Thermodynamic Considerations for the Use of Vanadium Alloys with Ceramic Breeder Materials*

C. E. Johnson, I. Johnson, and J. P. Kopasz

Argonne National Laboratory Chemical Technology Division

9700 S. Cass Ave. Argonne, IL 60439

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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To be published in the workshop Proceedings for The Fourth International Workshop on Ceramic Breeder Blanket Interactions

Kyoto, Japan

by a contmtor of the U. S. Government under contract No. W-31-104ENG-38. Accordingly, the u. s. Government retains a nonexclusive, royalty-free license to plblish or reproduce the published form of this contribution. or allow others to do SO, for

*Work Supported by the U.S. Department of Energy, Office of Fusion Energy, under Contract W-31-109-ENG-38

OM3TRISUTIQN OF THIS DOCUMENT IS UNLlMlTED R

Portions of this document nray be illegible in electronic image products. fmaees are produced from the best available original dollumaL

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THERMODYNAMIC CONSIDERATIONS FOR THE USE OF VANADIUM ALLOYS WITH CERAMIC BREEDER MATERIALS

C.E. Johnson, 1. Johnson, and J.P. Kopasz Chemical Technology Division Argonne National Laboratory

Argonne, IL 60439

introduction

Fusion energy is considered to be an attractive energy form because of its minimal environmental impact. In order to maintain this favorable status, every effort needs to be made to use low activation materials wherever possible. The tritium breeder blanket is a focal point of system design engineers who must design environmentally attractive blankets through the use of low activation materials. Of the several candidate lithium-containing ceramics being considered for use in the breeder blanket, Liz0 and Li2Ti03 are attractive choices because of their low activation. Also, low activation materials like the vanadium alloys are being considered for use as structural materials in the blanket. The suitability of vanadium alloys for containment of lithium ceramics is the subject of this study. Thermodynamic evaluations are being used to estimate the compatibility and stability of candidate ceramic breeder materials (Li20, Li2Ti03, and Li2Zt-03) with vanadium and vanadium alloys. This thermodynamic evaluation will focus first on solid-solid interactions. As a tritium breeding blanket will use a purge gas for tritium recovery, gas-solid systems will also receive attention.

Calculations

Initial efforts in this thermodynamic evaluation have focused on the reaction shown in equation (l) , which considers the oxidation of vanadium by lithium oxide.

Li20 + V = VO + 2 Li (1)

The vanadium/oxygen system is quite complicated, as seen in Figure 1 (taken from Hansen and Anderko [I 1). There is a region of limited solubility of oxygen in metallic vanadium, which increases as the temperature increases. This is followed by a body centered tetragonal phase of approximate composition V20. The NaCl-type cubic VO phase follows; it has a range of composition varying from 43% to 55% oxygen. This phase is followed by V203, which also has a variable composition range. This is followed by several phases whose compositions are not completely established, ending with VO2. The most oxygen rich phase is V205. The region of interest for this study is

,

WEIGHT PER CENT OXYGEN 5 10 15 20 25

2000

1800

1600

0 W O O 0

W

1

- a

1200 a tl CL r w c to00

800

600

400 0 10 20 30 40 50 60 Y ATOMIC P E R CENT O X Y G E N

Figure 1. Vandim-Oxygen Phase Diagram.

the a-region, which exhibits an oxygen content well below that required for stoichiometric vanadium oxide. The chemistry of this region depends strongly on the oxygen potential and the kinetics of the dissolution of oxygen into the vanadium. One must recognize the potential to form hypostoichiometric oxides that could influence the reaction potential. The initial calculations will focus on vanadium metal and lithium ceramic. Consideration of the alloy elements chromium and titanium which make up the candidate structural vanadium alloy and the condition of the breeder blanket under irradiation as related to the compatibility issue will be discussed in detail in later sections of this paper.

Equilibrium type calculations were carried out for the Li2ON system, a simple system that when fully understood will facilitate extension to other more complex systems. Table 1 calculated thermodynamic results for the change in enthalpy (AH), entropy (AS), and free energy (AG), as well as the equilibrium constant (K), for equation (1) at 500-1300 K. The equilibrium constant for this reaction ranges from for a typical blanket operating temperature regime (700-1 000 K). The equilibrium constants for an even wider temperature range (500-1 300 K) indicate little potential for oxidation of vanadium, or conversely, reduction of lithium oxide when these two materials are considered to be in a closed system. Also, because the structural materials surrounding the ceramic breeder will tend to be at a somewhat lower temperature than the average temperature of the blanket, this contributes to a potentially more favorable environment for compatibility of the ceramic and structural material.

to

Table 1

Thermodynamic Data for Chemical Reaction of Equation (1)

T, K AH, kJ AS, J AG, kJ K

500. 600. 700. 800. 900.

