In the system Pu-O, the partial pressures of the gaseous species Pu(g), PuO(g), and PuO2(g) vary with the O/Pu ratio and temperature as a large composition range exists for PuO2 _ x. The variation of the gaseous species calculated at 1970 K is shown in Figure 26(a) as a function of the O/Pu ratio using the CALPHAD model developed by Gueneau eta/.28 In the two-phase region [Pu2O3 + PuO2 _ x], the major species are PuO(g), Pu(g), and PuO2(g). With increasing O/Pu ratio in PuO2 _ x phase, the partial pressures of PuO(g) and Pu(g) decrease whereas PuO2(g) partial pressure slightly increases. Close to the stoichiometry, O(g) and O2(g) become the major gaseous species. The curve giving the total pressure of the gas as a function of the O/Pu ratio shows a minimum that corresponds to the congruent vapori­zation of plutonia occurring for a slightly hypostoi- chiometric oxygen composition.

In the U-O system, the vaporization is more com­plex due to the existence of both the hypo — and hyper­stoichiometric composition ranges of uranium dioxide. According to the measurements by Pattoret213 using mass spectrometry, reported in Figure 26(b), a sample with a composition UO2 _ x heated at 2250 K will tend to lose preferentially U via UO(g) to reach the

congruent composition UO2 _ x with a O/U ratio equal to 1.987 ± 0.01 corresponding to the minimum in total pressure, that is, the congruency. A sample with a com­position UO2 or UO2 + x will lose oxygen via UO3(g) to reach the same congruent composition. Above solid UO2, the largest contribution is from UO2(g).

2.02.5.1 U-Pu-O

The partial pressures of the different gaseous actinide oxide molecules were calculated by Rand and Markin52 over U0 85Pu0.15O2 ± x at 2000 K using the thermodynamic data on the solid and gas phases (Figure 27). UO3(g) is the predominant gas species in the hyperstoichiometric region. This is due to the fact that in the MOX, the oxygen potentials are higher than in UO2 (see Section 2.02.4.2.2). The composi­tion of the vapor is enriched in uranium and oxygen in comparison to the solid. It means that the solid will lose uranium and oxygen. In the hypostoichiometric region, the uranium species are less in the vapor than in the solid for O/metal ratio below 1.96. It means that the solid will preferentially lose plutonium. A review of the previous experimental studies on vaporization of (U, Pu)O2 oxides was reported by Viswanathan and Krishnnaiah.214 The authors derived a thermochemi­cal model to calculate the partial and total pressures

Figure 27 Calculated partial pressures over Uo.85Puo.15O2 ±x at 2000 K according to Rand and Markin.52

above MOX with up to 40% mol. PuO2. The different studies show that a quasi-congruent vaporization is reached where (O/Metal)vapor=(O/Metal)solid which corresponds to a slightly hypostoichiometric mixed oxide in oxygen like in the binary oxides UO2 and PuO2.

2.02.5.2 U-Pu-Am-O

Calculations ofthe same type were recently performed by Maeda eta/.215 above a mixed oxide with the compo­sition (U0.69Pu0.29Am0.02)O2 ±x at 2073 and 2273 K (see Figure 28). The results show that in the hyperstoichio­metric region, UO3(g) remains the predominant gas species. But for O/metal ratio below 1.96 corresponding to the congruent composition, the AmO(g) species becomes the majormolecule in the vapor. Very recently, an experimental study on vaporization of AmO2 and (Pu, Am)O2 oxides was performed by Gotcu-Freis

216,42

et a/. using mass spectrometry.