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ELSEVIER Journal of Nuclear Materials 247 (1997) 315-321

Study on the hydrogen solubility in zirconium alloys

Shinsuke Yamanaka *, Masanobu Miyake, Masahiro Katsura

Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565, Japan

Abstract

Pure zirconium and zirconium alloys of Zircaloy-2 (1.47 wt% Sn, 0.14 wt% Fe, 0.11 wt% Cr, 0.05 wt% Ni), Zircaloy-4 (1.52 wt% Sn, 0.21 wt% Fe, 0.11 wt% Cr) and Zr-1 wt% Nb alloy and Zr(O) alloys were selected as specimens. The hydrogen solubility measurement was performed in the temperature range 500-1050°C at a hydrogen pressure below 10 4

Pa. The hydrogen solubilities were different among alloys and the phase relationships were affected by the alloying element. The partial thermodynamic quantities of hydrogen in the alloys were derived from the experimental data and the effect of the alloying element was discussed. © 1997 Elsevier Science B.V.

1. Introduction 2. Experimental

Zirconium alloys such as Zircaloy have been widely used as the cladding materials of light water reactors. In recent years, hydrogen behavior in the cladding has been watched with keen notice because of high burn up of nuclear fuel required from the economic point of view. The renewed interest in hydrogen embrittlement of the cladding promotes reassessment of the zirconium-hydro- gen system.

The hydrogen solubility and the phase diagram for the zirconium-hydrogen binary system have extensively been studied [1-8] and reviewed [9-11]. The phase boundary between the alpha phase and the hydride phase was exam- ined for zirconium alloys below 500°C [12,13]. However, there exists insufficient data for the hydrogen solubility in zirconium alloys at higher temperatures and a little infor- mation is available for the effect of gaseous contaminants (mainly H20 and 0 2) on the hydrogen solubility [14]. The hydrogen solubility and the phase diagram are important factors for the practical uses of zirconium alloys in light water reactors under accidental as well as normal operating conditions. From this point of view, the hydrogen solubil- ity in substitutional and interstitial zirconium alloys has been therefore examined in the present study.

* Corresponding author. Tel.: +81-6 789 7887; fax: +81-6 873 3696; e-mail: yamanaka@nucl.eng.osaka-u.ac.jp.

Zircaloy-2, Zircaloy-4 and Zr-1 wt% Nb, the composi- tions of which are shown in Table 1, were chosen as substitutional alloys and Zr(O) alloys with an oxygen content of 0.025-0.02 O / Z r were chosen as interstitial alloys. Hydrogen gas (initial purity, 99.999%) was further purified by passing through a hot titanium bed and used for solubility measurement.

The hydrogen solubility was measured in the tempera- ture range 500-1050°C at a hydrogen pressure below 104 Pa using a modified Sieverts' apparatus.

3. Results and discussion

3.1. The effect of substitutional element

Figs. 1-8 show the equilibrium hydrogen pressure PH2 (Pa)-hydrogen concentration, C n ( H / Z r atom ratio), isotherms for pure zirconium, Zircaloy-2, Zircaloy-4 and Z r - I wt% Nb. The following phases appear: hcp a phase, bcc [3 phase and fcc &

The hydrogen solubility in pure zirconium is known to obey Sieverts' law in low hydrogen concentration region. For zirconium alloys, Sieverts' law also holds between the hydrogen pressure and the hydrogen concentration in both the a phase and 13 phase regions, as shown in Figs. 1-8. In our previous study [15], a comparison of Sieverts'

0022-3115/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0022-3115(97)00101-3

316 S. Yamanaka et al. /Journal of Nuclear Materials 247 (1997) 315-321

Table 1 Compositions of zirconium alloys used in the present study

Zr a Zircaloy-2 Zircaloy-4 Zr- 1Nb

S n - 1.47 wt% 1 . 5 2 wt% -

Fe 74 ppm 0.14 wt% 0,21 wt% 530 ppm Cr - 0.11 wt% 0.11 wt% - Ni - 0.05 wt% < 35 ppm - Nb - - - 0.97 wt% O < 50 ppm 0.125 wt% 0,125 wt% 0.143 wt% Zr bal. bal. bal. bal.

