## 1 Introduction  
The nuclear-power-plant-decommissioning procedure is mostly completed in five steps: removing fuel rods, decontaminating and cutting the system components, dismantling the reactor, treating waste, and restoring the soil. To dismantle reactor vessel internals (RVIs), they should be cut into pieces underwater, put in a basket, and pulled out of the water pool [1]. Mechanical-cutting and thermal-cutting methods are mainly used to dismantle RVIs and their components. Mechanical cutting includes disk saws, band saws, abrasive wheels, diamond-wire cutting, and shears. Thermal-cutting techniques include plasma arc, oxy-fuel, and gowing [1]. Recently, the thermal-cutting process that uses lasers as a heat source has been studied [2; 3; 4; 5; 6]. Due to its narrow-cut kerf-width characteristic, laser cutting has the merit of reducing the generation of secondary radioactive waste. In most thermal-cutting applications, the high-density heat which is induced melts materials while the assist gas removes molten materials. However, when cutting RVIs in underwater conditions, a large amount of radioactive aerosol can also be generated due to the use of high-pressure assist gas. During laser cutting, particles of a certain size are trapped in air bubbles and released into the air as they rise above the water's surface [3; 7]. Because of their fine size, the aerosols generated by thermal cutting are not only difficult to handle in the nuclear power dismantling process, but additional aerosol-collecting equipment must be used to protect workers from radiation exposure [8]. The plasma-cutting process was widely applied until the 1990s, but the total dismantling-process-management cost was increased due to large amounts of fine radioactive particle generation. Mechanical cutting methods have been gradually applied since the 2000s, due to the decrease in the generation of radioactive secondary waste [9; 10]. [PERSON] et al. [8] reported cutting speeds and aerosol-generation rates depending on the material thickness and cutting methods used. These data were obtained from the BR-3 nuclear power plant decommissioning project in Belgium. When compare to mechanical cutting, the thermal-cutting method enables high-speed material cutting, but also generates a larger number of aerosols. Furthermore, the thicker the material, the slower the cutting speed and the more aerosols produced [7,8]. The total cost of dismantling nuclear power plants can be reduced by applying a method that generates fewer aerosols while cutting at high speeds. In other words, the cutting speed should be fast and the use of assist gas should be minimized.  
Most underwater thermal-cutting studies focus more on the cutting tendency or the success of cutting, and less on reducing the generation of aerosols. To reduce assist gas usage, it is important to understand its flow phenomenon. Several studies on assist gas visualization of cutting nozzles have been conducted [11, 12, 13]. However, few studies have reported on underwater assist gas flow. It is difficult to observe the state of the process during actual underwater laser cutting. The reason is that, due to the debris and fine particles that are generated during the cutting process, the water becomes dark and cloudy, and it is difficult to accurately capture the assist gas flow pattern. In this study, to reduce the generation of radioactive aerosols during underwater laser cutting, several cutting process parameters are considered, such as the distance between the nozzle and the specimen, the cut kerf width, and the nozzle design. Through the underwater assist gas visualization experiment, the flow of the assist gas was observed and evaluated according to the process parameters. The effects of the process parameters on actual laser-cutting performance were also verified through an underwater cutting experiment.