Research and Development of Advanced Nuclear System

2-6Controlling Oxygen Content of U–Pu Mixed Oxide Fuel
－ Oxygen Potential Measurement and Application to Fuel Technologies －2.05

Temperature

at 1600 ℃
2.00

-300

-400

40％Pu

-600
1.80

333
100
22.7
1.95

PuO2-500

1500

PH2 /PH2 O30％Pu

1.85

20％Pu

1.90

1.95

1000

10％Pu

2.00

2.05

1.90

MOX pellet
02468

Temperature (℃)

-200

2000

O/M ratio

Oxygen Potential (kJ/mol)

-100

500

Time (h)O/M ratio
Fig.2-13 Oxygen potential of MOX at 1600 °C as functions ofO/M ratio and Pu content
The O/M ratio decreases with decreasing oxygen potential
and increasing Pu content. The O/M ratio can be controlled bycontrolling the oxygen potential and temperature.

Fig.2-14 O/M change of (U0.7Pu0.3)O2-x during heat treatment
The O/M ratio of MOX changes with PH2 /PH2O ratio. The O/M
ratio reaches 1.94 at 1650 °C in atmosphere with PH2 /PH2O = 333.
Photograph shows MOX pellet set in the measurement system.

Uranium–plutonium mixed oxide (MOX) has been used as amoisture was used for oxygen potential control. In this gas

fuel for sodium-cooled fast reactors; it has a fluorite structure

system, the oxygen potential is determined by the reaction

with an oxygen-to-metal (O/M) ratio of 2.00. UO2 can take

H2O⇔H2+1/2O2 and controlled by adjusting the PH2/PH2O

extra oxygen and exist in a hyper-stoichiometric composition

ratio. Many data (more than 1000 points) were obtained andwith a range of O/M > 2.00. Pu atoms are substituted for U

evaluated, and an equation representing the oxygen potential

atoms, and a MOX solid solution is formed. On the otherwas derived as a function of the O/M ratio, temperature, andhand, MOX can exist in a hypo-stoichiometric region ofPu content. On the basis of this equation, a technique for

O/M < 2.00 by forming oxygen vacancies. Changes in theevaluating the O/M ratio was developed.

O/M ratio significantly affect various properties of MOX,

Fig.2-13 shows the relationship between the oxygen

such as the thermal conductivity. Therefore, it is important topotential and the O/M ratio at 1600 °C. The oxygen

control the O/M ratio to develop fuel technologies.

potential increases with increasing Pu content. This

It is necessary to know the relationship among the O/M ratio,

technology for evaluating the oxygen potential has been

temperature, and oxygen potential to develop O/M control

applied to the fuel pellet fabrication process and irradiation

techniques. In this work, the oxygen potential (ΔGO2) of MOX

behavior analysis. The O/M change in (U0.7Pu0.3)O2-x during

was measured by the gas equilibrium method. The oxygen

heat treatment is plotted as a function of the PH2/PH2O ratiopotential is a quantity used in thermodynamics to describe thein Fig.2-14. The figure shows that the PH2/PH2O ratio affects

oxygen chemical stability in MOX and can be obtained using

the variation in O/M. Further, the chemical stability ofeq.(1).

fission products inside irradiated pellets can be evaluatedΔGO2 = RT lnPO2 ,⋅⋅⋅⋅⋅ (1)using this technology.

where R is the gas constant. A gas mixture of Ar/H2 to add
Reference
Kato, M., Oxygen Potentials and Defect Chemistry in Nonstoichiometric (U,Pu)O 2, Stoichiometry and Materials Science−When Numbers
Matter, 2012, p.203-218.
JAEA R&D Review 201339