Daisuke Hiroishi

Solubility of magnetite in high temperature water and an approach to generalized solubility computations

Magnetite solubility in pure water was measured at 423 K in a fully teflon-covered autoclave system. A fairly good agreement was found to exist between the experimental data and calculation results obtained from the thermodynamical model, based on the assumption of Fe3O4 dissolution and Fe2O3 deposition reactions. A generalized thermodynamical approach to the solubility computations under complex conditions on the basis of minimization of the total system Gibbs free energy was proposed. The forms of the chemical equilibria were obtained for various systems initially defined and successfully justified by the subsequent computations. A [Fe3+]T-[Fe2+]T phase diagram was introduced as a tool for systematic understanding of the magnetite dissolution phenomena in pure water and under oxidizing and reducing conditions.

Radiolysis of water at elevated temperatures—II. Irradiation with γ-rays and fast neutrons up to 250°C

Abstract Determination of G -values of water decomposition products in neutral water formed by irradiation with γ-rays and fast neutrons at elevated temperatures up to 250°C was carried out by a combination of the NaNO 2 , acetone + methanol, and HClO 4 + methanol solutions, which had been established at room temperature. From the measurement of the H 2 yields for these solutions, the G H2 , G H2 + G H , and G eaq− + G H + G H2 , respectively, have been determined. Consequently, the G H2 , G H and G eaq− were obtained. By using a mass balance relation, the value for water decomposition, G −H2O , was calculated. The yields of H 2 O 2 , formaldehyde and ethyleneglycol produced as a function of dose in the NaNO 2 and HClO 4 + methanol solutions were chosen to derive the G OH and G H2O2 by fitting the simulated calculation to the experimental results. In the γ-radiolysis, the G -values obtained in the present experiment are in good agreement with recently reported ones except for the G OH and G H2O2 . The G equ− for γ-radiolysis is in a good agreement with the reported values up to 200°C, but slightly lower at 250°C. The water decomposition gives a similar pattern with G eaq− yields. The G OH does not change significantly with temperature, which is contrary to the evaluation by other groups and the G H2O2 tends to increase with temperature. In the fast neutron radiolysis, although a similar change of the G -values with temperature was observed, the higher molecular products of G H2 and G H2O2 and the lower radical yields of G H and especially G eaq− were determined. It was made clear that the LET effect still remains even at elevated temperatures. Concerning the experimental results, it is suggested that the spur size would expand at higher temperatures and the fraction of the intra spur reactions decreases. Consequently, the amount of radicals diffusing out from the spur increases. To confirm this assumption, an additional measurement of H 2 yields in the 10 −3 mol · kg −1 HClO 4 solutions with different concentrations of methanol irradiated with γ-rays and fast neutrons was also made.

Fast neutron radiolysis of water at elevated temperatures relevant to water chemistry

Abstract A summary of the studies on the determination of the G-values of water decomposition induced by the irradiation with fast neutrons in a “YAYOI” reactor is reported. The same method was applied to γ and fast neutron radiolysis. At present, it seems that almost enough basic knowledge of water decomposition by irradiation at elevated temperatures has been accumulated for the quality control of the coolant.

Radiolysis of high temperature water

Abstract The yields of the primary products for water radiolysis were estimated in the temperature range of 25 to 250°C from the product analyses of three kinds of aqueous systems irradiated with γ-rays and farst neutrons. Model calculations were carried out using these primary yields to simulate the water radiolysis reactions in nuclear reactor.

Radiolysis of water at elevated temperatures. I: Irradiation with gamma-rays and fast neutrons at room temperature

