Kwangheon Park

Defect models for the oxygen potentials of gadolinium- and europium-doped urania

A defect model for the thermochemistry of the mixed rare-earth uranium oxides has been developed. The parameters of the model were determined by fitting the theory to experimental oxygen potential data. The basic defects and associated equilibria found previously for pure urania were used in the mixed oxide defect model. In addition, the model prohibits oxygen interstitial occupancy of six to ten sites neighboring a trivalent dopant ion and assumes vacancy-dopant clusters in an excess of oxygen vacancies. Successful modeling of the oxygen potential as a function of temperature and oxide composition was achieved by assuming gadolinium to be present as isolated ions in the lattice, but europium modeling required complexes consisting of six Eu3+ and one anion vacancy.

The behavior of quartz crystal microbalance in high pressure CO2

Abstract Quartz crystal microbalance (QCM) is a very useful in situ tool for measuring minute changes of mass of thin surface films with real-time monitoring. The effect of temperature deviation and pressure change, viscosity and roughness loading on the frequency shift were examined by measuring the frequency change of bare QCM under high-pressure CO 2 . From a contour analysis, the frequency shift from viscosity loading and that from roughness loading were obtained. Frequency shift from viscosity loading was about three to four times larger than that estimated by Kanazawas equation. The resonator, whose surface roughness was larger than the diffusion length, exhibited a frequency shift from roughness loading.

Metal extraction from the artificially contaminated soil using supercritical CO2 with mixed ligands.

Supercritical fluids have good penetrating power with a high capacity to dissolve certain solutes in the fluid itself, making it applicable for soil cleaning. Supercritical CO2 along with mixed ligands has been used for cleaning artificially contaminated soil. The extraction of metal from the soil was successful, and the molar ratio of ligands to the extracted metal was as low as 3. Complicated structures with a large surface area of the real soil seemed to cause the lower efficiency. Reduced efficiency was also observed over time after the sample preparation, indicating the possibility of chemisorption of the metal ion onto the soil. The use of supercritical CO2 with dissolved mixed ligands was sufficient to extract metal from the soil.

Xenon Diffusivity in Thoria-Urania Fuel

Abstract Postirradiation annealing tests were performed to obtain the 133Xe diffusion coefficients in uranium dioxide (UO2) and mixed thorium-uranium dioxide [(Th-U)O2] fuels. Specimens were a single-grained UO2, a polycrystalline UO2, and a polycrystalline (Th-U)O2. The (Th-U)O2 specimen was a mixture of 35% ThO2 and 65% UO2. Each 300-mg specimen was irradiated to a burnup of 0.1 MWd/t U. Postirradiation annealing tests were performed at 1400, 1500, and 1600°C, continuously. The xenon diffusion coefficients for the nearly stoichiometric single-grained UO2 agree well with the data of others. The xenon diffusion coefficients in the polycrystalline (Th-U)O2 are approximately one order lower than those in the polycrystalline UO2. The xenon diffusion coefficient in the (Th-U)O2 increases with the increasing oxygen potential of the ambient gas.

The stoichiometry and the oxygen potential change of urania fuels during irradiation

Abstract A defect model for irradiated UO 2 fuel solid-solution was devised based on the defect structure of pure urania. Using the equilibrium between fuel solid-solution and fission-products and the material balance within the fuel, the stoichiometry change of urania fuel was traced with burn-up. This tracing method was applied to high burn-up fuels. The oxygen potential of urania fuel turned out to increase slightly with burn-up. The stoichiometric change was calculated to be negligible due to the buffering role of Mo.

Thermodynamics of (Mg, Ce, U)O2+x(x≤≥0) solid solutions

Quaternary solid solutions, Mg v Ce z U 1 y z O 2+x (x ≥ ≤ 0), were prepared and equilibrated under various oxygen partial pressures. The solubility of magnesium in Ce 0.1 1 U 0.9 O 2+x was about y = 0.05, and above this concentration MgO precipitated, forming a two-phase mixture. In Mg y Ce z U 1 zO 2 with y ≤ 0.05, a part of the dissolved magnesium atoms occupy the interstitial 4b position of Fm3m. The ratio of the interstitial magnesium to the total magnesium increases from about 0.3 to 0.6 as the oxygen partial pressure decreases to 10 19 -10 21 atm. The rates of change of the lattice parameter, ∂a/∂y, ∂a/∂z and ∂a/∂x, for x greater than 0 were somewhat smaller in absolute values than literature values. For x < 0, the rates ∂a/∂z and ∂a/∂x were in good agreement with the literature values. However, ∂a/∂y was much smaller as absolute value due to interstitial magnesium. A plot of oxygen potential vs O/M ratio at 1000°C showed a marked shift to lower x for the Mg y Ce z U 1-y-z O 2-x solid solution. The shift rate was -0.01 per 1 mol% magnesium.

