Oxygen potentials of PuO 2-x
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Oxygen potentials of PuO2-x Akira Komeno1, Masato Kato1, Shun Hirooka1, Takeo Sunaoshi2 1 Advanced Nuclear System Research and Development Directorate, Japan Atomic Energy Agency, 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan 2 Inspection Development Company, 4-33 Muramatsu, Tokai-Mura, Ibaraki 319-1194, Japan ABSTRACT Oxygen potentials of PuO2-x were measured at temperatures of 1473 - 1873 K by thermogravimetry. The oxygen potentials were determined by in situ analysis as functions of oxygen-tometal ratio and temperature. The measurement data were analyzed on the basis of defect chemistry and an approximate equation was derived to represent the relationship among temperature, oxygen partial pressure, and deviation x in PuO2-x. INTRODUCTION It is well known that actinide dioxides such as PuO2, UO2 and (U,Pu)O2 with a fluorite structure, are non-stoichiometric oxides [1-3]. Their stoichiometry significantly affects various physical properties, and the effect of the oxygen-to-metal (O/M) ratio on physical properties has been investigated in them [3-5]. The O/M ratio is determined by thermodynamics data, specifically the oxygen potential ΔG o which is obtained by 2
ΔG O = RT ln Po (1) where R is the gas constant, T is absolute temperature and Po is oxygen partial pressure. Many studies on ΔG o of these oxides have been carried out and ΔG o values have been measured as parameters of temperature, O/M ratio and Pu content [6-15]. Data on PuO2-x are essential to represent thermodynamic data on (U,Pu)O2 ± x which is used for nuclear fuel, but the former data are limited. Data on PuO2-x were measured by four groups [16-19] and are shown in Fig.1 in the 2
2
2
2
2
Oxygen potential (kJ mol-1)
-350 -400 -450
Ref.[19] 1418K Ref.[19] 1318 K Ref.[17] 1370 K Ref.[18] 1368 - 1375 K Ref.[16] 1373 K
-500 -550 -600 -650 1.60
1.70
1.80 O/M (-)
1.90
2.00
Figure 1. Comparison of oxygen potential in PuO2-x.
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temperature range of 1318 K - 1418 K. The figure shows that Woodley’s [16] and Swanson’s [17] data are almost the same. In the low O/M region, the data of Markin et al. [18] are consistent with those of Woodley [16]. The data reported by Atlas and Schlehman [19], however, are scattered in a range of about ± 50 kJ/mol especially in the near stoichiometric region and they are higher than those of [16] and [18]. In this work, the oxygen potential data of PuO2-x were measured by thermo-gravimetry and potential was determined as functions of temperature and O/M ratio. EXPERIMENT Specimen preparation Plutonium oxide powder was dissolved in nitric acid solution, and small amount of americium contained in the oxide was removed by a cation exchange method. The acid solution eluant was evaporated to dryness, and plutonium oxide (purity of 99.999% estimated by calculation of separation capacity) was obtained by heat treatment in air. The oxide was pressed and sintered at 1973 K in air into a pellet, and then the pellet was sliced into a disc about 1mm thick as a measurement specimen. The specimen was annealed at
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