Mikio Kumagai

A study of the Ce(III)/Ce(IV) redox couple for redox flow battery application

Abstract In this study, the electrochemical behavior of the Ce(III)/Ce(IV) redox couple in sulfuric acid medium with various concentrations and the influence of the operating temperature were investigated. A change of the concentration of sulfuric acid mainly produced the following two results. (1) With an increase of the concentration of sulfuric acid the redox peak currents decreased. (2) The peak potential separation for the redox reactions increased with rising concentration of sulfuric acid from 0.1 to 2 M and then decreased with further increase of the concentration. Elevated temperature was electrochemically favorable for Ce(III)/Ce(IV) couple, which caused an increase of the peak currents for the redox reactions and a decrease of the peak potentials separation. Constant-current electrolysis shows that the current efficiency was 73% for the oxidation process of Ce(III) and 78% for the reduction process at 298 K, and could be improved by elevating the temperature. The open-circuit voltage of the Ce–V cell, after full charging, remained constant at 1.870±0.005 V for more than 48 h, and is about 29% higher than that of the all-vanadium batteries. The coulombic efficiency was approximately 87%, showing that self-discharge of the Ce–V battery was small. The preliminary exploration shows that the Ce(III)/Ce(IV) couple is electrochemically promising for redox flow battery (RFB) application.

Studies on the separation of minor actinides from high-level wastes by extraction chromatography using novel silica-based extraction resins

To develop an advanced partitioning process by extraction chromatography using a minimal organic solvent and compact equipment to separate minor actinides such as Am and Cm from nitrate acidic high-level waste (HLW) solution, several novel silica-based extraction resins have been prepared by impregnating organic extractants into the styrene-divinylbenzene copolymer, which is immobilized in porous silica particles (SiO2-P). The extractants include octyl(phenyl)-N, N-diisobutylcarbamoylmethylphosphine oxide (CMPO), di(2-ethylhexyl)-phosphoric acid (HDEHP), and bis(2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex 301). Compared to conventional polymer-matrix resins, these new types of extraction resin are characterized by rapid kinetics and significantly low pressure loss in a packed column. The results of separation experiments revealed that trivalent actinides and lanthanides can be separated from other fission products, such as Cs, Sr, and Ru in simulated HLW solution containing concentrated nitric acid by extraction chromatography using a CMPO/SiO2-P resin-packed column. Satisfactory separation between Am(III) and a macro amount of lanthanides from simulated HLW solution with pH 4 was achieved by using a newly purified Cyanex 301/SiO2-P resin. However, the Am(III) separation was very sensitive to the purity of Cyanex 301, and the improvement of its stability is an important task for practical utilization.

Development of the MAREC process for HLLW partitioning using a novel silica-based CMPO extraction resin

A new partitioning technology named “MAREC” process has been proposed for the separation of minor actinides (MA=Am, Cm) from high level liquid waste (HLLW) by extraction chromatography using a novel porous silica-based CMPO extraction resin. Separation experiments for simulated HLLW solutions containing typical fission product (FP) elements were carried out by a column packed with the CMPO/SiO2-P extraction resin. The experimental results showed that MA as a mixture with some heavy rare earths can be effectively separated from other FP elements by using two chromatographic columns. Furthermore, some specific FP elements such as Pd, Zr and Mo were also efficiently separated from the simulated HLLW. The separation behavior of the elements are considered to result from the difference of their adsorption and elution selectivity based on the complex formation with CMPO and the eluents such as DTPA and H2C2O4. These results revealed that the proposed MAREC process for HLLW partitioning is essentially feasible.

Properties and mechanism of molybdenum and zirconium adsorption by a macroporous silica-based extraction resin in the MAREC process

