Environmental regulations of the International Maritime Organization (IMO) are getting stricter, and the demand for replacing the fuel of ships with eco-friendly fuels instead of heavy oil in the shipbuilding and marine industries is increasing. Among eco-friendly fuels, LNG (liquefied natural gas) is currently the most popular fuel. This is because it is an alternative that can avoid the IMO's environmental regulations by replacing fuel. In PART 1, as a basic study of laser welding of high manganese steel materials, a fiber laser bead-on-plate experiment was conducted using nitrogen protective gas, and the effect of each factor on the penetration shape was analyzed through cross-sectional observation. In PART II, argon and helium shielding gases, not the nitrogen shielding gas used in PART I, were tested under the same experimental conditions and the effect of the shielding gas on penetration during laser welding was conducted.

Conversion to modern hydrogen energy is required, and research on liquefied hydrogen cargo containment systems is needed for large-capacity transport and storage. In this study, changes in the mechanical properties of the adhesive required for storage and transport in liquid hydrogen were confirmed. The lap shear test was performed by realizing cryogenic conditions in a small chamber using liquid nitrogen and liquid helium. There was an increase of 11.0% in the -180℃ condition compared to room temperature, and an increase of 1.8% in the -230℃ condition compared to the -180℃ condition was confirmed. In the case of shear strain, it is known that it decreases as the temperature goes down. As a result of the experiment, it was confirmed that the value at room temperature and the value at -180℃ reduced the shear strain by 5.0%, and -230˚ compared to the -180℃ condition. An increase of 1.5% was confirmed in the C condition. In the case of the specimen tested at -230℃, the deformation in the gripper part was larger than in other tests, and it is judged that the maximum shear strength and shear strain were affected. In addition, in this study, there is a limitation in the experiment at -230°C rather than 253°C, which is the boiling point of hydrogen

In modern times, where problems due to environmental pollution are continuously occurring, hydrogen is in the spotlight as the energy of the future. Hydrogen is an eco-friendly energy resource that does not even generate CO2, and is actively supporting research to utilize hydrogen energy at the national level. This study is a study on the cryogenic mechanical properties of the elements constituting the cargo hold during the transportation of liquid hydrogen. Among the various components, the evaluation of mechanical properties of the cryogenic adhesive under liquid helium conditions was confirmed. The related contents are summarized as follows. As a result of performing SSRT by curing the adhesive, it was confirmed that tensile strength and maximum strain were increased at cryogenic temperature (-230°C) compared to room temperature (25°C). It was confirmed that the adhesive-hardened specimen showed a brittle fracture mode at both room temperature and cryogenic temperature during tensile. Improvements in this study, such as pores occurring during adhesive curing, the use of standard specimens, and experiments at -253°C, the boiling point of hydrogen, exist, and are planned to be carried out in subsequent studies.

To conduct a nuclear fuel irradiation test, the inside of the nuclear fuel rod must be assembled along with the test fuel, several different parts, and sensors, and then filled with high-pressure and high-purityhelium gas. Therefore, it is necessary to develop helium gas filling techniques that can achieve exact TIG (Tungsten Inert Gas) spot welding at a pin-hole of the nuclear fuel rod to fill helium gas into the nuclear fuel test rod. However, previous apparatuses do not have repeatability for TIG spot welding as they lack an electrode position control jig to exactly fix a TIG electrode in a high-pressure chamber, and they consume a large amount of helium gas. Therefore, a TIG spot welding apparatus was developed to easily and accurately conduct TIG spot welding and significantly reduce the gas consumption. In addition, the optimum welding conditions of this welding apparatus were established through various weld tests.

The very high temperature gas reactor (VHTR) is one of the next generation nuclear reactors for its safety, long-term stability, and proliferation-resistance. The high operating temperature of over 800˚C enables various applications with high energy efficiency. Heat is transferred from the primary helium loop to the secondary helium loop through the intermediate heat exchanger (IHX). The IHX material requires creep resistance, oxidation resistance, and corrosion resistance in a helium environment at high operating temperatures. A Ni-based superalloy such as Alloy 617 is considered as a primary candidate material for the intermediate heat exchanger. In this study, the microstructures of Alloy 617 crept in pure helium and air environments at 950˚C were observed. The rupture time in helium was shorter than that in air under small applied stresses. As the exposure time increased, the thickness of outer oxide layer of the specimens clearly increased but delaminated after a long creep time. The depth of the carbide-depleted zone was rather high in the specimens under high applied stress. The reason was elucidated by the comparison between the ruptured region and grip region of the samples. It is considered that decarburization caused by minor gas impurities in a helium environment caused the reduction in creep rupture time.