This study aims to establish an optimized decommissioning strategy for radiation protection by quantitatively evaluating worker radiation dose and decommissioning costs for the dismantling of a reactor vessel head (RVH). A three-dimensional model was developed based on the actual geometry of the Kori Unit 1 RVH, and various decommissioning scenarios were analyzed using the DEMplus simulation tool, considering factors such as decontamination, cutting methods, decommissioning timing, and the application of remote cutting. In the baseline scenario with manual cutting at 10 years after replacement, individual dose was 45.38 mSv, exceeding the operator's internal limit of 20 mSv per year. Replacing manual cutting with remote robotic cutting reduced the collective dose by approximately 95.9%, with the individual dose decreasing to 1.66 mSv. Although delaying dismantling by 15 or 20 years produced additional dose reduction due to radioactive decay, its economic benefit was limited when accounting for long-term storage costs. Strengthening decontamination procedures showed varying effectiveness depending on the cutting method. This study provides an optimized execution strategy by comprehensively evaluating dose, cost, and work time across scenarios, offering practical reference data to support radiation protection and decision-making in future RVH decommissioning projects.

Weld seam offset is an inevitable consequence of automated welding systems, yet its effect on internal defect formation through molten pool thermal and fluid dynamics remains poorly understood. This study presents a three-dimensional computational fluid dynamics (CFD) investigation of the heat transfer and flow field evolution in V-groove gas metal arc welding (GMAW) under torch offset conditions of 0 mm, 1 mm and 2 mm, where the 0 mm condition serves as a reference case revealing the burn-through behavior resulting from excessive heat concentration at the root. A high-fidelity numerical model was established and validated against experimental non-destructive evaluation (NDE) results including radiographic testing (RT) and ultrasonic testing (UT) for the 1 mm and 2 mm offset conditions, with CFD-derived defect geometries shown to correspond directly with characteristic NDE signal patterns. The results demonstrate that although both offset conditions induce initial thermal asymmetry, the resulting molten pool dynamics differ fundamentally. Under the 1 mm offset, a transient clockwise circulation develops following biased droplet impingement but subsequently stabilizes into a quasi-symmetric thermal and flow field, restoring uniform solidification behavior. In contrast, the 2 mm offset sustains a persistent unidirectional circulation throughout the welding process, consistently directing convective heat transport toward the near-side bevel and maintaining the far-side wall near the solidus temperature. This thermally driven asymmetry governs side-wall lack of fusion, while a void entrapment mechanism driven by the asymmetric pool dynamics accounts for the trapped porosity and root defects observed experimentally. The findings provide a physically grounded thermal framework for understanding how offset magnitude translates into distinct defect morphologies in automated V-groove welding.

This study investigates the laser welding characteristics and mechanical properties of the Can-Cap butt joint in 600 mm ultra-large prismatic batteries, which are emerging as next-generation electric vehicle (EV) and energy storage system (ESS) platforms. To overcome the high reflectivity and thermal conductivity of aluminum alloys, a green laser with a wavelength of 515 nm was utilized to weld 0.7 mm thick Al3003-H14 sheets. The effects of welding patterns (straight and wobble) and scanning speeds on the weldability, cross-sectional defects, microstructural hardness, and tensile strength were experimentally evaluated. Precise joint alignment was achieved using the Auto Position Alignment (APA) function of the laser scanner. Optical microscopy of the etched cross-sections revealed that both patterns produced sound, fully penetrated weld beads free of internal porosity and hot cracks. Vickers hardness depth profiles indicated that the fusion zone (WZ) exhibited the highest hardness due to rapid-cooling-induced microstructure refinement, with the straight pattern showing higher hardness than the wobble pattern due to its faster cooling rate. Tensile testing demonstrated that the tensile strengths of the straight and wobble patterns reached 84.1% (138.8 MPa) and 77.1% (127.3 MPa) of the base metal, respectively. The straight pattern exhibited a 9.1% higher tensile strength compared to the wobble pattern, which is attributed to the narrower bead width, reduced heat-affected zone (HAZ), and higher joint hardness of the straight path. These experimental findings provide baseline datasets to validate computational fluid dynamics (CFD) melt pool simulations and advance intelligent laser autonomous manufacturing frameworks.

