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Defect Structure of Nonstoichiometric Plutonium Oxide
Nuclear fuel systems typically require decades tooptimize, since the assessment of behavior of nuclear fuel ismade based on empirical data and irradiation testing. As ageneral trend, it is tried to assess them on basic scientificunderstanding. In this work, an oxygen defect of plutoniumoxide has been studied by the first principle calculations,which predict the electron structure of materials without
experimental data.
Plutonium, which is generated in an operating nuclear
reactor due to transmutation of uranium, is one of importantmaterials as nuclear fuel. Plutonium is mainly used in thenuclear reactor in the chemical form of oxide. Plutoniumoxide has a wide range of non-stoichiometry, determined bytemperature and oxygen partial pressure. Non-stoichiometricplutonium oxide is represented by PuO2-x, where x is nonstoichiometry. The oxygen partial pressure is a key parameter,
which controls chemical reactions in the fuel pin.
It is very important to develop a model, which is able topredict well the correlation between the non-stoichiometry ofPuO2-x and the partial pressure of oxygen. In the design ofnuclear fuel, the model based on defect chemistry has beenused for calculation of oxygen potential. The defect chemistrymodel is essentially empirical one, since the parameters in
the defect chemistry model are adjusted by the experimentaldata. In this study, the first principle calculations were used tocheck the validity of the defect chemistry model.

Fig.1 Supercel for plutonium oxide with oxygen defect. Redand white spheres indicate plutonium and oxygen atoms,respectively.
The total energy and the electronic structure of PuO2-x
have been calculated by PAW method within the GGA. We
also used the DFT+U methodology to check localization off-electron. Figure 1 shows a supercell of 96 atoms containing
32 plutonium atoms, 63 oxygen atoms and one oxygen
vacancy. The calculations were performed with the HITACHI/
SR-8000 in Tohoku University. The calculated lattice
parameter and the enthalpy of formation of plutonium dioxide
are in good agreement with the experimental values. The40

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vacancy formation energy, which was defined as the energy
required for extraction of an oxygen atom from the perfectlattice, was calculated to be 4.20eV. This value is close tothat calculated from the experimental data.

Fig. 2 Differential charge densities near oxygen vacancy
Difference in the charge density between the defectmodel and the perfect model has been calculated to makethe effect of vacancy clear (Fig. 2). The six nearest neighbor
oxygen atoms moved from original site toward the oxygen
vacancy site. The two atoms of those were shown in Fig. 2. Itis also shown that the charge densities around the twonearest neighbor plutonium atoms are affected by the oxygenvacancy. The calculation results show that only the nearest
neighbor plutonium atoms are affected by the oxygenvacancy. This agree with the following picture used in theclassical defect chemistry, that is, the oxygen has twoelectrons provided by plutonium, which are left behind whenan oxygen atom is leaving lattice position. These twoelectrons are localized on two plutonium atoms, turning Pu+4
into Pu+3.
References[1] K. Konashi H. Matsui, Y. Kawazoe, M. Kato and S.
Minamoto, J. Phys. Soc. Jpn. 75 suppl., 143 (2006).
[2] K. Konashi, M. Kato, S. Minamoto, ‘First Principle PlaneWave Pseudopotential Calculation of Point Defects in PuO2X’, ANS 2005 Winter Meeting, Nov. 15-19, 2005, Washington,D.C., USA
Contact toKenji Konashi (International Research Center for Nuclear
Materials Science)e-mail:konashi@imr.tohoku.ac.jp

Advanced Nuclear Energy Materials