Hydrogen absorption properties of UThTiZr alloys

Journal of Alloys and Compounds, 213/214 (1994) 467-470 JALCOM 4093

467

Hydrogen absorption properties of U-Th-Ti-Zr alloys Takuya Yamamoto and Hideo Kayano Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311 13 (Japan)

Hadi Suwarno and Michio Yamawaki Faculty of Engineeringo University of Tokyo, Tokai, Ibaraki 31911 (Japan)

Abstract The hydrogen absorption properties of U - T h - T i - Z r alloys were investigated for the purpose of developing a new hydride nuclear fuel. The alloy specimens consisted of two phases before hydrogenation, namely a Th phase and a U - T i - Z r phase. The phases in the hydrogenated alloys depended strongly on their history. Upon hydrogenation at low hydrogen pressure above 1073 K, the ThZr2H 6 phase grew fairly well but not enough owing to the formation of the stable ternary hydride in the T i - Z r - H system. This also disturbed the formation of UTi2Hs. Although a homogeneous phase was consequently not achieved, hydrides without disintegration were obtained.

1. Introduction

U-Th mixed fuel has been researched for many years, especially as oxides, carbides and molten salt alloys. Because both Th and U form stable hydrides, they also seemed useful for a mixed fuel. In the TRIGA reactor [1] the use of U-ZrH~ has been shown to reduce the probability of a reactivity accident, called a prompt negative temperature coefficient of reactivity. Further, it was found by the authors [2] that U-Ti alloy forms monolithic UTi2Hs, which has the same crystal structure as T h Z r 2 H 6. In this study the hydrogen absorption properties of several alloys in the UTi2-ThZr2 system were examined in order to develop a new reactor fuel.

2.2. Hydrogen absorption-desorption experiments The hydrogen absorption properties of the prepared alloys were examined at various temperatures between 723 and 1173 K and at various hydrogen pressures below 105 Pa. The hydrogen concentrations of the specimens were calculated from the pressure change in an apparatus of known volume and also from the mass change after the experiments. The desorption properties were examined at 1073 K for the UThTi2Zr2 specimen. X-ray diffraction (XRD), SEM and EDS techniques were used to examine the phases in the specimens.

3. Results and discussion 2. Experimental details

3.1. Hydrogen absorption properties and phases in hydrides

2.1. Alloy preparation

The amount of hydrogen absorbed by the UThTi/Zr/ specimen under various equilibrium conditions is plotted in Fig. 1. Also shown is the temperature dependence of the equilibrium pressure in related M-H systems, where a line labelled MHx-MHy denotes the pressure of the plateau extending between MH~ and MI-Ir and a line labelled MHx indicates the equilibrium pressure of MHx outside any plateau region. These results show that the hydrogen concentration in a specimen depended strongly on the history of pressure and temperature conditions of the specimen. Figure 2 shows the X-ray diffraction patterns of several UThTizZr2 specimens. The ThZrzH7 +~ phase grew most in run 3, where the temperature was the highest of

An alloy specimen with a UTi2:ThZr2 molar composition of 1:1 (written as UThTi2Zr2 hereinafter) was prepared by melting the constituent elements in an argon are furnace. Melting was done five times, with the melt being turned upside down during each interval. Th-rich specimens U T i z ( T h Z r 2 ) ~ (x = 1.9 and 4.8) were also prepared by melting the UThTi2Zr2 alloy together with ThZr2 alloy. The initial phases in the UThTi2Zr2 alloy were identified by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). It consisted of two phases, namely a U-Ti-Zr solid solution and a Th phase containing a little dissolved Zr.

0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSD1 0925-8388(94)04093-J

468

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phase could grow. Furthermore, significant growth of that phase required a temperature higher than 1073 K, because Th and Zr existed in different phases before hydrogenation. A higher temperature of 1173 K was adopted for hydrogenation of the ThZr2-rich alloys UTi2(ThZr2)I.9 and UTi2(ThZr2)4.8. For these alloys the H/M ratios were 1.3 and 1.6 respectively at 105 Pa and 1073 K and 1.7 in both alloys at 105 Pa and 773 K. Figure 3 shows the XRD patterns of polished surfaces of specimens of (a) UThTi2Zr2Hg.7, (b) UTi2(ThZr2)l.9H14.4 and (c) UTiE(ThZr2)4.8H30.3. The fraction of ThZr2H7 phase increased with increasing Th and Zr content. However, other phases such as U, Th4H15 and Zr-Ti-H .were always observed. The ThO2 phase seemed to be formed from ThH2 upon polishing. The phases were also confirmed by SEM and EDS as shown in Fig. 4. The fraction of the bulky grey phase ThZrEH7 increased with increasing Th and Zr content. However, the bright U phase, the dark Zr-Ti-H phase and the Th4H~5 and ThO2 phases, which are slightly lighter than the bulk, never vanished. On the contrary,