Abstract To achieve effective separation of molybdenum and zirconium in the MAREC process, the adsorption properties and mechanism of Mo(VI) and Zr(IV) with a macroporous CMPO/SiO2‐P (CMPO: octyl(phenyl)‐N,N‐diisobutylcarbamoylmethylphosphine oxide) extraction resin have been studied. By investigating the influence of the aqueous concentrations of H+ and NO3 − on the adsorption of Mo(VI), the composition of complex of Mo(VI) and CMPO/SiO2–P is determined as H2MoO4 · 2CMPO/SiO2–P for dilute aqueous HNO3 and H2MoO3 (NO3)2 · 2CMPO/SiO2–P for concentrated aqueous HNO3, respectively. Similarly, the composition of Zr(IV) and CMPO/SiO2–P is determined as ZrO(NO3)2 · 2CMPO/SiO2–P or Zr(NO3)4 · 2CMPO/SiO2–P in 0.3–4.0 M HNO3, while ZrO2 · 2H2O · 2CMPO/SiO2–P is assumed for lower HNO3 concentration. Based on the compositions of Mo(VI) and Zr(IV) with CMPO/SiO2–P and the elution behavior of Mo(VI) and Zr(IV) by using 0.05 M diethylenetriaminepentaacetic acid (DTPA) at 0.01–1.0 M HNO3, a dynamic interconversion equilibrium between the complexes of Mo(VI) or Zr(IV) and CMPO/SiO2–P is demonstrated to take place in the elution process. To verify the adsorption mechanism, the adsorption and elution behavior of Mo(VI) and Zr(IV) with 0.05 M DTPA‐pH 2.0 was performed from a simulated high level radioactive liquid waste (HLLW) containing Pd(II), Gd(III), Y(III), Eu(III), Sm(III), Mo(VI), and Zr(IV). The results indicate that Mo(VI) and Zr(IV) not only can be efficiently eluted with 0.05 M DTPA‐pH 2.0, but also the elution efficiency is much better than that of 0.5 M H2C2O4 previously used in the MAREC process. The reverse equilibrium of complexes between Mo(VI) or Zr(IV) and CMPO/SiO2–P in high and low acidity was demonstrated, respectively.

Chromatographic Separation of Strontium (II) from a Nitric Acid Solution Containing some Typically Simulated Elements by a Novel Silica‐Based TODGA Impregnated Polymeric Composite in the MAREC Process

Abstract A new kind of macroporous silica‐based N,N,N′,N′‐tetraoctyl‐3‐oxapentane‐1,5‐diamide (TODGA) chelating polymeric adsorption material (TODGA/SiO2‐P) was developed and synthesized by impregnating TODGA molecules into the pores of ∼50 µm SiO2‐P particles to separate strontium effectively from high‐level liquid waste (HLLW). The adsorption of some typically simulated elements Na(I), K(I), Cs(I), Rb(I), Sr(II), Ba(II), and Ru(III) towards TODGA/SiO2‐P adsorbent was investigated by examining the influence of contact time and HNO3 concentration. It was found that with an increase in the HNO3 concentration, the adsorption of Sr(II) onto TODGA/SiO2‐P adsorbent increased quickly from 0.5 M to 2.0 HNO3 and then decreased. TODGA/SiO2‐P exhibited excellent adsorption ability and selectivity for Sr(II) over all of the tested elements, which showed almost no adsorption. Based on the batch experiments, the separation of Sr(II) from a 2.0 M HNO3 solution containing ∼5×10−3 M of the simulated elements was performed by TODGA/SiO2‐P packed column at 25°C and 50°C, respectively. Cs(I), K(I), Ru(III), Ba(III), Rb(I), and Na(I) were found to facilely elute out column and flow into effluent along with feed solution and 2.0 M HNO3 because of no adsorption. Sr(II) adsorbed towards TODGA/SiO2‐P was desorbed sufficiently by distilled water and separated completely from the simulated elements. Its recovery percentage was 100.2% at 25°C and 99.7% at 50°C. Furthermore, the leakage behavior of TODGA from its silica‐based adsorbent was investigated. It was found that the quantity of TODGA leaked was basically equivalent to its solubility in the corresponding HNO3 solution.

Separation of actinides from simulated spent fuel solutions by an advanced ion-exchange process

Abstract In order to develop an advanced ion-exchange process to recover U, Pu and other actinide elements from the spent fuels of light-water reactors (LWR), we have manufactured a new type of anion-exchanger characterized by a rapid adsorption–elution rate, high mechanical strength and relatively excellent radiation-resistance. The separation behavior of some actinide elements from simulated spent fuel solutions containing concentrated nitric acid was examined experimentally by ion-exchange chromatography using dilute nitric acid, uranous and thiourea as eluents. U(VI), Np(V), Np(IV) and Np(VI) showed similar adsorption–elution behavior and could be separated from most fission product elements (FPs) such as Cs(I), Sr(II), Mo(VI), Rh(III), Pd(II) and Tc(VII) and trivalent rare earths. Pu(IV) was strongly adsorbed by this anion-exchanger and was effectively eluted as Pu(III) using U(IV)–N 2 H 4 as reductive eluent. Am(III) was not adsorbed and mixed with the non-adsorptive FPs.

Preparation of Novel Silica-Based Nitrogen Donor Extraction Resins and Their Adsorption Performance for Trivalent Americium and Lanthanides

In this work, we have synthesized several 2,6-bis(5,6-dialkyl-1,2,4-triazine-3-yl)pyridine (R-BTP) ligands with different alkyl groups (C=1∼3) and prepared the novel silica-based extraction resins by impregnating the ligands into the SiO 2 -P support. Furthermore, the adsorption performance of Am(III) and Ln(III) from nitrate acidic solution onto these novel silica-based nitrogen donor extraction resins was investigated experimentally.