Total impurity concentration < 210 ppm.

constant and the enthalpy of solution has been made among zirconium alloys. In the c~ phase region (below 850°C), Zircaloy-4 had the largest solubility among the alloys and pure zirconium. The enthalpies of solution for the alloys were higher than that for pure zirconium. In the [3 phase region (above 950°C), the hydrogen solubilities in the alloys were smaller than that in pure zirconium. The enthalpies of solution for the alloys were appreciably higher than that for pure zirconium.

At higher hydrogen concentration, there exist disconti- nuities in the slope of isotherms below 850°C for pure zirconium, as evidenced by Fig. 1. The discontinuities indicate the phase boundaries: c~ phase/[3 phase, [3 p h a s e / 5 phase and a p h a s e / ~ phase. Above 900°C, the slope of the isotherm is found from Fig. 2 to vary gradu- ally with the hydrogen concentration, which suggests the existence of a single [3 phase. There is not a large discrep-

104

10 3

ca 2 = 10

10

1

(%) • - a ° o . o

875 . , - a o o . o

• ~ a m o . o 9O0

• ~xuo*O

9 2 5 • " oo *o

o • t~O,o

9 5 0 " "-a °*~

~oaoo .o

975 ,,-a~o,a

-,-aS~

1000 • , ~7~o

~oo

1025 , , ~ o o

l e - t~o

1050 , I ~o

0.01 0.1 C . (H/Zr)

Fig. 2, Equilibrium hydrogen pressure Puz-hydrogen concentra- tion, C H, isotherms for pure Zr. (Temperature: 875-1050°C.)

ancy between our results of the phase boundaries and the assessed phase diagrams in the literature [9-11]. The temperature of the eutectoid reaction: [3Zr= a Z r + 5ZrH 2 .~ appears to be slightly higher than the 550°C reported in the literature [9-11].

The discontinuities in the slopes of the isotherms for zirconium alloys were observed at the hydrogen concentra-

("4

10 3

102

10

1

0 .1

50O .'?~. o l 550 . -

• _ ~ . 600 o

- - a a •

650 ~ ~ 8° °°qS) ,~

II, K a @ o o oeooo :~ 700 • _\ , ,# oO . . . . * - "

750 ,_ .

800 o-- oo •a0mnu • • •~ua • • o • •

• _ a o

850 • ~a o o * " 4A-

o u . • •

' a • " i , i , , , , , d , , . . . . . . I . . . . IIHI I I

0.01 0.1 1 C n(H/Zr)

4 10

l o 3

-10 2

cq

~5 10

1

0 .1

(*c)

600

650

700

750 A ° •

e - • 800 •

° -A ° o • - A ° o

850 • - ae o o_ •o •

o o _ u

• o A i o

o

Q • u °

• - • o •

o o o = • n •

o - • •

6 • o •

o o o o

0.01 0.1 1 C H (H/Zr)

Fig. 1. Equilibrium hydrogen pressure PH2-hydrogen concentra- Fig. 3. Equilibrium hydrogen pressure P.2-hydrogen concentra- tion, C H, isotherms for pure Zr. (Temperature: 500-850°C.) tion, C H, isotherms for Zircaloy-2. (Temperature: 600-850°C.)

S. Yamanaka et al. / Journal o f Nuclear Materials 247 (1997) 315-321 317

c q

10 4

10 3

10 2

10

9 0 0 ~ mO •

mOmoa 4P

9 2 5 *omo • •

950

9 7 5

IO00 "c'o~

1 0 2 5 ~ a ~

1 0 5 0

0.01 0.1

C . (H/Zr) Fig. 4. Equilibrium hydrogen pressure PHz-hydrogen concentra- tion, C a, isotherms for Zircaloy-2. (Temperature: 900-1050°C.)

t ' N

104

103

102

10

1

,~ ,o a •

900 • =o ~ •

a moaa•

925

950 ®. q~ ' e , % o

9 7 5 . ~ o • ~ ,

o

1 0 0 0 ~ o ~

1 0 2 5 ~'~

o

1050

o,~ ~ o a

0.01 0.1

C - (H/Zr) Fig. 6. Equilibrium hydrogen pressure Pna-hydrogen concentra- tion, C a, isotherms for Zircaloy-4. (Temperature: 900-1050°C.)

tion above 0.04H/Zr. No plateau pressure was found in the isotherms for zirconium alloys. The regions above 0 .04H/Zr appear to correspond to o~ + [3 phase region. Fig. 9 shows the experimental results obtained for the phase boundaries between the ~ phase region and the

