Kune-Woo Lee

WASHING-ELECTROKINETIC DECONTAMINATION FOR CONCRETE CONTAMINATED WITH COBALT AND CESIUM

A great volume of radioactive concrete is generated during the operation and the decommissioning of nuclear facilities. The washing-electrokinetic technology in this study, which combined an electrokinetic method and a washing method, was developed to decontaminate the concrete generated in nuclear facilities. The results of only an electrokinetic decontamination for the concrete showed that cobalt was removed to below 1% from the concrete due to its high pH. Therefore, the washingelectrokinetic technology was applied to lower the pH of the concrete. Namely, when the concrete was washed with 3 M of hydrochloric acid for 4 hours (0.17 day), the CaCO₃ in the concrete was decomposed into CO₂ and the pH of the concrete was reduced to 3.7, and the cobalt and cesium in the concrete were removed by up to 85.0% and 76.3% respectively. Next, when the washed concrete was decontaminated by the electrokinetic method with 0.01M of acetic acid in the 1L electrokinetic equipment for 14.83 days, the cobalt and the cesium in the concrete were both removed by up to 99.7% and 99.6% respectively. The removal efficiencies of the cobalt and cesium by 0.01M of acetic acid were increased more than those by 0.05M of acetic acid due to the increase of the concrete zeta potential. The total effluent volume generated from the washing-electrokinetic decontamination was 11.55L (7.2ml/g).

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.

Behavior of radioactive elements during thermal treatment of nuclear graphite waste : Thermodynamic model analysis

Nuclear graphite waste retains various radioactive elements. This study investigated the behavior of the radioactive elements during a thermal treatment of the nuclear graphite waste, based on the thermodynamic equilibrium. Two typical thermal conditions were simulated: an excess air incineration and a hydrothermal oxidation. Tritium, 14C and 36Cl are expected to be present in the gas phase throughout the whole waste thermal systems. 133Ba, 90Sr, 22Na, and 137Cs were analyzed to be semi-volatile elements. Their behavior will be dependent upon the gaseous atmosphere as well as the operating temperature. Uranium species are expected to convert into a gas-phase UO2 at temperatures above 1,000°C under excess air incineration atmospheres. Other radioactive elements such as 59Ni, 60Co, 141Ce, 152Eu and 241 Am are non-volatile at temperatures up to 1,200°C, regardless of the gaseous atmosphere being simulated.

Analysis of Combustion Kinetics of Powdered Nuclear Graphite by using a Non-isothermal Thermogravimetric Method

This study investigated the combustion kinetics of nuclear graphite waste. The purpose of this study is to establish the combustion mechanisms and the combustion rate parameters to aid in the modeling and designing of a pilot-scale graphite waste combustion chamber

Volatility and leachability of heavy metals and radionuclides in thermally treated HEPA filter media generated from nuclear facilities.

The purpose of the present study was to apply thermal treatments to reduce the volume of HEPA filter media and to investigate the volatility and leachability of heavy metals and radionuclides during thermal treatment. HEPA filter media were transformed to glassy bulk material by thermal treatment at 900°C for 2h. The most abundant heavy metal in the HEPA filter media was Zn, followed by Sr, Pb and Cr, and the main radionuclide was Cs-137. The volatility tests showed that the heavy metals and radionuclides in radioactive HEPA filter media were not volatilized during the thermal treatment. PCT tests indicated that the leachability of heavy metals and radionuclides was relatively low compared to those of other glasses. XRD results showed that Zn and Cs reacted with HEPA filter media and were transformed into crystalline willemite (ZnO·SiO(2)) and pollucite (Cs(2)OAl(2)O(3)4SiO(2)), which are not volatile or leachable. The proposed technique for the volume reduction and transformation of radioactive HEPA filter media into glassy bulk material is a simple and energy efficient procedure without additives that can be performed at relatively low temperature compared with conventional vitrification process.

