A general view was given on the fundamental characteristics of supercritical water based on spectroscopic data. In situ Raman and FTIR spectroscopies were used for studying water in super- and sub-critical conditions, in which a feature of the OH symmetric stretching was understood. The results were compared with those in proton NMR studies reflecting the strength of the hydrogen bonding.

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The treatment of waste is becoming a important social problem. In this situation, various processes for treating waste utilizing characteristics of supercritical water are investigated. As one of such processes, a new oil conversion process in supercritical water has been developed, and a demonstration plant of 1 ton/day capacity has been constructed. We successfully liquefied scrap of plastic materials of electric cable made of cross-linked polyethylene and obtained the liquefaction yield of more than 80%.

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In this paper, we describe specific features of reactions in supercritical water. Around the critical point, phase behavior and reaction rate or kinetics varies significantly. In higher temperature region, contribution of noncatalytic reaction becomes significant even for hydrolysis, dehydration, or other heterolitic reactions. Complex contribution of these factors and the change of these effects sometimes lead to the appearance of extraordinary reactions. Previously reported researches on organic and inorganic synthesis reactions in supercritical water are summarized, referring to these key features, and controllability of reactions around the critical point is pointed out based on these results. Especially for the hydrothermal synthesis of metal oxide fine particles, the method of which we have proposed and developed, detailed explanation and discussion has been made. Key features of the method (control of phase, reactions and crystallization) are summarized and then applications of this method to the continuous production of magnetic materials (BaO₆ Fe₂O₃), Phosphor (YAG : Tb), and lithium ion battery materials (LiCoO₂) are explained, with discussing the possibility of creating new materials.

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There is a need to destroy both military and civilian hazardous waste and an urgency, mandated by public concern over traditional waste handling methodologies, to identify safe and efficient alternative technologies. One very effective process for the destruction of such waste is supercritical water oxidation (SCWO). By capitalizing on the properties of water above its critical point, (374°C and 221 atm for pure water), this technology provides rapid and complete oxidation with high destruction efficiencies at typical operating temperatures. Nevertheless, corrosion of the materials of fabrication is a serious concern and the practicality of SCWO may be limited by the ability to control corrosion. This paper reviews the literature on the degradation characteristics of a number of candidate materials of fabrication, including iron, nickel, and titanium-base alloys, ceramics, and noble metals. In addition, a number of potential methodologies for reducing corrosion damage in SCWO systems have been reviewed.

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Effects of annealing conditions on stress corrosion cracking (SCC) of SM 400B carbon steel has been investigated in 1M sodium bicarbonate solution at 343K. A potentiostatic slow strain rate test equipped with a charge coupled device camera system was employed to evaluate SCC susceptibility from the viewpoint of the crack behavior. Fixing an annealing time of 3.6 ks, no SCC appeared on the specimens annealed below 973 K, but intergranular SCC occurred beyond 1073 K. In the case that the SCC occurred, an increase in annealing temperature led to a decrease in crack initiation time and an increase in crack propagation rate. Fixing an annealing temperature of 1173 K, it is found that the SCC occurred in the annealing time range from 0.6 to 86.4 ks and that the fracture strain showed maximum at 3.6 ks. Analyzing the crack behavior, crack initiation time and crack propagation rate were found to be maximum and minimum at 3.6 ks, respectively. Thus, it was concluded that the maximum fracture strain at 3.6 ks resulted from minimum SCC susceptibility evaluated by initiation as well as propagation of the cracks. On the other hand, the observation of microstructure of the specimen revealed that the specimen surface suffered from decarburization during the annealing treatment beyond 1073 K, and the SCC always occurred on the specimen showing the decarburized structure.

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Effects of potential and concentration of bicarbonate on stress corrosion cracking of annealed SM 400 B carbon steel has been investigated in bicarbonate solutions at 343 K. The surface of annealed specimen had decarburized layer of about 0.5 mm thickness. A potentiostatic slow strain rate testing apparatus equipped with a charge coupled device camera system was employed to evaluate SCC susceptibility from the viewpoint of the crack behavior. In a constant bicarbonate concentration of 1 M, cracks were observed in the potential range from -800 to 600 mV_Ag/AgCl. and especially, the initiation and the propagation of the cracks were accelerated at -600 mV. At a constant potential of -600 mV, cracks were observed in the concentration range from 0.001 to 1 M, and the initiation and the propagation of the cracks were suppressed as the concentration decreased. Polarization curves for the decarburized surface were measured with two different scan rates. High SCC susceptibility may be expected in the potential range where the difference between the two current densities is large. It was found in this system that the potential with the maximum difference in the current density was -600mV for 1 M bicarbonate solution, and the potential increased with a decrease in the concentration of bicarbonate. This means that an applied potential of -600 mV provides the highest SCC susceptibility for 1 M bicarbonate solution, and that the SCC susceptibility decreases as the concentration decreases. These findings support the dependence of the actual SCC behavior on the potential and the concentration of bicarbonate.

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The effect of the sulfides on corrosion and the observation of the microstructure of the inclusions especially, the sulfides, were investigated to elucidate the mechanism of the grooving corrosion resistance in Cu-Ti added high frequency electric resistance welded steel pipes. Main conclusions are as follows. (1) The constitution of the sulfides changes from MnS to Cu_xS+MnS and Ti₂S by the addtion of Cu and Ti and the relative small sulfides less than 0.1μm increase. (2) MnS overall covered with Cu_xS precipitates in Cu-Ti added steel. Cu_xS in Cu-Ti added steel more precipitates than that in Cu-Ti free steel because of the difference in Cu content. (3) Cu_xS+MnS and Ti₂S are not corroded in 3% sodium chloride solution (NaCl) from the in-situ observation. Therefore, it was supposed that the grooving corrosion of the welded portion because the constitution and the morphology of the sulfides varied by the addition of Cu and Ti. That is, it is caused by preventing MnS itself from dissolving for the environment by the formation of Cu_xS.

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Understanding of short crack behaviors is essential to construct lifetime prediction models for light water reactor components. There is, however, no established method of directly measuring short crack behaviors. Therefore we need some interpolation or extrapolation technique to precisely evaluate the short crack growth rate. In this study, constant load tests were conducted to investigate a short crack growth rate on primary water stress corrosion cracking (PWSCC) for different mill annealed alloy 600 samples in primary water at 350°C. Maximum crack length was measured for each sample and then divided by relevant test duration to evaluate its crack growth rate. On the other hand, to determine the accurate crack growth rates, a crack growth simulation model which allowed for mechanical effects of grain boundaries on a crack kinked at the grain boundary triple point was developed. With this simulation model, the crack growth processes in the tests were reproduced using a computer and thereby the crack growth rate was evaluated.

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The corrosion behavior of stainless and carbon steels in high temperature and high pressure (HT/HP) water systems with addition of Zr and Si compounds was investigated. The corrosion of stainless and carbon steels was inhibited by addition of a Zr compound. This is due to formation of a stable film by Zr oxide and then the increase of protective ability of the film. Only corrosion of stainless steel was depressed by addition of a Si compound. It is thought that this is owing to depression of outer-diffusion and dissolution of CrO₄^2- by adsorption of SiO₃^2- on a steel surface and then stabilization of the film containing a Cr component.

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