•   Introduction
•   To sustain human habitat in the 21st century and beyond, topics on Energy, Environment, Water, and Sustainability, called EEWS, are the most essential issues. Higher fossil fuel prices, threats to energy security, and environmental problems including global warming have made the world face severe challenges. However, we are changing these challenges to opportunities by exploring new research areas and innovative technologies. As a member of the nuclear engineering department, we consider nuclear power to be the basis of such explorations. Nuclear energy is a proven technology that is cost-effective, safe, stable, and has no direct greenhouse gas emissions. Beyond that, the clean and intensive energy from nuclear energy could be used to produce hydrogen, distil saltwater on a massive scale, and advance the next generation of vehicles. Therefore, increasing the role of nuclear energy in meeting future energy demands is being considered all over the world.
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    Water-splitting nuclear hydrogen production - The Once-through Hybrid Sulfur process
•   Water-splitting nuclear hydrogen production is expected to help to resolve major global energy challenges when high energy efficiency and low cost hydrogen production become possible. Among ‘sulfur-based’ water-splitting thermo-chemical cycles which are recognized as high priority candidates for research and development worldwide, we are focusing on the Hybrid Sulfur cycle (HyS), first proposed by Westinghouse Electric Corp. and has been researched under the name of Nuclear Hydrogen Initiative (NHI) established by the U.S. DOE-NE. The Once-through Hybrid Sulfur (Ot-HyS) process, proposed by us, produces hydrogen using the same Sulfur dioxide Depolarized water Electrolysis (SDE) process found in the original Hybrid Sulfur cycle (HyS). In this process, the Sulfuric Acid Decomposition (SAD) process in the HyS procedure is replaced with the well-established sulfur combustion process. A flow sheet for the Ot-HyS process was developed and simulated using Aspen Plus with appropriate thermodynamic models. It was demonstrated that the Ot-HyS process has higher net thermal efficiency, as well as other advantages, over competing benchmark processes. The net thermal efficiency of the Ot-HyS process is 47.1% (based on LHV) and 55.7% (based on HHV) assuming 33.3% thermal-to-electric conversion efficiency of a nuclear power plant with no consideration given to the work for the air separation. Hydrogen produced through the Ot-HyS process would be used as off-peak electricity storage, to relieve the burden of load-following and could help to expand applications of nuclear energy, which is regarded as a ’sustainable development’ technology. Because of its competitive advantages, such as a higher net thermal efficiency, less technical challenge and favorable sulfur statistics, the Ot-HyS process could play an important role in securing a bridge to the sustainable energy future during the short-term transitional period.
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    Water Desalination using a Capacitive De-Ionization method
•   It is estimated that one fifth of the world's population does not have access to safe drinking water. This proportion is expected to increase due to population growth relative to water resources. Further demand in the long term will come from the need to make hydrogen from water. Fresh water is a major priority in sustainable development. Where it cannot be obtained from streams and aquifers, desalination of seawater or mineralized groundwater is required. Most desalination today uses fossil fuels, and thus contributes to increased levels of greenhouse gases. Nuclear energy is being used for desalination and has the potential for much greater use because desalination is energy-intensive and nuclear desalination is generally very cost-competitive with using fossil fuels. In particular, the Capacitive De-Ionization (CDI) method consumes less energy than other desalination methods. So we are now trying to improve the performance of the CDI method through experiments that modify the flow path in order to achieve the lower-energy-use seawater desalination.
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    Reactor Safety Analysis and CHF Experiments
•   As one of the goals of the Generation IV reactor, 'Safety and Reliability' is an interesting issue. In aspect of the safety during Design Basis Accidents (DBAs) and severe accidents, the research for the reactor safety system was performed through system design innovation and the experimental research was conducted to investigate the safety limit.
•   (1) DBA: Simplified Design of Safety Injection System
•   Simplified designs will reduce the time for licensing, construction period, and cost of maintenance even while maintaining the highest standards of protection against DBAs. Using the system simplicity concept, we proposed a conceptual design of SITs which are pressurized by steam. And, for the feasibility study, MARS code was used. 

  (2) Severe accident: The CHF for IVR-ERVC Strategy  In-Vessel Retention External Reactor Vessel Cooling (IVR-ERVC) is a strategy that mitigates the consequences during severe accident in which the core melts and relocates to the lower head of the reactor vessel. The IVR-ERVC strategy of APR1400 consists of flooding the reactor cavity so that the decay heat from the molten corium is removed by cooling the external vessel wall. To know the limit of coolability of external vessel wall cooling, the critical heat flux (CHF) on the reactor vessel outer wall was measured using the two-dimensional slice test section for various experimental conditions. The CHF experiments were performed under flow boiling condition using the experimental water loop.


  CHF Enhancement using Nanofluids  One of the key research areas in the cooling systems of nuclear reactors, nuclear fusion reactors, thermal power plants and others is the removal of high heat flux to guarantee the efficiency, performance and safety. In process of removing high heat flux, nucleate boiling is a very effective heat transfer mechanism, however it is well known that there exists a critical value of heat flux at which nucleate boiling transitions to film boiling shows very poor heat transfer behavior. Critical heat flux(CHF) is a main constraint to the design process because it can generate the damages or deformations of material. We have been studying the critical heat flux enhancement using nanofluids, to improving economic and safety performance of cooling system. To clarify the boiling mechanism and enhancement of CHF in nanofluids, experimental researches have been performing on the conditions of pool boiling and flow boiling.


  Design of an Innovative Power Conversion System for a Sodium-cooled Fast Reactor - Supercritical binary gas mixture Brayton cycle  Conventional Nuclear Power Plants (NPP) are able to use only 3~4 % of uranium resources. To expand the utilization of uranium fuel and reduce the High-Level Waste (HLW), a new concept of NPP has been under development, such as fast reactors. Fast reactors convert useless uranium to useful resources. Among various types of fast reactors, the Sodium-cooled Fast Reactor (SFR) is one of strongest candidates for the next generation of nuclear reactors. The SFR has a primary loop with liquid sodium as a coolant. However, most of the existing designs of SFRs have a steam Rankine cycle as a power generation cycle. This has the potential of the occurrence of a sodium-water reaction which can deteriorate the safety of a SFR. To prevent any hazards from a sodium-water reaction, an SFR with the Brayton cycle using Supercritical Carbon dioxide (S-CO2) as working fluids can be used as an alternative approach that improves upon the current SFR design. The S-CO2 cycle is more sensitive to the critical point of working fluids than general Brayton cycles. This is because compressor work significantly decreases at slightly above the critical point. For this reason, the critical point acts as a limitation of the lowest operating condition of the cycle. By changing the critical point, total cycle efficiency can be enhanced. Modifying the critical point of the working fluid can be done by adding other gases to CO2. In order to design safe and more efficient power conversion systems for SFRs, we are going to evaluate the improvement using multi-component gas as a coolant using a developed Brayton cycle code and turbomachinery code. In particular, critical point measurement of binary gas mixtures will be conducted to validate the simulated results.