10 4

-"-102

t ' - I ~ 10

1

0.1

('C)

600

650

700

750

800

850

• a

0

- - • O R

0 • • o " i

• - o

o

~0 • " a 0

0

• • 0

0 A 0

0

0 0

. . . . . . . . I , , 1 , , , , , I i , , , , , , d

0.01 0.1 1 C H (H/Zr)

+ f3 phase region for the zirconium alloys (O : Zry-2, zx: Zry-4, O: Zr - lNb) , together with the zirconium-hy- drogen system (the dashed line and []). The dotted line indicates the phase boundaries of the Z r - H system [10] and the broken line indicates the reported values for zirconium alloys [12,13]. The phase boundary slightly

1 0 4

10

C q

10

1

0.1

(*C) • _

6® • .-: 6 5 0 • - • o •

• • a

700 • - • ~ •

• L • a io

7 5 0 • - • o • • o •

• a a • • S 0 0

• A ° • o o o o

• _ n •

8 5 0 • • o o

- a ~o • _ o • a o • _ o

• • a = • o

• a •

o

i i i i i H I I i i . . . . . . I i i , i l i i l l t i

0.01 0.1 1 C . (H/Zr)

Fig. 5. Equilibrium hydrogen pressure PH2-hydrogen concentra- Fig. 7. Equilibrium hydrogen pressure P,%-hydrogen concentra- tion, C u , isotherms for Zircaloy-4. (Temperature: 600-850°C.) tion, C a , isotherms for Zr-1 wt% Nb. (Temperature: 600-850°C.)

318 S. Yamanaka et al . / Journal of Nuclear Materials 247 (1997) 315-321

cv~ g~

10 3

10 2

10

CC)

900

925

950

, omo A~ •

.olo ~ e

,olo ~ e

~I~AAe

975 o

1000 , ~ t ,

1025 *'i~ o ~qD

1050 't~

0.01 , , , , , , , , I , , , , , , , ,

0.1

C H (H/Zr)

Fig. 8. Equilibrium hydrogen pressure PH2-hydrogen concentra- tion, CH, isotherms for Zr-1 wt% Nb. (Temperature: 900- lO5OOC.)

differs among alloys and the addition of alloying elements such as Sn, Fe, Cr, Ni and Nb appears to increase the stability of the a phase. The effect of each alloying on phase stability has not been clarified in the present study. Our data appear to be consistent with low temperature data sets for zirconium alloys [12,13]. Though the isotherms for the alloys shown in Figs. 1-8 suggest that the phase regions of 13, 13, + ~ and 8 may exist in the systems, the phase boundaries at higher hydrogen concentrations were not able to be determined in the present study.

The hydrogen solubility obviously changed with the kind of alloy. Assuming that hydrogen atoms have access to tetrahedral interstitial sites in a zirconium metal lattice, we applied the following solubility equation [16] to the experimental data to analyze the hydrogen solubility in the zirconium alloys:

In( C H T T / 4 / / ( /3 -- C H ) PI/22AH 2 )

= - ( H H - E d ) / k T + SH/k.

In this equation, H H and S H are the partial molar enthalpy and the partial molar excess entropy of hydrogen ,referring to the standard state of hydrogen atoms at rest in a vacuum, E d is one half of the dissociation energy of the hydrogen molecule, k the Boltzmann constant and An2 the value related to partition functions of hydrogen gas. The value o f / 3 is the number of interstitial sites of a given kind available to hydrogen per zirconium atom. For the 13 phase, /3 = 6 and for the a phase, /3 = 2. In our previous study [15], the values of partial molar thermodynamic

800

700

600

500

400

0

\ / " 0 Clb\

\ \ \ \

02x t2a

I

1 I I ~zx • t

/ Z ..... J "

/ , / .

/..,.L/-

/;;,"

O. 02 O. 04 O. 06 O. 08 0.1 C (H/Zr)

Zr Zry, Zr-Nb Zr Zry-2 Zry-4 Z r - l N b Reported value Reported value

0 O A • . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 9. Phase boundaries between ~ and ~ + 13 for pure Zr and Z r

alloys. Experimental values: - o - : Z r , O : Z r y - 2 , A : Z r y - 4 , O :

Z r - l N b . Reported values: - . - , Z r [ 1 0 ] , - - - , zirconium alloys [ 1 2 , 1 3 ] .