1000. 1100. 1200. 1300.

175.870 177.098 178.006 178.681 1 79.1 54 179.441 179.549 179.481 179.220

51.051 53.301 54.705 55.609 56.169 56.473 56.578 56.520 56.31 2

150.345 145.1 18 139.71 2 134.193 128.602 122.968 1 17.314 11 1.658 106.01 5

1.960E-016 2.31 9E-013 3.747E-011 1.727E-009 3.432E-008 3.770E-007 2.684E-006 1.378E-005 5.494E-005

Similar calculations have been carried out for reactions resulting in the formation of V203 and V205. These calculations show an even stronger resistance to formation of the vanadium oxide, with equilibrium constants in the range of 1 O-20 to 1 0-90 for a similar temperature regime.

300

4-

A /P'

k' >' A'

Cr/Cr 0 k' 2 3 x '

A' Af'

t

400 600 800 1000 1200 1400

Temperature, K

Figure 2. Ellingham Diagram for Oxide Couples.

The various competitive reactions have been organized in an Ellingham diagram (see Figure 2), wherein the oxygen potential for each metaVmetal oxide couple is plotted against the temperature. This plot includes the elements iron, chromium, vanadium, titanium, zirconium, and lithium, which are all components of in the candidate ceramics and structural materials. From a thermodynamic perspective, the diagram shows that Fe, V, and Cr are unable to reduce lithium oxide. Zirconium is able to reduce lithium oxide above 1300 K, while temperatures have to be increased above 1400 K before titanium is able to reduce lithium oxide. Thus, this thermodynamic analysis suggests a very low probability for vanadium oxide formation, under breeder blanket conditions.

In extending this thermodynamic evaluation, we next consider what happens should reduction of lithium oxide take place, enabling the formation of a vanadium oxide in the blanket system. The reaction sequence shown in Eqs.2 and 3 are used to illustrate the possibilities that formation of a vanadium oxide brings for the formation of lithium vanadate, e.g., LiV03. The formation of lithium vanadate may proceed as follows:

5 Liz0 + 2 V = V205 + 10 Li (2)

V2O5 + Lie0 = 2 LiVO3 (3)

Or the sequence may be summarized in the following manner,

3 Li20 + V = LiVO3 + 5 Li (4)

The equilibrium constant for Eq. 4 varies from 1 0-l8 to 1 O-20 over the temperature range of interest, indicating that the stabilization energy recovered in the formation of the ternary oxide is not high enough to ensure the formation of a lithium vanadate. The Ellingham diagram in Figure 3 shows the stability relationships for the vanadium oxides, lithium vanadate, and lithium oxide in Eqs. 2-4. This figure indicates that the vanadium oxides are much less stable than lithium oxide, with little chance for reduction of lithium oxide or formation of lithium vanadate. Also, the diagram shows that the stabilization energy recovered in the formation of lithium vanadate is insufficient for it to compete with the thermodynamic stability of lithium oxide. Thus, it is unlikely that formation of a vanadium oxide or a lithium vanadate, should it occur, will have any impact on the compatibility between vanadium and lithium oxide.

The preceding calculations have considered vanadium as a pure metal, while the favored structural material is an alloy containing 3 wt. % Cr and 3 wt. o/o Ti. We thus examined the overall stability of this alloy in an oxidizing environment. The Ellingham diagram shown in Figure 2 illustrates the stability relationships of the components in the alloy. There it can be seen that Cr2O3 is significantly less stable than Li20, so no reaction is expected. The situation is somewhat less favorable for titanium in that for temperatures above 1500 K, Ti0 may be more stable than Li20. However, at the low concentration of titanium present in the alloy and the reasonable assumption that the

,.

