Theoretical investigation of the impact of grain boundaries and fission gases on UO2 thermal conductivity

Du, Shiyu¹  Andersson, Anders D.¹  Germann, Timothy C.¹  Stanek, Christopher R.¹ 

[1] Los Alamos National Laboratory

关键词: ANISOTROPY;    ATOMS;    BUBBLES;    FISSION;    FISSION PRODUCTS;    GASES;    GRAIN BOUNDARIES;    KAPITZA RESISTANCE;    METRICS;    MICROSTRUCTURE;    NUCLEAR FUELS;    PELLETS;    PERFORMANCE;    PHYSICAL PROPERTIES;    SYMMETRY;    TEMPERATURE GRADIENTS;    THERMAL CONDUCTIVITY;    URANIUM DIOXIDE;   

DOI  :  10.2172/1039681
RP-ID  :  LA-UR-12-21046
PID  :  OSTI ID: 1039681
Others  :  TRN: US1202272

美国|英语

来源: SciTech Connect

【 摘 要 】

Thermal conductivity is one of the most important metrics of nuclear fuel performance. Therefore, it is crucial to understand the impact of microstructure features on thermal conductivity, especially since the microstructure evolves with burn-up or time in the reactor. For example, UO{sub 2} fuels are polycrystalline and for high-burnup fuels the outer parts of the pellet experience grain sub-division leading to a very fine grain structure. This is known to impact important physical properties such as thermal conductivity as fission gas release. In a previous study, we calculated the effect of different types of {Sigma}5 grain boundaries on UO{sub 2} thermal conductivity and predicted the corresponding Kapitza resistances, i.e. the resistance of the grain boundary in relation to the bulk thermal resistance. There have been reports of pseudoanisotropic effects for the thermal conductivity in cubic polycrystalline materials, as obtained from molecular dynamics simulations, which means that the conductivity appears to be a function of the crystallographic direction of the temperature gradient. However, materials with cubic symmetry should have isotropic thermal conductivity. For this reason it is necessary to determine the cause of this apparent anisotropy and in this report we investigate this effect in context of our earlier simulations of UO{sub 2} Kapitza resistances. Another source of thermal resistance comes from fission products and fission gases. Xe is the main fission gas and when generated in sufficient quantity it dissolves from the lattice and forms gas bubbles inside the crystalline structure. We have performed studies of how Xe atoms dissolved in the UO{sub 2} matrix or precipitated as bubbles impact thermal conductivity, both in bulk UO{sub 2} and in the presence of grain boundaries.