Youichi Enokida

Dissolution Behavior of Uranium Oxides with Supercritical CO2 Using HNO3-TBP Complex as a Reactant

Dissolution behavior of U3O8 and UO2 using supercritical CO2 medium containing HNO3-TBP complex as a reactant was studied. The dissolution rate of the oxides increased with increasing the HNO3/TBP ratio of the HNO3-TBP complex and the concentration of the HNO3-TBP complex in the supercritical CO2 phase. A remarkable increase of the dissolution rate was observed in the dissolution of U3O8 when the HNO3/TBP ratio of the reactant was higher than ca. 1, which indicates that the 2:1 complex, (HNO3)2TBP, plays a role in facilitating the dissolution of the oxides. Half-dissolution time (t½ ) as an indication of the dissolution kinetic was determined from the relationship between the amount of uranium dissolved and the dissolution time (dissolution curve). A logarithmic value of a reciprocal of the t½ was proportional to the logarithmic concentration of HNO3, CHNO3, in the supercritical CO2. The slopes of the (l/t½ ) vs. ln CHNO3 plots for U3O8 and UO2 were different from each other, indicating that the reaction mechanisms or the rate-determining steps for the dissolution of U3O8 and UO2 are different. A principle of the dissolution of uranium oxides with the supercritical CO2 medium is applicable to a method for the removal of uranium from solid matrices.

New Method for the Removal of Uranium from Solid Wastes with Supercritical CO2 Medium Containing HNO3-TBP Complex

This study aims at the development of a method for the decontamination of uranium from the solid wastes containing uranium oxides, UO 2 or U 3 O 8 , using supercritical CO 2 medium containing HNO 3 -TBP complex (TBP: tri-n-butylphosphate).

Cleaning of materials contaminated with metal oxides through supercritical fluid extraction with CO2 containing TBP

Abstract Studied was an application of supercritical fluid extraction (SFE) to cleaning of the materials contaminated with metal oxides. By using supercritical CO 2 including chemical complexes of tri- n -buthylphosphate (TBP) and HNO 3 , lanthanide was recovered from oxide mixtures placed in a pressure vessel. The lanthanide oxide was dissolved with TBP-HNO 3 and extracted with TBP in supercritical CO 2 . This cleaning method reduces volume amount of liquid waste arising from treatment of the contaminated materials. Being applied to a mixture of Gd 2 O 3 and SrO, we found that Gd was selectively recovered and Sr was not found in the recovered solution. Gd was also selectively recoverd when a mixture of Gd 2 O 3 and ZrO 2 was treated. This selective nature of decontamination is an additional advantage of the SFE with TBP-HNO 3 complex. This method would be applicable to the selective cleaning of materials contamined with radioactive oxides such as UO 2 /PuO 2 .

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:

Preliminary Experiments on Hydrogen Isotope Separation by Water-Hydrogen Chemical Exchange under Reduced Pressure

A pressure-reducing method is used effectively in a water distillation process to enhance the equilibrium separation factor. The feasibility of the technique is established through application to a water-hydrogen chemical exchange process using a prototype separation column. Isotope separation experiments examining the water-hydrogen chemical exchange reaction are performed for column pressures of 12–101 kPa, and the separation factors for hydrogen and deuterium are obtained. The HETP (Height Equivalent to a Theoretical Plate) values were distributed in the range of 6 to 15 cm. By reducing the pressure in the column, the process temperature can be lowered without reducing the molar fraction of water vapor in the gas stream. It confirmed that the separation factors under reduced pressure are larger than under atmospheric pressure. This fact demonstrates the effectiveness of reduced pressure in water-hydrogen chemical exchange processes.

Selective recovery of neodymium from oxides by direct extraction method with supercritical Co2 containing TBP–HNO3 complex

A method for the direct extraction of a metal utilizing supercritical CO2 containing tri-n-butylphosphate (TBP)–HNO3 (TBP), which is the TBP solution of TBP–HNO3 complex, has been developed for the recovery of Nd from its oxide. This method was applied using a flow reactor to several oxides (Nd2O3, ZrO2, MoO3, and RuO2), and binary mixtures of the oxides (Nd2O3–ZrO2, Nd2O3–MoO3, and Nd2O3–RuO2). Neodymium (Nd) was extracted almost quantitatively from 0.01 mol Nd2O3 powder with supercritical CO2 containing TBP–HNO3 (TBP) at 313K and 12 MPa, while Zr, Mo, and Ru were hardly extracted from their oxides and remained in the reactor. Nd was extracted quantitatively and selectively also from the binary mixtures of the oxides. From these results, it is found that the supercritical CO2 extraction process using TBP–HNO3 (TBP) as reactant is feasible for the selective recovery of lanthanides directly from the various oxide mixtures.

Solvent Extraction of Np (V) with CMPO from Nitric Acid Solutions Containing U (VI)

The extraction of Np(V) in the presence of U(VI) or Nd(III) was studied with 0.2 M CMPO- decalin and with 0.2 M CMPO-1.4 M TBP-n-dodecane. Distribution ratios of Np(V) decreased with increasing the concentration of Nd(III), which was considered to be explained by the decrease in the concentration of free CMPO. On the other hand, distribution ratios increased as the concentration of U(VI) increased. The absorption spectrum of Np(V) in organic solution showed a new absorption peak which was probably attributable to the formation of cation-cation complexes of Np(V) and U(VI). Assuming the formation of cation-cation complexes, the dependence of distribution ratio of Np(V) on the concentration of U(VI) was found to be semi-quantitatively explained.

Effect of development speed on separative performance of displacement chromatography using porous ion exchange resin for Li isotope separation

Abstract A series of displacement chromatography experiments using a porous ion exchange resin for Li isotope separation was performed using several development speeds. Chromatographic development at a high speed is preferable for practical use, but the degree of separation becomes smaller with increasing development speeds. The height equivalent to a theoretical plate (HETP) value, which is an important factor of separative performance, increased exponentially when the development speed was increased. As a new measure to take account of both HETP and development speed, a separation per unit time (SPUT) was defined. A maximum SPUT value was achieved when the development speed was of the order of 300 cm h−1 in the system using the porous ion exchange resin TITEC-H1.

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.