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  • J. Ind. Eng. Chem., Vol. 13, No. 3, (2007) 406-413

    Development of a Washing System for Soil Contaminated with Radionuclides Around TRIGA Reactors

    Gye-Nam Kim, Wang-Kyu Choi, and Chong-Hun Jung, Jei-Kwon Moon

    Korea Atomic Energy Research Institute, Duckjin-dong, Yuseong-gu, Daejon, Korea

    Received November 10, 2006; Accepted January 17, 2007

    Abstract: The purpose of this study was to develop a soil washing system, and to define the most suitable ex-perimental conditions for operation of its individual components, for decontaminating the radioactive soil around a TRIGA (Training, Research, Isotope, General Atomic) reactor in Korea. Analysis results indicated that the main radionuclides were Cs137 and Co60, the soil particle size ranged from 0.063 to 1.0 mm, and the ra-dioactive concentration was the strongest in soil particles smaller than 0.063 mm, as predicted. Meanwhile, ox-alic acid was found to be the most efficient chemical agent for washing, especially for the removal of cobalt. A scrubbing time of 4 h was optimal for obtaining a removal efficiency of more than 75 % for 137Cs and 60Co. A mixing ratio of the soil weight to the volume of the oxalic acid solution of 1:10 was observed to be best for washing, and it was estimated to be reasonable for two scrubbing cycles with 1.0 M oxalic acid to avoid the generation of excessive amounts of waste solution. The removal efficiency with hydro-cyclone was 30 % high-er than that without hydro-cyclone. Vertical plates and alum acted as important factors to reduce the sed-imentation time. The waste-solution could be reused after passing it through a column of strong acid resin up to five times.

    Keywords: soil washing, remediation, cobalt, cesium, oxalic acid

    Introduction

    1)

    The South Korean government has operated two re-search reactors, TRIGA Marks I and II, in Seoul for 30 years. The sites around these research reactors are con-taminated with radionuclides arising from their long-term operation . The main reason for the site contamination is the leakage of drainage boxes and the loss of radioactive sources at the site. KAERI (Korea Atomic Energy Research Institute) excavated the highly radionuclide- contaminated soil at the sites around the research reactors in 1988 and placed it in 4,000 sets of 200-L wastedrums; these drums have been in a radioactive waste storage fa-cility at KAERI since 1988. Firstly, 50 drums of soil were selected according to the effective dose rates of their soil drum surface and they were pulled out. The ef-fective dose rate indicates the degree of the effect that ra-diation has on the human body. Then, the soil in each drum was taken out and classified by three sizes of

    To whom all correspondence should be addressed.(e-mail: kimsum@kaeri.re.kr)

    sieves. Next, it was necessary to develop a technology for decontaminating the TRIGA soil contaminated with radionuclides. For the past decades, the need to develop economical techniques to decontaminate large contam- inated areas has grown. Soil washing and soil flushing are effective for decontaminating soils of a high hydraul-ic conductivity. Meanwhile, electro kinetic remediation [1,2] offers the possibility of in situ and ex situ re-mediation of contaminants in cases where conventional techniques are unfeasible [3,4]. In this study we developed a soil washing system for the removal of 137Cs and 60Co from TRIGA soil and in-vestigated the optimum experimental conditions for oper-ation of the equipment to maximize the removal effi-ciency of 137Cs and 60Co. The radio-waste storage facility at KAERI can take custody of about 10,000 drums of ra-dio-waste; it is storing more than 9,000 drums at present. Therefore, it should secure storage space for future con-current radio-waste through a release of its current ra-dio-waste, whose disposal is possible after treatment. Thus, it is necessary that the radioactive soil be

  • Development of a Washing System for Soil Contaminated with Radionuclides Around TRIGA Reactors 407

    Table 1. Particle Classification and Radionuclide Concentrations

    Surface effective dose rate of waste drum

    Soil Particle Size Volume (%) Co-60 (Bq/kg) Cs-137 (Bq/kg)

    0.05 mR/h above(7 %)

    >1.0 mm 28.3 38.9-233.7 1.5-886.0

    0.0631.0 mm 61.2 155.2-1079.3 16.3-6700.2

    1.0 mm 48.5 2.6-24.0 0.3-35.1

    0.0631.0 mm 46.5 6.3-217.8 13.9-436.7

    1.0 mm 52.4 5.3-15.2 2.5-7.0

    0.0631.0 mm 43.1 14.1-69.1 25.4-53.0

    0.063 mm) of the TRIGA soil can be removed by a soil wash-ing method; in contrast, the 137Cs and 60Co contaminants in the fine particles (

  • Gye-Nam Kim, Wang-Kyu Choi, Chong-Hun Jung, and Jei-Kwon Moon408

    Table 2. Contents of the TRIGA SoilComponent Content (%)

