Atomistic Simulations of Mass and Thermal Transport in Oxide Nuclear Fuels

Technical Report · Mon Jun 04 00:00:00 EDT 2012

DOI:https://doi.org/10.2172/1043009· OSTI ID:1043009

Andersson, Anders David ^[1]; Uberuaga, Blas P. ^[1]; Du, Shiyu ^[1]; Liu, Xiang-Yang ^[1]; Nerikar, Pankaj ^[2]; Stanek, Christopher R. ^[1]; Tonks, Michael ^[3]; Millet, Paul ^[3]; Biner, Bulent ^[3]

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1. Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
2. IBM, Bangalore (India)
3. Idaho National Laboratory (INL), Idaho Falls, ID (United States)

In this talk we discuss simulations of the mass and thermal transport in oxide nuclear fuels. Redistribution of fission gases such as Xe is closely coupled to nuclear fuel performance. Most fission gases have low solubility in the fuel matrix, specifically the insolubility is most pronounced for large fission gas atoms such as Xe, and as a result there is a significant driving force for segregation of gas atoms to grain boundaries or dislocations and subsequently for nucleation of gas bubbles at these sinks. The first step of the fission gas redistribution is diffusion of individual gas atoms through the fuel matrix to existing sinks, which is governed by the activation energy for bulk diffusion. Fission gas bubbles are then formed by either separate nucleation events or by filling voids that were nucleated at a prior stage; in both cases their formation and latter growth is coupled to vacancy dynamics and thus linked to the production of vacancies via irradiation or thermal events. In order to better understand bulk Xe behavior (diffusion mechanisms) in UO_2±x we first calculate the relevant activation energies using density functional theory (DFT) techniques. By analyzing a combination of Xe solution thermodynamics, migration barriers and the interaction of dissolved Xe atoms with U, we demonstrate that Xe diffusion predominantly occurs via a vacancy-mediated mechanism, though other alternatives may exist in high irradiation fields. Since Xe transport is closely related to diffusion of U vacancies, we have also studied the activation energy for this process. In order to explain the low value of 2.4 eV found for U migration from independent damage experiments (not thermal equilibrium) the presence of vacancy clusters must be included in the analysis. Next a continuum transport model for Xe and U is formulated based on the diffusion mechanisms established from DFT. After combining this model with descriptions of the interaction between Xe and grain boundaries derived from separate atomistic calculations, we simulate Xe redistribution for a few simple microstructures using finite element methods (FEM), as implemented in the MOOSE framework from Idaho National Laboratory. Thermal transport together with the power distribution determines the temperature distribution in the fuel rod and it is thus one of the most influential properties on nuclear fuel performance. The fuel thermal conductivity changes as function of time due to microstructure evolution (e.g. fission gas redistribution) and compositional changes. Using molecular dynamics simulations we have studied the impact of different types of grain boundaries and fission gas bubbles on UO₂ thermal conductivity.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE Laboratory Directed Research and Development (LDRD) Program; USDOE Office of Nuclear Energy (NE), Nuclear Fuel Cycle and Supply Chain. Fuel Cycle Research and Development Program
• DOE Contract Number: AC52-06NA25396
• OSTI ID: 1043009
• Report Number(s): LA-UR--12-21891
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
36 MATERIALS SCIENCE
ACTIVATION ENERGY
ATOMS
BUBBLES
DIFFUSION
DISLOCATIONS
FINITE ELEMENT METHOD
FISSION
FUEL RODS
FUNCTIONALS
GASES
GRAIN BOUNDARIES
IRRADIATION
MICROSTRUCTURE
Materials Science(36)
NUCLEAR FUELS
NUCLEATION
OXIDES
POWER DISTRIBUTION
SEGREGATION
SOLUBILITY
TEMPERATURE DISTRIBUTION
THERMAL CONDUCTIVITY
THERMAL EQUILIBRIUM
THERMODYNAMICS
TRANSPORT
VACANCIES