Oxygen potentials of Am and Np-bearing MOX

Hirooka, Shun ; Matsumoto, Taku; Kato, Masato   ; Sunaoshi, Takeo*

Oxygen potentials of (UPuAmNp)O pellet sintered in the atmosphere of Ar/H/HO mixture gas were measured at 1600C and 1650C by the gas equilibrium method. The oxygen partial pressure of the atmosphere was accurately adjusted by changing HO content in the gas and monitored with oxygen sensors. Simultaneously, weight change originated by oxidation or reduction was measured by thermo-gravimetry method and O/M was calculated by the weight change. The measured data showed that the substitution of minor actinide with U increased the oxygen potential, indicating that O/M could be lowered by adding minor actinide if temperature and oxygen partial pressure of the surrounding gas was preserved. The present data was analyzed together with the previous Am-MOX data and led to the results that containing 1% of Am is equivalent to 4% of Pu on the effect of oxygen potential. Similarly, 1% of Np is equivalent to 0.2% of Pu. These effects were reflected to the defect formation energies and the relation among O/M, temperature, content ratio and oxygen partial pressure was derived. This relation can evaluate O/M of MA-MOX more accurately and, therefore, is the useful fuel technology for fuel design and evaluating irradiation behaviors.