-220

PureZr []

Zry2 " ~ O

-O--O--O--O-O - - % ~ , Zr 4

"5 " ~ \ z,lNu ~_~ -260 -o-o--o-~o o~-\.~ •

~ ' e -'~',

-300

I I 0.01 0.1

CH(H/Zr)

Fig. 10. Change in the partial molar enthalpy H n value with the hydrogen concentration C H.

S. Yamanaka et al. /Journal of Nuclear Materials 247 (1997) 315-321 319

quantities were estimated for the or phase of the alloys and found to be constant irrespective of the hydrogen concen- tration. The partial molar quantities for the ct phase dif- fered among the alloys. The results for the 13 phase obtained in the present study are indicated in Figs. 10 and 11. As can be seen in Figs. 10 and 11, the partial molar enthalpy and the partial molar excess entropy are constant below 0.02 H / Z r and decrease with increasing hydrogen concentration. The values of the partial molar quantities vary with the kind of alloy and there is a marked differ- ence in the concentration dependence among the alloys.

For both the ct and 13 phases, both the partial molar enthalpy and excess entropy for dilute solution regions increase with the addition of an alloying element. For the ct phase, the magnitude of the changes in the partial molar quantities is slightly larger for Zircaloy-4 than for the other alloys, while for the 13 phase, it is larger for Z r -Nb than for the other alloys. Hydrogen-hydrogen interaction ap- pears to cause the reduction in partial molar quantities for the 13 phase and the magnitude of the interaction seems to strongly depend on the kind of alloy. The positive changes in the partial molar quantities with addition of alloying elements cannot be explained by vibrational or dilatomet- ric contribution. Further analysis is required to understand the effect of each alloying element on the hydrogen solu- bility in zirconium.

-5..4

Temperature (%)

1050 1000 950

/ f

-5.6 A,v .** or./3 / / . / / z ] / /

/ /~/"

I"7~" , /

--5.6 ,'~a / •

.../ •

-6.0 2 t';~ *

-6.2 ¢.,' ~, * b • DIll c

0 • • * f

-6~ L L i t i 7.4 76 28 B,O 8,2 8./*

I041T (K -1 )

Fig. 12. Temperature dependence of Sieve~s' constant K H for

J3Zr(O) alloys. (a) Pure Zr, (b) 0.0025 O/Zr, (c) 0.0058 O/Zr, (d) 0.0102 O/Zr, (e) 0.0154 O/Zr and (f) 0.0203 O/Zr.

3.2. The effect of interstitial element

In our previous work [16], thermodynamic studies have been performed on the hydrogen solubility in the hcp ct Zr(O) alloy. The hydrogen solubility for the hcp c~ phase first increased with the oxygen content and then decreased

40 PureZr

Zry2 - 0 - 0 - - 0 - 0 0 - - 0 0 - - 0 - 0 - - 0 - 0 © - - 0 ~ . • Z~4

~ X k Zrl Nb

20--~-~-~'~ ~'~\'\

- %

I I

0.01 0.,1 C~H/Zr)

Fig. 11. Change in the partial molar excess entropy S H value with the hydrogen concentration CH.

at higher oxygen contents. The presence of interstitial oxygen varied the enthalpy of solution.

All the solubility data for the bcc 13Zr(O) closely obey Sieverts' law. The temperature dependence of Sieverts' constant obtained for the 13Zr(O) alloys is shown in Fig. 12. It is found from Fig. 12 that linear relationships between the natural logarithm of Sieverts' constant and the reciprocal temperature hold for 13Zr(O) alloys with oxygen contents below 0.01 O /Zr . There is no linear relationship for alloys with higher oxygen contents. This suggests that the Zr(O) alloys below 0.01 O / Z r is the single 13 phase and the Zr(O) alloys with higher oxygen contents are in the two phase region of ct + 13. The solubility of hydrogen first decreases with the oxygen content and then increases slightly. This trend for 13Zr(O) differs from that for ctZr(O). The enthalpy of solution first decreases and then increases as the oxygen content increases.