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the three runs. As shown in Fig. 1, hydrogen pressures were kept below the plateau pressure of ZrTi-ZrTiH2 till the H content reached UThTi2Zr2H3. 8 and further kept below the plateau pressure of ZrH-ZrH~.4 or Th-ThH2 till the H content reached UThTi2Zr2Hs in both runs 2 and 3. Under these conditions the ThZr2H5

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T. Yamamoto et aL / Hydrogen absorption properties of U-Th-gT-Zr alloys

469

(0) (b) Fig. 4. Backscatteredelectron imagesof hydrogenatedU-Th-Ti-Zr alloysobservedby SEM: (a) UThTi2Zr2Hg.7;(b) UTi2(ThZr2)4.sH30.3. the increase in Th and Zr content made formation of the UTi2H5 phase more difficult. UThTi2ZrzH9.7 contained a small fraction of the grey UTi2H5 phase intercalated into the U phase as shown in Fig. 4(a). UTiE(ThZr2)4.sH30.3 shown in Fig. 4(b) has no UTiEH5 phase, since all the Ti was probably bound tightly in the Ti-Zr-H ternary hydride phase.

3.2. Isothermal desorption experiment Figure 5 shows the desorption pressure-composition isotherm of the 1:1 specimen obtained at 1073 K. There are clearly four distinct plateau regions. Each plateau indicates that two solid phases are involved in the desorption equilibrium. Comparing with the plateau pressure data compiled in Fig. 2 and ref. 6 and considering the phases observed using XRD and SEM, the plateaux can be attributed to the equilibrium 1 05

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4. Conclusions

The hydrogen absorption and desorption properties of three kinds of U-Th-Ti-Zr alloys were examined. The ThZr2H7 phase grew well when the alloy was held at a temperature 1073 K under a sufficiently low hydrogen pressure that neither ThH2 nor ZrH2 could be formed in it. However, owing to the easy formation of Zr-Ti-H phase, a homogeneous hydride was not obtained. Nevertheless, the absence of disintegration and the rather low hydrogen desorption pressure showed that it can be used as a nuclear fuel.

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reactions ThZr2-ThZr2H3, ZrxTil _:ZrxTil_xH, Zr~Til _xH-ZrxTil _~H1.5 and Th-ThH2 from the lowest to the highest respectively. The highest plateau was slightly higher than that for pure ThH2. X-Ray diffraction showed that the crystal lattices of ThH2_/and ThaH15 were slightly larger than those of the pure ones. These facts suggest that Ti or Zr was probably dissolved in the phase. From the obtained properties the UThTi2Zr2 alloy can accommodate 0.93H/M at 1073 K at a hydrogen pressure of 105 Pa, whereas for U-ZrH~ fuel 1.6H/Zr can be held in the hydride. This low capacity is due to the presence of U and Ti which show a high hydrogen desorption pressure. As described above, increasing the ThZr2/UTi2 ratio made the H/M ratio at 1073 K competitive with that of U-ZrHx alloy, since Ti inhibited homogenization rather than promoting it. Further, the alloy never disintegrated during hydrogenation. Therefore, by decreasing Ti, it will be a good material for mixed hydride fuel.

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Fig. 5. Desorption isotherm of UThTi2Zr2H, at 1073 K.

Acknowledgments 6

We would like to thank Dr. Kenji Yamaguchi and Ms. Futaba Ono for valuable discussions and technical support.

470

T. Yamamoto et al. / Hydrogen absorption properties of U-Th-Ti-Zr alloys

References 1 M.T. Simnad, F.C. Foushee and G.B. West, Nucl. Technol., 28 (1976) 31. 2 T. Yamamoto, H. Kayano and M. Yamawaki, Sci. Rep. RITU ,4, 35 (1991) 275. 3 W. Bartscher, J. Rebizant and J.M. Haschke, Z Less-Common Met., 136 (1988) 385.

4 V.Y. Labaton, E.V. Garner and E. Whitehead, J. Inorg. NucL Chem., 24 (1962) 1197. 5 J.W. Ward, in A.J. Freeman and C. Keller (eds.), Handbook on the Physics and Chemistry of the Actinides, Elsevier, Amsterdam, 1985, Chap. 1. 6 N. Pessall and A.D. McQuillan, Trans. Metall. Soc. AIME, 224 (1962) 536.