[en] The critical heat flux (CHF) is one of the most important thermal hydraulic parameter in thermal hydraulic design and safety analysis. The CHF enhancement allows higher limit of operation condition so that heat transfer equipment can be operated safely with more margins and can have more economics. Nano-fluid is one of the most potential methods which can enhance the CHF and many researchers are working on. This study consists of two parts. First part is dispersion stability of Al₂O₃ Nano-fluids and second part is flow boiling CHF experiment with Al₂O₃ Nano-fluids. In the first part, zeta potentials of Al₂O₃ Nano-fluids with varying time and concentration were measured by ELS-Z2 of Otsuka Electronics to find out the effect of time and concentration on the zeta potentials. The experiments were conducted at concentration of 0.001 vol% ∼ 0.5 vol% and time of 1 hour ∼ 3 days. All measurements of zeta potentials were in the range of 30 mV ∼ 60 mV. The zeta potentials of Al₂O₃ Nano-fluids slightly increased as time increased within 1 day. The zeta potentials of Al₂O₃ Nano-fluids slightly increased as particle concentration increased from 0.001 vol% to 0.5 vol%. The pHs of Al₂O₃ Nano-fluids were also measured. In the second part, flow boiling CHF enhancement experiments using Al₂O₃ Nano-fluids with three different concentrations (0.001 vol%, 0.01 vol%, and 0.1 vol%) were conducted under atmospheric pressure, at mass flux of 100∼300 kg/m²s, at inlet temperature of 50∼75 .deg. C. The CHFs of Al₂O₃ Nano-fluids were enhanced in flow boiling for all experiment conditions up to about 70%. Maximum CHF enhancement (70.24%) was shown at 0.01 vol% concentration, 50 .deg. C inlet temperature and 100 kg/m²s of mass flux. Inner surfaces of test section tube were observed by FE-SEM and Zeta potentials of Al₂O₃ Nano-fluid were measured before and after the CHF experiments

[en] Nano-fluids are considered to have strong ability to enhance CHF. Most CHF experiments using nano-fluids were conducted in pool boiling conditions. However there are very few CHF experiments with nano-fluids in flow boiling condition. In the present study, flow boiling CHF experiments using bare round tube with Al₂O₃ nano-fluid and Al₂O₃ nano-particle deposited tube with DI water were conducted under atmospheric pressure. CHFs were enhanced up to ∼ 80% with Al₂O₃ nano-fluid and CHFs with Al₂O₃ nano-particle deposited tube were also enhanced up to ∼ 80%. Inner surface of test section tube were observed by SEM and AFM after CHF experiments

[en] In-vessel Retention (IVR) is one of the key severe accident management strategies that have been applied currently for advanced light water reactors such as APR1000 or APR1400. The concept of IVR consists of external cooling of the reactor vessel by flooding the reactor cavity to remove the decay heat from the molten core through the lower head of the vessel. However, the heat removal process is limited by the occurrence of critical heat flux (CHF) at the reactor vessel outer surface that may lead to a sharp increase of local temperature, damaging the integrity of the reactor vessel. In order to obtain higher power of nuclear reactors and to assure the achievement of the IVR capability during accident conditions, an enhancement of CHF at the outer surface of the vessel is required. The potential use of nano-fluids to increase the CHF is among the main IVR enhancing approaches. In this study, Al₂O₃ and CNT nano-fluids with different concentrations have been used as the potential coolant to enhance IVR capabilities. The dispersion stability of the nano-fluids was verified by zeta potential measurements. The results showed effects of time, concentration and pH on the stability of nanofluids. Three types of nano-fluids were selected as the candidates to apply for the IVR. A series of experiments have been performed in this study to understand the pool-boiling critical heat flux behavior on downward facing surfaces submerged in a pool of nano-fluids at very low concentration. The inclination angle was changed from horizontal to vertical to investigate the effect of orientation on CHF enhancement which is needed for the application in IVR

[en] The CHF enhancement allows higher limit of operation condition so that heat transfer equipment can be operated safely with more margins and can have more economics. Nano-fluid is one of the most potential methods which can enhance the CHF. The flow boiling CHF enhancement experiments using Al₂O₃Nano-fluids with three different concentrations (0.001 vol%, 0.01 vol%, and 0.1 vol%) were conducted under atmospheric pressure, at low mass flux of 100∼300 kg/m²s, at inlet temperature of 50∼75 .deg. C. The CHFs of Al2O3 Nano-fluids were enhanced in flow boiling for all experiment conditions up to about 70%. Maximum CHF enhancement (70.24%) was shown at 0.01 vol% concentration, 50 .deg. C inlet temperature and 100kg/m²s of mass flux. The Zeta potentials of Al₂O₃ Nano-fluid were measured before and after the CHF experiments

[en] Studies regarding CHF enhancement using nanofluid have been conducted by many researchers. Few of them have been conducted under flow boiling condition. In our previous work, we could not find the concentration effect of nanofluid on the flow boiling CHF. We concluded that the effect of nanofluid on the CHF may already be saturated at low concentration because of long boiling time (more than 15 minutes), and there may be a minimum deposition layer of nanoparticles to enhance the flow boiling CHF. In the present study, we conducted flow boiling CHF experiments using nanofluid under different heating rate to examine the minimum deposition layer. After the flow boiling CHF experiments, the inner surfaces of the test section tube were explored by FESEM

