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Melting Point of Uranium Dioxide

Recommendation

The recommended value for the melting temperature of UO_2.00 is:

Tm = 3120 +/- 30 K (IPTS-68 scale)

This value has been recommended by Rand et al. [1] from their analysis of fourteen experimental studies (over a period of 20 years) of the melting temperature of UO₂. This recommendation of Rand et al. has been accepted by international agreement and was recommended in our assessment of UO₂ properties in 1981 [2,3] and by Harding, Martin, and Potter [4] in their 1989 review of material properties for fast reactor safety.

In their review of experimental measurements on the melting of UO₂, Rand et al. noted that the range in the values for the melting temperature decreased with time. Measurements prior to 1965 were reviewed by Hausner [5]. Measurements since 1965 include measurements by Latta and Fryxell [6], Lyon and Baily [7], and Bates [8]. Measurements have been made using a 'V'-filament method and by thermal arrest methods. The latter method is more reliable since the sample is encapsulated and vaporization is not a problem. Of the thermal arrest data, those of Latta and Fryxell appear to be the best. Their value, 3138 +/- 15 K, agrees within experimental errors with the value reported by Lyon and Baily, 3113 +/- 20 K.

In their 1985 review of experimental data on the melting of irradiated oxide fuels, Adamson et al. [9] comment that the 'V'-filament method appears to give consistently low melting temperatures when applied to variable-stoichiometry oxides such as UO_2+/-x and (U,Pu)O_2+/-x. They attribute the low melting (solidus) temperatures obtained with the 'V'-filament technique, which use small uncontained samples, to pronounced compositional changes that arise from rapid incongruent vaporization and oxygen exchange with the supporting atmosphere (Ar or He) and/or tungsten support. The compositional changes cause changes in the surface emissivity which lead to measurement errors. Adamson et al. [9] comment that measurements made by Bates8 and by Christensen [10,11] on unirradiated samples of stoichiometric UO₂ gave melting temperatures in the range of 3063-3073 K which are approximately 50 K lower than its true melting point. The measurements of Bates [8] and Christensen [10,11] on irradiated UO₂ gave solidus temperature changes from zero to +130 K for low burnup (< 1%) and - 120 K for high burnup (6 to 11%). These data were rejected by Adamson et al. in their assessment because of the unreliability of the 'V'-filament measurements. Adamson et al. conclude that the effect of burnup on the melting behavior is not large. They developed a model for mixed oxide fuel that predicts variations in the solidus as a function of burnup. For burnups up to 10%, the solidus of (U_0.75 Pu_0.25 O₂) is reduced by 22 K.9

In recent experimental measurements of the heat capacity of liquid UO₂ using laser heating of a 0.5 to 0.8 mm diameter UO₂ sphere, Ronchi et al. [12] made several measurements of the freezing temperature of UO₂ on different samples. For specimens in an inert gas atmosphere with up to 0.1-bar of oxygen, they obtained melting points in the interval 3070 +/- 20 K. Higher melting temperatures (3140 +/- 20 K) were obtained for samples in an inert gas atmosphere without oxygen. The variation in melting temperature is in accord with the expected lower oxygen to uranium (O/U) ratio in the latter samples. The O/U ratio of the samples used in these experiments was not determined but the experimenters cannot exclude a slight oxidation up to O/U=2.03.

The melting point of UO₂ given in MATPRO [13] is 3113.15 K. This temperature is based on measurements by Brassfield et al. [14] and the equations for the solidus and liquidus boundaries of the UO₂-PuO₂ phase diagram given by Lyon and Baily [7]. Properties in the MATPRO library are used in the SCDAP/RELAP5 code.

The uncertainty in the recommended temperature of UO₂ is +/-1% (1s) The experimental results of Latta and Fryxell [6] and of Lyon and Baily [7] are well within this uncertainty.

1. J. K. Fink, M. G. Chasanov, and L. Leibowitz, Properties for Reactor Safety Analysis, ANL-CEN-RSD-80-3, Argonne National Laboratory Report (1981).

2. J. K. Fink, M. G. Chasanov, and L. Leibowitz, Thermophysical Properties of Uranium Dioxide, J. Nucl. Mater. 102 17-25 (1981).

3. M. H. Rand, R. J. Ackermann, F. Gronvold, F. L. Oetting, A. Pattoret, Rev. Int. Des Hautes Temperatures et des Refractories 15, 355 (1978).

4. J. H. Harding, D. G. Martin, and P. E. Potter, Thermophysical and Thermochemical Properties of Fast Reactor Materials," Commission of European Communities Report EUR 12402 (1989).

5. A. Hausner, J. Nucl. Mater. 15, 179 (1965).

6. R. E. Latta and R. E. Fryxell, J. Nucl. Mater. 35, 195 (1970).

7. W. L. Lyon and W. E. Bailey, The Solid-Liquid Phase Diagram for the UO₂-PuO2 System, J. Nucl. Mater. 22, 332 (1967).

8. J. L. Bates, J. Nucl. Mater. 36, 234 (1970).

9. M. G. Adamson, E. A. Aitken, and R. W. Caputi, Experimental and Thermodynamic Evaluation of the Melting Behavior of Irradiated Oxide Fuels, J. Nucl. Mater. 130, 349-365 (1985).

10. J. A. Christensen, R. J. Allio, and A. Biancheria, Trans. American Nuclear Society 7, 390 (1964); also United States Report WCAP-6065 (1965).

11. J. A. Christensen, Irradiation Effects on Uranium Dioxide Melting, United States Report HW-69234 (1964).

12. C. Ronchi, J. P. Hiernaut, R. Selfslag, and G. J. Hyland, Laboratory Measuremnt of the Heat Capacity of Urania up to 8000 K: I. Experiment, Nuclear Science and Engineering 113, 1-19 (1993).

13. J. K. Hohorst (Editor), SCDAP/RELAP5/MOD2 Code Manual, Vol 4: MATPRO- A Library of Materials Properties for Light-Water-Reactor Accident Analysis," Section 2.1 Melting Temperature, NUREG/CR-5273 (1990).

14. H. C. Brassfield et al., Recommended Property and Reactor Kinetics Data for Use in Evaluating a Light-Water-Coolant Reactor Loss-of-Coolant Incident Involving Zircaloy-4 or 304-SS-Clad UO₂, GEMP-482 (April 1968).