Behavior of Phosphate in Uranium-Bearing Organophosphorus Solvents in the Pyrolysis and Combustion Reactors

This study investigated the behavior of phosphate in uranium-bearing organo phosphorus solvents under pyrolysis and combustion conditions. The volatilization characteristics of phosphate and the reaction between uranium oxide and phosphate oxide were established based on the results of a thermogravimetric analysis of tributyl phosphate (TBP) and uranium-bearing TBP-dodecane solutions. Uranium dioxide, UO2, functioned as a sorbent for the capture of vaporized phosphate oxide, P4O10(g). A significant fraction of phosphate remained in the form of uranium pyrophosphate (UP2O7) and uranium metaphosphate (U(PO3)4). The P4O10(g) capturing capacity of UO2 was 0.96-0.84 g/g for pyrolysis and 0.60 g/g for combustion, respectively. The relatively low phosphate-capturing capacity of combustion was due to the dissociation of P4O10(g) from (U(PO3)4) at flame temperatures higher than 1300K. Pyrolysis appears to be a better method than combustion for a thermal destruction of uranium-bearing TBP solutions while affording minimized problems associated with the condensation of corrosive phosphoric acid.

VOLUME REDUCTION OF DISMANTLED CONCRETE WASTES GENERATED FROM KRR-2 AND UCP

As part of a fundamental study on the volume reduction of contaminated concrete wastes, the separation characteristics of the aggregates and the distribution of the radioactivity in the aggregates were investigated. Radioisotope 60 Co was artificially used as a model contaminant for non-radioactive crushed concrete waste. Volume reduction for radioactively contaminated dismantled concrete wastes was carried out using activated heavy weight concrete taken from the Korea Research Reactor 2 (KRR-2) and light weight concrete from the Uranium Conversion Plant (UCP). The results showed that most of the 60 Co nuclide was easily separated from the contaminated dismantled concrete waste and was concentrated mainly in the porous fine cement paste. The heating temperature was found to be one of the effective parameters in the removal of the radionuclide from concrete waste. The volume reduction rate achieved was above 80% for the KRR-2 concrete wastes and above 75% for the UCP concrete wastes by thermal and mechanical treatment.

ABRASIVE BLASTING TECHNOLOGY FOR DECONTAMINATION OF THE INNER SURFACE OF STEAM GENERATOR TUBES

The inner surfaces of bundled inconel tubes from steam generators in South Korean nuclear power plants are contaminated with cobalt and abrasive blasting equipment has been developed to efficiently remove the cobalt. The principal parameters related to the efficient removal using this equipment are the type of abrasive, the distance from the nozzle, and the blasting time. Preliminary tests were performed using oxidized inconel samples which enabled the simulation of cobalt removal from the radioactive inconel samples. The oxygen in the oxidized samples and the cobalt in the radioactive inconel were removed more effectively using the blasting distance, blasting time, and a silicon carbide abrasive. Using the developed abrasive blasting equipment, the optimum decontamination conditions for radioactive inconel samples were blasting for more than 6 minutes using silicon carbides under 5 atmospheric pressures.

AN ANALYSIS OF THE EFFECT OF HYDRAULIC PARAMETERS ON RADIONUCLIDE MIGRATION IN AN UNSATURATED ZONE

A One-Dimensional Water Flow and Contaminant Transport in Unsaturated Zone (FTUNS) code has been developed in order to interpret radionuclide migration in an unsaturated zone. The pore-size distribution index (n) and the inverse of the air-entry value (α) for an unsaturated zone were measured by KS M ISO 11275 method. The hydraulic parameters of the unsaturated soil are investigated by using soil from around a nuclear facility in Korea. The effect of hydraulic parameters on radionuclide migration in an unsaturated zone has been analyzed. The higher the value of the n-factor, the more the cobalt concentration was condensed. The larger the value of α-factor, the faster the migration of cobalt was and the more aggregative the cobalt concentration was. Also, it was found that an effect on contaminant migration due to the pore-size distribution index (n) and the inverse of the air-entry value (α) was minute. Meanwhile, migrations of cobalt and cesium are in inverse proportion to the Freundich isotherm coefficient. That is to say, the migration velocity of cobalt was about 8.35 times that of cesium. It was conclusively demonstrated that the Freundich isotherm coefficient was the most important factor for contaminant migration.