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Abstract  

Photodissolution tests of UO2 sintered pellets were carried out in 3M nitric acid solution and at about 50 °C under UV irradiation. The light source was a Hg-lamp emitting a light of 254nm wavelength. In the products, chemicals such as H2O2 and NO2 ion were detected during photodissolution of the UO2 sintered pellets. Based on this result, a new dissolution mechanism of UO2 in nitric acid solution by photochemical reaction was suggested in this study.

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Abstract  

The applicability of vitrification technology to treat radioactive incineration ash was studied, especially in terms of leaching characteristics, by using several glassy waste forms which are fabricated with simulated incineration ash and base-glass at different mixing ratios. The ISO leaching test has been conducted for 820 days. Two semi-empirical models were applied to find out the dominant leaching mechanism of glass elements. Dissolution associated with diffusion was the dominant leaching mechanism and the elemental leaching characteristic depended upon its solubility in water. A theoretical leaching prediction model was applied to observe the long-term leaching behavior of major glass elements and surrogate nuclides. Diffusion coefficients and dissolution rate constants, the main parameters in the long-term prediction model, of glass elements and surrogates were obtained using short- and long-term experimental data. The model was found to be useful in predicting the long-term behavior of such elements in order to assess the stability of glassy waste forms.

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Journal of Radioanalytical and Nuclear Chemistry
Authors: Kwang-Wook Kim, Kee-Chan Song, Eil-Hee Lee, In-Kyu Choi, and Jae-Hyung Yoo

Abstract  

The change of Np oxidation state in nitric acid and the effect of nitrous acid on the oxidation state were analyzed by spectrophotometry, solvent extraction, and electrochemical methods. The Np extraction with 30 vol.% TBP was enhanced by the adjustment of the Np oxidation state using a glassy carbon fiber column electrode system. The knowledge of electrolytic behavior of nitric acid was important because the nitrous acid affecting the Np redox reaction was generated during the adjustment of the Np oxidation state. The Np solution used in this work consisted of Np(V) and Np(VI) but no Np(IV). The ratio of Np(V) in the range of 0.5M5.5 M nitric acid was 32%19%. The electrolytic oxidation of Np(V) to Np(VI) in the solution enhanced the Np extraction efficiency about five times higher than without electrolytic oxidation. It was confirmed that the nitrous acid in a concentration of less than about 10–5 M acted as a catalyst to accelerate the chemical oxidation reaction of Np(V) to Np(VI).

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Abstract  

The electrochemical redox behavior of nitric acid was studied using a glassy carbon fiber column electrode system, and its reaction mechanism was suggested and confirmed in several ways. Electrochemical reactions in less than 2.0M nitric acid was not observed. However, in more than 2.0M nitric acid, the reduction of nitric acid to nitrous acid occurred and the reduction rate was slow so that the nitric acid solution had to be in contact with an electrode for a period of time long enough for an apparent reduction current of nitric acid to nitrous acid to be observed. The nitrous acid generated in more than 2.0M nitric acid was rapidly and easily reduced to nitric oxide by an autocatalytic reaction. Sulfamic acid was confirmed to be effective to destroy the nitrous acid. At least 0.05M sulfamic acid was necessary to scavenge the nitrous acid generated in 3.5M nitric acid.

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Abstract  

In order to remove U, Tc, and Np, which are positioning materials or target nuclides for transmutation, from the high-level radioactive waste, conditions of co-extraction and sequential stripping of the nuclides were studied by using 30 vol.% TBP. On the basis of the experiments performed on each element of U, Tc, and Np, a combination of co-extraction of U, Tc, Np Tc stripping Np stripping U stripping was suggested. To enhance the Np extraction yield, the electrolytic oxidation of Np(V) was required at the co-extraction step. For the stripping of Tc 5M HNO3, of Np the electrolytic reduction of Np(VI) to Np(V), and of U 0.3M sodium carbonate were used. Phase ratios (O/A or A/O) were recommended to be of 2-3, for co-extraction and for stripping.

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Abstract  

The formation of precipitates by hydrazine was experimentally examined in the simulated high level liquid waste (HLLW), which was composed of 9 elements (Nd, Fe, Ni, Mo, Zr, Pd, Ru, Cs, Sr). Palladium was precipitated over 90% above 0.05M of hydrazine concentration and at 2M HNO3, while all of the other elements were hardly precipitated. The elements of Pd and Zr were precipitated 93% and 70% in the simulated solution in which the concentrations of Zr and Mo were decreased from 0.069M to 3.45·10–3M and 6.9·10–3M, respectively, and the acid concentration was decreased to about 0.5M after denitration. In a Pd solution of 0.5M and 2M HNO3, the precipitation yield of Pd increased with hydrazine concentration and reached over 98% at 0.1M. The precipitation yield of Pd at 0.5M HNO3 was higher than at 2M HNO3. The Pd precipitate, formed by adding hydrazine to an acidified solution, was an amorphous compound consisting of Pd, hydrazine, nitrate and hydrate.

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Journal of Radioanalytical and Nuclear Chemistry
Authors: Dong-Yong Chung, Eung-Ho Kim, Young-Joon Shin, Jae-Hyung Yoo, Cheong-Song Choi, and Jong-Duk Kim

Abstract  

The decomposition rate of oxalate by hydrogen peroxide has been investigated by a KMnO4 titration method. The rate equation for decomposition of hydrogen peroxide in the aqueous phase is 1n([H2O2]/[H2O2]0)=–k1·t, where k1=0.2, for [H+]<2M, k1=0.2+0.34([H+]–2), for [H+]>2M. As the acidity increases over 2M, an acid catalysis effect appeard. The new rate equation proposed for the decomposition of oxalate by hydrogen peroxide is

\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$- \frac{d}{{dt}}X_{[OX]} = k_2 [H_2 O_2 ]_0 (1 - X_{[OX]} )(e^{ - k_1 t} - \frac{{[OX]_0 }}{{[H_2 O_2 ]_0 }}X_{[OX]} )$$ \end{document}
The rate constant for decomposition of oxalate, k2, increased with nitric acid concentration and the effect of hydrogen ion concentration was expressed as k2=a[H+]n, where the values fora andn were a=1.54, n=0.3 at [H+]<2M, a=0.31, n=2.5 at [H+]>2M, respectively.

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