The weldability and the relationship between microstructure and tensile properties at 298 and 110 K produced by laser-arc hybrid welding (LAHW) in high-Mn steel welds were thoroughly investigated. In the laser zone of LAHWs using filler wire, more Mn vaporization in the laser zone was observed than at the arc zone of LAHWs. The arc zone showed a decrease in Mn content of similar to 0.6 wt%, while the laser zone showed a decrease of similar to 0.9 wt%. The arc and laser zones of the LAHWs showed stacking fault energies (SFEs) of 17.8 and 17.3 mJ/m(2), respectively. The tensile deformation of LAHWs at 298 K was conducted with a deformation twins mode, while it was shifted to deformation twins + epsilon-martensite transformation at 110 K. The yield strength was slightly higher in the laser zone, which had a finer grain size compared to the arc zone. The formation of epsilon-martensite with deformation twins preceded necking during tensile testing, therefore increasing the yield strength at 110 K. In terms of performance, the LAHW process demonstrated a 25% increase in productivity compared to the conventional submerged arc welding (SAW) process, with a yield strength exceeding 400 MPa, comparable to that of SAW. These findings indicate that LAHW is a highly effective welding method for high-Mn steels, particularly in cryogenic applications.

In welding and additive manufacturing processes, thermal behavior significantly affects process quality and structural performance due to the use of localized heat sources. While finite element analysis (FEA) and computational fluid dynamics (CFD) have been widely used to analyze thermal and fluid phenomena, conventional heat source models such as the double ellipsoidal model often require complex parameter optimization and fail to fully capture asymmetric or variable weld pool geometries under different welding conditions. In this study, a novel heat source model, termed the traveling steady pool (TSP) model, is proposed to address these limitations. Unlike conventional models where the heat input is centered around the arc or laser, the proposed model treats the stabilized molten pool itself as the source of heat transfer through its lower boundary. By analyzing bead cross-sections and molten pool lengths from high-speed camera images, the pool shape is approximated using simple first-order polynomials, enabling the construction of a simplified TSP (S-TSP) model without relying on CFD. The model was implemented in FEA and applied to welding under various tilt angles (0°, 15°, 30°, 45°). Results demonstrate that the S-TSP model accurately reproduces the fusion zone (FZ) and heat-affected zone (HAZ), offering a practical and efficient alternative for thermal analysis and weld quality prediction in complex welding environments.

A selective laser melting (SLM) process was adopted to construct SLM structures for a venturi-type nozzle in a pressurized dissolution microbubble generator. The SLM structures with stainless steel 316L were built on the distribution plate of a venturi-type nozzle with different geometries to investigate the effects of the sidewall angle of the SLM structure and empty space at the center of the distribution plate. The microbubble generation test was conducted for 30 s using a pressurized dissolution microbubble generator, and venturi-type nozzles with and without the SLM structures were installed in the water tank. The generated microbubble size ranged from 1 to 110 mu m. A greater number of microbubbles was generated for the SLM structure-assisted nozzles. The largest number of generated microbubbles was 70,490 with a microbubble size of 31.82 mu m. Computational fluid dynamics simulation revealed that the SLM structures induced turbulent vortexes by impeding the water flow along the radial direction. Consequently, a low-pressure region developed in the water flow, and the liquid phase of water changed to the vapor phase. Eventually, microbubbles were generated by the SLM structures via vortexinduced cavitation.

