UN核燃料烧结致密化过程的相场模拟

Phase-Field Simulation of the Densification Process During Sintering of UN Nuclear Fuel

引入了平移和旋转等作用过程,建立了烧结致密化过程的相场模型,分析了平流通量刚体运动对烧结颈的形成、平衡二面角和烧结致密化过程的影响。模拟结果表明,引入平流通量刚体运动在烧结初期加快了烧结颈的形成,而在烧结后期作用不明显;烧结相场模型中是否引入平流通量不影响晶界平衡二面角的取值;致密化过程的气孔收缩分为3个阶段,分别是烧结初期的表面扩散主导阶段、平流通量取代表面扩散的阶段和致密化完成阶段;增大平移迁移率能加快致密化过程、增加致密化程度,但超过一定阈值后其作用达到饱和;颗粒完成致密化收缩后会形成稳定的三叉晶界(夹角120°),多晶UN烧结模拟形貌演化中三叉晶界的形成和气孔收缩致密化的行为与实验结果一致。

UN is a candidate fuel for light water reactors and fast reactors due to its high density,high thermal conductivity,and high melting point.The highly densified UN particles are desirable to strengthen the fuel structure and delay the release of fission gas.However,the mechanism of densification during sintering is still unclear from the view point of existing experimental results.Therefore,it is essential to simulate the densification process during sintering using the phase-field(PF)method.In the present work,the rigid body action of translation and rotation was introduced in the PF model.This work analyzed the effects of the advection flux of rigid body motion on the formation of the sintered neck,the equilibrium dihedral angle,and the densification during sintering.The simulation results showed that the introduction of advection flux of rigid body motion accelerated the formation of the sintering neck in the early stage of sintering,while such an effect was not obvious in the later stage.The equilibrium dihedral angle of the model with advection flux was consistent with that of the model,which only contained surface diffusion.The densification stomatal shrinkage was divided into three stages:surface diffusion dominated stage,advection flux dominated stage,and final densification progress.The increase in translational mobility accelerated the densification speed and increased the final density after densification,although this effect reached saturation after a certain threshold.Stable trigeminal grain boundaries(GBs)with 120°were formed when densification was completed.The characteristics of the sintered morphology of polycrystalline UN,such as trigeminal GBs,pore shrinkage,and densification,were consistent with the experimental results.