.37/T

˚D ,aged = 0.205 × 10

␮mol/(msPa0.5 ) 0.5

␮mol/(msPa

)

(6) (7)

Comparing the Eqs. (6) and (7) with (4) and (5), it is obvious that the pre-exponential factors of permeability of H2 and D2 are strongly lowered by tritium aging, nearly one order of magnitude. Meanwhile, the permeation activation energies are reduced at a certain extent. But the difference of activation energies between H2 and D2 for aged membrane are similar to virgin. 3.2. The aging effect on permeation separation factor The permeation separation experiments were done with 1:1 H–D gas mixture. According to Eq. (3), we obtained the permeation separation factor for virgin and aged membrane separately, as shown in Fig. 3. Evidently, tritium aging make the permeation separation factor rise somewhat. 3.3. Discussion As a common property of tritium aging effects for palladium and its alloy [12–14], the plateaus of hydrogen absorption/desorption isotherms decrease, and ˛ and ˇ single phase regions shift towards greater stoichiometries, and a tritium heel which is hard to desorb under routine desorption temperature appears. The quantity of the heel depends on species of the membrane material and increases with the aging time, mostly, is in the same order of magnitude with the 3 He accumulated in alloy crystal. Atoms as tritium heel are very difficult to diffuse in palladium alloy, and its formation may arise from three modes. The first mode is that tritium atoms are strongly trapped to aging-induced crystal defects such as matrix vacancies and dislocation loops or networks, grain boundaries, etc. The second mode is the combination of tritium atoms with He bubbles, which make their potential energy decrease. The third mode is deep potential trap around the He atom trapping the tritium atoms [16].

2222

L. Guangda et al. / Fusion Engineering and Design 86 (2011) 2220–2222 1.46

In a word, theoretical qualitative analysis suggest that tritium aging will lower the pre-exponential factor ˚0 and decrease the permeation activation energy EP of hydrogen in palladium alloy membranes. As the reduction of ˚0 is dominative, the hydrogen permeability is decreased seriously even though the reduction of EP make it increase.

1.44 1.42

virgin membrane

1.40

aged membrane

1.38 1.36 1.34

α

1.32

4. Conclusion

1.30 1.28 1.26 1.24 1.22 1.20 1.18 1.16 560

580

600

620

640

660

680

700

720

740

T, K Fig. 3. Comparison of permeation separation factor to H/D mixture for aged and virgin membrane.

For Pd alloy membrane, tritium aging will cause its hydrogen isotopes’ permeability decrease. The effect primarily arise from the heels forming in aging process, resulting in parts of the hydrogen atoms strongly absorbed in crystal matrix and difficult to permeate. For the aged Pd8.5Y0.19Ru membrane with 3 He concentration of 0.042 (He/M), the permeability is lowered to less than half of virgin membrane in the temperature range between 573 K and 723 K. The lower the operation temperature is, the more seriously the permeability decreases, whereas the separation factor for protium–deuterium gas mixture increase somewhat. Acknowledgments

Because the operation temperature is quite high, the quantity of “heel” is rather large comparing with the solubility of hydrogen in aged membrane alloy. The changes of parameters in the permeability expression after tritium aging can be analyzed with the permeation rule: ˚ = KD

(8)

Here K is the Sievert’s constant √related to hydrogen solubility C in membrane alloy, that is C = K P; and D is the hydrogen diffusion coefficient in the alloy and can be expressed as: D = D0 e(−ED /RT )

(9)

The thermodynamic expression of K is: K = e−(HC→0 −TSC→0 /RT ) = e−(HC→0 /RT )+(SC→0 /R)

(10)

In Eq. (9) and (10), ED is the activation energy of hydrogen diffusion in the alloys. D0 is diffusion constant. HC→0 and SC→0 is the changes of enthalpy and entropy when hydrogen dilutedly dissolve in the alloy. Therefore, the permeability can be expressed as: ˚ = D0 e(SC→0 /R) e(−HC→0 /RT −ED /RT )

(11) e(SC→0 /R)

