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Acta Metall Sin  2020, Vol. 56 Issue (9): 1295-1303    DOI: 10.11900/0412.1961.2019.00440
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Phase Field Simulations of the Sintering Process of UO2
SUN Zhengyang1,2, YANG Chao3, LIU Wenbo1,2()
1 School of Nuclear Science and Technology, Xi‘an Jiaotong University, Xi'an 710049,China
2 Shaanxi Key Laboratory of Advanced Nuclear Energy and Technology, Shaanxi Engineering Research Center of Advanced Nuclear Energy, Xi’an Jiaotong University, Xi’an 710049, China
3 Advanced Research Institute of Multidisciplinary Science, Beijing Institute of Technology, Beijing 100081, China
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Abstract  

UO2 is widely used as fuel in various nuclear reactors, and the sintering of UO2 ceramic powder under high temperature is one of the most important processes during the preparation of UO2 fuel. However, sintering is a very complicated process which is controlled by many simultaneous mechanisms. The phase field method was used to simulate the sintering process of UO2 ceramic powder in the present work. In the modified phase field model, the influence of three anisotropic diffusion mechanisms, including surface diffusion, grain boundary diffusion and lattice diffusion, on the microstructure evolution during sintering was considered, and the effect of the interface energy between different ceramic particles on the sintering morphology was also considered. Based on the experimental conditions and thermodynamic parameters, the sintering process of UO2 ceramic powder at 2000 K was simulated. The simulation results showed that the initial morphology of the ceramic powder affects the sintering kinetics; large grains grow more easily, and small grains disappear at the last stage of sintering; the GB diffusion mechanism is the dominant mechanism during the sintering; the equilibrium dihedral angle between GB and phase boundaries can be strongly affected by the GB energy. In addition, the sintering process of the polycrystalline UO2 ceramic powder was also simulated, and the simulation results were in good agreement with the experimental results.

Key words:  phase field model      UO2      sintering      diffusion mechanism     
Received:  19 December 2019     
ZTFLH:  TG148  
Fund: National Natural Science Foundation of China(11705137);China Postdoctoral Science Foundation(2019M663738);China Postdoctoral Science Foundation(2018T111053);State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University(KF201713);Innovative Scientific Program of CNNC
Corresponding Authors:  LIU Wenbo     E-mail:  liuwenbo@xjtu.edu.cn

Cite this article: 

SUN Zhengyang, YANG Chao, LIU Wenbo. Phase Field Simulations of the Sintering Process of UO2. Acta Metall Sin, 2020, 56(9): 1295-1303.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00440     OR     https://www.ams.org.cn/EN/Y2020/V56/I9/1295

Fig.1  Schematic of phase field model (The red and blue circles represent diffuse boundaries between grain boundaries, and between pore and grain, respectively. ρ—concentration field variable, η1orientation field variable of grain 1, η2orientation field variable of grain 2)
Physical parameterValueRef.
Ds8.8258×10-11 m2·s-1[31]
Dgb7.8998×10-13 m2·s-1[32]
Dl7.8998×10-15 m2·s-1[34]
γs0.6 J·m-2[34]
γgb0.3 J·m-2[34]
δ6 nm[34]
Table 1  Physical parameters of UO2 at 2000 K[31,32,34]
ParameterValueParameterValue
A?17M?gb67.5
B?7M?l0.675
κ?η6.75L?1
κ?ρ20.25Δx=Δy1
M?s7541Δt2×10-5
Table 2  Non-dimensional parameters used in the present simulation
Fig.2  Phase-field modeling of the microstructure evolution of two equal circle grains
Fig.3  Phase-field modeling of the microstructure evolution of two equal hexagons
Fig.4  Logarithmic neck growth curves of two different shapes (l—neck length, t—time step)
Fig.5  Fitting curve of late neck growth
Fig.6  Phase-field modeling of the microstructure evolution of two unequal circles
Fig.7  Larger grain's area and neck growth curves as a function of time
Fig.8  Phase-field modeling of the microstructure evolution of a double grains system with only surface diffusion (a1~a3), and with surface diffusion, boundary diffusion and lattice diffusion (b1~b3) mechanism
Fig.9  Equilibrium dihedral angles generated using different grain boundary energies (Φ—equilibrium dihedral angle)
Fig.10  Phase-field modeling of the microstructure evolution of multi-grain system
Fig.11  Growth curve of grain A in Fig.10
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