Thermal cutting processes that can be applied to dismantling nuclear power plants include oxygen cutting, plasma cutting, and laser cutting. According to the global trend, research projects are being carried out in various countries to upgrade laser cutting, and many studies are also being conducted in Korea with plans to apply laser cutting processes when dismantling nuclear power plants. However, with the current technology level of the laser cutting process, the maximum thickness that can be cut is limited to 250 mm. Therefore, in this study, a laser-oxygen hybrid cutting process was implemented by adding a laser heat source to the oxygen cutting process that can cut carbon steel with a thickness of 250 mm or more (RV, beam, column, beam, etc.) when dismantling the nuclear power plant. This has the advantage of improving the cutting speed and reducing the cutting width Kerf compared to conventional oxygen cutting. In this research, the laser-oxygen hybrid cutting process consisted of laser cutting to which Raycus’ 8 kW Fiber Laser power source was applied and oxygen cutting to which hydrogen was applied with Fuel Gas. The oxygen torch was placed perpendicular to the test piece, and the laser head was irradiated by tilting 35° to 70°. The effects of cutting directions on quality and performance were studied, and cutting paths were selected by comparing cutting results. Thereafter, it was confirmed that there is an optimal laser output power according to the cutting thickness by studying the effect on the cutting surface quality by changing only the laser output power under the same cutting conditions. The results of this study are expected to be helpful in the remote cutting process using laser-oxygen hybrid cutting when dismantling domestic nuclear power plants in the future.

Laser cutting has been recognized as one of key techniques in dismantling nuclear power plants as it has several advantages such as a remote operation and a reduced secondary waste. However, it generates a significant amount of aerosols that can pose a health risk to workers and further induce environmental pollution during the cutting operation. Thus, understanding the aerosol characteristics generated by the laser cutting is crucial for implementing an effective cutting operation and reducing the exposure to these hazardous particles. In this work, we established a methodology to collect the aerosols and investigate their properties in the laser cutting operation. We built an integrated laser cutting system for aerosol analyses, consisting of a high-power laser cutting module, a metal sample holder, an aerosol collector, and a closed chamber. We expect that this system will offer an opportunity for in-depth understanding of the aerosol properties, by connecting it with desired type of aerosol analysis platforms, and further safe dismantling operation of the nuclear power plants.

Since nuclear power plant (NPP) dismantling carries the possibility of radiation exposure from a hazardous environment, it’s important to minimize that by using a remote manipulator et al. However, due to complexity of nuclear facilities, it’s necessary for operators to increase their proficiency by operating in advance in a virtual environment. In this research, we propose a virtual manipulator system using a haptic device for NPP’s reactor vessel internals (RVI) dismantling which can realistically manipulate.

Currently, dismantling technology for decommissioning nuclear power plants is being developed around the world. This study describes the cutting technology and one of the technologies being considered for the RV/RVI cutting of Kori Unit 1. The dismantling technology for nuclear power plants include mechanical and thermal methods. Mechanical cutting methods include milling, drill saw, and wire cutting. The advantages of the mechanical method are less generating aerosol and less performance degradation in water. However, the cutting speed is slow and the reaction force is large. Thermal cutting methods use heat sources such as plasma arcs, oxygen, and lasers. The advantages of thermal method are fast cutting speed, low reaction force and thick material cutting. On the other hand, they have problems with fume and melt. Among them, the cutability of the oxygen cutting method is better in carbon steel than in stainless steel. In order to cut the RV/RVI of the Kori Unit 1, the applicability of fine plasma, arc saw, and band/ wheel saw is being reviewed. For RV cutting, the applicability of arc saw and oxy-propane is being considered Because RV is mostly made of carbon steel. However, since the flange is cladded with stainless steel, the use of mechanical methods such as wire saws should be considered. In the case of RVI, since it has a complicated shape and is made of stainless steel, it seems necessary to review various cutting methods. In addition, it will be necessary to minimize radiation exposure of workers by cutting underwater cutting.

The remote dismantling system proposed in this paper is a system that performs the actual dismantling process using the process and program predefined in the digital manufacturing system. The key to the successful applying this remote dismantling system is how to overcome the problem of the difference between the digital mockup and the actual dismantling site. In the case of nuclear facility decommissioning, compensation between the virtual world and the real world is difficult due to harsh environments such as unsophisticated dismantling sites, radiation, and underwater, while offline programming can be proposed as a solution for other industries due to its sophisticated and controllable environment. In this paper, the problem caused by the difference in the digital mockup is overcome through three steps of acquisition of 3D point cloud in radiation and underwater environment, refraction correction, and 3D registration. The 3D point cloud is acquired with a 3D scanner originally developed in our laboratory to achieve 1 kGy of radiation resistance and water resistance. Refraction correction processes the 3D point cloud acquired underwater so that the processed 3D point cloud represents the actual position of the scanned object. 3D registration creates a transformation matrix that can transform a digital mockup of the virtual world into the actual location of a scanned object at the dismantling site. The proposed remote dismantling system is verified through various cutting experiments. In the experiments, the cutting test object has a shape similar to the reactor upper internals and is made of the same material as the reactor upper internals. The 105 successful experiments demonstrate that the proposed remote dismantling system successfully solved the key problem presented in this paper.

In the dismantling process of a reactor coolant system (RCS) piping, a radiation protection plan should be established to minimize the radiation exposure doses of dismantling workers. Hence, it is necessary to estimate the individual effective dose in the RCS piping dismantling process when decommissioning a nuclear power plant. In this study, the radiation exposure doses of the dismantling workers at different positions was estimated using the MicroShield dose assessment program based on the NUREG/CR-1595 report. The individual effective dose, which is the sum of the effective dose to each tissue considering the working time, was used to estimate the radiation exposure dose. The estimations of the simulation results for all RCS piping dismantling tasks satisfied the dose limits prescribed by the ICRP-60 report. In dismantling the RCS piping of the Kori-1 or Wolsong-1 units in South Korea, the estimation and reduction method for the radiation exposure dose, and the simulated results of this study can be used to implement the radiation safety for optimal dismantling by providing information on the radiation exposure doses of the dismantling workers.