Kinetics of precipitation of U4O9 from hyperstoichiometric UO2+x

doi:10.1016/j.jnucmat.2007.03.054

Journal of Nuclear Materials

Volume 366, Issue 3, 1 July 2007, Pages 297-305

https://doi.org/10.1016/j.jnucmat.2007.03.054 Get rights and content

Abstract

For safe and reliable operation of fission reactors in space, the phase diagrams and reaction kinetics of systems used as nuclear fuels, such as U–O, U–N, U–C, are required. Diffraction allows identification of phases and their weight fractions as a function of temperature in situ, with a time resolution of the order of minutes. In this paper, we will provide results from a neutron diffraction experiment studying the U–O system. Using the neutron diffractometer HIPPO, the decomposition of UO_2+x into UO₂ and U₄O₉ as a function of temperature was investigated in situ. From the diffraction data, the participating phases could be identified as UO_2+x, UO₂ and U₄O_8.94 and no stoichiometric U₄O₉ was found. Results of the experiment were used to improve existing thermodynamic models. The presented techniques (i.e., neutron diffraction and thermodynamic modeling) are also applicable to the other systems mentioned above.

Introduction

Uranium-based fuels have been used for space reactor applications for many years. In particular, uranium dioxide (UO₂) has played an important role as fuel for the TOPAZ space reactors. Therefore, a sound understanding of the U–O system is required for the design, operation and disposal of reactor fuels for space applications.

Oxidation of UO₂ (due to a reaction with air or steam, for example) may result in the formation of higher oxides, such as U₄O₉, U₃O₇ and U₃O₈[1] depending on the temperature and oxygen partial pressure. The change in density, crystal structure and chemical behaviour accompanying the introduction of these higher phases may have adverse effects on the mechanical and chemical performance of the fuel. Therefore, it is important to understand the stability of the various phases present.

Many investigators have studied the U–O system, most recently summarized by Guéneau et al. [2], Chevalier et al. [3] and Lewis et al. [4] as UO₂ has traditionally been a popular fuel for both military and commercial water-cooled power reactors. Therefore, the U–O phase diagram is fairly well established. There still exists, however, some uncertainty around the non-stoichiometry of U₄O₉. Recent attempts to model the U–O system [3] approximate U₄O₉ as stoichiometric. Many investigators, however, have shown that U₄O₉ is a narrowly non-stoichiometric phase (U₄O_9−y). Experiments have shown that three phases exist: α-U₄O_9−y (below ∼80 °C), β-U₄O_9−y (between ∼80 °C and ∼550 °C) and γ-U₄O_9−y (above ∼550 °C) [5]. Fig. 1 shows a summary of the experimental data [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] on the location of the phase boundaries for U₄O_9−y as well as the proposed fields for the three phases of U₄O_9−y overlaid on the current thermodynamic treatment for this system [20]. In addition, the kinetics of precipitation of U₄O₉ from UO_2+x are not well known, only that quenching of UO_2+x is required to prevent the formation of U₄O₉[6], [19].

Section snippets

Description of experiment

To investigate the non-stoichiometry in U₄O₉ and the kinetics of precipitation of U₄O₉ from UO_2+x, an in situ neutron diffraction experiment was performed using the high-pressure preferred orientation (HIPPO) neutron diffractometer [21], [22] at the Los Alamos Neutron Science Center (Los Alamos National Laboratory, New Mexico).

The reason neutron diffraction is used instead of X-ray diffraction (XRD) for the study of the U–O system is due to the much deeper penetrating depth into UO₂ with

Data analysis

Rietveld refinement [23] of the diffraction patterns was performed using the General Structure Analysis System (GSAS) software [24]. Diffuse scattering parameters were included to account for the silica glass liner (container). The crystal structures used in the refinement were stoichiometric UO₂ space group Fm3m from Hutchings [25] and UO_2.234 space group I $\bar{4}$ 3d from Kim et al. [26]. The observed patterns also matched the I $\bar{4}$ 3d structure reported by Copper and Wills [27]. Many other crystal

Discussion of results

One result from this experiment of particular interest is the weight fraction of UO₂ at the beginning of the experiment (Fig. 5) and its accompanying lattice parameter (Fig. 6). On the initial heat up of the sample, the UO₂ phase weight fraction is much higher (∼100% instead of ∼30%) and its lattice constant is lower than would be expected from the phase diagram and published lattice parameter data for stoichiometric UO₂ (this can also visually seen from the patterns in Fig. 3). These results

Conclusions

The excellent agreement of refined lattice parameters and weight fractions from the neutron diffraction experiment with published values has demonstrated that time-of-flight neutron diffraction can be a very useful tool for studies of the U–O system, even at high temperatures. The neutron diffraction experiment has demonstrated that, at approximately 300 °C, a powder sample of UO_2+x will reach thermodynamic equilibrium in less than 1 h and U₄O_9−y can begin to precipitate at only 100 °C. In

Acknowledgements

The authors would like to acknowledge the helpful discussions with S. White (RMC) and experimental support of D. Williams and O. Gourdon (LANL). The sample for this study was provided by J. Dunwoody (LANL). The work was funded in part by the National Science and Engineering Research Council of Canada (NSERC).

This work has benefited from the use of the Los Alamos Neutron Science Center at the Los Alamos National Laboratory. This facility is funded by the US Department of Energy under Contract

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Kinetics of precipitation of U4O9 from hyperstoichiometric UO2+x

Abstract

Introduction

Section snippets

Description of experiment

Data analysis

Discussion of results

Conclusions

Acknowledgements

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Inorg. Nucl. Chem.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

Solid State Commun.

Nucl. Instrum. and Meth. A

J. Nucl. Mater.

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Kinetics of precipitation of U₄O₉ from hyperstoichiometric UO_2+x