Atsuhiko Terada

Thermochemical water-splitting cycle using iodine and sulfur

Research and development on the thermochemical water-splitting cycle using iodine and sulfur, a potential large-scale hydrogen production method, is reviewed. Feasibility of the closed-cycle continuous water splitting has been demonstrated by coupling the Bunsen reaction, thermal decomposition of hydrogen iodide and that of sulfuric acid. Studies are in progress to realize efficient hydrogen production. Also, development of chemical reactors made of industrial materials has been carried out, especially those used in the corrosive process environment of sulfuric acid vaporization and decomposition.

Development of Hydrogen Production Technology by Thermochemical Water Splitting IS Process Pilot Test Plan

Japan Atomic Energy Agency (JAEA) has been conducting a study on a thermochemical IS process for hydrogen production. A pilot test of IS process is under planning that covers four R&D subjects: (1) construction of a pilot test plant made of industrial materials and completion of a hydrogen production test using electrically-heated helium gas as the process heat supplier, (2) development of an analytical code system, (3) component tests to assist the hydrogen production test and also to improve the process performance for the commercial plant, (4) a design study of HTTR-IS system. Development of innovative chemical reactors is in progress, which are equipped with a ceramic heat exchanger. In the design of the IS plant, it is important to establish the system for “design by analysis”. Therefore, we have developed a multiphase flow analysis code that can analyze systems in which chemical reactions occur.

Characterization and storage of radioactive zeolite waste

For the safe storage of zeolite wastes generated by the treatment of radioactive saline water at the Fukushima Daiichi Nuclear Power Station, this study investigated the fundamental properties of herschelite adsorbent and evaluated its adsorption vessel for hydrogen production and corrosion. The hydrogen produced by the herschelite sample is oxidized by radicals as it diffuses to the water surface and thus depends on the samples water level and dissolved species. The hydrogen production rate of herschelite submerged in seawater or pure water may be evaluated by accounting for the water depth. From the obtained fundamental properties, the hydrogen concentration of a reference vessel (decay heat = 504 W) with or without residual pure water was evaluated by thermal–hydraulic analysis. The maximum hydrogen concentration was below the lower explosive limit (4%). The steady-state corrosion potential of a stainless steel 316L increased with the absorbed dose rate, but the increase was repressed in the presence of herschelite. The temperature and absorbed dose at the bottom of the 504 W vessel were determined as 60 °C and 750 Gy/h, respectively. Under these conditions, localized corrosion of a herschelite-contacted 316L vessel would not immediately occur at Cl− concentrations of 20,000 ppm.

Flowsheet Study of a Multistage Flash Desalination System for Cogeneration With High Temperature Gas-Cooled Reactor

High temperature gas-cooled reactor (HTGR) produces not only electricity but also high temperature heat for a variety of heat applications. In the Middle East countries, large demand exists for cogeneration of pure water and electricity from desalination plant coupling with power station. Desalination with an HTGR direct cycle gas turbine system can efficiently meet this demand because such system can produce pure water from using only the waste heat of power generation via secondary pressurized water loop. The waste heat of up to 248MWt is available for desalination from the reactor system of 600 MWt thermal power. In this paper, heat and mass balance was calculated for a new concept of desalination system which is shown to increase use of waste heat by incrementing the number of thermal loading steps at the heat recovery section. Calculation is performed at steady water production condition to clarify the optimum steps of incremental loading based on engineering considerations. As the result, it was found that heat transfer area of heat recovery section in case of brine heater number 3 was 28% smaller than that of number 2.Copyright

Experimental study on heat transfer and pressure drop in mercury flow system for spallation neutron source

In the design of MW-class spallation target system, using mercury to produce practical neutron applications, keeping the highest level of safety is vitally important. To establish the safety of spallation target system, it is essential to understand the thermal hydraulic properties of mercury. Through thermal hydraulic experiments using a mercury experimental loop, which flows at the rate of 1.2 m3/hr maximum, the following facts were experimentally confirmed. The wall friction factor was relatively larger than the Blasius correlation due to the effects of wall roughness. The heat transfer coefficients agreed well with the Subbotin correlation. Furthermore, for validation of the design analysis code, thermal hydraulic analyses were conducted by using the STAR-CD code under the same conditions as the experiments. Analytical results showed good agreement with the experimental results, using optimized turbulent Prandtl number and mesh size.

