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Research Papers

An Finite Element Method Study of the Thermal Conductivity of Polycrystalline UO2

[+] Author and Article Information
Enze Jin

State Power Investment Central
Research Institute,
Beijing 102209, China

Chen Liu

Beijing Research Institute of Chemical
Engineering and Metallurgy,
Beijing 102209, China

Heming He

State Power Investment Central
Research Institute,
Beijing 102209, China
e-mail: heheming@snptc.com.cn

1Corresponding author.

Manuscript received September 8, 2016; final manuscript received June 17, 2017; published online July 31, 2017. Assoc. Editor: Guoqiang Wang.

ASME J of Nuclear Rad Sci 3(4), 041006 (Jul 31, 2017) (10 pages) Paper No: NERS-16-1099; doi: 10.1115/1.4037189 History: Received September 08, 2016; Revised June 17, 2017

A finite element method (FEM) is applied to investigate the thermal conductivity of polycrystalline UO2. The influences of microstructure are especially important for UO2 due to the severe structural changes under irradiation conditions. In this study, we have investigated the influences of microstructures on the thermal conductivity of polycrystalline UO2 using FEM. The temperature profile of fuel pellet with different microstructures during service is also investigated. The thermal conductivity increases with increasing grain size. The grain size distribution has obvious influence on the thermal conductivity especially when there are pores in the polycrystal. The influences of porosity and pore size are very sensitive to the position of the pores. The results obtained in this study are useful for the prediction of property changes of UO2 fuel in pile and important to gain some design guidance to tune the properties through the control of the microstructure.

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Figures

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Fig. 1

(a) The finite element model of polycrystalline UO2 used in the present study. The circular holes in the model represent pores. (b) In the 2D finite element model, grains are set to connect with each other via a narrow gap. (c) Simple illustration of the computational procedure to calculate temperature profiles of UO2 pellet during service.

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Fig. 2

(a) Polycrystal simulation model with different grain size: 5 μm, 10 μm, 20 μm, and 40 μm. (b) The effective thermal conductivity κeff with different grain size. (c) Effective thermal conductivity of polycrystalline UO2 at 1000 K with different grain size. The solid line represents the effective thermal conductivity calculated by Eq.(7), where the value of κ0 is 3.94 W/m·K, and Rk is 1.14 × 10−7 m2 K/W.

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Fig. 3

(a) Radial temperature profile of UO2 pellets (radium = 0.4 cm) with different grain sizes and (b) temperature distribution inside the UO2 pellet

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Fig. 4

(a) Polycrystalline UO2 models with different topological entropies:3.5, 2.96, and 2.31. The grain size is 5 μm, and the porosity is zero. (b) Thermal conductivity of polycrystalline UO2 with different topological entropies. (c) Enlarged view of thermal conductivity of polycrystalline UO2 with different topological entropies.

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Fig. 5

(a) Polycrystalline UO2 models with grain growth directions of 0 deg, 30 deg, 60 deg, and 90 deg. The dimension of each grain is approximately 40 μm × 10 μm. (b) Thermal conductivities of polycrystalline UO2 models with grain growth directions of 0 deg, 30 deg, 60 deg, and 90 deg. The thermal conductivity of polycrystal with no certain grain growth direction (d = 20 μm) is also shown for comparison.

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Fig. 6

(a) Polycrystalline UO2 models with porosity of 3%, 5%, and 7%. Pores are distributed randomly in the polycrystals. (b) Thermal conductivities of polycrystalline UO2 with different porosities. (c) Comparison of thermal conductivity curves of polycrystalline UO2 with 5% porosity computed in this study (solid line) and Eq. (8) (the shaded region). (d) The differences of thermal conductivity between polycrystals with grain size of 40 μm and 5 μm.

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Fig. 7

(a) Polycrystalline UO2 models pores with in different locations and (b) thermal conductivities of polycrystalline UO2 with different pore locations

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Fig. 8

(a) Polycrystalline UO2 models pores with in different pore sizes. Pores are distributed randomly in the polycrystals. (b) Thermal conductivities of polycrystalline UO2 with different pore sizes. The solid lines represent the polycrystals that all pores are randomly distributed, and the dashed lines represent the polycrystals that all pores are along GBs. (c) Radial temperature profile of UO2 pellets with different pore sizes. Pores are along the GBs. (d) Temperature distribution inside the UO2 pellet.

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Fig. 9

Thermal conductivities of polycrystals with different topological entropies under the condition of pore existence. The dashed lines represent the polycrystals that all pores are randomly distributed, and the solid lines represent the polycrystals that all pores are along GBs.

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