[en] The stem thrust required to unwedging a gate valve is influenced by the pressure and temperature when the valve is closed and by the changes in these conditions between closure and opening. 'Pressure locking' and 'thermal binding' refer to situations where pressure and temperature effects cause the unwedging load to be much higher than normal. A model of these phenomena has been developed. The effects of pressure and temperature are analyzed to determine the change in this disk-to-seat 'interference'. Flexibilities or stiffness of the disk and body strongly influence the unwedging thrust. Calculation and limited comparison to data have been performed for the RHR motor operated valve designs and scenario. Pressure changes can increase the unwedging thrust when bonnet pressure exceeds the pressure in the adjacent piping and temperature changes can increase the unwedging thrust when a temperature change after closure produces and increase in the disk-to-seat interference

[en] In vessel retention of molten corium through external reactor vessel cooling (IVR-ERVC) is a strategy to manage severe accident of nuclear power plant. This IVR-ERVC strategy is adopted by some operating nuclear power plants and proposed for some advanced LWR such as AP600 and AP1000. It gives sufficient thermal margin for AP600. However it is not sure that the IVR-ERVC provides sufficient thermal margin for large power reactors (more than 1000MWe) without additional enhancements. In this study, CHF experiments were conducted using 2-D silce under realistic severe accident condition (additives of coolant, material of heater, etc.) to find additional thermal margin of IVR-ERVC. Through this study, CHF experiments for IVR-ERVC using a small scale two-dimensional slice test section were conducted. Basically, the effect of additives and heated material on CHF was investigated. Through the experiments, we found that the CHFs of BA, TSP and BA+TSP were all enhanced except the CHFs of BA at radius of 150mm. These enhancements can give more margins for the IVR-ERVC strategy under severe accident conditions

[en] For realization of fusion power plant, various technical subjects should be overcome in the future. Among them, fusion-specific safety standards and licensing regulations should be developed at a proper time in order to avoid additional delay in construction of the plant. Although these needs are clearly expected from licensing of fusion DEMO (DEMOnstration) plant, there are no study on regulation concepts of fusion DEMO plant since technology and design of fusion power plant is being developed even now. It is a big dilemma to initiate this research in such situation to prepare for meeting regulatory demands expected in 2030s. For the early stage of this research, safety objectives, which are the ultimate goals for safety and the foundation of safety infrastructure, should be considered. Since these are very inclusive and implicative to be top tier of safety document system, it can be generally discussed and expected for fusion power regardless of phase of fusion technology development. In the present study, first, existing safety objectives for fissile nuclear power are reviewed. Then examples of safety goal for fusion facilities including ITER licensing are summarized to read the trend of safety goals for the fusion facilities. Finally, based on these reviews and reasonable considerations, safety objectives for fusion power plant in the future are proposed

[en] After Fukushima Daiichi nuclear accident, safety of nuclear power and public acceptance as well become the most crucial issue. Not only how to make the system safe but also how to establish society acceptance is big problem. Actually, this kind of problem is closely linked to how to regulate nuclear power plant. In the not too distant future, innovative nuclear reactors including sodium-cooled fast reactor (SFR), high temperature gas-cooled reactor (HTGR) and fusion reactor will be realized, which have more safety features than water-cooled fissile reactors. Even they have inherent safety features; however, buildup of such as innovative power plant is impossible without providing persuasive regulations based on reasonable regulatory framework. As we have seen from lessons of Fukushima accident, failure of reasonable regulation and occurrence of terrible accident could spoil realization of plan. For the case of fusion power plant, Korea has domestic plan to begin construction of fusion DEMO (DEMOnstration) plant at 2022. Although the needs of regulation for fusion power are clearly expected from licensing of fusion DEMO plant, there are no study on regulation concepts of fusion DEMO plant since technology and design of fusion power plant is being developed even now. Expedite study on development of regulatory framework for fusion power is positively necessary to prepare for meeting regulatory demands at a proper time. In the present study, first, domestic plan to realize innovative reactor plant including fusion power plant are organized. In addition to this, previous study on development of regulatory framework for innovate reactors are reviewed. Technology-neutral safety approach for establishment of regulatory frame has been come into spotlight and has being developed to utilize the methodology for deriving safety requirement regardless of reactor design. Technology-neutral safety approach for new reactor designs is also reviewed. Then, inherent safety features of fusion power to be expected are suggested based on the report of 'Safety and Environmental Assessments of Fusion Power: SEAFP', which is resulted from European fusion development agreement (EFDA). Finally, feasibility of technology-neutral safety approach for fusion power is explored

[en] The patterning of high-resolution-featured deep-nanoscale structures with a high aspect ratio (AR) has received increasing attention in recent years as a promising technique for a wide range of applications, including electrical, optical, mechanical and biological systems. Despite extensive efforts to develop viable nanostructure fabrication processes, a superior technique enabling defect-free, high-resolution control over a large area is still required. In this review, we focus on recent important advances in the designs and processes of high-resolution nanostructures possessing a high AR, including hierarchical and 3D patterns. The unique applications of these materials are also discussed. (paper)