## 1 Introduction  
As of December 2020, 192 nuclear reactors have been permanently shut down, and 313 reactors have been connected to the power grid for more than 30 years [1]. Hence, the global nuclear decommissioning market is expected to grow in the future. The decommissioning of nuclear power plants is conducted in the following order: preparation for decommissioning, decontamination, cutting, waste treatment, and environmental restoration [2]. In particular, cutting technology is a key factor in the decommissioning of nuclear power plants [3]. Cutting technologies for nuclear decommissioning can be roughly divided into mechanical cutting technologies, such as saw cutting, and thermal cutting technologies, such as plasma and laser cutting [4, 5]. Among them, laser cutting technology is attracting attention as a next-generation nuclear power plant dismantling technology because it has various advantages [6, 7, 8, 9, 10, 11]. Laser cutting has a faster cutting speed than saw cutting and produces less aerosol than plasma cutting, which generates a large amount of it [12, 13]. Considering that remote dismantling is performed in an underwater environment, the advantage of laser cutting is that it can be remotely controlled and automated [14]. When repairing and dismantling inside reactors, underwater welding and cutting techniques are used to provide radiation shielding [15, 16]. Underwater laser cutting has some process differences compared to cutting in air [17]. When conducting underwater laser cutting, a local dry cavity is required. The high-pressure assist gas that prevents the laser beam from being absorbed by water forms a local dry cavity and secures the path of the laser beam. Local dry cavities are also formed when machining with underwater laser welding [18; 19; 20] or laser cladding [21; 22] used in the construction and repair of nuclear reactors [23; 24]. In addition, underwater laser cutting requires greater laser power than cutting in air. The reason that the underwater cutting ability is lower than in air is that the specimen is rapidly cooled by the surrounding water [25]. Decommissioned reactor structures are disposed of in a radioactive-waste facility through a process involving storage and transportation after being decontaminated, packaged, and subjected to various treatment steps to prevent the spread of contaminants. In this process, in order to increase the efficiency of the storage space of waste, it is necessary to reduce the load volume by increasing the cutting quality when dismantling the structure. Therefore, the cutting surface quality is also important when nuclear power plants are decommissioned using underwater cutting with a laser heat source. However, only a few studies have been conducted on the cutting surface quality of thick stainless steel plates during underwater laser cutting. [PERSON] et al. studied the optimization of parameters such as cutting speed, focal position, and focal length to achieve high-edge-quality laser cutting on a 10 mm-thick stainless steel plate [26]. Although their study provided meaningful results in terms of laser cutting parameters and cutting quality, their findings are not applicable to the thick stainless steel plates used in nuclear power plants. Meanwhile, [PERSON] et al. studied the effects of cutting parameters considering C-Mn steel with thicknesses of 60-70 mm; however, they did not conduct experiments in underwater conditions [27]. [PERSON] et al. studied supersonic and dual nozzles [28; 29], while [PERSON] et al. studied the step-like cutting method [30] and oblique cutting to increase the maximum laser cutting speed and thickness [31]. However, these studies did not discuss the cutting surface quality under optimal conditions. Therefore, in this study, underwater laser cutting experiments on a 50 mm-thick stainless steel specimen were conducted using a fiber laser with a maximum power of 20 kW to improve the cutting surface quality and increase the spatial efficiency of waste storage containers.