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52 FIG. 4. Average UO2 absorption coefficient from measurements of Bober et al. , Determination of the optical constants of liquid UO2 from reflectivity measurements, Proc. Eighth Symp. on Thermophysical Properties, Gaithersburg, MD, 1981, Vol II, pp. 234-244, ASME (1982). , Bestimmung der optischen Konstanten von geschmolzenen Kernbrennstoffen, J Nucl. Mater. 124, 120-128 (1984). , Study of the spectral reflectivity and emisssivity of liquid ceramics, High Temp. - High Pressures 12, 161-168 (1980).

Figure 5 shows that the data of Engel [13] (300 to 1000 K) appear to have a systematic error because they differ from other data by a normalization. Thus the data of Engel have not been included in the combined analysis. The variances for the data of Popov et al. and that of Engel from a smoothed curve through all the data are about a factor of 20 higher than the variances for data included in this analysis. A combined fit of the enthalpy and heat capacity data [1-9], which are listed in Table 5, has been made using a nonlinear weighted χ2 minimization procedure.

In his thorough data assessment, Martin examined the macroscopic expansion data for possible zero errors and made corrections to the data of Lambertson and Hanwerk [6], the data of Brett and Russell [9], and the data of Murray and Thackery [10]. He found good agreement between the data from macroscopic length changes and lattice parameter measurements so that these data could be combined in the final analysis. The good agreement between data from macroscopic measurements by Conway et al. and the lattice parameter measurements of Hutchings [2] led Martin to conclude that at least up to 2523 K, the contribution to the macroscopic expansion due to Schottky defects is negligible.