高温气冷堆是第四代核电的重要方向之一。由于固有安全性，模块式高温气冷堆事故安全的重点是裂变产物和放射性源项。尤其地，与裂变产物输运紧密耦合的石墨粉尘是学界和业界长期关注的重点。准确了解石墨粉尘在事故时的输运沉积行为对反应堆安全分析和源项评估具有重要意义。当前对于石墨粉尘输运沉积现象的理解仍然有局限，主要挑战是石墨颗粒高度非球形带来的其动力学研究的困难。本文通过实验和数值方法，研究石墨颗粒非球形效应对颗粒-壁面和颗粒-流体相互作用的影响，建立石墨气溶胶动力学物理模型。进而与湍流场建模结合，研究高温气冷堆进水事故超压泄放时，安全壳内石墨气溶胶的输运沉积规律。首先，针对石墨颗粒-壁面相互作用，发展了基于高速显微摄影技术的单颗粒碰壁实验台，提出了近壁区流体曳力和气膜阻尼的修正方法，获得了微米级石墨颗粒的临界粘附速度和恢复系数。基于表面粘附和局部耗散导致的能量损失，建立了石墨颗粒碰壁动力学模型。以球形颗粒为参照，阐释了非球形效应对入射角、反弹角和姿态等的影响规律。进而，建立了适用于非球形粘附性颗粒碰壁过程的有限元方法。基于Hamaker理论将粘附力引入动态碰撞过程，将阻尼耗散与局部应变率关联，成功解析表面粘附与阻尼耗散间的耦合效应，并与大量经典实验数据高度吻合。开展数值实验获得物性、几何和碰撞参数对能量耗散的影响规律。引入粘附数、无量纲粘附力和无量纲松弛时间，构建了粘附性颗粒碰壁临界粘附速度和恢复系数显式关联式。获得了粒径和径厚比对非球形石墨颗粒临界粘附速度的影响，与碰壁实验结果吻合良好。然后，针对石墨颗粒-流体相互作用，本文搭建微米颗粒自由沉降实验台，直接测量了多种非球形核级石墨颗粒的自由沉降速度和曳力系数。将曳力系数与颗粒雷诺数关联，通过修正经典斯托克斯定律，建立了曳力系数统计模型。最后，本文基于欧拉-拉格朗日方法，将建立的非球形石墨气溶胶动力学模型与多相计算流体力学结合，阐释了非球形效应对颗粒沉积的影响机制，获得了HTR-PM超压泄放时安全壳内石墨气溶胶的动态输运沉积规律，定量揭示了安全壳的粉尘滞留能力。通过大规模数值模拟，确定了HTR-PM600超压泄放出口方向和位置的优化设计，并应用到工程实践中。本文还提出采用池水洗技术主动抑制高温堆石墨粉尘的思路，并通过实验初步获得了石墨颗粒的水洗效率和去污因子。
The high temperature gas-cooled reactor (HTGR) is an important option for the next generation nuclear reactor. Owing to the inherent safety, the main foucs of the safety analysis is on the fission products (FP) and source term. Particularly, the problem of graphite dust, which is tightly coupled with the transport of FP, has been a long-term focus of HTGR academic and industry. It is of great importance for the source term evaluation to understand the underlying physics that dominates the transport and deposition of graphite dust. However, the understanding on the dynamics of graphite dust is still quite limited, due to the challenge of highly irregular shape of particles.In this thesis, the effects of irregular shape on particle-wall and particle-fluid interactions are experimentally and theoretically investigated. The dynamic models of graphite aerosol are established. These models are further combined with the multi-phase CFD modeling, for investigating the transport and deposition characteristics of graphite aerosol in the containment during the overpressure relief in the water-ingress accident of HTGR.First, for the graphite particle-wall interaction, the single particle-wall collision experiment is performed based on high-speed photomicrography technique and a new correction for the effects of near-wall drag and gas-film damping. The critical sticking velocity and restitution coefficient of graphite particles are measured. According to the energy loss casued by the surface adhesion and local viscoelastic dissipation, we present a dynamic model for graphite particle-wall collision. By comparing to the experimental results of spherical particles, the effects of highly irregular shape on the incident angle, rebound angle and particle orientation are investigated.Second, we develop a shape-independent finite element model (FEM) to numerically simulate the adhesive non-spherical particle-wall collision. At the sub-particle scale, the surface adhesion is introduced based on local contact status and Hamaker theory, and the energy dissipation is directly correlated with the local strain rate. The close coupling between the surface adhesion and the viscoelastic damping is successfully resolved, and the simulation results agree well with various classic experimental data in wide impact parameters. Then, the effects of the material properties, geometry and collision parameters on the energy dissipation are systemically investigated. By the dimensional analysis, we introduce the adhesion number, dimensionless adhesion force and dimensionless relaxation time and present new explicit correlations for the critical sticking velocity and restitution coefficient of small adhesive particles. Further, we use the FEM to investigate the effects of particle size and diameter-to-thickness ratio on the critical sticking velocity of irregular graphite particles, and the results agree well with the single paricle experimental measurement.Third, for the graphite particle-fluid interaction, we directly measure the terminal settling velocity and drag coefficient of several types of nuclear particles via the free settling experiment. The drag coefficient is correlated with the particle Reynolds number. By modifying the classic Stokes law, a statistical model for the drag coefficient of graphite particles is proposed.Finally, based on the Eulerian-Lagrangian method, the new dynamic models of irregular graphite particles are incorporated into a multi-phase computational fluid dynamics model. We discuss the effect of the highly irregular shape on the particle depsotion. The dynamic transport behaviors and deposition distribution of the graphite aerosol in the containment are resolved during the overpressure relief of water-ingress accident of HTR-PM. The results show that the containment has a good ability to retain the graphite dust. By plant-scale numerical simulations, the optimal parameters of the direction and location of the overpress relief of HTR-PM600 are determined, which have been appied in the engineering practice. Moreover, we proposed a new pool scrubbing route for active control of graphite dust during over-pressure relief of HTGR. The preliminary decontamination factor and retention efficiency of graphite particles are obtained, which suggests that the pool scrubbing could be an efficient technology for retaining the graphite dust in the future.