Study of hydrogen mitigation for safe storage of spent cesium adsorption vessels

Hydrogen mitigation is one of the important issues for safe storage of spent cesium adsorption vessels that were used in the contaminated-water treatment facility at the Fukushima Daiichi Nuclear Power Station because the production and accumulation of hydrogen is induced by the radiolysis of residual water in the vessel. In the present study, an experimental examination was performed using a miniature acrylic vessel to simulate an upper section of the vessel using particle image velocimetry to clarify the internal flow of hydrogen-mixed gas and to verify the analytical results by the ANSYS Fluent code. As a result, weak upflow and circulating flow at the stepped section were successfully visualized, and the validity of the analytical results was confirmed by the flow patterns. Additionally, the practicality of a recombination catalyst for hydrogen and oxygen was considered as a passive autocatalytic recombiner in hydrogen mitigation. A catalytic reaction test was conducted to evaluate its effectiveness. The results showed that the catalyst retains activity under the humid condition assumed in the real vessel.

Core Design Study of Small-Sized High Temperature Reactor for Electricity Generation

A High Temperature Gas-cooled Reactor (HTGR) has several features different from conventional light water reactors such as inherent safety characteristics, high thermal efficiency and high economy. On the other hand, one of disadvantages of the HTGR with a prismatic core is to require rather long-term and expensive refueling, resulting in relatively long maintenance period and high cost. To solve the disadvantage, the present study challenges the core design of a small-sized reactor for long refueling interval by increasing core size, fuel loading and fuel burn up compared with the High Temperature engineering Test Reactor (HTTR). The preliminary burn-up calculation suggested that approximately 6 years of long refueling interval was found to be reasonably achieved. A refueling interval longer than 6 years may be possible by decreasing further power density, subsequently larger core size with operational reactor power of 120MWt, but this idea was not taken by the requirement of the reactor that the core size shall be accommodated reasonably in the core with double size of the HTTR at maximum.Copyright

Experimental Test Plan of Air Ingress for HTGR

Primary pipe rupture is an important design base accident in the high temperature gas cooled reactor (HTGR). When a primary pipe of the HTGR ruptures, helium coolant gas in the reactor blows out into the reactor confinement structure and the reactor primary system depressurizes. After the pressures of the reactor and the confinement equalizes, air is expected to enter the reactor core from the breach. Consequently, the core graphite structures may be oxidized by the air and the complicated natural convection of multi component gas mixtures with chemical reactions would take place inside the reactor. Hence, it is necessary to investigate the air ingress process, the natural convection of multi component gas mixtures in order to understand the effect and develop mitigation of the air ingress. JAEA has performed analysis and fundamental experiments about air ingress from the rupture of one or more main coolant pipes on the lower body of the RPV. These studies showed the air ingress phenomena in the depressurized reactor and proposed a new passive mechanism of sustained counter air diffusion (SCAD) that has been shown effective in preventing major air ingress through natural circulation in the reactor. In the present plan, JAEA will construct an experimental reactor mockup including reactor core, the SCAD system, pressure vessel, coaxial pipe and so on. The core is made of graphite or ceramics and heated by electric heaters to allow for test operation up to 1200°C. Present status of these activities will be presented. Based on the analytical results and know-how obtained through the bench test, a 1/8 scale air ingress mockup test, which intends to simulate the accident condition of GTHTR300, is being planned with a conceptual design as the next step of the air ingress experiment evaluation in JAEA. In the design of mockup experimental facility, it is important to reproduce flow phenomena in a reasonable scale from the viewpoint of construction cost. We designed the internal structure to reproduce mixing performance of multi-component flow involving ingress phenomena especially in the guillotine breaks of primary coolant pipe. Complex flow pattern with gas oxidized chemical interaction in the graphite porous structure of the HTGR core will be characterized. Preliminary analytical results especially natural circulation flow patterns induced by density and concentration difference obtained with a CFD model agreed well with that measured by bench experiments, which showed natural circulation pattern in a simplified reactor.Copyright

