Decontamination Experiment of Plant Soil Using Soil Washing Method

In Korea, there are not detailed analysis data on soil contamination in nuclear power plant(NPP) and suitable treatment criteria of contaminated soils. When the soils of NPP were contaminated, they have been simply collected and kept in radioactive waste building. But contamination of soils in NPP is inevitable and enormous amounts of contaminated soils can be produced during decommissioning. Therefore, generation of soil wastes should be minimized using proper decontamination methods. For this purpose, we investigated several decontamination methods and selected soil washing method as the best decontamination method for the contaminated soil in NPP. To achieve design data of decontamination equipment using soil washing method, several lab-scale experiments were performed using contaminated soils. Based on the results of experiment, decontamination equipment which is composed of particle separator and soil washer has been manufactured and installed. Contaminated soils were used to perform verification experiment of this equipment, and we proved its decontamination capability. Decontamination efficiency above 80% can be acquired by verification experiments according to the combination of experiment conditions. INTRODUCTION Radioactive soil can be generated during normal operation in nuclear power plant (NPP). Small amounts of radioactive particles released from NPP are deposited onto soils inside NPP and concentrated in particular regions due to the migration of nuclide by ground water or rain. Therefore the rise of contamination level by long-lived radionuclide can be caused in particular region after long operation of NPP. It is possible that this situation can be considered as careless release of radioactive material. When the soils of NPP were contaminated, they have been simply collected and kept in radioactive waste building. In Korea, there are not detailed analysis data on soil contamination in nuclear power plant and suitable treatment criteria of contaminated soils. But contamination of soils in NPP is inevitable and decommissioning of nuclear power plant can produce large amounts of contaminated soils. Therefore, generation of soil wastes should be minimized using proper decontamination methods. For this purpose, we investigated the characteristics of several available decontamination methods.[1-6] Also the characteristics of radioactive contaminated soils such as radiation level, radionuclide, particle size and radiation level according to the particle size of soils were investigated.[7] Among several decontamination methods, particle separation method and soil washing method, which uses water as decontamination agent, were selected as the best decontamination process based on the investigation results. Based on the results of the lab scale experiments, we designed the soil decontamination equipment composed of soil separator, soil washer and water circulator and then installed. Water, citric acid, HNO3, Na2CO3, strong acids and strong bases are also tested as decontamination agent in laboratory. Above chemical agent showed higher decontamination efficiency greater than 90%. But they WM’05 Conference, February 27 March 3, 2005, Tucson, AZ did not show sufficient efficiency in case of highly radioactive fine sand and generated secondary liquid waste which is difficult to treat. This results show that decontamination using chemical agent is inappropriate. Therefore we selected water as decontamination agent. The verification experiment was performed to prove the decontamination capability of this equipment using 5 drums of contaminated soils. Soils were separated into several groups according to the radiation level before verification experiments. And the soil groups were separated according to the particle size. Particle sizes of soils are classified into three categories; ≥ 2.0 mm, 2.0 ~ 0.5 mm and ≤ 0.5 mm. The experiments to obtain the decontamination efficiencies of the equipment were performed according to particle size, radiation level, ratio of soil and decontamination agent and decontamination time. Materials and Methods Decontamination Equipment To develop decontamination equipment for contaminated soil, the characteristics of several available decontamination processes were investigated and soil washing process was selected as the best process for contaminated soil. To acquire design data for soil decontamination equipment, several lab-scale experiments were performed using contaminated soil.[8] Based on the acquired data, soil washing decontamination equipment using water, which can use chemical agent in particular situation, was manufactured and installed in NPP. And verification experiments for this equipment were performed. The decontamination equipment was composed of particle separator using vibration screen, soil washer and liquid waste circulator. Fig. 1. is the process of this equipment. Fig. 1. Process of soil decontamination Excavated soil should be separated according to the radiation level or the possibility of decontamination. The volume of soil waste can be reduced by separation. Contaminated soil was separated according to particle size and analyzed using HPGe detector. Major nuclides were Co-60, Cs-134 and Cs-137. Radioactivity was not detected in particles above 4.76 mm, coarse sand, and the radioactivity was increased with the decrease of particle size. Main contaminant was Cs-137. In this study, Cs-137 was selected as target nuclide for decontamination. WM’05 Conference, February 27 March 3, 2005, Tucson, AZ Separated soil was decontaminated using soil washer. Washed soil was dried and analyzed using HPGe detector. And decontamination efficiency was acquired. Generated liquid waste was sent to the liquid waste circulator composed of MF filter and ion exchange resin. Water passed through circulator was sent to soil washer and re-used as decontamination agent. Materials and Methods For the verification experiment, 5 contaminated soil drums were opened and classified according to the conditions of soil. Large stone and various wastes with large size such as tree were excluded. Soil was sufficiently dried for smooth particle separation. Dried soil was separated by particle separator. Particle sizes of soil were classified into three categories; ≥ 2.0 mm, 2.0 ~ 0.5 mm and ≤ 0.5 mm. The volume of soil below 0.5 mm occupied about 30% of total volume and did not use in the experiments. Two kinds of soil(≥ 2.0 mm, 2.0 ~ 0.5 mm) were analyzed using HPGe detector and re-classified according to radiation level. For convenience, we classified each soil groups as high and low radiation groups according to radiation level. To acquire decontamination efficiency of equipment, the decontamination experiments were performed according to radiation level, particle size, ratio of water to soil, and decontamination time as shown in table 1. The contaminated soils, decontaminated soils and the liquid waste were analyzed. And the decontamination efficiencies were acquired. Table I. Experimental Conditions Radiation Level Particle Size Ratio (Water:Soil) Time 20:1 40 min ≥2.0 20:1 60 min 20:1 40 min 40:1 60 min Low Level 0.5 2.0 20:1 60 min 20:1 40 min ≥2.0 20:1 60 min 20:1 40 min 40:1 60 min High Level 0.5 2.0