It is known that water exists in supercritical state above 374 C and 22.1 MPa, where gas phase and liquid phase are merged into a single phase. In supercritical state the density is controllable by changing the pressure. The properties such as the ionic product, solubilities of salts, gas and organic compounds, and dielectric constant of supercritical water are very different from those of water at room temperature. Recently much attention has been paid to supercritical water because many possible applications such as synthesis of functional materials, waste oxidation, and biomass conversion have been proposed by using above peculiar properties of the supercritical water. Thus, much intensive work is in progress all over the world. In addition, a new concept of the nuclear reactor using the supercritical water as a coolant has been proposed and selected as one of the forth generation nuclear reactors by DOE, USA. Therefore, the radiation chemical study of the supercritical water seems inevitably important but little work has been done so far. Then, we started the radiation chemistry study of supercritical water by pulse radiolysis and gamma-radiolysis seven years ago. We have observed hydrated electron as a first target. We also measured inorganic radicals, metal ions, organic radicals in high temperature and supercritical water by a pulse radiolysis technique. It was found that the absorption spectra of the transient species are dependent on temperature. Some transients show red-shift and some bands are blue-shifted with increasing temperature. A G-value evaluation of water decomposition products was also done by using methyl viologen or 4,4’-bipyridyl (BPY) as a scavenger and it was revealed that the values are significantly dependent not only on temperature but also on density in supercritical water. An extended study on the behavior of solvated electrons in different alcohols at high temperatures and super critical state is also in progress. In the present lecture, I would like to present our recent experimental results.
Department of Nuclear Engineering and Management, School of Engineering, University of Tokyo