200

400

600

800

1000

1200

A -&-------

A’ >--- A/ LNO,

A==’ I’

k’ VNO A / =/=-

B--

I I I I I I I I I I 1 1 l I I I I I I I I I I I I

400 600 800 1000 1200 1400

Temperature, K

Figure 3. Ellinaham Diagram for Lithium Vanadate Svstem

titanium is fully dispersed throughout the alloy, it appears unlikely that the titanium can become enough of a chemical potential sink for a chemical reaction to occur. Also, at these low concentrations and with the solute assumed to be fully dispersed, the kinetic mechanism to achieve chemical reaction appears improbable.

The approach taken to evaluate the ternary lithium ceramics Li2ZrO3 and Li2Ti03 was to consider that these materials were composed of the binary oxides Lie0 and ZrO2 and Liz0 and TiO2. This approach of separation into binary components facilitates assessing whether either of the components making up the ternary ceramic may, in any fashion, interact with vanadium (alloy) to form a corrosion layer. We have already shown that lithium oxide cannot be reduced by vanadium. Also, as can be seen from the oxygen potential data for Ti02 and 3 0 2 in Figure 2, the vanadium metal will not likely reduce either the zirconium oxide or the titanium oxide. Therefore, it appears safe to conclude that the lithium-containing ceramics are compatible with vanadium metal or vanadium alloy for the expected range of blanket operating temperatures.

A possible problem may arise when these ceramics are placed in a neutron environment, where transmutation of lithium takes place. When tritium is released from the ceramic, it is believed that the tritium is likely to be released as a condensable "tritiated water" (TzO), which may react with the lithium oxide to form LiOH (where H has been used instead of T):

Li2O(s) + H20(g) = 2 LiOH(s,l) (5)

The LiOH is soluble to a small extent in the lithium oxide, and a solid solution is thus likely to form. Also important to this evaluation is to estimate the expected partial pressure of tritiated water in an operating blanket. Such information can be used to determine the potential for formation of lithium tritoxide, LiOT(g). The tritoxide species can be very reactive, so its behavior in this system must be thoroughly understood. Using the relationship found in Eq. 5 and data from the BEATRIX 11, Phase 1, irradiation [2] wherein a large mass of lithium oxide was irradiated, we calculated that the lithium tritoxide pressure ranged from 1 x l 0-l6 atm at 500 K to 2x1 0-7 atm at 1000 K. The partial pressure for LiOT can be mathematically expressed as

Even at these low partial pressures, a case could be made that interaction of tritiated water with vanadium will form the vanadium oxides:

Several reactions must be considered when computing the extent of vanadium oxidation: the partial pressure of T20 over the Li2O-LiOT solid solution, the solubility of LiOT in Li2O(s), and the vaporization of LiOT(g). The transmutation of lithium into

tritium liberates the oxygen that was joined with the lithium. Since the excess oxygen is generated initially as excess oxide ion within the Li20 crystal lattice, to react with the vanadium, it must be transported from the crystal to the surface of the vanadium by means of some volatile species, e.g., H20(g). This transport step should greatly limit the rate of oxidation of vanadium.

The reactivity of the Cr and Ti components in this environment should also be considered. At the previously stated partial pressures of lithium tritoxide (hydroxide), no reaction with the chromium is expected. That may not be the case for the titanium additive. At unit activity for both the titanium metal and lithium hydroxide, the reaction

LiOH + Ti = Ti0 + LiH (9)

goes strongly to the right, with an equilibrium constant of 8 x IO8 at 500 K and 5 x lo4 at 1000 K. However, at an activity of 0.3 for the titanium and a partial pressure of

atm for lithium hydroxide, no reaction is expected on the basis of our thermodynamic calculations.

to

The SOLGAS computer code was used to carry out equilibrium calculations that would simulate the transmutation (Le., neutron irradiation in a breeder blanket) of lithium to tritium for both the Liz0 and the Li2ZrO3 blankets. For the case of the Li2O blanket, it was assumed that the LiOH (LiOT) was either in solid solution in the Li20 or in the gas phase. The purge gas was assumed to be a mixture of He, H2, and the oxygen containing species formed by the liberation of oxygen resulting from the transmutation of lithium. The result of this equilibrium calculation was that essentially all of the liberated oxygen ended up as VO. A very small amount of the oxygen formed Hz0, 0 2 , and LiOH(g). A small amount of the LiOH was present in solid solution in the Li20. However, these “sinks” for oxygen were not very competitive with vanadium. For up to 2% transmutation of the lithium, only VO was formed. The higher oxides of vanadium and the lithium vanadate (LiVO3) were not formed.

