Relative oxygen potential measurements of (U,Pu)O2 with Pu = 0.45 and 0.68 and related defect formation energy

Using a thermogravimetric analyzer and an oxygen sensor, relative oxygen potentials of mixed oxide (MOX) fuels with Pu contents of 45% and 68% were measured at 1473 K – 1873 K; the relative oxygen potentials provide the relation among the equilibrium oxygen-to-metal (O/M) ratio, temperature, and oxygen partial pressure of the flowing gas. To evaluate the effect of the Pu content on the relative oxygen potential of MOX and related defect formation energy, this work aims to bridge the gap in the results available in existing reports, where Pu contents of 0%-46% and 100% have been studied [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The measured data indicate that the trend of increasing relative oxygen potential with increasing Pu content, which can occur as a consequence of more Pu cations changing from being tetravalent to trivalent, reaches a saturation at a Pu content of 68%, in particular in the region of O/M < 1.96. As the formation of defects related to oxygen results in oxidation and reduction of nonstoichiometric MOX and the intrinsic ionization is the dominant defect formation process, the relation between the equilibrium O/M ratio and the oxygen partial pressure was well reproduced by introducing the equilibrium constants of the defect formation process. The highest concentration of electron and hole defects caused by the intrinsic ionization was observed in MOX with a Pu content of 68%, and the related properties, such as electrical conductivity and specific heat, also exhibited the highest values upon increasing the defect concentration at high temperature.