Thermochemistry of defects and phase stability in plutonium dioxide (PuO[2-x]).
- Marius
- Petrica
Based on a thermochemical model of defect formation, the concentration of various types of defects and the non-stoichiometry of PuO{sub 2-x} are calculated as functions of temperature and partial pressure of oxygen. The model is able to predict oxygen diffusivity for temperatures in the (900, 1400) C range and oxygen pressure (1,10{sup -25}) atm. Comparison with experimental data shows that the model describes non-stoichiometry and oxygen diffusivity well. The calculated free energy of non-stoichiometric plutonia is used to derive the high oxygen, high temperature, region of the Pu-O phase diagram, which is still subject of controversy. The thermochemistry of PuO{sub 2+x}, if any, is also discussed. Thermochemical models of PuO{sub 2-x} are necessary for prediction of oxidation-reduction of Pu-based alloys and for the design of better oxide nuclear fuels. The type and defect formation mechanism determine the alloy performance while kinetic properties of point defects govern radiation tolerance and fission gas release. Oxygen diffusion is also important in optimizing surface properties. Together with the phase stability, diffusion governs the non-stoichiometry of the alloy. Modeling the defects and the mobility of oxygen opens a path to the modeling of the more complex phenomena, such as ageing. Although the Ce-O and Pu-O phase diagrams are still controversial in the high temperature region, there is a consensus regarding the existence of fluorite (f.c.c.) phases in the region x < 0.3 and 1000 < T < 2000 K. A thermochemical model of defect formation in CeO{sub 2-x} and PuO{sub 2-x} was developed and implemented in a computer program able to predict the concentration of various types of defects and the non-stoichiometry as functions of temperature and partial pressure of oxygen. The model is based on five types of defects: polarons, singly and doubly-charged oxygen vacancies, singly-charged metal-oxygen vacancy complexes, and neutral oxygen vacancy complexes. The same program was used to calculate the oxygen chemical and self-diffusivity in CeO{sub 2-x} and PuO{sub 2-x} for temperatures of (1200, 1700) K and oxygen pressures (1,10{sup -25}) atm. The model is currently used to determine the oxygen chemical potential as a function of oxygen partial pressure and temperature, as part of a new calculation of the Ce-O and Pu-O phase diagrams. The approach will be extended to UO{sub 2+x}, taking into account the specific defect types.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 977824
- Report Number(s):
- LA-UR-04-5860; TRN: US1003732
- Resource Relation:
- Conference: Submitted to: 11th Symposium on Thermodynamics of Nuclear Materials, Karlsruhe, Germany, Sept. 6-10, 2004.
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ALLOYS
COMPUTER CODES
DEFECTS
DIFFUSION
FISSION
FLUORITE
FREE ENERGY
KINETICS
NUCLEAR FUELS
OXIDES
OXYGEN
PARTIAL PRESSURE
PHASE DIAGRAMS
PHASE STABILITY
PLUTONIUM DIOXIDE
POINT DEFECTS
POLARONS
RADIATIONS
SURFACE PROPERTIES
THERMODYNAMICS
TOLERANCE
VACANCIES