Droplet impaction in nuclear installations and safety analysis: Phenomena, findings and approaches

Droplet impaction on the wall surface is ubiquitous in natural and industrial applications. Understanding this phenomenon is also significant to nuclear installation design and safety analysis. In this study, we review the phenomena of droplets impaction (or impingement) on the wall surface and approaches to study them in nuclear engineering. Firstly, we examine the droplet impaction phenomenon in four types of nuclear installations: moisture separator, spray system, steel pipes and fuel channel. In moisture separator, the secondary droplets produced by the droplet splashing reduce the separation efficiency. Further, the study of heat transfer between the impaction droplet and the wall is mostly concerned with the spray system and the fuel channel. In addition, the continuous droplet impaction might induce erosion of the steel pipes. Secondly, we discuss four accidents where the droplet impaction happens. A large amount of droplets is generated during LOCA (Large break loss-Of-Coolant Accident) through the liquid leaking in the installations. The consequence is that deposited droplets would be suddenly vaporized to form a high-pressure vapor, leading a high explosion risk. In accident of fuel-coolant interaction, droplets can be either water or metal, both of which will generate lots of heat after impaction. Additionally, the droplet impaction in the SG (steam generator) tube leakage accident is mostly found in the analysis. Furthermore, fuel droplet impaction in nuclear power plants is considered in analysis of aircraft crash aimed at nuclear facilities. Thirdly, two types of simulation methods-mesh-free and mesh-based simulation methods-are reviewed for droplet surface modeling. We find that the Moving-Particle Semi-implicit (MPS) method is a popular meshfree method used in nuclear engineering. For the mesh-based method, Volume of Fluid (VOF) is often coupled with the level-set method to both keep good conservation property and capture the free surface with high resolution. Finally, we offer some future directions. This review will acquaint readers with the state-of-the-art work on the droplet impaction in nuclear engineering.