20.08.2015   Thoria-based Nuclear Fuels

UO2

The melting point of UO2 given in MATPRO [28] is 3,113.15 K. This temperature is based on the measurement made by Brassfield et al. [29] and the equations for the solids and liquids boundaries of the UO2-PuO2 phase diagram given by Lyon and Bailey [30]. The recommended values by ORNL [31] for UO200 are

3.120 ± 30 K and for PuO2 is 2,701 ± 35 K. This value for UO2 has been recommended by Rand et al. [32] from their analysis of 14 experimental studies of the melting temperature of UO2. This recommendation of Rand et al. [32] was accepted internationally and was recommended in the assessment of UO2 prop­erties by Harding et al. [33] in their 1989 review of material properties for fast reactor safety. The melting point of UO2, according to Latta et al. [34] is 3,138 ± 15 K, which is considerably higher than that of Christensen (3,073 K) [35]. Belle and Burman [12] recommended the melting temperatures of UO2 as

3.120 K, with an error of probably ±30 K.

In recent experimental measurements of the heat capacity of liquid UO2 using laser heating of a UO2 sphere, Ronchi et al. [36] 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 3,070 ± 20 K. Higher melting temperatures (3,140 ± 20 K) were obtained for samples in an inert gas atmosphere without oxygen. The variation in melting temperature is in accordance with the expected lower oxygen-to-uranium (O/U) ratio in the latter samples. The melting point of UO2 drops on variation O/M ratio around stoichiometry: for example, if the melting point of stoichiometric UO2 is 3,138 K, its value drops to 2,698 K at an O/U ratio of 1.68 and to 2,773 K at an O/U ratio of 2.25. The effect of irradiation is lowering of the melting point of UO2. At a burnup of 1.5 x 1021 fissions/cm3, it has been reported [12] that the melting point drops to 2,893 K. Typical values of melting points for UO2 and PuO2 obtained by various authors are given in Table 2.

Table 2 Melting point of UO2 and PuO2 determined by various authors [31]

UO2 (in K)

PuO2 (in K)

Remarks

Lyon and Baily [30]

3,113 ± 20

2,663 ± 20

Aitkens and Evans [146]

3,120 ± 30

2,718 ± 35

Latta et al. [34]

3,138 ± 15

Adamson et al. [147]

3,120 ± 30

2,701 ± 35

BU: 0.4 MWD/kg

MATPRO [148]

3,113.15

2,647

BU: 3.2 MWD/kg

Komatsu [149]

3,138 ± 15

2,718

BU: 0.7 MWD/kg

Rand et al. [32]

3,120 ± 30

Ronchi et al. [36]

3,075 ± 30

Christensen [37]

3,073

Belle and Berman [12]

3,120 ± 30

Belle and Berman [12]

2,893

BU: 1.5 x 1021 fissions/cm3

BU burnup