Cheong-Song Choi

The influence of AUC powder characteristics on UO2 pellets

The AUC powders produced from three different pilot scale conversion plants at KAERI (the Korea Advanced Energy Research Institute) were investigated in terms of the correlationship between the precipitation conditions and their powder characteristics, as well as their effect on the characteristics of UO2 powder and on the sintered pellets. The UO2 powder characteristics, such as the tap density, the specific surface area, and the flowability, etc. were found to be closely related to the AUC powder characteristics. As a result, to obtain desirable nuclear fuel, the internal strain of AUC precipitates accumulated during the precipitation step should be minimized, and the tap density and the specific surface area of UO2 powder should be 2.85–3.05 g/cm3 and 4.5–5.5 m2/g, respectively, for optimum performance in compacting and sintering processes.

A new bed-collapsing technique for measuring the dense phase properties of gas-fluidized beds

Abstract A new bed-collapsing technique, the “optimum dual-drainage method”, is proposed to determine the dense phase properties in gas-fluidized beds of fine powders. The effects of gas flow resistance of the distributor and the windbox volume on the bed collapse of UO 2 , ammonium uranyl carbonate (AUC) and alumina powders have been determined in a 0.07-m I.D.×2.5-m high Plexiglas column. In the single-drainage method, the measured voidage and gas velocity in the dense phase increase with increasing windbox volume, gas flow resistance of distributor and gas velocity whereas in the optimum dual-drainage method, the dense phase properties are found to be independent of distributor resistance and windbox volume. It has been found that the period of bed expansion after the bubble escape stage on the bed-collapse curve varied with the type of powders, and the bed expansion was most pronounced in the bed of UO 2 powders in the single-drainage method. Momentum transfers from the windbox to the bed have been correlated with the pertinent experimental variables ( U o , V w , K d ).

DECOMPOSITION OF OXALATE BY HYDROGEN PEROXIDE IN AQUEOUS SOLUTION

AbstractThe 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

A Photo-induced Dissolution of UO2 Sintered Pellets in a Simulated Solution

- \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]} )

Dissolution of UO2 by Photochemical Reaction

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.

Thermal decomposition kinetics of ammonium uranyl carbonate

The partitioning of americium and neodymium by precipitation with oxalic acid was investigated in the simulated radwaste, which was composed of 10 elements of alkali, akaline earth, and transition metals in nitric acid solution. The effect of concentrations of oxalic acid and nitric acid in the simulated waste on the precipitation yield and purity of Nd and Am was examined in this study. As a result, the precipitated fraction of each element was increased with increasing concentration of oxalic acid and decreasing concentration of nitric acid. At an initial concentration of 0.5 M nitric acid and 0.5 M oxalic acid, both Am3 and Nd3+ were precipitated over 99% and other elements almost remained in the simulated solution. It was also found that Am was completely coprecipitated with Nd-oxalate precipitates, and Zr4- caused the coprecipitation of Cs2, Sr2+, and Pd2+.

Neptunium Oxalate Precipitation from the Simulated Radioactive Liquid Waste

The objective of this study is to improve the established dissolution technique of UO2 target by using a photochemical reaction. Photo-dissolution tests of UO2 sintered powder and pellets were carried out in a simulated nitric acid solution at about 50 °C under UV irradiation. The simulated solution consists of 2 M nitric acid containing elements such as Cs, Sr, Zr, Ru, Mo and Nd. The light source is a Hg-lamp emitting 254 nm wavelength. As results, in the dark reaction, UO2 sintered pellets were hardly dissolved, whereas UO2 was rapidly dissolved after 7 hours of dissolution time in the UV irradiation. The very low dissolution rate in the dark reaction was due to surface characteristics of sintered pellets: UO2 sintered pellet is very dense and has extremely low specific surface area. However, the dissolution rate of UO2 sintered pellet was considerably increased in the simulated solution under UV irradiation. This was attributed to the fact that ruthenium and molybdenum ions in the simulated solution could accelerate the dissolution of UO2 under UV irradiation. Additionally, when the pulverized sintered UO2 powder was used, the dissolution rate of UO2 increased more rapidly than that of UO2 sintered pellet.

REMOVAL OF PALLADIUM PRECIPITATE FROM A SIMULATED HIGH-LEVEL RADIOACTIVE LIQUID WASTE BY REDUCTION BY ASCORBIC ACID

Abstract A new master plot developed from the differential method is proposed, which can give information about the reaction mechanism and the activation energy simultaneously from non-isothermal TG data. The applicability of the master plot is confirmed by comparing the results of kinetic analysis with those published for the thermal decomposition of CaCO3 in an air atmosphere. The results are in good agreement with those previously published.