## 4 Conclusions  
In this study, the underwater laser cutting of 50 mm-thick STS304L specimens was performed while varying the key parameters of the laser beam to obtain a high-quality cutting surface. The technique employed is applicable for the decommissioning of nuclear power plant structures. The following conclusions were drawn by analyzing the changes in the kerf width, drag line, and roughness of the laser-cut specimens with respect to the cutting parameters:  
(1) To understand the effect of the focal position of the laser beam on the cutting quality, underwater laser cutting was performed at focal positions \(f_{\rm d}=-20\) mm and \(f_{\rm d}=-30\) mm. A larger front-surface kerf was observed when the diameter of the laser beam irradiated on the specimen surface was relatively large at \(f_{\rm d}=-30\) mm. This facilitated the discharge of the molten metal owing to the relatively large inflow of the assist gas, resulting in a generally satisfactory cut formation;  
(2) Underwater laser cutting was performed while varying the laser power \(P\) from 7 kW to 13 kW. The results indicated that the front-surface kerf width changed minimally as the laser power increased, but the rear-surface kerf became wider owing to the increased melting caused by the increased heat input. However, sufficient molten metal could not be discharged, causing an increased drag line roughness. Thus, the best cutting quality was obtained when \(P=9\) kW;  
(3) The cutting speed during underwater laser cutting considerably influences the heat input and the mass flux of the assist gas; as the cutting speed increases, the heat input and  
Figure 10: Cross-section and topographic color distribution of cutting surface according to the cutting speed; (**a**) \(v=30\) mm/min, (**b**) \(v=50\) mm/min, (**c**) \(v=90\) mm/min, (**d**) \(v=130\) mm/min (Processing conditions: \(f_{\rm d}=-30\) mm, \(P=9\) kW, \(d_{\rm N}=3\) mm, \(P_{\rm G}=10\) bar, Nozzle type = Normal plate). the mass flux of the assist gas decrease, the rear-surface kerf width decreases, and the drag line roughness increases because the molten metal is not smoothly discharged. (4) Consequently, underwater laser cutting was possible up to a cutting speed of \(v=130\) mm/min under \(f_{\rm d}=-30\) mm and P = 9 kW; however, the best-quality cutting surface was obtained under \(v=30\) mm/min. In the future, we plan to study the effects of changes in the assist gas pressure, stand-off distance, and nozzle shape during underwater laser cutting, as well as the cutting characteristics of stainless steel with increasing thickness. Conceptualization, K.K. and J.-D.K. ; methodology, K.K. and J.-D.K. ; validation, K.K., M.-K.S. and J.-D.K. ; investigation, K.K. and S.-J.-L.; resources, S.-J.L., D.S., J.S. and J.-D.K. ; writing--original draft preparation, K.K. ; writing--review and editing, M.-K.S. and J.-D.K. ; visualization, K.K. ; supervision, J.-D.K. ; project administration, J.-D.K. ; funding acquisition, J.-D.K. All authors have read and agreed to the published version of the manuscript. This research received no external funding. This paper was supported by the National Research Council of Science and Technology (Project number: NK232A, 2021, KOREA) and by the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korean Government (MOTIE) (P0008763, The Competency Development Program for Industry Specialist), and is a revised and expanded version of a paper entitled \"Fundamental study for underwater cutting of 50 mm thick stainless steel using fiber laser in nuclear decommissioning\" presented at the International Symposium on Marine Engineering and Technology 2021. No conflict of interest. No conflict of interest. References  
* [1] International Atomic Energy Agency. _Nuclear Power Ractors in the World_; International Atomic Energy Agency: Vienna, Austria, 2021; ISBN 9789201244215.