Dong-Yong Chung

A Conceptual Process Study for Recovery of Uranium Alone from Spent Nuclear Fuel by Using High-Alkaline Carbonate Media

Abstract This work studied a conceptual process to recover uranium alone from spent nuclear fuel using high-alkaline carbonate media with hydrogen peroxide for the purposes of reducing the volume of high-level active waste and recycling of uranium from the spent fuel with greatly enhanced proliferation resistance, environmental friendliness, and operational safety. The transuranium (TRU) elements were evaluated to be undissolved and precipitated together with other fission products during the oxidative leaching of uranium from the spent fuel. The leaching ratio of uranium dioxide to TRU dioxide from spent fuel in the carbonate solution with H2O2 was estimated to be more than about 108. Only Cs, Tc, Mo, and Te among the major fission products in the spent fuel were dissolved together in the carbonate solution. In the carbonate solution with H2O2, UO2 was dissolved in the form of uranyl peroxo-carbonato complex ions, which could be recovered in the form of uranium peroxide precipitate with a very low solubility by acidification of the solution in a succeeding step. All the inorganic salts of Na2CO3, NaOH, and HNO3 used in the process suggested could be almost completely recovered and recycled into the process again without any generation of secondary wastes.

Equilibrium, kinetic and thermodynamic study of cesium adsorption onto nanocrystalline mordenite from high-salt solution.

In this study, the equilibrium, kinetics and thermodynamics of cesium adsorption by nanocrystalline mordenite were investigated under cesium contamination with high-salt solution, simulating the case of an operation and decommissioning of nuclear facilities or an accident during the processes. The adsorption rate constants were determined using a pseudo second-order kinetic model. The kinetic results strongly demonstrated that the cesium adsorption rate of nano mordenite is extremely fast, even in a high-salt solution, and much faster than that of micro mordenite. In the equilibrium study, the Langmuir isotherm model fit the cesium adsorption data of nano mordenite better than the Freundlich model, which suggests that cesium adsorption onto nano mordenite is a monolayer homogeneous adsorption process. The obtained thermodynamic parameters indicated that the adsorption involved a very stable chemical reaction. In particular, the combination of rapid particle dispersion and rapid cesium adsorption of the nano mordenite in the solution resulted in a rapid and effective process for cesium removal without stirring, which may offer great advantages for low energy consumption and simple operation.

Evaluation of the Behavior of Uranium Peroxocarbonate Complexes in Na–U(VI)–CO3–OH–H2O2 Solutions by Raman Spectroscopy

In this work, the formation of uranium species and their stabilities in Na-U(VI)-CO(3)-OH-H(2)O(2) solutions at different pHs are studied by Raman spectroscopy. In this solution, the UO(2)(O(2))(CO(3))(2)(4-) species was formed together with three other uranium species of UO(2)(O(2))(2)(2-), UO(2)(CO(3))(3)(4-), and a speculated uranium species of the uranyl carbonate hydroxide complex, UO(2)(CO(3))(x)(OH)(y)(2-2x-y), which showed remarkable Raman peaks at approximately 769, 848, 811, and 727 cm(-1), respectively. The UO(2)(O(2))(CO(3))(2)(4-) species disappeared at pH conditions where bicarbonate dominated, and its Raman peak could be clearly observed in only a narrow pH range from approximately 9 to 12. When the pH of the solution increased further, the UO(2)(O(2))(CO(3))(2)(4-) species changed to UO(2)(CO(3))(3)(4-) and the UO(2)(CO(3))(x)(OH)(y)(2-2x-y) species. Moreover, the UO(2)(O(2))(CO(3))(2)(4-) species continuously decomposed into uranyl tricarbonate in the carbonate solution at an elevated temperature because of the instability of the peroxide ion, O(2)(2-), in alkaline conditions.