The effects of adsorbates on Zircaloy oxidation in air and steam

The effects of adsorbates on the oxidation of Zircaloy-4 in air and steam are studied by the measurement of the weight gain of specimens. The effect of LiOH is dependent on surface condition, temperature and type of atmosphere. LiOH works as a mineralizer stabilizing monoclinic phases. LiOH only affects the specimens of pickled surface. LiOH enhances oxidation at low temperature, but retard oxidation at high temperature. NaCl enhances the oxidation, where nonuniform stresses on the surface and embrittlement of oxide by Cl are the cause. The effects of fluorides on oxidation are also measured. NaF is most harmful and KF follows next. LiF does not effect the oxidation of Zry.

Decontamination of uranium-contaminated waste oil using supercritical fluid and nitric acid.

The waste oil used in nuclear fuel processing is contaminated with uranium because of its contact with materials or environments containing uranium. Under current law, waste oil that has been contaminated with uranium is very difficult to dispose of at a radioactive waste disposal site. To dispose of the uranium-contaminated waste oil, the uranium was separated from the contaminated waste oil. Supercritical R-22 is an excellent solvent for extracting clean oil from uranium-contaminated waste oil. The critical temperature of R-22 is 96.15 °C and the critical pressure is 49.9 bar. In this study, a process to remove uranium from the uranium-contaminated waste oil using supercritical R-22 was developed. The waste oil has a small amount of additives containing N, S or P, such as amines, dithiocarbamates and dialkyldithiophosphates. It seems that these organic additives form uranium-combined compounds. For this reason, dissolution of uranium from the uranium-combined compounds using nitric acid was needed. The efficiency of the removal of uranium from the uranium-contaminated waste oil using supercritical R-22 extraction and nitric acid treatment was determined.

Kinetic Study of Containment Retention Factor (CRF) for Large Dry Containment Under Radiation Load

The Nuclear Power Plants (NPPs) have been built on the concept of Defense in depth. The severe accident causes the failure of fission product barriers and let the fission products to escape into environment. The containment is the last barrier to the fission products. Thus, the containment is installed with engineering safety features (ESFs) i.e. spray system, heat removal system, recirculation filtration system; containment filtered venting system (CFVS), and containment exhaust filtration system. In this work, kinetic study of the containment retention factor (CRF) has been carried out for a large dry PWR containment considering 1000 MWe PWR. The computational modeling and simulation have been carried out by developing a kinetic code in MATLAB, which uses the fractions of activity airborne into the containment after the accident. The Kinetic dependency of CRF on containment filtration systems, spray system with caustic and boric acid spray has been carried out. For noble gases, iodine and aerosols, the CRF increases with the increase in exhaust rate. While, CRF for iodine first increases then start reducing with containment spray flow rate. The Kinetic dependency of CRF has also been studied for boric and caustic spray.Copyright

Modelling and simulation of containment retention factor for can-type containment

Severe accidents cause the failure of fission product barriers and allow the fission products to escape into the environment. The containment is the last barrier for fission products. Thus, containment spray system, heat removal system, recirculation filtration system, Containment Filtered Venting System (CFVS) and containment exhaust filtration system are installed in the containment to mitigate the exposure to environment. In this work, modelling and simulation of the Containment Retention Factor (CRF) have been carried out for the can-type PWR containment considering a 1000 MWe PWR. Computational modelling and simulation have been carried out by developing a kinetic code in MATLAB, which uses the containment-specific activity available after the accident. The kinetic dependency of CRF in terms of containment filtration systems and spray system with caustic and boric acid spray has been carried out. For noble gases, iodine and aerosols, the CRF increases with the increase in exhaust rate, while with the containment spray flow rate, the CRF for iodine first increases and then starts reducing with significant magnitude.