Analysis of oxygen potential of (U 0.7Pu 0.3)O 2±x and (U 0.8Pu 0.2)O 2±x based on point defect chemistry

Abstract

Stoichiometries in (U 0.7Pu 0.3)O 2± x and (U 0.8Pu 0.2)O 2± 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 (U 0.7Pu 0.3)O 2± x and (U 0.8Pu 0.2)O 2± 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 (U 0.7Pu 0.3)O 2.00 and (U 0.8Pu 0.2)O 2.00 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.