Biosorption of uranium(VI) from aqueous solution by biomass of brown algae Laminaria japonica

The uranium(VI) adsorption efficiency of non-living biomass of brown algae was evaluated in various adsorption experimental conditions. Several different sizes of biomass were prepared using pretreatment and surface-modification steps. The kinetics of uranium uptake were mainly dependent on the particle size of the prepared Laminaria japonica biosorbent. The optimal particle size, contact time, and injection amount for the stable operation of the wastewater treatment process were determined. Spectroscopic analyses showed that uranium was adsorbed in the porous inside structure of the biosorbent. The ionic diffusivity in the biomass was the dominant rate-limiting factor; therefore, the adsorption rate was significantly increased with decrease of particle size. From the results of comparative experiments using the biosorbents and other chemical adsorbents/precipitants, such as activated carbons, zeolites, and limes, it was demonstrated that the brown algae biosorbent could replace the conventional chemicals for uranium removal. As a post-treatment for the final solid waste reduction, the ignition treatment could significantly reduce the weight of waste biosorbents. In conclusion, the brown algae biosorbent is shown to be a favorable adsorbent for uranium(VI) removal from radioactive wastewater.

Evaluation of the stability of uranyl peroxo-carbonato complex ions in carbonate media at different temperatures

This work studied the stability of peroxide in uranyl peroxo carbonato complex ions in a carbonate solution with hydrogen peroxide using absorption and Raman spectroscopies, and evaluated the temperature dependence of the decomposition characteristics of uranyl peroxo carbonato complex ions in the solution. The uranyl peroxo carbonato complex ions self-decomposed more rapidly into uranyl tris-carbonato complex ions in higher temperature carbonate solutions. The concentration of peroxide in the solution without free hydrogen peroxide represents the concentration of uranyl peroxo carbonato complex ions in a mixture of uranyl peroxo carbonato complex and uranyl tris-carbonato complex ions. The self-decomposition of the uranyl peroxo carbonato complex ions was a first order reaction, and its activation energy was evaluated to be 7.144×10(3) J mol(-1). The precipitation of sodium uranium oxide hydroxide occurred when the amount of uranyl tris-carbonato complex ions generated from the decomposition of the uranyl peroxo carbonato complex ions exceeded the solubility of uranyl tris-carbonato ions in the solution at the solution temperature.

CO-SEPARATION OF Am AND RARE EARTH ELEMENTS FROM A HIGHLY ACIDIC RADWASTE SOLUTION BY A SOLVENT EXTRACTION WITH (DIMETHYLDIBUTYL TETRADECYLMALONAMIDE-DIHEXYLOCTANAMIDE)/N-DODECANE

This study was carried out to investigate the high-acidity co-separation of Am and RE from a simulated radwaste solution by a solvent extraction using a mixture of Dimethyldibutyltetradecylmalonamide (DMDBTDMA, as an extractant) and dihexyl octanamide (DHOA, as a phase modifier) diluted with n-dodecane (NDD). All the experiments were conducted as a batch type. First, the environmentally friendly DMDBTDMA and DHOA composed of only CHON atoms were self-synthesized. Then, the conditions for the prevention of a third phase, generated in the organic phase were examined. In addition, the effects of the concentration of nitric acid, DHOA, oxalic acid and H 2 O 2 on the co-extraction of Am and RE were elucidated. Consequently, the optimum condition of (0.5M DMDBTDMA+0.5M DHOA)/NDD-0.3M C 2 H 2 O 4 -4.5M HNO 3 and O/A=2 was obtained through experimental work. Under this condition, the extraction yields were found to be about 80% for Am more than 70% for RE such as La, Eu, Nd, Ce, etc., 3% for Cs and Sr, 69% for Fe and less than 11% for Mo and Ru. For the co-extraction of Am and RE, Fe should be removed in advance or prevented from a co-extraction with Am by controlling the different extraction rates of Am and Fe. About 95% of the Am and RE in the organic phase were stripped using a 0.5M HNO 3 .

