Recently, numerous studies have investigated underwater laser cutting as a method for dismantling complex structures inside nuclear reactors and minimizing the generation of secondary waste during decommissioning activities. To optimize the packaging, transport, and storage of nuclear dismantling waste, as well as to reduce both the safety risks to workers associated with secondary waste production and the additional time and costs required for post-processing, the performance and quality of underwater laser cutting have become critical factors. In this study, experiments were conducted using two types of laser cutting heads designed to modify the characteristics of the laser beam in order to enhance the performance and quality of underwater laser cutting. The results showed that increasing the depth of focus and beam diameter caused the cuff-shaped profile of the laser-cut section to become straighter. In particular, drag-line analysis revealed that an increased injection of auxiliary gas into the cutting zone improved the overall quality of the cut surface.

Despite pre-cleaning with vaporizing press oil, aluminum alloys, widely employed in industry, often retain oil residues and cause various problems during or after machining and assembly operations. Laser cleaning has emerged as a n eco-friendly, non-contact, and highly controllable surface treatment technique that selectively removes contaminants from metal surfaces. In this study, the cleaning technology was applied to vaporizing press oil on an A3003 aluminum alloy surface and was systematically investigated with variable pulsed laser processing parameters. The removal characteristics were analyzed by FTIR to identify the disappearance of lubricating oil characteristics as C-H vibrational peaks and by SEM-EDS to assess microstructural and compositional changes. The results demonstrated that complete removal of lubricant residues without substrate damage was achieved under the optimized condition of -3 mm defocus distance, 75 % overlap ratio, three scans, and 100 W average power. However, when the overlap ratio exceeded 75% or the laser power exceeded 200 W, excessive thermal accumulation induced localized burning and carbonization, resulting in reduced cleaning efficiency. The optimized parameters derived in this study provide a fundamental guideline for the development of environmentally sustainable laser cleaning processes for aluminum alloy heat-ex changer components.

This study aims to investigate the effect of the laser beam overlap rate on the mechanical properties of Al3003 aluminum alloy arc weldment with laser peening. To determine the optimal laser beam overlap rate for laser peening of the weldment, peening experiments were conducted on bead-welded and butt-welded specimens with varying overlap rates, and the effect of the beam overlap rate was analyzed. As the overlap rate increased, the residual stress changed from tensile to compressive, with the highest level of compressive residual stress at the overlap rate of 75%. Laser peening was performed on the aluminum weldment of the prototype, applying the optimal peening conditions identified earlier. As a result of comparing the residual stress, hardness, and tensile strength of the weld before and after laser peening, it was found that the tensile residual stress in the weldment was improved to a compressive residual stress of about −50 MPa or more. The hardness and tensile strength of the weld increased after peening, and the mechanical properties were also improved.

The reactors, in which underwater laser cutting is applied, are usually more than 10[Formula: see text]m high, and the inside of the reactor is filled with coolant. It is necessary to investigate underwater laser cutting by constructing a pressurized water tank that simulates a 10-meter-deep environment in consideration of the 10-meter-deep environment where underwater cutting will be performed during actual reactor dismantling. Therefore, this study aims to first examine the effect of water pressure on laser cutting in an underwater environment and then analyze the quality of the cut part according to the cutting direction and nozzle diameter.

Every nuclear facility has a limited operating life, at the end of which they must be shut down and decommissioned. For decommissioning, the removal of radioactive contaminants and demolition should be considered. Before nuclear power plants are decommissioned, the primary system, which is the most heavily contaminated, is decontaminated first, followed by decontamination of the surfaces of containment containers and buildings. This study examined the removal characteristics of Ni-ferrite coated on the stainless steel (STS304) specimen surface using an eco-friendly laser that generates little secondary waste, and derived the optimal efficiency conditions for laser decontamination. In this study, simulated oxide film specimens were fabricated by coating the STS304 substrate using non-radioactive Ni-ferrite. The experimental equipment for laser decontamination was a Q-switched fiber laser with an average power of 100 W. The laser beam was irradiated on the surface of the specimen according to the beam overlap rate. In addition, the characteristics of the decontaminated area and removal of the coating layer were checked using digital 3D microscopy, scanning electron microscope (SEM), and X-ray diffraction (XRD). The results of laser decontamination experiment indicated the possibility of coating layer removal with fewer scans as the x- or y-axis directions beam overlap rate increased. However, the surface was discolored when excessive overlaps occurred. This study provided basic data for laser decontamination applicable to the removal of Ni-ferrite oxide films on STS304.

