Shinji Yasui

Vaporization Behavior of Cs in Plasma Melting of Simulated Low Level Miscellaneous Solid Wastes.

A DC transfer type plasma with a maximum output of 60 kW was used to melt 10 kg of carbon steel and 3 kg of granular oxide mixtures ( Fe2O3, Al2O3, SiO2 and CaO) containing non-radioactive cesium nitrate, for measuring Cs vaporization. These materials are the main components of low level miscellaneous solid wastes. The vaporization rate constant values obtained by approximating Cs vaporization by a first order reaction rate formula were in the range of (4.01–7.1)×10-6 (m/s). The vaporization rate constant of Cs did not depend on the initial Cs content, and increased with the increase of slag basicity ( %CaO/%SiO2). Since the vaporization rate constant was unaffected by the viscosity of the slag on which the Cs transfer rate in the slag depends, the Cs vaporization phenomenon from the slag can be estimated to be controlled by the vaporization step from the slag surface.

Distribution properties of lightning surge current induced from earth electrodes in household wiring

In the modern information society, lightning surges represent a serious risk to home electronics and telecommunication equipment. In this study, the distribution properties of a lightning surge current from the earth through indoor wiring were investigated by using model circuits of common home electronics. The distribution properties of the surge current were significantly affected by the conditions of the grounding systems, protection devices, and the circuits of home electronics. To protect the electrical circuits from the lightning surge, it is necessary to adopt a common grounding system and to introduce appropriate protection devices.

Deposition of SiO2 Thin Films on Polycarbonate by Atmospheric-Pressure Plasma

Low-temperature, atmospheric-pressure SiO2 thin film deposition was achieved using dielectric barrier discharge plasma with a high-voltage pulse. Experiments were conducted using diethoxydimethylsilane as the silicon source at different oxygen concentrations, gas flow rates, and deposition times. Hard, transparent thin-coated films were obtained at high oxygen concentrations because of the capture of carbon by oxygen during deposition. A deposition rate of 60 nm/min was achieved with the pulsed plasma at a duty ratio of 4%, gap length of 1 mm, and peak voltage of 13 kV without cooling or heating of the polycarbonate substrate. A usable SiO2 hard-coat film with a thickness of 1.6 µm and an indentation hardness of 1480 N/mm2 was obtained after a deposition time of 30 min in oxygen.

Vaporization Rate of Cesium from Molten Slag in a Plasma Melting Furnace for the Treatment of Simulated Low-Level Radioactive Wastes

Abstract The vaporization phenomena of cesium (Cs) from molten slag have been investigated in a plasma melting process for simulated radioactive waste materials. A direct current transfer-type plasma with a maximum output of 50 kW was used to melt carbon steel and granular oxide mixtures (Fe2O3, Al2O3, SiO2, CaO, and MgO) containing nonradioactive cesium nitrate, to measure Cs vaporization. These materials are the main components of low-level miscellaneous solid wastes. The vaporization rate of Cs from the molten slag during the plasma melting was observed and was compared with the vaporization rate obtained in an electric resistance furnace. The apparent vaporization rate of Cs was found to follow the first-order rate equation with respect to the molten slag’s Cs content, and its rate constant values varied (3.5 to 21.0) × 10-6 m/s varying with the chemical composition of the miscellaneous solid wastes. These rate constants were about one order larger than those obtained in the electric resistant furnace and also the diffusion coefficients of basic elements in the molten slag. These results suggest that the vaporization rate of Cs is controlled by the vaporization step from the free molten slag furnace to the gas phase and depends predominantly on the thermodynamic properties of the molten slag.

Cause investigation of diode rectifier trouble by lightning in traction substation

On August 17th, 2012, a diode rectifier at Nagano Traction Substation was broken. The broken rectifier was dismantled and investigated in detail and, as a result, it was estimated that the overvoltage caused by the thunder lightning caused the dielectric breakdown in the rectifier. To confirm the assumption of the broken process, the field measurement at Nagano Traction Substation and FDTD (finite-difference time-domain) calculation analysis at Nagoya Institute of Technology were carried out. As a result, it was indicated that the direct lightning to the elevated bridge over the substation caused the transient overvoltage of the grounding system of the substation and resulted in the breakdown within the rectifier. Because the concrete bridge pier is located adjacent to the substation, the surge voltage from the elevated bridge is able to influence on the potential of the substation mesh. In this paper, the investigation process to determine the cause of the trouble will be shown in detail.