Wire arc-based directed energy deposition (DED) is a highly productive additive manufacturing (AM) technique; however, excessive heat input often results in distortion and irregular bead geometry, leading to increased surface waviness and necessitating extensive post-processing. To address these challenges, this study introduces a novel 3D weaving path aimed at enhancing wetting behavior and minimizing micro-scale waviness in wire arcbased DED. The weaving motion promotes metal spreading by adjusting the wetting area, thereby reducing the contact angle and improving surface smoothness. High-speed imaging and computational fluid dynamics (CFD) simulations were utilized to investigate molten metal behavior during deposition. Experimental results revealed that the 3D weaving path reduces surface waviness by more than 70 % compared to conventional stringer paths, significantly lowering the required machining depth. Additionally, mechanical property evaluations confirmed that the proposed approach maintains consistent hardness and tensile strength, ensuring structural integrity. These findings demonstrate the potential of 3D weaving path technology to enhance the efficiency and precision of large-scale metal AM, reducing post-processing demands and improving manufacturability.

Numerical simulation is essential in solving complex problems in welding and additive manufacturing (AM). However, conventional finite element analysis (FEA) models require repeated parameter tuning due to their inability to capture fluid behavior, while computational fluid dynamics (CFD) is often impractical for industrial applications. To address this, a Traveling Steady Pool (TSP) model was developed based on the observation that the molten pool stabilizes after a critical time. The steady pool geometry was extracted from CFD results, segmented into longitudinal sections, fitted with polynomial functions, and interpolated to construct a threedimensional profile. This geometry was applied in FEA as a moving Dirichlet boundary at the melting temperature. The TSP model was validated against conventional heat source models and experimental data, including thermal images and bead cross-sections. Thermo-elasto-plastic simulations under various inclination angles further confirmed its predictive capability in deformation analysis. The model also provided physical insights into key phenomena, such as the enlarged CGHAZ at 45 degrees and minimal deformation at 30 degrees, based on the heat transfer characteristics of the molten pool geometry.

Many industries, such as automobile, aerospace, heavy industries, etc, have been interested in welding automation due to cost reduction and improving productivity. For welding automation, seam tracking plays an important role for the quality of welds. If the seam is misaligned, it can cause troubles such as weld defects and poor appearance. Most of previous studies have analyzed the influence on the bead shape and weld defects by current, voltage, welding speed, but there is lack of research regarding the effects of misaligned seam, called offset. Thus, in this study, fillet welds were conducted with flux-cored arc welding, which has high welding efficiency and low cost. To find out the guideline of offset, various offsets were intentionally applied to the welding experiments, and the changes in bead shape were observed and weld defects were detected for various combination of offset and welding speed. In addition, the grades of weld defect in each condition were evaluated by ISO 5817, which illustrates the international standard of weld defects. Based on the results, the guidelines of offset was suggested to avoid weld defects.

Steam generator (SG) replacements in South Korea began with the Kori No. 1 unit in 1998 due to performance degradation. Currently, 20 steam generators have been replaced in total. While additional decommissioning of dozens of steam generator will be required soon due to life-expiration of several nuclear power plants, there has been no actual dismantling performance of steam generators yet, and the replaced decommissioned steam generators are currently stored in intermediate storage facilities. To minimize waste volume and facilitate site reuse, it’s necessary to proactively dismantle steam generators. These components are less radioactively contaminated and easier to dismantle compared to primary equipment like reactors. Additionally, securing related dismantling technology is essential for managing future replacements or equipment that has been stored. Establishing a process scenario about where and how the steam generator will be safely dismantled is important. It is necessary to analyze the advantages and disadvantages of each scenario to study the timing, location, and method of dismantling, and to develop an optimal process scenario through analysis of worker radiation exposure and dismantling costs. For this purpose, simulations were conducted on the radiation dose to workers according to the timing and method of dismantling, using 3D dismantling simulation software developed by Cyclife Digital Solutions, a subsidiary of French EDF, and the results were reviewed by mathematically modeling and analyzing the radiation doses exposed to workers over the years using an exponential decay model.

