092-802-2942

092-802-2942

Ph.D. in Engineering

Physical Chemistry of High Temperature Melts and Development of Novel Sensing Technology for High-temperature Processing

01years06months

　Inorganic base materials (metals, glass, and ceramics), which are commonly found in our surroundings and boast a huge industrial scale, are all produced through manufacturing processes at high temperatures. Through joint research with related companies and universities, the Nakashima Laboratory is working to unravel the complex phenomena that occur at high temperatures through various experiments and observations, and to provide guidelines for the smooth and efficient control of the high-temperature manufacturing processes of base materials.
　In addition to mass-produced base materials, we are also engaged in the research and development of a wide range of high-functional glasses and ceramics. For example, we are collaborating with a number of companies on materials and analytical methods that are opening up new markets, such as ultra-high temperature heat resistant ceramics (UHTCs) with melting points exceeding 3000°C and impedance tomography, which provides 4-dimensional visualization of the inside of materials melted at high temperatures.
　As described above, we provide an environment where students can hone and develop their research and presentation skills in a wide range of fields through meetings with companies and other universities, including those overseas.

Research Interests

• Property Evaluations of Oxide Melts characterized by AC Field
  keyword : oxide melt, structure, suspension, foam, emulsion
  2016.04～2026.03.
• Partial liquid phase bonding of advanced ceramics and diffusion phenomena at the bonding interface
  keyword : PTLP bonding, refractory metal, UHTCs, Borides, Carbides
  2008.01.
• Fabrication of translucent alumina and yttria ceramics
  keyword : translucent, pressure-less sintering
  2007.03～2013.03.
• Evaluation of viscosity and crystallization behavior of oxide melts at high temperature
  keyword : silicate, multi-phases, viscosity, non-newtonian fluid, electric capacity
  2006.04.

