20.08.2015   Thoria-based Nuclear Fuels

Density of Liquid UO2

The recommended equation for the density of liquid uranium dioxide is based on the in-pile measurements of the vapor pressure, density, and isothermal com­pressibility of liquid (U, Pu)O2 by Breitung and Reil [62]. Measurements of density as a function of enthalpy and as a function of temperature were obtained from the melting point to 7,600 K. The equation of Breitung and Reil for the

Table 6 Densities of ThO2 and UO2 as a function of temperature [12]

Temperature (K)

ThO2 (g/cm3)

U 2 (g/cm3)

298

10.00

10.96

473

9.95

10.90

673

9.89

10.83

873

9.83

10.76

1,073

9.78

10.69

1,273

9.72

10.62

1,473

9.66

10.55

1,673

9.60

10.48

density of UO2 and (U, Pu)O2 for mole fractions of Pu < 0.25 is in good agree­ment with the equation for the density of UO2 from experiments by Drotning [63], which had been recommended in the 1981 assessment by Fink et al. [64].

The recommended equation for the density of UO2 as a function of temperature is:

p = 8.860 — 9.285 • 10-4 • (T — 3120), (16)

where, density (p) is in g/cm3 and temperature (T) is in K.

No data exists for the volume change of ThO2 on melting, but some information is available on UO2. Christensen [65] measured the density of UO2 between 1,553 and 3,373 K by high-temperature radiography. The density of solid and liquid at 3,073 K were 9.67 and 8.74 g/cm3, respectively. The accepted value of the density of liquid UO2 at the melting point is 8.74 ± 0.016 g/cm3. The volume increase on melting was 10.6 %.

Finally, the burnup also affects the density by the change in the porosity. At low burnup (<15 GWd/t), density increases by the fuel densification process; at the higher burnup, density decreases (porosity increases) due to the fuel swelling [66].