In plasma arc cutting (PAC), the characteristics of the gas flow exiting the nozzle significantly influence the cut quality. The high-pressure jet in PAC forms complex shockwave structures when it is incident on the metal to be cut. In this study, the flow behavior inside the kerfs of various geometries derived from an actual PAC workpiece is assessed. The shape of the cutting front in the kerf varies with the changing cutting speed. A high cutting speed yields a curved cutting front, resulting in unwanted gas flow behavior and adversely influencing the cutting performance. In this study, a computational fluid dynamics simulation model was used to analyze the effects of a curved cutting front on the gas flow behavior during the PAC process. The gas flow patterns obtained from the numerical simulations were qualitatively compared with the Schlieren experiment results. The analysis results indicated that the curvature of the cutting fronts generated oblique shockwave structures that significantly reduced the flow velocity. In particular, the weak shock structures throughout the curved cutting front gradually decreased the flow velocity. The critical flow velocity was realized in the kerf with a highly curved cutting front, beyond which the vertical penetration of the material was not possible. The shear stress lines concurred with the striation patterns on the kerf walls, thereby validating the numerical analysis results.

The study analyzed the vaporization characteristics of Mn during the laser welding of high-manganese (Mn) cryogenic steel, focusing on the effect of the focal position. The volume of fluid method and transient computational fluid dynamics simulations were used to analyze the molten pool behavior. The keyhole formation process and keyhole wall temperature induced by the focal position were investigated to understand the mechanism behind the varying amounts of Mn vaporization depending on the degree of laser penetration. The reduced defocus distance of laser beam (least defocused; LD) produced the high energy density and complete penetration, less multiple reflection of laser beam, low keyhole-wall temperature, therefore exhibiting the minor Mn vaporization. The electron probe microanalysis results and the analytical model used in this study were highly congruent, confirming the most significant Mn vaporization in the partial-penetrated welds of keyhole formation (medium defocused; MD).

This study performed computational fluid dynamics (CFD) simulation to investigate the flow patterns of molten pool during welding with a transient heat transfer. The influence of gravity changed the flow patterns of the molten pool, which determined the molten pool length and cooling time from 800℃ to 500℃(t 8/5 ) under the same heat input. In the flat position, the upward flow caused by arc forces and the inward flow generated by Marangoni convection resulted in a conflict between flows, and a molten pool length of 12.1 mm. In the overhead position, the volume of the molten pool was continuously drawn by gravity, eliminating the conflict between the inward and the upward flow. Therefore, convective heat transfer accelerated toward the edge of the molten pool, leading to an increased molten pool length (15 mm). In the vertical downward position, gravity pushed the molten pool toward the welding direction. The resultant flow pattern resulted in a rapid cooling rate and reduced the upward flow of the molten pool, leading to a short molten pool length (8.8 mm). The t 8/5 obtained from the CFD were coupled with thermodynamic simulations to predict the microstructures of the coarse-grain heat-affected zone.

Vaporization modeling of high-Mn steel during laser welding, along with the effects of Mn vaporization on the tensile properties at room and cryogenic temperatures, were investigated for various laser defocusing (LD) conditions. The LD10 exhibited a full penetration keyhole, while LD20 showed a partial penetration keyhole. The vaporization modeling based on computational fluid dynamics (CFD) analysis confirmed that a significant Mn flux occurred in LD20, which corresponded with the most substantial loss of Mn observed in the partially penetrated weld. The room temperature tensile testing showed a slight increase in the yield strength and a minor reduction in tensile elongation for LD20, which could be attributed to the stress-induced epsilon-martensitic transformation. In cryogenic tensile testing, LD20 exhibited a higher yield strength with more martensitic transformation, including some stress-induced alpha '-martensitic transformation observed. Through the vaporisztion modeling, significant Mn vaporization in LD20 was found to decrease the stacking fault energy, which enhanced stress-induced martensite transformation and yielding stress during tensile deformation at 110 and 298 K.

