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 PO2PO2 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 PO2PO2. The analysis results showed that x   was proportional to PO2±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, ΔH¯O2 and ΔS¯O2, 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.