Abstract As a first step for the determination of G values of water decomposition products at elevated temperatures, an experimental method has been developed and the values for the radiolysis with 60Co γ-rays at room temperature have been determined. The method is based on the combination of three kinds of aqueous solutions containing: (a) 2×10 -3 mol kg -1 NaNO 2 ; (b) 10 -3 mol kg -1 acetone+ 10 -2 mol kg -1 methanol; and (c) 10 -3 mol kg -1 HClO 4 +10 -2 mol kg -1 methanol, where the H 2 yields are expected to be proportional to the values of G H2 , G H2 + G H , and G H2 + G H + G e āq , respectively. The NaNO 2 and HClO 4 + methanol solutions were chosen to derive the G OH and G H 2 O 2 by fitting the simulated calculation to the experimental results. The values of G H 2 , G H , G e āq , G OH and G H 2 O 2 for for γ radiolysis were determined, respectively, as 0.45, 0.63, 2.75, 3.12 and 0.58. Consequently, the value for water decomposition, G -H 2 O , was calculated as 4.28. All the G -values obtained in the present experiment are in good agreement with established values except for the G OH and G H 2 2 . Beside the G values, the rate constants for the reactions of 2( Ċ H 2 OH)→(CH 2 OH) 2 , 2( Ċ H 2 OH)→CH 2 O+CH 3 OH and Ċ H 2 OH+H 2 O 2 →CH 2 O+H 2 O+ Ȯ H were determined as 2.25×10 9 , 1.5×10 8 and 5×10 4 M -1 s -1 , respectively. The same procedure has been applied to determined the values for fast neutron radiolysis of water at room temperature and the values of 1.07, 0.58, 0.43, 0.86, 1.15 and 3.15, respectively for G H 2 , G H , G e āq , G OH , G H 2 O 2 and G -H 2 O were obtained. It was revealed that the fast neutron radiolysis brings smaller radical yields, larger molecular ones and smaller net water decomposition due to the high LET of the fast neutrons.

Experimental verification of photostability for free- and bound-amino acids exposed to γ-rays and UV irradiation

The photo-stability of simulated free and bound amino acids exposed to photon sources of 1.2–1.3 MeV γ-rays from a 60C° source and ultra-violet (UV) irradiation from a 10 eV deuterium lamp was examined. The free amino acids in aqueous solution were drastically decreased while the bound amino acids in aqueous solution were slightly decreased with the duration of γ-rays and UV irradiation. The apparent half-lives (t1/2) of bound serine and threonine under γ-rays irradiation were more than 10.6 and 27.9 times as stable as the free amino acids, respectively. The photo-stability of bound amino acids was greater than that of free amino acids under irradiation by high-energy photons. These results strongly suggest that bound, but not free, amino acids could be found in extraterrestrial environments. Radio- or photo-alteration processes are mainly decomposition pathways, with the production of secondary amino acids via decomposition of the α-carboxylic group; β-alanine and γ-aminobutyric acids, α-decarboxylation products of aspartic acid and glutamic acid, respectively, were detected.

Hydrolysis of zinc ion and solubility of zinc oxide in high-temperature aqueous systems

Hydrolysis constants of the zinc ion were measured at 25, 50, 75, 185, 200, and 225°C through the direct measurement of pH using pH sensors, especially of the yttria-stabilized zirconia membrane-ty...

Solubility of Nickel and Cobalt Oxides in High-Temperature Water

AbstractThe solubility of NiO and CoO in high-temperature water is measured at temperatures between 373 and 523 K in a special batch autoclave system. Great importance has been given to deriving reliable thermodynamical data for the related ionic species at elevated temperatures. The experimental results for NiO seem to be consistent with the previously proposed free-energy data set for Ni2+. Because of the lack of experimental information on the free energy of formation (ΔG°f) for Co2+, the Criss-Cobble approach is used to calculate the values. The estimates are compared with the experimental results. It is concluded that the application of the Criss-Cobble procedure to Co2+ ions is not correct above 423 K. Based on the experiments, a new set of ΔG°f values for Co2+ is proposed.

Radiation induced CO2 reduction in an aqueous medium suspended with iron powder

Abstract Radiation induced CO 2 conversion to CO and hydrocarbon has been studied using a solution suspended with iron powder and saturated with CO 2 . Due to the corrosion enhanced by γ-rays, H 2 has become the most dominant product of all. Corresponding to the rapid H 2 formation, a little hydrocarbon containing less than 4 carbon atoms has been produced in the initial period of irradiation, whereas CO yield has gradually increased to a considerable amount over a prolonged irradiation period. The CO yield has also shown a marked increase associated with introducing Ni 2+ or Cu 2+ into the solution. It is understood that 5.6 hydrogen atoms are consumed in the termination reaction of H 2 formation.

Solubility of zinc ferrite in high-temperature oxygenated water

Abstract The solubility of zinc ferrite was measured at 423 K, 473 K, and 523 K in an oxygenated water system, which is rather similar to the chemical condition of boiling water reactors. Thermodynamic analysis was performed by a procedure minimizing standard Gibbs free energy of the system at the final state. From both the analysis and the experimental results it was concluded that the dissolution process of ZnFe2O4 in conditions where no redox reaction occurs is described by combination of ZnFe2O4 dissolution and Fe2O3 precipitation equilibria. By fitting to the experimental results, thermodynamic data of ZnFe2O4 are re-analyzed at 423 K, 473 K, and 523 K.