Molten pool behavior plays an important role during bead formation in wire–arc direct energy deposition (DED) processes. However, this pool is susceptible to external forces, particularly the changes in direction of gravity when the depositional position is changed. In this study, a 3D simulation of the wire–arc DED was performed using computational fluid dynamics (CFD) considering various depositional positions. The experiments in − 30°, 0°, and 30° positions were performed using a tilting table to visualize the influence of the components of gravity. The volume of fluid method was used to represent the deposition and solidification process of the molten pool. A high-speed camera and cross-sections of the bead were used to evaluate the CFD model by comparing the molten pool lengths and bead geometries; the simulation and experimental results were similar. Based on these verified results, flow patterns, temperature distributions, and solid fractions were used to elucidate the changes in each position (such as a 14.3

The effects of cutting speed and current on plasma cut quality were investigated. Regression modeling was used to establish relationship between process parameters and responses. Analysis of variance was performed to determine the significance of independent variables influencing the kerf geometry. The analysis showed that the proposed regression model fits the experimental data fairly well and the selected variables influence kerf length difference and top kerf width substantially. However, while the bottom kerf width was significantly influenced by cutting speed, it was not affected by the current. The analysis results were compared with the experimental data and were found to be in good agreement. The images captured with high-speed camera using appropriate band-pass filter revealed that the plasma flows under the workpiece were affected by the process parameters. The tail length and the direction angle of the plasma flow dictated the shape of the kerf geometry.

The autogenous manual gas tungsten arc welding (GTAW) process was used for cladding austenitic stainless steel 316L using a single pass with various contact tip-to-work distances (CTWDs). Immersion and electrochemical tests were used to evaluate the corrosion resistance of the welded specimens, and a microstructural analysis was conducted to investigate the chemical composition of the molten pool and the heat-affected zone of welding. The key findings of this study indicate that the corrosion resistance improved under a CTWD of 5 mm due to the optimal distribution of ferrite and a refined microstructure. Additionally, the highest hardness was observed in specimens with a CTWD of 3 mm, attributed to the increased ferrite content in the weld metal. As the CTWD increased, the ferrite fraction decreased, and the hardness also diminished. However, in the CTWD 7 mm case, the higher heat input influenced the microstructure and molten pool shape significantly through the Marangoni effect, resulting in a lower corrosion resistance. These results suggest that optimizing the CTWD can enhance the corrosion resistance of welded 316L stainless steel.

This study investigated how welding affects the thermal deformation of square cells produced for casks, which are dry storage containers for spent nuclear fuel. We aimed to minimize structural deformation by utilizing STS316L as the material for the square cells. We explored a method of subdividing the square cells and joining them through butt welding. Keeping the upper plate thickness constant, GTA butt welding was conducted while varying the column’s wall thickness, followed by measurement with a laser vision sensor. The heat conduction and thermal strain were then calculated using a finite element analysis (FEM). Both experimental and analytical results confirmed that there was significant thermal deformation in the cases of thick-walled columns due to variations in heat conduction distribution, with the resulting deformation patterns depending on thickness.

In decommissioning a nuclear power plant, radioactively contaminated metal structures must be dismantled and cut to a disposable size. When cutting NPPs structures, functional methods for cutting objects need to determine. The thermal cutting method is mainly used because of its high cutting speed compared to other cutting methods. In previous research, the distribution of aerosols generated during thermal cutting shows a high concentration in the range of 1 ㎛ or less. The effect of fine particles(1 ㎛ or less) with high concentrations should be prevented for workers' safety. In this study, Aerosol distribution in real-time measures verified the characteristics of the aerosol generated during cutting. Metal cutting used plasma arc, and flame-cutting methods. The aerodynamic diameter distribution of generated aerosols during the cutting process was measured using high-resolution aerosol measuring equipment in real time. This study analyzed the changes in the generation of real-time aerosols by the cutting method. And It was confirmed that there was a difference in the distribution of aerosol diameters generated during cutting and post-cutting. The results show distribution characteristics through high-resolution aerosol measurements.