Kayo Sawada

Removal of platinum group metals contained in molten glass using copper

Removal of platinum group metals (PGMs) such as Pd, Ru, and RuO2 from molten glass by using various amounts of liquid Cu was done as a basic study on a new vitrification process for a high-level radioactive waste. We prepared two types of borosilicate glasses containing PGMs and Cu, respectively. These glasses were mixed together and heated at 1,473 K for 4 h in Ar atmosphere. More than 95% of Pd were removed as a spherical metal button composed of Pd-Cu alloy when Cu was added in an amount 0.5 times the weight of Pd. Nearly 95% of Ru was also removed as a spherical button with 2.5–5 times as much Cu addition as Ru in weight. Ruthenium oxide was reduced to metallic Ru by a reaction with Cu in the molten glass. The removal fraction was increased by increasing the amount of Cu and reached 63% when Cu addition was 7.5 times as much as RuO2 in weight. By addition of Si as a reducing agent, nearly 90% of Pd and Ru were removed with Cu and Si metal composites even under O2:Ar = 20:80 (v/v) condition.

Stoichiometric Relation for Extraction of Uranium from UO_2 Powder using TBP Complex with HNO_3 and H_2O in Supercritical CO_2

The stoichiometry of UO2 dissolution with a tri-n-butylphosphate (TBP) complex with HNO3 and H2O was experimentally determined in supercritical carbon dioxide (SF-CO2) at 25 MPa, 323 K to estimate the required amount of the complex for a process design calculation. The molecular ratio of TBP:HNO3:H2O was determined as 1.0:1.8:0.6 when prepared as an organic phase after vigorous mixing of concentrated HNO3 and TBP. The HNO3 consumption was approximately 4 times the amount of extracted uranium. In designing a supercritical fluid extraction process, the following equation can be used to describe the overall dissolution of UO2 using the TBP complex with HNO3 and H2O in SF-CO2:

Liquid Metal Extraction for Removal of Molybdenum from Molten Glass Containing Simulated Nuclear Waste Elements

Laboratory-scale experiments for removing Mo and MoO3 from molten borosilicate glass were performed using liquid Cu as an extractant. Removal of Mo from the simulated HLW glass containing oxides of Nd, Fe, Zr, Mo, Sn, Ni, Sr, Cd, Ru, and Se was also performed, and the fractions of these elements transferred into Cu were examined. Mixtures of Cu anda ternary SiO2-B2O3-Na2O glass containing metallic Mo or MoO3 were heated in an alumina crucible at 1,673K in an Ar environment. The amounts of Mo and MoO3 added to 10 g of the ternary glass were fixed at 0.1 and 0.15 g, respectively. As for the glass containing metallic Mo, more than 90% of Mo was extracted into liquid Cu. Spherical Cu metal buttons containing Mo formed on the bottom of the crucible when Cu was added at more than 10 times that of Mo on a mass basis. Removal of Mo from the glass containing MoO3 was also achieved by the addition of Si as a reducing agent for the reduction from MoO3 to Mo. The fraction of Mo extracted into liquid Cu depended on the molar ratio of Si to Cu added to the glass. The fraction increased up to 84% with an increase in the molar ratio of Si/Cu. However, the excess addition of Si may enhance the chemical interaction between the metal phase and the glass phase, and some of the metal phase containing Mo remained in the glass phase without forming a metal button. The optimum molar ratio of Si/Cu that produces the highest removal fraction was found to be approximately 0.5. Almost the same removal fraction of 88% was obtained from the simulated HLW glass under the condition of Si/Cu = 0.5. Nearly 100% of Ru was extracted into Cu with Mo, while Sr, Zr, and Nd were hardly extracted and remained in the glass.

Distribution Coefficients of U(VI), Nitric Acid and FP Elements in Extractions from Concentrated Aqueous Solutions of Nitrates by 30% Tri-n-butylphosphate Solution

The distribution coefficients of uranium, nitric acid and 8 elements simulating fission products were obtained from the concentrated aqueous solutions of nitrates, whose nitrate ion concentrations were 4–15 mol dm−3. The relationships among the distribution coefficient of uranium, the nitrate ion concentration in the aqueous phase, and the concentration of tri-n-butylphosphate uncombined with nitrates in the organic phase were obtained from the data.

Decontamination of Radioactive Contaminants from Iron Pipes using Reactive Microemulsion of Organic Acid in Supercritical Carbon Dioxide

The ferrite fixed on the iron pipes was decontaminated by a reactive microemulsion in supercritical carbon dioxide (SC-CO2). The specimens were prepared by treating the iron pipes with steam at 1,273 K for 2 min. The specimen was not dissolved in 3 mol·dm−3 HNO3 because its surface was covered with ferrite, while the original iron pipe was easily dissolved. This difference was used for determination of the fraction of ferrite. The fraction of ferrite covering the iron pipes was 1.5±0.3 wt%. A microemulsion containing organic acid was prepared using a fluorinated reagent, pentade-cafluorooctanoic acid (PFOA), and a non-fluorinated surfactant, polyoxyethylene (2) nonylphenyl ether (NP-2) and citric acid. In the former system, PFOA acted as a surfactant as well as an acid. By observation of the phase equilibrium, the microemulsion was found to be stabilized when the molecular ratio of water to surfactant, the w value, was 5.0 for the PFOA+H2O+SC-CO2 system, and 8.7 for the NP-2+citric acid+SC-CO2 system at 25 MPa, and 323 or 353 K. Although the removal fractions of the ferrite were 0 and 1% for the PFOA and NP-2 system, respectively, at 25 MPa, 323 K, they increased to 92 and 56% at 25 MPa, 353 K.