Functionalized Mesoporous Silica Membranes for CO2 Separation Applications

Mesoporous silica molecular sieves are emerging candidates for a number of potential applications involving adsorption and molecular transport due to their large surface areas, high pore volumes, and tunable pore sizes. Recently, several research groups have investigated the potential of functionalized mesoporous silica molecular sieves as advanced materials in separation devices, such as membranes. In particular, mesoporous silica with a two- or three-dimensional pore structure is one of the most promising types of molecular sieve materials for gas separation membranes. However, several important challenges must first be addressed regarding the successful fabrication of mesoporous silica membranes. First, a novel, high throughput process for the fabrication of continuous and defect-free mesoporous silica membranes is required. Second, functionalization of mesopores on membranes is desirable in order to impart selective properties. Finally, the separation characteristics and performance of functionalized mesoporous silica membranes must be further investigated. Herein, the synthesis, characterization, and applications of mesoporous silica membranes and functionalized mesoporous silica membranes are reviewed with a focus on CO2 separation.

Study of cerium-promoted rhodium alumina catalyst as a steam reforming catalyst for treatment of spent solvents

This study attempted to develop an appropriate catalyst used for the steam reforming of gasified spent solvents. Rh2O3/ CeO2-Al2O3 catalysts with various CeO2 loadings were prepared and heated at different temperatures of 500 and 800 °C, and their surface properties were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), temperature programmed reduction (TPR) and Brumauer-Emmett-Teller (BET) analyses. Rhodium existed in the form of Rh2O3, regardless of the sample composition as well as the heating temperature. In the tested range of cerium addition (up to 12 times the rhodium mass), no significant changes in BET surface areas and binding energy corresponding to Rh 3d5/2 were observed. Instead, the addition of cerium led to a greatly enhanced dispersion of rhodium nanoparticles, and no agglomeration of rhodium was observed for samples heated even at 800 °C. Honeycomb monolith rhodium catalysts promoted with cerium were fabricated and tested for the steam reforming of a gasified spent solvent, mainly consisting of butylene (C4H8). The test results suggested that a cerium-promoted rhodium catalyst could be used as an appropriate reforming catalyst for treating low-quality non-methane hydrocarbons without the formation of coke at high temperatures of up to 900 °C.

A Concept for an Emergency Countermeasure Against Radioactive Wastewater Generated in Severe Nuclear Accidents Like the Fukushima Daiichi Disaster

Abstract This work studied a concept of prompt countermeasure to minimize the accumulation of radioactive wastewater generated in severe nuclear accidents like the Fukushima Daiichi accident. A sequential precipitation process for the removal of Cs, Sr, I, and residual nuclides of Co, Mn, Sb, and Ru was suggested as a way to embody this concept. The process was confirmed to be possible as an effective and rapid emergency treatment for radioactive wastewater using many experiments with non-radioactive and active nuclides. Cobalt ferrocyanide—impregnated chabazite zeolite, Ba-impregnated 4A zeolite, and Ag-impregnated 13X zeolite were chosen as adsorbents for Cs, Sr, and I in this work had very high selectivities and fast adsorption rates with decontamination factors (DFs) on the order of 102 to 103. The adsorbent powders were rapidly settled in solution within 5 to 10 min by adding a coagulant of ferric ions. The residual nuclides could be removed by coprecipitation using ferric ion and flocculation using anionic polyacrylamide with DFs of more than 100 within 10 min.

Evaluation of the stability of precipitated uranyl peroxide and its storage characteristics in solution

This work studied the stability of uranyl peroxide, which can be obtained as the final product of several processes to treat uranium mixture waste and uranium ore, in solution using various temperature, pH, and ionic strength conditions. The change in concentration of dissolved uranium and hydrogen peroxide from uranyl peroxide, the form of the dissolved uranium species, and the change in morphology of dissolved uranyl peroxide were investigated for 100 or more days. Uranyl peroxide was stable in distilled water at elevated temperatures, but dissolved in other solutions at temperatures higher than 40 °C; a greater amount of uranyl peroxide dissolved in more acidic conditions at elevated temperatures. Uranyl ions that dissolved from uranyl peroxide were able to be recovered as uranyl peroxide in the solution where the dissolution occurs by adding hydrogen peroxide. After the precipitation of uranyl peroxide, the uranyl concentration in the supernatant is low enough for the supernatant to be recycled or released into the environment. The morphologies of the partially dissolved uranyl peroxide and the re-precipitated uranyl peroxide from dissolved uranyl ions were different from that of the initial uranyl peroxide.