In order to clarify the influence of interstitial oxygen, the solubility data for ctZr(O) alloys were analyzed by a dilute solution model proposed for the ctZr(O) alloy in our previous study [17]. The changes in the solubility and in the enthalpy of solution with addition of interstitial oxygen were discussed in terms of the difference in partial molar enthalpy and excess entropy between pure metal and alloy, which are defined as, ~ H = H H - H ° and ~ S = S H S° where H H and S H are the partial molar enthalpy and the partial molar excess entropy of hydrogen in 13Zr(O) and H ° and S ° are the values for pure 13 zirconium. Figs. 13

320 S. Yamanaka et al. / Journal of Nuclear Materials 247 (1997) 315-321

and 14 show the changes in 8H and 8S values with the composition of the alloy. As shown in Figs. 13 and 14, the BH and BS first decrease with the oxygen content and then markedly increase at higher oxygen contents. The change in the solubility in [3Zr(O) alloys is attributable to a net result from the variation in 8H and 8S. The effects of interstitial oxygen on partial molar quantities are signifi- cantly different from bcc [3Zr(O) to hcp aZr(O).

On the assumption that the enthalpy and entropy differ- ence BH and BS were given in the form of 8H = BH ~ + 8 H d + 8H ~ and 8S = ~S ~ + 8S d + ~S c where 8H v and

8S v are the differences in vibrational contribution between the alloy and pure zirconium, 8H d and 8S d the lattice dilatation terms and 8H ~ and 8S c the configurational terms, we analyzed the changes in 5H and ~S with the oxygen content in [3Zr(O). The results for the analysis are shown in Figs. 13 and 14. Detailed methods for the analysis were published elsewhere [17].

As shown in Figs. 13 and 14, the ~H v is small and

negative and 8S ~ is small and positive. The values of 8H ~ and 8S v obtained for [3Zr(O) are larger than those for

o~Zr(O). As evidenced by Figs• 13 and 14, the volume expansion due to interstitial oxygen results in a negative enthalpy change and the value of 8S d is negligibly small. There is no marked difference in the volume effect be- tween [3Zr(O) and oLZr(O). The configurational enthalpy

b . . . . . . . . . i . . . . . . . . . i . . . . . . . . .

4 ~ L, o

3

" 2 .-

I - "

0

-1

. . . . . . . . . i . . . . . . . . . i . . . . . . . . .

-2 0 0.005 0.01 0.01 5

Composition [O/Zr]

Fig. 13. Theoretical analysis of the change in the 6H value with the oxygen content of ~Zr(O). (2): experimental values.

. . . . . . . . . . i . . . . . . . . . i . . . . . . . . .

. f ~ . i

E /

-1 . . . . . . . . . . . . . . . . . . . . . . . . . . ":"': 0 O.OOS 0.01 0.01S

Composition [O/Zr]

Fig. 14. Theoretical analysis of the change in the 6S value with the oxygen content of /3Zr(O). A : experimental values.

term 8H c increases with the oxygen content, as shown in Fig. 13. Fig. 14 indicates that 8S c shows a negative value and decreases with the oxygen content. The trend in 8H c and ~S c for 13Zr(O) is almost the same as c~Zr(O). Although the theoretical curves shown by solid lines in Figs. 13 and 14 demonstrate the tendency for the changes in the experimental ~H and 8S values with oxygen addi- tion into f3Zr(O), the absolute values of 8H and ~S were different from one another•

Although we have attempted to account for the influ- ence of interstitial oxygen on hydrogen solubility in zirco- nium, it is not yet fully clarified. To explain the hydrogen solubility in 13Zr(O), we have to take into account other contributions such as the dual site occupation of hydrogen in the bcc lattice. The dual site occupation of hydrogen appears to result in increases in ~H c and 8S c.

4. Conclusion

The hydrogen solubility in pure zirconium and zirco- nium alloys of Zircaloy-2, Zircaloy-4 and Zr-1 wt% Nb and Zr(O) alloys has been studied to elucidate the influ- ence of substitutional and interstitial elements on hydrogen solubility in zirconium. The solubility measurement was

S. Yamanaka et al. / Journal of Nuclear Materials 247 (1997) 315-321 321

carried out at the temperatures between 500-1050°C at a hydrogen pressure below 104 Pa. The hydrogen solubili- ties were different among zirconium alloys and the phase relationships were affected by the alloying element. The influence of the alloying element was discussed on the basis of the thermodynamic properties of hydrogen in zirconium alloys derived from the experimental data.

Acknowledgements

The present study has been performed under the aus- pices of the Japan Atomic Energy Research Institute.

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