Formation of Diamond-Like Carbon Thin Films by Using Surface Discharge Plasma under Atmospheric Pressure

The deposition of diamond-like carbon (DLC) thin films was conducted by using surface discharge plasma under atmospheric pressure. The high-frequency surface discharge plasma was generated by applying a pulsed voltage between electrodes. The main radicals generated by the surface discharge were H, CH, and C2 by using H2, CH4, and He as plasma gases. The optical emission intensities of these radicals increased with increasing pulse width and decreasing pulse cycle of the applied pulse voltage. By increasing the length of the surface discharge, the ratios Hβ/Hα and C2/Hα increased. It was demonstrated that a hard DLC film can be obtained at a lower deposition rate under the experimental conditions.

Gas-Solid Reaction Properties of Fluorine Compounds and Solid Adsorbents for Off-Gas Treatment from Semiconductor Facility

We have been developing a new dry-type off-gas treatment system for recycling fluorine from perfluoro compounds present in off-gases from the semiconductor industry. The feature of this system is to adsorb the fluorine compounds in the exhaust gases from the decomposition furnace by using two types of solid adsorbents: the calcium carbonate in the upper layer adsorbs HF and converts it to CaF2, and the sodium bicarbonate in the lower layer adsorbs HF and SiF4 and converts them to Na2SiF6. This paper describes the fluorine compound adsorption properties of both the solid adsorbents—calcium carbonate and the sodium compound—for the optimal design of the fixation furnace. An analysis of the gas-solid reaction rate was performed from the experimental results of the breakthrough curve by using a fixed-bed reaction model, and the reaction rate constants and adsorption capacity were obtained for achieving an optimal process design.

Observation of lightning current in the soil by rocket-triggered lightning

In recent years, there has been a significant increase in the damage to electrical and electronic devices owing to lightning surge-induced overvoltage/overcurrent, in Japan. In particular, the lightning surge overcurrent that flows through grounding lines can cause serious damage to electronic equipment. To understand the phenomenon of the lightning surge current propagation in the ground poles, it is important to understand the lightning current propagation in the soil. We have developed a detection circuit for determining the lightning current distribution in the soil and have measured the lightning current caused by rocket-triggered lightning in the soil. We have observed the lightning current in the soil owing to a negative precursor discharge at a depth of 0.1 m, 30 m from the triggered flash point.

Formation of diamond-like carbon thin films using barrier-type surface discharge plasma under atmospheric pressure

We studied the deposition of diamond-like carbon (DLC) thin films using barrier-type surface discharge plasma under atmospheric pressure. The main radicals generated by the barrier-type surface discharge using H2, CH4, and He as the plasma gases were Hα, Hβ, and CH. The emission intensities increased as the ratio of CH4 in the mixed gas decreased, and the mixed gas ratios of 2% CH4, 18% H2, 80% He were appropriate for the generation of the barrier-type surface discharge. The gas flow rate and applied voltage required to achieve a suitable plasma state for deposition of the DLC films varied depending on the polarity of the applied pulse. When a negative pulse is used, homogenous films can be obtained on the silicon wafer under the entire hole of the electrode; however, the deposition rate becomes very low in the range of 1.8–5.8 nm/min because the surface streamer plasma is very weak. On the other hand, using a bipolar and a positive pulse, a relatively high deposition rate in the range of 10–30 nm/min can be achieved on the silicon wafer under the central part of the electrode, although the thickness of the DLC films becomes nonuniform at the edge part of the electrode. The appropriate conditions of the DLC film deposition in this study were the pulse voltages of 6–8 kV and a gas flow rate of 1500 mL/min when using bipolar- and positive-pulse voltages. The relatively hard DLC films (6–8 GPa) were obtained under these conditions.

Sterilization of Fusarium oxysporum by treatment of non-thermalequilibrium plasma in nutrient solution

Fusarium wilt of spinach due to F. oxysporum infection is one of the most destructive root diseases in hydroponics in factories using the nutrient film technique. We investigated new technologies for the sterilization of microconidia of F. oxysporum by using a non-thermalequilibrium plasma treatment method in nutrient solution. Specifically, we investigated the sterilization capabilities of five types of gas (air, O2, N2, He, and Ar) used for plasma generation. The highest sterilization capability was achieved by using O2 plasma. However, ozone, which causes growth inhibition, was then generated and released into the atmosphere. The sterilization capability was lower when N2 or air plasma was used in the nutrient solution. It was confirmed that sterilization can be achieved by plasma treatment using inert gases that do not generate ozone; therefore, we determined that Ar plasma is the most preferable. In addition, we investigated the sterilization capabilities of other factors associated with Ar plasma generation, without direct plasma treatment. However, none of these other factors, which included Ar bubbling, pH reduction, increased temperature, hydrogen peroxide concentration, and UV radiation, could completely reproduce the results of direct plasma treatment. We assume that radicals such as O or OH may contribute significantly to the sterilization of microconidia of F. oxysporum in a nutrient solution.