Current and Past Project

• Nuclear power is one of Japan's major power sources, and is attracting increasing global interest due to demand for deoxygenation and stable energy supplies. One of its problems is the disposal of high-level radioactive waste. Japan plans to use high-temperature melting furnaces to immobilize the glass and then store it underground for interim storage, but since the past accident, Japan has not been able to acquire this technology on its own. The goal of this project is to develop a machine learning method to predict and control the physical properties of liquid waste and glass melts. First, by combining high-temperature experiments with machine learning, robust prediction of the physical properties of waste liquid-glass melts and solidified products will be achieved. Next, we address the optimization problem of determining the constituents of the glass that can incorporate more types and quantities of liquid waste.
• This study aims to synthesize hydroxyapatite from artificial steel slag as a sole raw material without adding other main catalysts, and to develop functional photocatalytic composites that promote lignin degradation and hydrogen production by inducing its visible light responsiveness, joining different crystals, and imparting magnetism, as appropriate by DFT structural relaxation calculations. Improve the photocatalytic activity by doping with different elements and adding electron trapping level by atom deficiency. By developing photocatalytic composites that can be mass-produced, we will link the carbon cycle with the iron and hydrogen cycles controlled by the steelmaking process, a key industry, and contribute to both the global environment and the economy toward the realization of a carbon-negative society.
• 　Eleven years have passed since the severe accident at the Fukushima Daiichi Nuclear Power Station (hereinafter referred to as "1F") of Tokyo Electric Power Holding Co. Eleven years have passed since the severe accident at the Fukushima Daiichi Nuclear Power Station (1F), and measures to remove fuel debris are now being considered in a manner consistent with the in-vessel damage status and debris distribution at each unit. The understanding of the in-vessel conditions and the refinement of accident scenarios are expected to serve as basic data for safe and as rapid decommissioning as possible. On the other hand, as the situation inside the accident reactor is gradually clarified through the progress of the 1F internal investigation and 1F sample analysis, it is becoming clear that the actual situation inside the 1F reactor is different from the "estimation from typical accident conditions" evaluated based on the analysis of the Three Mile accident in the United States, which was more than assumed in the past. The accident scenario analysis "Forward Analysis" using accident progression analysis codes, etc., which has been conducted in the past, has reached its limits in refining the understanding of such 1F-specific in-pile conditions. Therefore, there is a need for "Backward Analysis," which is based on field knowledge to identify critical accident events that have not been sufficiently studied in the past, to accurately understand and model the phenomena, and to acquire and verify more accurate and appropriate data through material experiments.
  　In this study, as urgent issues in the current situation in the 1F reactor, we are investigating the Cs migration pathway during the accident to determine the cause of the high dose under the shield plugs in Units 1F Units 2 and 3, the state of Cs deposition and attachment to structural materials, and the characterization of metal-rich debris that is estimated to have melted down from the RPV to the PCV in Units 1F Units 2 and 3. Backward Analysis focusing on the characterization of metal-rich debris (evaluation of oxidation characteristics at the time of dissolution), which is presumed to have been dissolved from the RPV to the PCV in Units 1F2 and 3. In "Research Topic (1) Reducing the uncertainty of Cs distribution evaluation," the chemical environment and mass transport during the accident in the RPV and PCV were evaluated based on the results of MAAP analysis based on the most probable scenario of the accident progression in Units 2 and 3, and the changes in Cs chemical forms were preliminarily evaluated. In addition, thermal analysis tests were conducted in a steam atmosphere using Cs-containing samples, and the reaction tendency of Cs with steel and other materials was preliminarily investigated. In order to predict the influence of residual Cs in fuel debris on the flowability of oxide melts containing concrete components and their Cs release behavior during the MCCI process, we measured the viscosity of CeO2(-ZrO2)-CaO-Al2O3-SiO2 melts at high temperatures and also measured the viscosity of Cs2O-Fe2O3 aerosol formation phenomenon. These results indicate that even a small amount of residual Cs dramatically reduces the viscosity of oxide melts containing concrete components, but has only a very limited effect on accident propagation events because the Cs is immediately released due to increased flowability. Furthermore, the Cs2O-FeO system has high chemical affinity and melts easily, and depending on the in-core conditions, the reaction of residual Cs from the fuel debris with molten stainless steel may have generated a Cs-Fe-O aerosol, which was not previously expected and may have contributed to the high-dose conditions above the RPV. In "Research Topic (2) Evaluation of Oxidative Transformation of Metal Debris," the oxidative transformation of simulated metal debris was preliminarily evaluated by thermal analysis tests in a steam atmosphere. Preliminary experiments on molten Fe-Zr metal melts and ZrO2 equilibrium, which is the reaction basis of molten metal debris and oxide debris, were initiated, and the validity of the Zr activity measurement method necessary to determine the unoxidized Zr reaction contribution ratio in the metal melts, an important factor in clarifying the pressure vessel failure mechanism, was confirmed. Furthermore, we determined the conditions for measuring the viscosity of metal debris, which is indispensable for predicting the flow of solid-liquid coexisting metallic materials that will be discharged at the time of pressure vessel failure. Through these efforts, we established a method for extracting and measuring elemental parameters for analyzing the process of metallic debris remelting in the lower plenum and melting steel materials while being oxidized and transformed by water vapor, leading to the pressure vessel failure. In "Research Topic (3) Comprehensive Evaluation," the results of each research topic were comprehensively considered to identify events and scenarios to be studied intensively in this study. In order to reduce the uncertainty of Cs migration behavior in high dose situations, which has been an issue in the field, we focused on the Cs reaction in high temperature and steam depleted conditions and the migration to the top of the RPV focusing on the aerosol formation process during the release from the fuel debris, and on the Cs reaction and the aerosol formation process during the remelting of metallic debris. The pressure vessel failure event associated with the remelting of metallic debris is important to verify the elemental reactions in each of oxidation and transformation by water vapor (gas-liquid reaction) and steel melting (solid-liquid reaction), which are in a competitive relationship.
  As described above, in FY2021, we will start the development of the necessary infrastructure and preliminary experiments to promote the experimental research and simulation study on each elemental technology method, as well as to develop a mutual understanding of the two issues of Backward Analysis extracted in this study through mutual collaboration with Forward Analysis specialists. In addition, we discussed the policy to improve the understanding of the phenomena and the in-core situation through mutual collaboration with Forward Analysis experts on the two issues of Backward Analysis identified in this study. As a result, we were able to share our understanding of the chemical state changes and migration pathways of fuel debris and Cs with reference to the most probable scenario of accident progression, and we made a prospect for future research development.
  In addition, this study aims to utilize the cooperative relationship with St. Petersburg University in Russia, which is well known for thermodynamic evaluation of multi-systems including FP-derived oxides, to improve the evaluation for reducing uncertainty in estimating the in-core contamination status. In Russia, as part of nuclear safety research against the background of the 1F accident, the advancement of Cs evaporation transfer models and MCCI analysis methods for accident progression is being considered. Therefore, in order to expand the knowledge on the adsorption and re-evaporation of Cs once evaporated into ex-vessel debris, Cs2O -SrO-Al2O3-SiO2 system including Cs oxides and concrete components. On the Japanese side, in addition to the Cs physical property data at high temperatures among the data acquired at St. Petersburg University, the objective is to evaluate the effect of Sr on the FP contamination status in the reactor by utilizing the physical property data of Sr with moderate evaporability, which we do not directly conduct.
  　In FY2021, we determined the test conditions to start with data acquisition for the Cs2O-Al2O3 and SrO-Al2O3 binary systems, which are the subsystems of the multi-system system we are targeting. Next, as preliminary experiments, two samples were prepared for the Cs2O-Al2O3 system and three samples for the SrO-Al2O3 system, and phase identification of the microstructures in the fabricated samples was conducted by powder XRD analysis.
  　In addition, a literature survey was conducted on how to conduct high-temperature experiments (up to 2700 K) on Cs- and Sr-containing multinary systems for full-scale tests scheduled to begin in FY2022. The thermodynamic evaluation experiment at St. Petersburg University is based on high-temperature mass spectrometry using a Knudsen cell. The sample is held at elevated temperatures up to the target temperature, and the data from the ionization of the gas produced by the instrument is analyzed to determine the vapor pressure of the gassed compound. Therefore, it was reported that preliminary experiments determine the compound form that is generated and ionized when the target system is gasified.
  