The results for the Li2ZrO3 blanket were similar to those found for the Liz0 blanket. There was no solid solution of Li20 and LiOH formed. Some of the liberated oxygen from the transmutation ended up as a separate Zr02 solid phase. The oxygen potential of the system was set by the equilibrium between V and VO:

VO(s) = V(s) + 112 0 2 (1 0)

While these equilibrium calculations indicate that essentially all of the oxygen liberated by the transmutation of lithium ends up as VO, in a real breeder blanket system the transport kinetics will play a very important role. For example, the liberated oxygen is most likely transported as T20. The partial pressure of T20 is very low in these systems so the rate of reaction with vanadium could also be low. In addition, the oxidation of vanadium by T20 vapor would be expected to be different from the oxidation with pure oxygen.

The extent of oxidation of vanadium will depend upon the kinetics of the reaction. The difficulty here is that while there is a modest data base, the overall interpretation poses some difficulty. Several authors [4-91 have reported investigations on the oxidation of vanadium at low oxygen pressure, the oxidation proceeding according to a parabolic rate law. Yamawaki; et ai. report that the Arrhenius plot of the parabolic rate constants shows a discontinuity at about 800K. This discontinuity is attributed to the change in the substrate composition. There is some indication that small additions of chromium reduce the oxidation rate, while additions of titanium to the V-Cr alloy seem to be a bit faster than that for V-Cr. One report [9] indicates that neutron irradiation on single-crystal vanadium remarkably accelerates the oxidation process in any phase at temperatures above 870 K.

Despite the promising results to date, efforts to develop vanadium alloys with a stronger oxidation resistance are needed before vanadium alloys can be considered for blanket operations. Recent studies [IO] have shown that the reaction of an oxygen-rich layer in the vanadium alloy with calcium dissolved in liquid lithium produces a thin layer of calcium oxide (CaO), which acts as a protective coating on the alloy. The CaO coating is extremely stable and should present a benign surface to the low concentrations of moisture expected in an operating blanket. This concept needs further investigation to ensure that the integrity of the surface is maintained under thermal cycling and neutron irradiation. Self-healing of CaO layers is excellent above 700 K.

Conclusion

The use of vanadium alloy with either Liz0 or LiZTi03 appears to be a very attractive for the breeder blanket design concept because of its low activation. Our study assessing the compatibility between lithium containing ceramics and vanadium alloys indicates that vanadium alloy has potential for use as a structural material in the design of ceramic breeder blankets for fusion reactors. There is no apparent compatibility problem between the ceramics and vanadium metal or vanadium alloys. However, compatibility problems may exist when small amounts of moisture are present in the system. The partial pressure of "moisture" generated during irradiation of lithium ceramics in a breeder blanket could be very close to that required thermodynamically for oxidation of vanadium to occur. However, there is still a paucity of data on the kinetics of the oxidation of vanadium. To resolve this issue, laboratory and in-reactor compatibility experiments need to be pursued.

References

[I] M. Hansen and K. Anderko, "Constitution of Binary Alloys", McGraw-Hill, New York, NY, pp 1073-1 076 (1 958)

[2] 0. D. Slagle and G. W. Hollenberg, BEATRIX ll Phase 1 Data Summary Report, PNL 10279, January (1995).

[3] G. Eriksson, Chemica Scripta. 8 (1975) 100.

[4] G. Busch and A. Tobin, J. Nucl. Mater. 141-143 (1986) 599-603.

[5] B. A. Loomis and G. Wiggins, J. Nucl Mater. 1228~123 (1984) 693-697.

[6] M. Yamawaki, T. Igarashi,T. Yoneoka, and M. Kanno, Nippon Kinsoku Gakkaishi 44 (I 980) 425-430.

r/] T. Yoneoka, M. Yamawaki, and M. Kanno, Nippon Kinsoku Gakkaishi 44 (1980) 1384-1 392.

[8] T. Yoneoka, T. Igarashi, M. Yamawaki, M. Kanno, Nippon Kinsoku Gakkaishi 44 (1 980) 125 1 -1 258.

[9] L.N. Larikov, N.P. Ptotnikova, and V.N. Primushko, Zashch. Met. 15 (1979) 730- 733.

[I 01 J.-H. Park and T. F. Kassner, Proceedings Seventh international Conference on Fusion Reactor Materials (ICFRM), Obninsk, Russia, September 25-29, 1995.