Analysis of oxygen potential of (U0.7Pu0.3)O2±x and (U0.8Pu0.2)O2±x based on point defect chemistry

Stoichiometries in (U0.7Pu0.3)O2±x and (U0.8Pu0.2)O2±x were analyzed with the experimental data of oxygen potential based on point defect chemistry. The relationship between the deviation x of stoichiometric composition and the oxygen partial pressure P O 2 was evaluated using a Kröger–Vink diagram. The concentrations of the point defects in uranium and plutonium mixed oxide (MOX) were estimated from the measurement data of oxygen potentials as functions of temperature and P O 2 . The analysis results showed that x was proportional to P O 2 ± 1 / 2 near the stoichiometric region of both (U0.7Pu0.3)O2±x and (U0.8Pu0.2)O2±x, which suggested that intrinsic ionization was the dominant defect. A model to calculate oxygen potential was derived and it represented the experimental data accurately. Further, the model estimated the thermodynamic data, Δ H ¯ O 2 and Δ S ¯ O 2 , of stoichiometric (U0.7Pu0.3)O2.00 and (U0.8Pu0.2)O2.00 as −552.5 kJ·mol−1 and −149.7 J·mol−1, and −674.0 kJ · mol-1 and −219.4 J · mol−1, respectively.