The Application of an Advanced Ion Exchange Process to Reprocessing Spent Nuclear Fuels, (I) Separation Behavior of Fission Products from Uranium

In order to develop an advanced ion exchange process for the reprocessing of spent nuclear fuels, a novel anion exchanger, AR-01 with the resin embedded in porous silica beads and benzimidazoles as functional groups has been manufactured. Adsorption behavior of various fission product elements (FPs) and uranium in nitric acid medium were investigated experimentally using this anion exchanger. Separation performance of FPs from U(VI) in simulated spent fuel solutions was demonstrated by column chromatography utilizing dilute HNO3 and thiourea as eluents. Most FPs such as Cs(I), Sr(II), Mo(VI), Rh(III) and trivalent rare earths showed negligibly slight adsorption and could be separated from U(VI) satisfactorily. Cerium(IV) was strongly adsorbed, but was gradually reduced to non-adsorptive Ce(III) by the anion exchanger. Zirconium(IV) presented weak adsorption and its a part mixed with U(VI) in the column experiments. Ruthenium(III) exhibited quite strong adsorption in a broad HNO3 concentration range as the...

Synthesis of a Novel Silica‐Based Macroporous Polymer Containing TODGA Chelating Agent and Its Application in the Chromatographic Separation of Mo(VI) and Zr(IV) from Diethylenetriaminepentaacetic Acid

Abstract A novel silica‐based N,N,N′,N′‐tetraoctyl‐3‐oxapentane‐1,5‐diamide (TODGA) chelating polymeric material (TODGA/SiO2‐P) was synthesized by impregnating TODGA into ∼50 µm of spherical SiO2‐P particles for separating Mo(VI) and Zr(IV) from a HNO3 solution containing 0.05 M diethylenetriaminepentacetic acid (DTPA) in minor actinides recovery by extraction chromatography (MAREC) process. The adsorption of TODGA/SiO2‐P for Mo(VI) and Zr(IV) was investigated by examining the influence of HNO3 and DTPA concentrations. It was found that HNO3 concentration from 0.3 M to 9 M either with or without 0.05 M DTPA had a strong effect on the adsorption of Mo(VI) and Zr(IV) towards TODGA/SiO2‐P. In the absence of 0.05 M DTPA, both the adsorption of Mo(VI) and Zr(IV) decreased with an increase in the concentration of HNO3 from 0.3 M to 3 M and then gradually increased. In the presence of 0.05 M DTPA, Zr(IV) showed scarcely any adsorption towards TODGA/SiO2‐P below 2 M HNO3 and was gradually adsorbed from 2 M to 3 M HNO3 while Mo(VI) showed almost no adsorption. In excess of 3 M HNO3, the adsorption curve of Mo(VI) or Zr(IV) fully overlapped with that of the absence of 0.05 M DTPA. This resulted from the dissociation of Mo(VI) or Zr(IV) from its complex with DTPA because of full protonation of DTPA in high HNO3 concentration and the strong complexation taking place simultaneously with TODGA/SiO2‐P. In terms of the batch experiments, the separation of Mo(VI) and Zr(IV) from a 3 M HNO3 solution containing 0.05 M DTPA was performed by employing the TODGA/SiO2‐P packed column at 50°C. Molybdenum(VI) was found to leak out of the column along with 3 M HNO3. Zirconium(IV) adsorbed by TODGA/SiO2‐P was effectively eluted by 0.5 M H2C2O4. The recovery was 101.6% for Mo(VI) and 94.8% for Zr(IV). This work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) under the framework of the “Development of Innovation Nuclear Technologies.”

Development of a novel redox flow battery for electricity storage system

A novel cylindrical battery which uses carbon fibres with high specific surface area as electrodes and a porous silica glass with high chemical stability as membrane has been fabricated. The results obtained from electrolysis of 0.5 M K3Fe(CN)6–0.5 M KCl and of 85 mM V(IV)–1 M H2SO4 indicate that the cell possesses excellent electrolytic efficiency. As a redox flow battery (RFB) its performance was investigated by employing all-vanadium sulfate electrolytes. The results of the cyclic voltammetry measurements indicate that at a glassy carbon electrode the electrochemical window for 2 M H2SO4 solution could reach 2.0 ∼ 2.4 V. Constant current charging–discharging tests indicate that the batteries could deliver a specific energy of 24 Wh L−1 at a current density of 55 mA cm−2. The open-circuit cell voltage, after full charging, remained constant at about 1.51 V for over 72 h, while the coulombic efficiency was over 91%, showing that there was negligible self-discharge due to active ions diffusion through the membrane during this period.