    SiO2 68.1Al2O3 16.7K2O 8.27

    Fe2O3 2.2P2O5 1.63CaO 1.51Na2O 0.97MgO 0.21

    Table 3. Physico-Chemical Characteristics of the TRIGA SoilDry bulk density (g/ cm3) 1.20

    Porosity (%) 43.6Water content (%) 12.00

    pH 4.30

    Figure 1. Process diagram of the soil washing system for re-storation of TRIGA soil.

    developed in this study. A component of the TRIGA soil was analyzed by SRS-303 XRF (X-ray fluorescence, made in Siemens, Germany); the results are shown in Table 2. The soil sample dried for ca. 12 h at 110 oC in an oven was meas-ured for several soil parameters. Each parameter was ob-tained by the following equations; the results are shown in Table 3.

    n=1-

    =

    Here, n is the porosity, b is the bulk density, s is the particle mass density, is the water content, Vw is the volume of water, and VT is the total unit volume. Meanwhile, b is the oven-dried mass of the sample div-ided by its field volume.

    Figure 2. Each element of the soil washing system developed for restoration of TRIGA soil.

    Development of the Soil Washing System

    The soil washing system was manufactured under con-sideration of the contamination characteristics of the soil; it consists of individual elemental equipment, namely, a soil hopper, sieve, screw feeder, scrubber, mixing tank, hydro-cyclone, sedimentation, a waste-solution treatment equipment, reagent box, and a control plate, as shown in Figures 1 to 5. The sieve divided the contaminated soil into three sizes. The screw feeder transports the sieved soil to the scrubber. The two scrubbers washed the trans-ported soil consecutively with impellers. The hydro-cy-clone separated the washed soil from the waste-solution. Sedimentation removed the fine particles in the waste solution. A column of strong acid resin purified the waste solution.

    Experiment

    To develop a soil washing system for the removal of ra-dionuclides from TRIGA soil, several experiments were executed with individual elemental equipment for washing. To obtain a higher removal efficiency of the ra-dionuclides from the TRIGA soil, some parameters were optimized through the following experiments.

    Selection of a Suitable Size for a Soil Washing The size of the contaminated TRIGA soil particles ranged from very fine to more than 1.0 mm. The volume of the soil particles larger than 1.0 mm was 2852 %, and the volume of the soil particles smaller than 0.06 mm was less than 410.5 %, but the volume of the medium sized particle of the soil was 4361 % as shown in Table 1. Meanwhile, the radioactivity was strongest in the soil particles smaller than 0.063 mm, as predicted.

  • Development of a Washing System for Soil Contaminated with Radionuclides Around TRIGA Reactors 409

    Figure 3. Soil washing equipment and screw feeder equipment.

    Selection of a Suitable Chemical Agent Experiments were performed with many chemical agents for the selection of a suitable agent to decontami-nate the soil contaminated with 137Cs and 60Co. The TRIGA soil of a size 0.0631.0 mm was decontami-nated with many chemical agents, namely, H2O, citric acid, citric acid+HNO3, NH4NO3, FeCl3, (COOK)2H2O, (NH4)2SO4, H2C2O4H2O, NaOH, and Na3PO4 sol-utions to compare their removal efficiency of the radio-nuclides from the TRIGA soil.

    Optimization of the Scrubbing Time To optimize the scrubbing time of the TRIGA soil, the contaminated soil and 0.5 M oxalic acid washing sol-ution were placed in the scrubber and then the soil was scrubbed for 30 min. The waste solution was removed from the soil mixed with a washing solution and the washed soil was dried for 23 days. 20 g of the dried soil was sampled; its radioactivity was measured by MCA. Next, the radioactivity of each washed soil was measured by the same method after washing for 1, 2, 3, 4, and 6 h.

    Optimization of the Mixing Ratio of the Soil Weight to the Volume of Oxalic Acid Solution Experiments were executed with different mixing ratios of the soil weight and the volume of the oxalic acid sol-ution, namely, 1:7.5 and 1:10. The removal efficiencies of the radionuclides from the soil were measured. Simultaneously, the removal efficiencies of the radio-nuclides were measured according to the change in the concentration of the oxalic acid (M).

    Optimization of the Number of Scrubbing Repetitions and Chemical Agent Concentration: The TRIGA soil having a particle size of 0.0631.0 mm was used for this experiment. Because the number of scrubbing repetitions plays an important role in enhanc-ing the removal efficiency of the radionuclide, an experi-ment was executed with different numbers of scrubbing cycles to obtain the optimum removal efficiency of the radionuclides by the least number of scrubbing repeti-tion. As the removal efficiency was changed with the concentration of the chemical agent, several washing ex-periments were executed within the range of 0.21.0 M

    Figure 4. Design of the vertical plate for a sedimentation tank.

    of oxalic acid and 13 scrubbing cycles to determine the repetition number of scrubbing cy

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