Si Hyung Kim’s research while affiliated with Korea Atomic Energy Research Institute and other places

The electrical conductivity (σ) of (CeyU1–y)O2+x (y=0.05,0.15,0.25) has been measured as a function of oxygen partial pressure (P02) at elevated temperatures by a d.c. 4-probe method. From the Po2-dependence of o, Ce ions were found to exist as Ce3+ on U sites and, consequently, to act as electron acceptors. From the electrical conductivity and the charge carrier concentration, hole mobility was calculated; activation energy for hopping was obtained from the temperature dependence of the hole mobility. Furthermore, redox reaction kinetics of (CeyU1-y)O2+x was investigated via conductivity relaxation experiment; surface reaction was found to be predominant over chemical diffusion and the surface reaction rate constants increased with decreasing P02.

Sintering behavior and microstructure were characterized for the pellets of U3O8-5wt%CeO2 powder mixture when sintered in Ar and CO2 at 1500°C and in N2-7vol%H2 at 1700 °C, respectively, and compared with those for UO2-5wt%CeO2 powder mixture. Densification was achieved only to about 90%TD in Ar and CO2 atmospheres while it was about 96%TD in N2-7vol%H2. Average grain size was about 11.5 um for both CO2 and N2-7vol%H2, in contrast to the sintering of UO2-CeO2 mixture in which the average grain size for CO2 and N2-7vol%H2 is about 18.0 ?m and 8.5 ?m, respectively. Sintering in various atmospheres and by use of the additives was also attempted for comparison. The differences in densification and grain growth are discussed in terms of oxide phases involved and microstructure evolution during sintering with and without additives in the different atmospheres.

The effects of a powder treatment, the sintering temperature and the sintering time on the grain growth Of UO2 pellets were investigated in air to obtain UO2 pellets with large grains. Air could be used for sintering because an oxidation path above 1803 K does not pass through a two-phase (UO2+x + U3O8-z) region. The UO2 pellets sintered by the CO2-air-CO2-H-2 process consisted of a single grain or some large grains in the order of several millimeters. (c) 2008 Elsevier B.V. All rights reserved.

A heterogeneous type burnable absorber needs a diluent material to adjust its gadolinium concentration. TiO2, ZrO2 or Al2O3 was added to Gd2O3, separately and GdxMyOz (M = Ti, Zr or Al) pellets were fabricated by a powder process. Pellets with a single Gd2TiO5 or Gd2Ti2O7 were fabricated and their phases were confirmed by XRD. Thermal properties of the GdxMyOz were measured and their thermal conductivities were determined. The thermal expansion was largest in Gd2Ti2O7, and it decreased with a gadolinium concentration increase. GdAlO3 had the highest thermal conductivity, next were the GdxTiyOz phases in a reverse order of the gadolinium concentration increase. The selected candidate pellets of GdxTiyOz were irradiated in the HANARO reactor, and post-irradiation examinations of the pellets were carried out in a hot cell. The examination results indicate that the thermal properties of the BP pellets should be considered to ensure an in-reactor integrity.

The oxidation behavior of U–10 wt% Mo alloy was studied using an XRD and a thermogravimetric analyzer in the temperature range from 473 to 773 K in air. It was found from the XRD study that U–10 wt% Mo alloy was completely converted to U3O8 at temperatures above 673 K. The oxidation rate of U–10 wt% Mo was lower than that of uranium. The activation energy for oxidation of U–10 wt% Mo was determined to be 66.64 kJ/mol in the temperature range of 473–773 K and it was higher than that of uranium.

Pellets admixed with zinc stearate showed density decrease of 3%T.D. during resintering in H2 atmosphere, but density change was below 0.4%T.D. in the pellets admixed with Acrawax and stearic acid when all the powder compacts were sintered in CO2 atmosphere. Experiments revealed that the density of the pellets doped with zinc compounds and sintered in CO2 also displayed a density decrease of 3%T.D. after resintering. Therefore, it was considered that the major cause of the large swelling of the pellets sintered in CO2 atmosphere after resintering was Zn residues.

The relation between sintered densities and porosities in UO pellets is investigated. The open porosity decreases linearly up to about 95% T.D.,(theoretical density) as the sintered density increases whereas, above 96% T.D., sintered UO pellets do not have any open pores. The fraction of open porosity to the total porosity also decreases linearly as the sintered density increases, though the slope is lower than that of open porosity and, above 95% T.D., the fraction decreases rapidly to approach a zero.

Horizontal rotary ball milling has been demonstrated to be a useful method for reducing the particle size of ceramic powder in remote operation in shielded hot cells. Techniques, equipment and operating parameters, such as milling media, media wear and rotor speed were investigated with Al2O3 powder to evaluate its performance prior to contamination with nuclear fuel material. The established operating parameters were then verified with UO2 powder, which had been produced by a thermal process to make fuel pellets. The sintering of the milled UO2 powder showed the higher sintered densities obtainable by the milling, and the milling process seemed to be an important factor in improving the powder characteristics.

An innovative nuclear fuel concept for the utilization as energy resources and for the incineration of excess Pu arisings as well as for an effective transmutation of minor actinides (MA's; Am, Np and Cm) is discussed from the aspect of material technology. Stabilized cubic phase ZrO2 and other potential candidate materials for the Inert Matrix are compared in terms of the material properties and other behaviors such as the behavior against irradiation with the relevant information currently available. Strategies for the use of the Inert Matrix Fuel concept in various countries are discussed and compared for their options in nuclear fuel cycle technology.