Kazunori Suzuki

Electrochemical Properties of Uranyl Ion in Ionic Liquids as Media for Pyrochemical Reprocessing

We have proposed a new reprocessing process by using ionic liquids (ILs) instead of molten salts of alkali chlorides in pyrochemical process. In the proposed process, spent nuclear fuels are dissolved in ILs by using Cl2 as an oxidant, and UO2 2+ and PuO2 2+ ions in ILs are recovered as UO2 and PuO2 by electrochemical reduction. In order to examine applicability of ILs as media for reprocessing, we have studied electrochemical behavior of UO2 2+ in 1-butyl-3-methylimidazolium chloride (BMICl), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4), and 1-butyl-3-methylimidazolium nonafluorobutanesulfonate (BMINfO). Electrochemical properties of uranyl chloride dissolved into ILs were examined by cyclic voltammetry. In BMICl, an almost reversible redox couple was observed, and the formal potential and the diffusion coefficient were evaluated as _0:758V vs. Ag/AgCl and 4:8 × 10−8 cm2s−1, respectively. On the other hand, the electrochemical reactions of UO2 2+ in BMIBF4 and BMINfO were irreversible. In BMINfO, some reduction peaks and one sharp oxidation peak were observed in the range of −0:6∼–0:2V and around 0.85V vs. Ag/AgCl, respectively. The reduction and oxidation peaks were assigned to multi step reduction of UO2 2+ to U(IV) via U(V) and/or direct reduction of UO2 2+ to U(IV), and the oxidative dissolution of the resulting U(IV) compounds, respectively. The electrochemical reduction of UO2 2+ in BMINfO at −1:0V vs. Ag/AgCl produced the deposits on a carbon electrode as a cathode. Analyses of the deposits with the scanning electron microscope and the energy dispersive X-ray spectrometer indicated that the deposits are compounds containing uranium, oxygen, and chlorine. As a result, it is expected that the UO2 2+ in IL can be recovered electrolytically as uranium compounds such as UO2 and uranium oxychlorides.

Solubility of Sn(IV) Oxide in Dilute NaClO4 Solution at Ambient Temperature

The solubility of Sn(IV) oxide was determined in a dilute NaClO 4 solution with pH 2 through 12 at ambient temperature. Both oversaturationn and undersaturation experiments were carried out in an inert gas gloveboxn where the concentration of the oxygen and carbon dioxide were less than 1n ppm. The solubility of Sn(IV) oxide was 3×10 -8 mol/1 at neutraln pH, and increased at pH >7.5. Equilibrium constants of soluble reactionsn were calculated from the experimental data, using curve fitting method. The study suggests that the solubility of Sn(IV) oxide would be higher thann that provisionally used in current safety assessments of HLW disposaln sites.

Study of Sorption and Diffusion of 137Cs in Compacted Bentonite Saturated with Saline Water at 60°C

The effect of compaction of bentonite on the sorption behavior of 137Cs was studied for the safety assessment of the high level radioactive waste. The diffusion coefficients (effective De and apparent Da ) and the distribution coefficient for sorption Kd for 137Cs in compacted and dispersed bentonite saturated with saline water were investigated at 60°C by four different sorption and diffusion experiments: the in-diffusion, through-diffusion, reservoir-depletion and batch sorption experiments. The system of the through-diffusion experiment was carefully designed to maintain the boundary conditions of constant concentration at each end of the specimen. De and Da were found to be reproducible and showed good consistency among three of the diffusion experiments (through-diffusion, in-diffusion and reservoir depletion). Kd of 137Cs in compacted bentonite determined from the three types of diffusion experiments was in good accordance with that determined by the batch sorption experiment for dispersed bentonite.

Experiments on the leak path factor of ruthenium volatilized from high-level liquid waste tanks in a reprocessing plant in case of the boiling and drying accident

In case of the boiling and drying accident of a high-level liquid waste (HLLW) tank, a large amount of ruthenium (Ru) will be volatilized. In order to suppress the release of radioactive materials to the environment, the vapor may be led to the neighboring cells in which it will contact with the cell walls to be partially condensed. To understand the behavior of Ru in this situation, we have prepared an experimental apparatus. It consists of a small tank in which 60 mL simulated HLLW is heated to dryness, a 9.6 L stainless steel box which mimics the neighboring cell accepting the vapor from the small tank, and a condenser where the vapor coming out from the box is cooled to collect the condensate. The results show that more than 99% of the volatilized Ru is removed from the vapor in the box if its temperature is below about 120 °C.

Effects of Compaction and Temperature on Sorption and Diffusion of Cs and HTO in Compacted Bentonite Saturated with Saline Water

The sorption and diffusion of Cs and tritiated water (HTO) in compacted bentonite was investigated at temperatures from 30 to 60 deg. C. The apparent (D{sub a}) and effective (D{sub e}) diffusion coefficients were determined by in-diffusion and through-diffusion experiments with a constant boundary concentration maintained. The temperature dependence of De and Da obeyed an Arrhenius-type equation, allowing determination of the activation energy for diffusion of Cs and HTO. The D{sub e} value of Cs was three times the D{sub e} of HTO, which is considered to be a result of surface-excess diffusion. Cs may be concentrated near the surface of the negatively charged clay, thus giving a large diffusive flux. The activation energies for Cs diffusion were 21.4{+-}2.8 kJ/mol and 37.3{+-}1.5 kJ/mol as determined based on D{sub e} and D{sub a}, respectively. This difference was due to the temperature dependence of the distribution coefficient K{sub d} of Cs. (authors)

Adsorption of Sn(IV) on goethite in 0.01 M NaCl solution at ambient temperature

The adsorption of soluble Sn(TV) species on goethite (FeOOH), a naturallyn occurring mineral, was investigated in 0.01 M NaCl solution at ambientn temperature. The sequential spike method was used to obtain the distributionn coefficient (Kd). Kd values of higher than 1×10 7 ml/g at neutraln pH, and 3×10 5 ml/g at pH 11 were obtained. It is observed that Kdn values decrease with increasing pH. In the desorption experiment, then sequential extraction method was applied. The extractants such as 0.1 Mn NaOH, Tamms acid oxalate and Coffins reagent were used. The resultn indicates that the adsorption consists of both reversible and irreversiblen reactions, and that the most of fractions in adsorption of Sn on goethiten correspond to the irreversible adsorption or a stable fixation. Moreover, itn is suggested that the reversible adsorption converts to the irreversiblen adsorption with time.

Absorption Study of the Tc(IV) on Rocks and Minerals Under Simulated Geological Conditions

The absorption of the technetium on rocks and minerals have been studied by using freshly quarried rocks to provide reducing conditions, without the addition of reductive reagents. The initial {sup 99}Tc concentration in this study was about 1 {times} 10{sup {minus}8} mol/l (0.62 Bq/ml). Technetium was not absorbed on tuff or granite under oxidizing conditions, but higher Kd values of Tc were obtained for granite under reducing conditions. The Tc absorbed on granite was not desorbed under reducing condition. This indicates the absorption reaction is irreversible.

Leaching Studies of Radionuclides from Solidified Wastes with Theremosetting Resin

This paper reports on studies of the leachability of Co–60 and Cs–137 from simulated LWR radwastes solidified with thermosetting resin and evaluates the effects of chemical fixation on leachability. It is concluded that insolubilization by a nickel-ferrocyanide compound offers an effective chemical fixation of these radionuclides and is a recommended pretreating method for radwastes that are to be solidified.