The corrosion products formed on the internal surfaces of the primary system are the major factor that generates radioactive contaminants such as oxide films and cruds during decommissioning, causing radiation exposure to workers. Thus, decontamination of nuclear facility surfaces is crucial to ensure the safe completion of decommissioning work. This study applied laser decontamination, which enables precise work while minimizing the deformation of the material surface through the control of process parameters, and investigated the characteristics of the decontaminated area. Specimens coated with Ni-ferrite on an aluminum alloy (A6061) substrate were fabricated and used in this experiment. Laser decontamination was performed using a low-power Q’switching fiber laser while varying the laser beam overlap rate and number of scans. The results of laser decontamination differed by the process parameters and material characteristics. It is determined that high decontamination efficiency can be achieved if appropriate conditions are applied.

According to a recent report on nuclear power plants, 686 nuclear reactors exist worldwide, of which 172 have been permanently closed, and 20 have been completely dismantled. The design life of nuclear power plants worldwide is being reached, and the number of decommissioned reactors being closed because of economic and political reasons is expected to increase. Therefore, the growth of the decommissioning market locally and globally is inevitable. In this review paper, the growth potential of the nuclear power plant-decommissioning market and examples of cutting technology research and development in Korea and overseas is discussed. Among the applied cutting processes, mechanical cutting accounts for the majority; however, underwater processing is required for structures at intermediate or higher levels at which radioactivity has progressed for shielding and other purposes. Accordingly, process development for thermal cutting is actively progressing. Representative studies on thermal cutting process are summarized, and the current state of technological development of the laser-cutting process is presented.

Facilities and radioactive waste, containing nuclear materials, can cause serious harm to the human body and ecosystem by releasing radioactive materials when nuclear power plants are decommissioned. Hence, the decontamination process that lowers the radioactive level is essential. In this study, to alternate the existing decontamination process, the removal characteristics according to the process parameters were confirmed using a laser decontamination beam. This experiment was performed on a STS304 specimen coated with Ni-ferrite using portable Q' switching fiber laser with an average power of 100 W. It was confirmed whether the coating layer was removed by changing the laser beam pattern, de-focused distance, and energy density. Based on the laser decontamination experiment, the applicability of the decontamination process was confirmed for removing contaminants formed in nuclear facilities.

Various studies are being conducted on underwater cutting using lasers for nuclear power plant dismantling environment. The depth of pressure vessels for most reactors is more than 10 m. When cutting a structure contained in a reactor with 10 m or more depth, pressure is applied, and the shape of the cut due to the cutting gas in this environment needs to be identified. Though several studies exist on laser cutting of stainless steel underwater, laser cutting research in pressurized underwater environment is insufficient. This study investigated the relationship between the cutting kerf width and the assistant gas flow at a depth of 10 m or more in underwater environments. Laser cutting was performed on 80 mmt stainless steel in a pressurized underwater environment. The conditions for obtaining sound cut surface quality were derived by analyzing the kerf shape and roughness of the test piece according to the assistant gas pressure. Additionally, the relationship between the kerf width and the assistant gas flow was established by visualizing the flow of gas through Schlieren analysis and by analyzing the shape of the drag line according to the kerf width and assistant gas flow.