So, the expression of pre-exponential factor is ˚0 = D0 and the expression of permeation activation energy is EP = ED + HC→0 . Firstly, “heel” means parts of hydrogen atoms are deeply trapped and hard to diffuse. It will make the macroscopical diffusion constant D0 and the entropy change SC→0 minish. Therefore, ˚0 should decrease with aging time. This is the reason of serious reduction of the pre-exponential factor in Eqs. (6) and (7) comparing with Eqs. (4) and (5). Secondly, due to the HC→0 is a minus value, that is Ep = ED +|HC→0 |. As usual, to make strong trapped atoms move need more energy, but un-trapped atoms may be easier to move because the crystal lattice is larger and the interaction between atoms is weaker. Considering the fact that the most of diffusion atoms are un-trapped atoms, the macroscopically diffusion energy ED should be reduced. Besides, the P-c isothermal of Pd alloy tritides decrease with aging time means the released dissolution heat (the absolute value of HC→0 ) will increase. Evidently, the permeation activation energy will decrease with aging time.

This work was supported by outstanding researcher fund of China Academy of Engineering Physics (no. 2008-8). The author would like to thank Dr. Ao Bingyun for his beneficial discussion on this paper and Dr. Ye Xiaoqiu for his careful examination on this article input. References [1] D.T. Hughes, I.R. Harris, Hydrogen diffusion membranes based on some palladium–rare earth solid solution alloys, Zeitschrift fur Physikalische Chemie Neue Folge, Bd. 117 (1979) 185–193. [2] T. Hayashi, S. Konishi, H. Nakamura, Recent tritium experiments of the JAERI fuel cleanup system (JFCU) at the tritium systems test assembly (TSTA), Fusion Technol. 21 (1992) 1979–1982. [3] Luo Deli, Shen Cansheng, Meng Daqiao, Hydrogen isotope separation factors on palladium alloy membranes, Fusion Sci. Technol. 41 (3) (2002) 1142. [4] Yasuo Suzuki, Shojikimura, Separation and concentration of hydrogen isotopes by a palladium alloy membrane, Nucl. Technol. 103 (1993) 93–100. [5] M. Ohno, T. Morisue, O. Ozaki, Comparsion of gas membrane separation cascades using conventional separation cell and two-unit separation cells, J. Nucl. Sci. Technol. 15 (5) (1987) 376–382. [6] D.L. Luo, Y.F. Xiong, J.F. Song, et al., Hydrogen isotope separation factor measurement for single stage hydrogen separators and parameters for a large-scale separation system, Fusion Sci. Technol. 48 (1) (2005) 156–158. [7] A. Basile, V. Violante, F. Santella, et al., Membrane integrated system in the fusion reactor fuel cycle, Catal. Today 25 (1995) 321–326. [8] R.H. Drake, Recovery of tritium from tritiated waste water cost-effectiveness analysis, [R] LA-UR-97-3767, June 1996. [9] V. Tebus, G. Arutunova, V. Bulkin, et al., Investigation of palladium alloy properties degradation during long-time tritium exposure, J. Nucl. Mater. 271–272 (1999) 345–348. [10] V. Tebus, L. Rivkis, E. Dmitrievskaia, et al., Evolution of a defect structure of Pd–Ag alloys during tritium exposure, J. Nucl. Mater. 307–311 (2002) 966–970. [11] V.G. Klevtsov, I.E. Boitsov, A.I. Vedeneev, et al., The impact of tritium on the structure and properties of Pd-membranes, Fusion Eng. Des. 49–50 (2000) 873–877. [12] K.L. Shanahan, J.S. Holder, Tritium aging effects in a Pd0.94 Rh0.05 Co0.01 foil, J. Alloys Compd. 348 (2003) 72–75. [13] S. Thiébaut, J. Demoment, B. Limacher, et al., Aging effects on palladium pressure-composition isotherms tritium storage, J. Alloys Compd. 36–40 (2003) 356–357. [14] S. Thiébaut, M. Douilly, S. Contreras, et al., 3 He retention in LaNi5 and Pd tritides: dependence on stoichiometry, 3 He distribution and aging effects, J. Alloys Compd. 446–447 (2007) 31–34. [16] Ping Feilin, Jiang Gang, Zhang Lin, et al., First principles for effect of He on LaNi5 tritium storage, At. Energy Sci. Technol. 39 (6) (2005) 487–491 (in Chinese).