Recovery of Metals from Simulated High-Level Radioactive Waste Glass through Phase Separation

The recovery of metals from simulated high-level radioactive waste (HLW) in a glass form using the phase separation of borosilicate glass was studied in order to satisfy possible future demands such as the adoption of some new treatments for the waste in the glass or use of the vitrified elements as resources. The simulated HLW glass was separated into SiO2-rich and B2O3-rich phases at 973 and 873K when the ternary mass ratio of SiO2:B2O3:Na2O was adjusted to 68:27:5 by the addition of SiO2 and B2O3 to the simulated HLW glass. Annealing at the lower temperature of 878K promoted the distribution of the elements in the B2O3-rich phase. Approximately 90% of the Ni, Zn, Fe, Nd, Te, Zr, and Mo were distributed in the B2O3-rich phase, and these elements, except Zr, were almost completely leached into 1 mol dm−3 nitric acid at 363 K. The leaching of Zr was also achieved using 1.5 mol dm−3 sulfuric acid after the nitric acid leaching. The leached fractions of the glass-network components, such as B and Al, were lower than the other elements at approximately 70–80% due to the existence in the SiO2-rich phase. An increase in the concentration of CaO in the glass in the range of 2–5 wt% inhibited the distribution of elements into the B2O3-rich phase. The increase in the concentration of CaO also changed the structure of the B2O3-rich phase from the continuous unit shape to the discontinuous spherical shape. Consequently, the leaching fraction of every element dramatically decreased. The structure of the B2O3-rich phase returned to continuous when the concentration of CaO was more than 10 wt%. From this glass, the leaching of 69% Zr was possible using only nitric acid without any sulfuric acid.

Generation of Nitrous Acid by Ultrasound Irradiation in the Organic Solution Consisting of Tri-n-butylphosphate, Nitric Acid and Water

The generation of nitrous acid (HNO2) by ultrasound was investigated for an organic solution consisting of tri-n-butylphosphate, nitric acid (HNO3) and water under several conditions. The amount of HNO2 was found to increase by a factor of 40 upon ultrasound irradiation, and the concentration of HNO2 reached the steady-state after long time irradiation. Although the concentration of HNO3 was low, the rate of generation of HNO2 in the organic solution was higher than that in aqueous solution equilibrated with the organic solution. The formation rates of HNO2 were greatly enhanced with an increase in concentration of HNO3 in the organic solution. The concentration of HNO2 was higher in the absence of a gas phase, because the HNO2 was transferred from the solution into the gas phase as a form of NOx. In the presence of a gas phase, a higher concentration of HNO2 was obtained at a lower temperature, due to the higher solubility of NOx in the solution.

Chlorination of antimony and its volatilization treatment of waste antimony-uranium composite oxide catalyst

Abstract For the waste antimony-uranium composite oxide catalyst, the chlorination of antimony and its volatilization treatment were proposed, and evaluated using hydrogen chloride gas at 873–1173 K. During the treatment, the weight loss of the composite oxide sample, which resulted from the volatilization of antimony, was confirmed. An X-ray diffraction analysis showed that uranium oxide, U3O8, was formed during the reaction. After the treatment at 1173 K for 1 h, almost all the uranium contained in the waste catalyst was dissolved by a 3 M nitric acid solution at 353 K within 10 min, although that of the non-treated catalyst was less than 0.1%. It was found that the chlorination and volatilization treatment was effective to separate antimony from the composite oxide catalyst and change uranium into its removable form.

Densities of Supercritical Fluids Containing CO2 and Tri-n-butylphosphate

The densities of supercritical fluids consisting of CO2 and 1) tri-n-butylphosphate (TBP), 2) a TBP solution containing HNO3, and 3) UO2(NO3)2·2TBP were experimentally measured using a variable-volume-type view cell at 313, 323 and 333 K. The densities of the fluids increased with an increase in pressure, and decreased with an increase in temperature. The densities were expressed as a function of pressure for each mole fraction of TBP in the supercritical fluids consisting of CO2 and the organic solution at 15–30 MPa. When the mole fraction of TBP in the fluid of CO2 and UO2(NO3)2·2TBP was less than 0.01, the molar volume of the fluid calculated from its densities was smaller than that of CO2 due to negative partial molar volume of the complex, which showed characteristics of a supercritical fluid dissolving solutes.