• Specialty steels and alloy steels are important products of Japan with excellent strength, toughness, corrosion resistance, etc. Japan's great strength lies in its ability to stably supply products with few impurities and defects. Many special steels and alloy steels use chromium, which is more easily oxidized than iron, and the slag from the melting of chromium steel contains chromium oxide. It is necessary to ensure the safety and security of the products for long-term use in changing environments and to develop new applications. To this end, it is ideal to develop a smelting process that produces slag with a low concentration of chromium oxide and easy processing, while achieving a highly efficient smelting reaction, to clarify the conditions under which trivalent chromium in the slag is stable, and to reduce the content of chromium oxide in the slag to a level where it is not a problem.
  Reduction of chromium oxide in the slag while conducting highly efficient refining through slag control to obtain slag with low chromium oxide concentration that is easy to process. A cascade process that maximizes the reducing power of aluminum, silicon, carbon, and other elements is used to achieve highly efficient reduction of chromium oxide in the slag, recover almost all of the chromium, and obtain a chromium-oxide-free slag. In Japan, about 80% of ferrochrome, or 590,000 tons of net chromium content, is used in stainless steel (JOGMEC), and it is reported that 8.3% of chromium is discharged outside the system in the conventional process (Kato et al: Nippon Steel Technical Report, 414 (2019), 124). If this process is realized, it will have a significant ripple effect on chromium supply and demand.
  
• n the metal materials industry, the by-product of the oxide melt melted at high temperature does not form a uniform liquid phase, but forms a complex fluid in which oxide solids and the like are dispersed. Therefore, the applicants have constructed a prediction model for the rheological properties of these high-temperature oxide suspensions by combining high-temperature experiments and machine learning. However, at present, the effect of the repulsive force due to the "charge" at the interface between the oxide solid and the oxide melt having a huge relative permittivity on the macro-rheological properties cannot be considered. Therefore, in this application, the interfacial charge between the high-temperature oxide melt and the solid is quantitatively measured using the AC impedance method, and the change in rheological characteristics due to the charge is measured with high accuracy. Furthermore, the rheological characteristic prediction model that has been constructed will be improved in accuracy and robustness from the viewpoint of interfacial charge.
• In the steel refining process, the slag by-product is not basically a homogeneous melt, but a complex mixture of solids such as untreated CaO and coal, gases such as CO gas generated by the reaction between hot metal and slag, and liquids such as hot metal entrained by forming slag, forming a high temperature fluid. These fluids form a complex mixture of high-temperature fluids, and there are a number of problems that are closely related to them. For example, the non-negligible amount of grain iron loss caused by the forming slag produced in the hot metal pretreatment process, the un-tailings CaO caused by excessive input of refining agents to ensure steel quality, and the increasing density of the forming slag to add value to the discharged slag, these problems are caused by the following factors The problems are thought to be caused by the lack of understanding of the behavior of the second phase in the slag matrix at high temperatures. In this study group, we will form a research platform that contributes to solving the above-mentioned problems by visualizing the flow and mass transfer phenomena of multi-phase slag through high-temperature experiments using new methods and computational science, including machine learning.
• Transient-Liquid-Phase Bonding of UHTCs using Refractory-metal-based Interlayer for High-efficiency Energy-generation Applications
  

Academic Activities Membership in Academic Society

• The Iron and Steel Institute of Japan
• The Minerals, Metals & Materials Society (TMS)
• The American Ceramic Society
• Japan Society of Thermophysical Properties
• The Japan Institute of Metals
• The Ceramic Society of Japan
• The Mining and Materials Proceessing Institute of Japan

Educational Activities