Review
A review of the thermophysical properties of MOX and UO2 fuels

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Abstract

A critical review of the thermophysical properties of UO2 and MOX fuels has been completed, and the best correlations for thermophysical properties have been selected. The properties reviewed are solidus and liquidus temperatures of the uranium/plutonium dioxide system (melting and solidification temperatures), thermal expansion and density, enthalpy and specific heat, enthalpy (or heat) of fusion, and thermal conductivity. Only fuel properties have been reviewed. The selected set of property correlations was compiled to be used in thermal-hydraulic codes to perform safety calculations.

Introduction

The excess weapon-grade plutonium available in the Russian Federation (RF) and the United States from the reduction in strategic nuclear weapons will be mainly disposed of by burning it as mixed-oxide (MOX) fuel in existing reactors. These reactors will be primarily VVER-1000s in the Russian Federation and pressurized-water reactors (PWRs) in the United States.

Safety analyses are required by the respective national regulatory bodies to prove that MOX fuel can be burned safely in these reactors. These safety analyses require calculations with safety codes that need the appropriate thermophysical properties of the MOX fuel.

The MOX fuel contains between 3% and 5% PuO2 blended with natural or depleted uranium dioxide (in the proportion of 95–97%). The PuO2 replaces the enriched fraction of 235U oxide in the regular UO2 fuel. Uranium and plutonium dioxides are isostructural, and they form solid solutions. The small fraction of PuO2 in the MOX fuel will change the thermophysical properties of MOX fuel slightly when compared to regular UO2 fuel. Nevertheless, appropriate thermal properties for the MOX fuel need to be selected.

There is a considerable body of data and correlations in the open literature covering thermal properties of UO2. An excellent review by Fink in 2000 presents the best correlations for UO2 fuel [1]. The work of Fink is also available at the International Nuclear Safety Center (INSC) database of Argonne National Laboratory on the World-Wide Web [2].

However, for MOX and PuO2 fuels the available data are not extensive, and no recent reviews are available. An open-literature review of MOX and PuO2 properties was completed in 1982, also by Fink [3]. There is a report on thermophysical properties [4] prepared in 1997 by an International Working Group for the International Atomic Energy Agency (IAEA) compiling available data and correlations for materials of water-cooled reactors (both light and heavy water). It considers UO2, MOX, and other fuels (e.g., uranium metal and uranium–aluminum alloys) and also cladding, absorbers, and structural materials. The report lists available correlations and data without evaluating their accuracy or merits and without making recommendations. The data covered in the report have been compiled and stored in a computer database named THERSYST, developed by the Institute for Nuclear Technology and Energy Systems (IKE) at the University of Stuttgart, Germany.

PuO2 or MOX properties are not available on the INSC database [2] except for the solidus and liquidus temperatures of the uranium/plutonium dioxide system. Therefore, there is a need to search for the available MOX thermophysical properties, to complete a critical review of the properties and compare them with UO2 properties, and to select the best sets of properties/correlations to be used by codes to perform safety calculations.

In this paper, a comprehensive review of the open literature on fuel properties has been completed with emphasis on MOX fuel. The PuO2 and UO2 fuel properties were also reviewed and compared to MOX properties. The PuO2 properties were studied when they were required to calculate MOX properties. This is the case for most properties that follow the law of mixtures. Finally, the best correlations for both MOX and UO2 fuel have been selected.

The properties reviewed are:

  • 1.

    solidus and liquidus temperatures of the uranium/plutonium dioxide system (melting and solidification temperatures),

  • 2.

    thermal expansion and density,

  • 3.

    enthalpy and specific heat,

  • 4.

    enthalpy (or heat) of fusion,

  • 5.

    thermal conductivity.

Only fuel properties are covered in this work.

For each property, a review of the available data and correlations is presented followed by recommendations for the best values of the property and/or the best correlations. Also, the variables that influence the property are described. Variables considered are: fuel composition (if UO2 or MOX, and in the case of MOX, PuO2 mole fraction, y), temperature (T), porosity (p) or fraction of the theoretical density (% TD), burnup (B), and oxygen-to-metal (O/M) ratio or deviation from stoichiometry (x=2−O/M). Gadolinium in the fuel has not been considered, but some correlations consider it as part of the fuel composition. It is anticipated that these variables can accommodate fuel property variations caused by different fuel manufacturing processes, including different grain sizes and fuel microstructures.

Section snippets

Review of available data and correlations

There is a large amount of data on the melting temperature of fuels. Table 1 presents values in chronological order reported since 1960. The UO2 fuel melts at a higher temperature than PuO2, with values for UO2 ranging from 3003 K [6] to 3149 K [8]. Melting temperatures for PuO2 range from 2511 K [7] to 2718 K [11], [17]. The UO2–PuO2 system follows the solid–liquid phase diagram of Fig. 1. MOX fuel melts at a temperature between that of pure UO2 and PuO2.

The data also show that burnup and/or

Review of available data and correlations

Table 2 shows the relevant references and data for thermal expansion and density of fuel. The UO2,PuO2, and MOX fuels have very similar thermal expansions [21], [22] as shown in Fig. 4. MATPRO [18] employs a different linear thermal expansion coefficient for UO2 than [21], [22], as shown in Fig. 5. The density of PuO2 is higher than the density of UO2. Density values in Table 2 are for fully dense fuel with zero porosity or 100% theoretical density (TD) at 273 K.

Recommendation

The UO2, PuO2, and MOX fuels

Review of available data and correlations

Table 3 shows in chronological order some of the available data and correlations on enthalpy and heat capacity for UO2, PuO2, and MOX. Because of the large amount of UO2 data, only selected data are presented in the table. The majority of the UO2 data can be found in the data sources employed in developing the different UO2 correlations.

We start with the review of the UO2 correlations presented in Table 3. In 1972 Kerrisk and Clifton [31] developed correlations for enthalpy and heat capacity

Review of available data

Table 7 shows in chronological order the available data on the heat of fusion of UO2, MOX, and PuO2 fuels. Few experimental data have been found for UO2, only one value for MOX, and no experimental values for PuO2. The reported values for PuO2 are all calculated from equations. Epstein [48] has two different calculations for the heats of fusion of UO2 and PuO2. His calculated heats of fusion for PuO2 are lower than for UO2. However, Adamson calculated higher heats of fusion for PuO2 than for UO2

Review of available data and correlations

The thermal conductivity is a property that does not follow the law of mixtures. Therefore, PuO2 values of thermal conductivity were not reviewed because they cannot be used to calculate the thermal conductivity of MOX.

There is a large body of data and correlations on thermal conductivity of UO2 fuel. The data are not so extensive for MOX fuel. Table 8 shows in chronological order the most relevant references that cover mainly correlations and reviews of other works.

The data reviewed show that

Conclusions

A comprehensive review of thermophysical properties of fuels, with emphasis on MOX fuel, has been completed. The properties reviewed are solidus and liquidus temperatures of the uranium/plutonium dioxide system, thermal expansion and density, enthalpy and specific heat, enthalpy of fusion, and thermal conductivity. Available properties for UO2 and MOX fuels have been analyzed and compared, and the best set of correlations for both UO2 and MOX fuels has been selected. These correlations can be

Acknowledgements

This paper has been a collaborative effort of the `Kurchatov Institute', Russian Federation, and Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725.

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