Laser welding was applied to the pipe-flange joint of the media coupling for supplying fuel to the submarine fuel cell. Welding was performed on a flat plate that simulated circumferential welding, and partial penetration welding was applied. Welding conditions were selected to satisfy the bead width and penetration depth required for the prototype through the bead welding experiment. Subsequently, laser power control was applied to the beginning and ending of welding to suppress welding defects in the overlapping area where the beginning and ending of the weldment that porosities, cracks, and craters mainly occur. The cross-section, microstructure, heat-affected zone, and hardness characteristics of the butt weldment were analyzed, and a sound weldment with suppressed porosities, cracks, and craters was obtained.

A basic study was conducted to investigate the correlation between assist gas pressure and the amount of secondary waste generated during underwater laser cutting for nuclear decommissioning. The assist gas pressure, i.e. the main parameter, was changed from 2 to 15 bar. Cutting quality was evaluated based on the shape of the cutting cross-section and the roughness of the cutting surface. In addition, the weight loss closely related to the generated secondary waste was calculated, and the schlieren method was used to check the gas flow according to the assist gas pressure. Satisfactory cutting quality could be obtained at a minimum assist gas pressure of 10 bar, and the weight loss related to secondary waste was less. When the cutting quality was good, the weight loss was less, and secondary waste generated was minimum.

In the automobile industry, improving the properties of the mold improves productivity and reduces the defect rate of the product. In this study, a laser heat source was used to improve the properties of the mold. To improve the surface strength of the mold steel, laser cladding was applied to form a surface alloy layer. In addition, an experiment was performed to improve the tensile residual stress of the cladding layer by applying laser shock peening. A continuous wave high-power fiber laser was used for laser cladding, and a high-peak Nd:YAG pulsed laser system was used for laser peening. As a result, an alloy layer harder by more than 1.5 times than the base material was formed on the surface of the mold steel through laser cladding, and the tensile residual stress of the laser cladding layer with peening applied was improved by about 50%.

With nuclear power plants worldwide approaching their design lifespans, plans for decommissioning nuclear power plants are increasing, and interest in decommissioning technology is growing. Laser cutting, which is suitable for high-speed cutting in underwater environments and is amenable to remote control and automation, has attracted considerable interest. In this study, the effects of laser cutting were analyzed with respect to relevant parameters to achieve high-quality underwater laser cutting for the decommissioning of nuclear power plants. The kerf width, drag line, and roughness of the specimens during the high-power laser cutting of 50 mm-thick stainless steel in an underwater environment were analyzed based on key parameters (focal position, laser power, and cutting speed) to determine the conditions for satisfactory cutting surface quality. The results indicated that underwater laser cutting with a speed of up to 130 mm/min was possible at a focal position of 30 mm and a laser power of 9 kW; however, the best-quality cutting surface was obtained at a cutting speed of 30 mm/min.

While dismantling nuclear power plants, the reactor vessel internal is cut underwater using mechanical and thermal cutting. In laser thermal cutting, assist gas must be used to remove melted metal; consequently, a large number of radioactive aerosols can be generated. To reduce the generation of aerosols, the assist gas pressure should be lowered. However, below the pressure limit, the molten metal is not well-removed from the cut surface and fails to cut. In this study, an assist gas visualization experiment was performed to find a condition for the gas to flow well inside the cut surface, even at low pressures. The top kerf width, nozzle type, distance between nozzle and specimen, and assist gas pressure were selected as process parameters, and in the case of large top kerf width condition, assist gas was able to penetrate deeply. In the actual laser-cutting experiment, the laser beam focus position was set to −20 mm and −30 mm. In the case of −30 mm, the top kerf width was widened due to the characteristics of the laser beam profile, and cutting was successful even though the assist gas pressure was lowered by 20%.

In this study, a laser cleaning method was developed in order to replace the existing manual work-based technologies. The experimental investigation analyzed the cleaning properties of the paint and the oxide layer on the steel surface according to the laser beam scanning method. Experiments showed that in the cleaning process, the line beam patterns with a moving stage showed differences in heat input depending on the area, and the square area beam patterns obtained with control of a Galvano scanner showed uniform cleaning performance. The results of this study demonstrated that through the prosed laser cleaning technique, oxide layers as well as paint on steel surfaces can be removed with excellent precision, which is expected to be useful in the development of eco-friendly surface cleaning techniques.