Fracture Strength Estimation of SiC Block for IS Process

The Japan Atomic Energy Agency has been conducting R&D on thermochemical water-splitting Iodine-Sulfur (IS) process for hydrogen production to meet massive demand in the future hydrogen economy. A concept of sulfuric acid decomposer was developed featuring a heat exchanger block made of SiC. Recent activity has focused on the reliability assessment of SiC block. Although knowing the strength of SiC block is important for the reliability assessment, it is difficult to evaluate a large-scale ceramics structure without destructive test. In this study, a novel approach for strength estimation of SiC structure was proposed. Since accurate strength estimation of individual ceramics structure is difficult, a prediction method of minimum strength in the structure of the same design was proposed based on effective volume theory and optimized Weibull modulus. Optimum value of the Weibull modulus was determined for estimating the lowest strength. The strength estimation line was developed by using the determined modulus. The validity of the line was verified by destructive test of SiC block model, which is small-scale model of the SiC block. The fracture strength of small-scale model satisfied the predicted strength.Copyright

Development Program of IS Process Pilot Test Plant for Hydrogen Production With High-Temperature Gas-Cooled Reactor

At the present time, we are alarmed by depletion of fossil energy and effects on global environment such as acid rain and global warming, because our lives depend still heavily on fossil energy. So, it is universally recognized that hydrogen is one of the best energy media and its demand will be increased greatly in the near future. In Japan, the Basic Plan for Energy Supply and Demand based on the Basic Law on Energy Policy Making was decided upon by the Cabinet on 6 October, 2003. In the plan, efforts for hydrogen energy utilization were expressed as follows; hydrogen is a clean energy carrier without carbon dioxide (CO2 ) emission, and commercialization of hydrogen production system using nuclear, solar and biomass, not fossil fuels, is desired. However, it is necessary to develop suitable technology to produce hydrogen without CO2 emission from a view point of global environmental protection, since little hydrogen exists naturally. Hydrogen production from water using nuclear energy, especially the high-temperature gas-cooled reactor (HTGR), is one of the most attractive solutions for the environmental issue, because HTGR hydrogen production by water splitting methods such as a thermochemical iodine-sulfur (IS) process has a high possibility to produce hydrogen effectively and economically. The Japan Atomic Energy Agency (JAEA) has been conducting the HTTR (High-Temperature Engineering Test Reactor) project from the view to establishing technology base on HTGR and also on the IS process. In the IS process, raw material, water, is to be reacted with iodine (I2 ) and sulfur dioxide (SO2 ) to produce hydrogen iodide (HI) and sulfuric acid (H2 SO4 ), the so-called Bunsen reaction, which are then decomposed endothermically to produce hydrogen (H2 ) and oxygen (O2 ), respectively. Iodine and sulfur dioxide produced in the decomposition reactions can be used again as the reactants in the Bunsen reaction. In JAEA, continuous hydrogen production was demonstrated with the hydrogen production rate of about 30 NL/hr for one week using a bench-scale test apparatus made of glass. Based on the test results and know-how obtained through the bench-scale tests, a pilot test plant that can produce hydrogen of about 30 Nm3 /hr is being designed. The test plant will be fabricated with industrial materials such as glass coated steel, SiC ceramics etc, and operated under high pressure condition up to 2 MPa. The test plant will consist of a IS process plant and a helium gas (He) circulation facility (He loop). The He loop can simulate HTTR operation conditions, which consists of a 400 kW-electric heater for He hating, a He circulator and a steam generator working as a He cooler. In parallel to the design study, key components of the IS process such as the sulfuric acid (H2 SO4 ) and the sulfur trioxide (SO3 ) decomposers working under-high temperature corrosive environments have been designed and test-fabricated to confirm their fabricability. Also, other R&D’s are under way such as corrosion, processing of HIx solutions. This paper describes present status of these activities.Copyright