While producing gas fuel supply pipes for duel fuel (DF) engines, a welding process is essential. Accordingly, specimen management before and after welding is crucial to obtain highly reliable weldments. In this study, we developed an environmentally friendly laser cleaning technology to address a toxic work environment and environmental pollution problems caused by chemical cleaning technology utilized in post-welding treatment of gas fuel supply for DF engines. An experiment was conducted by implementing surface laser cleaning of the butt and fillet weldment specimens according to process parameters. Conditions of process parameters were identified for facilitating laser cleaning and used in prototype production. The prototypes were processed through laser and chemical cleaning, and the quality of the end products was compared. The results indicated that the proposed method satisfactorily cleans the prototype surface without generating a toxic work environment and environmental pollution problems. Moreover, the roughness of approximately 5 μm was achieved on the laser cleaned surface. This is considered to be able to increase the adhesion of the paint compared to the smooth chemical cleaned surface during the painting for anticorrosion of the product.

Dissimilar laser welding has been researched to combine the excellent anticorrosion and high strength properties of Ti and the low weight and cost of Al. However, when welding dissimilar Al and Ti sheets, many kinds of intermetallic compound are easily generated. Therefore, intermetallic compounds and differences in material properties make joining such dissimilar metals very difficult. Previous studies clarified that ultra-high welding speed could suppress the weld defects. To elucidate the mechanism of Al and Ti dissimilar laser welding, material behavior of the weld fusion zone and components of fume generated during the ultra-high speed welding process were observed and analyzed using energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), high speed cameras, and a spectrometer. The results show that the atom movement of Al and Ti in the weld plume affects the behavior of elemental components distributed in the weld fusion zone.

The alloy aluminum of 5000 series will have relatively high strength through solid solution strengthening of Mg. However, when laser welding the 5000 series aluminum alloys, the Mg is selectively evaporated by welding heat due to its low melting and vaporization points, resulting in a reduction in the strength of welds. Therefore, laser welding application is difficult because of such a reduction in strength[Formula: see text] [M. Peel, A. Steuwer, M. Preuss and P. J. Withers, Acta Mater. 51, 4791 (2003); A. Haboudou, P. Peyre, A. B. Vannes and G. Peix, Mater. Sci. Eng. A 363, 40 (2003)]. In this paper, welding experiments were carried out using laser and laser–arc hybrids on aluminum alloy 5083 with 8-mm thickness. Tensile testing, scanning electron microscopy (SEM) analysis and electron probe microanalyzer (EPMA) analysis were carried out on specimens on which the laser and laser–arc hybrid weldings were performed. According to the tensile test results, the tensile strength during laser–arc hybrid welding was greater than 85% of the parent metal. In addition, EPMA analysis showed that the strength of the laser–arc hybrid welds was maintained by supplementing the optional evaporating Mg element, since they use the filler wire.

In the shipbuilding and marine industry, laser cleaning is considered an eco-friendly technology because it can significantly reduce the generation of secondary waste when removing the paint and oxide layer on a steel surface during ship repair work. Accordingly, various studies are being conducted worldwide for the application of laser cleaning. However, studies on the removal of various types of paint used on ships for anticorrosion and cosmetic effects and on the control of the scanning mode of laser cleaning application parts, such as square and circular, are limited. Therefore, in this study, the effects of the scanning mode and major process parameters were analyzed for four types of specimen coated with different paints and with various thicknesses. Consequently, it was possible to remove the paint on the steel surface completely by using handheld-type laser cleaning equipment with an average power of 100 W. In addition, the working time was reduced and the cleaning performance was improved by determining the scanning mode suitable for the laser cleaning application part and the optimal process parameters for each specimen.