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

Transcription

1 J. Ind. Eng. Chem., Vol. 13, No. 3, (2007) 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 experimental 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 Cs 137 and Co 60, the soil particle size ranged from to 1.0 mm, and the radioactive concentration was the strongest in soil particles smaller than mm, as predicted. Meanwhile, oxalic 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 137 Cs and 60 Co. 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 % higher than that without hydro-cyclone. Vertical plates and alum acted as important factors to reduce the sedimentation 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 research reactors, TRIGA Marks I and II, in Seoul for 30 years. The sites around these research reactors are contaminated 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 radionuclidecontaminated 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 facility at KAERI since Firstly, 50 drums of soil were selected according to the effective dose rates of their soil drum surface and they were pulled out. The effective dose rate indicates the degree of the effect that radiation 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. ( 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 contaminated areas has grown. Soil washing and soil flushing are effective for decontaminating soils of a high hydraulic conductivity. Meanwhile, electro kinetic remediation [1,2] offers the possibility of in situ and ex situ remediation of contaminants in cases where conventional techniques are unfeasible [3,4]. In this study we developed a soil washing system for the removal of 137 Cs and 60 Co from TRIGA soil and investigated the optimum experimental conditions for operation of the equipment to maximize the removal efficiency of 137 Cs and 60 Co. The radio-waste storage facility at KAERI can take custody of about 10,000 drums of radio-waste; it is storing more than 9,000 drums at present. Therefore, it should secure storage space for future concurrent radio-waste through a release of its current radio-waste, whose disposal is possible after treatment. Thus, it is necessary that the radioactive soil be

2 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 0.05 mr/h above (7 %) mr/h (60 %) 0.02 mr/h below (33 %) Soil Particle Size Volume (%) Co-60 (Bq/kg) Cs-137 (Bq/kg) >1.0 mm mm <0.063 mm >1.0 mm mm <0.063 mm >1.0 mm mm <0.063 mm decontaminated for its release from the radio-waste storage. There are in situ and ex situ methods for the restoration of sites contaminated with radionuclides or heavy metals. The in situ method is a field remediation process and the ex situ one involves excavating the contaminated soil and then decontaminating it at another place. The remediation method of this study, which is an ex situ method, extracts radionuclides and heavy metals from contaminated soil with a chemical agent [5,6]. The DOE (Department of Energy) in the USA has studied a soil washing method for restoration of the Hanford site and the site around the BNL (Brookhaven National Laboratory) [7,8]. Because the TRIGA soil has a specific size distribution, radionuclide types, and concentrations, it is necessary to develop a soil washing system suitable for its specific contamination characteristics. Because a large quantity of waste solution is generated during soil washing [9], it is necessary to establish a recycling method for the waste solution, such as ion exchange [10,11] or chemical sedimentation [12,13] for reduction of the waste solution volume. The main goal of this study was to clean up radioactive soil having particle sizes larger than mm. In this study, the contamination characteristics of the TRIGA soil were investigated. A soil washing system was manufactured with consideration of the contamination characteristics. To increase the removal efficiency of cesium and cobalt in the TRIGA soil, optimum chemical agent and washing conditions were determined through several experiments with individual elemental equipment. A method to reduce the volume of the waste solution is also suggested. Analysis of the Contamination Characteristics A total of 50 soil-waste drums were selected from 4,000 soil-waste drums in the KAERI radioactive-waste storage facility with consideration of the surface effective dose rate of each drum. A soil sample of 10 kg was sampled from each of the selected drums and dried for more than a week. The dried soil was sieved with 1-mm (No.16) and mm (No.230) sieves by an ABTS-200 Sieve- Shaker for 30 min per soil sample. Table 1 shows the soil volumes per particle size in the ranges of more than 1.0 mm, mm, and less than mm. The radioactivity level of the soil samples was measured by an MCA (Multi-Channel Analyzer) with a standard tube of 50 cc, QCY48 (Amersham), manufactured by KRISS (Korea Research Institute Standards and Science). Radionuclides measured by MCA were Am-241, Cd- 109, Co-57, Ce-139, Hg-203, Sn-203, Sr-85, Cs-137, Y-88, and Co-60. In total, 45 soil samples were measured by MCA with a sensitivity of 0.1 Bq. The time required to measure the radioactivity level of the soil sample by MCA was estimated to be more than 8 h. The TRIGA soil was mainly contaminated with cesium and cobalt. The smaller the size of the soil particle, the higher the radioactive concentration in the soil particles, as shown in Table 1. Also, the contamination concentrations of 137 Cs were higher than those of 60 Co. The radioactive concentrations of 137 Cs and 60 Co in parts of the contaminated soil were higher than the target concentration level (400 Bq/kg). The 137 Cs and 60 Co in the large particles (>0.063 mm) of the TRIGA soil can be removed by a soil washing method; in contrast, the 137 Cs and 60 Co contaminants in the fine particles (<0.063 mm) were barely removed by this method. In particular, the removal efficiency of 137 Cs from fine particles (<0.063 mm) by soil washing was extremely low. Therefore, a related technology for decontaminating particles larger than mm was

3 408 Gye-Nam Kim, Wang-Kyu Choi, Chong-Hun Jung, and Jei-Kwon Moon Table 2. Contents of the TRIGA Soil Component Content (%) SiO Al 2O K 2O 8.27 Fe 2O P 2O CaO 1.51 Na 2O 0.97 MgO 0.21 Table 3. Physico-Chemical Characteristics of the TRIGA Soil Dry bulk density (g/ cm 3 ) 1.20 Porosity (%) 43.6 Water content (%) ph 4.30 Figure 1. Process diagram of the soil washing system for restoration 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 o C in an oven was measured for several soil parameters. Each parameter was obtained by the following equations; the results are shown in Table 3. n=1- θ= Here, n is the porosity, ρ b is the bulk density, ρ s is the particle mass density, θ is the water content, V w is the volume of water, and V T is the total unit volume. Meanwhile, ρ b is the oven-dried mass of the sample divided 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 consideration 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 transported soil consecutively with impellers. The hydro-cyclone 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 radionuclides from TRIGA soil, several experiments were executed with individual elemental equipment for washing. To obtain a higher removal efficiency of the radionuclides 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 %, and the volume of the soil particles smaller than 0.06 mm was less than %, but the volume of the medium sized particle of the soil was % as shown in Table 1. Meanwhile, the radioactivity was strongest in the soil particles smaller than mm, as predicted.

4 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 decontaminate the soil contaminated with 137 Cs and 60 Co. The TRIGA soil of a size mm was decontaminated with many chemical agents, namely, H 2 O, citric acid, citric acid+hno 3, NH 4 NO 3, FeCl 3, (COOK) 2 H 2 O, (NH 4 ) 2 SO 4, H 2 C 2 O 4 H 2 O, NaOH, and Na 3 PO 4 solutions to compare their removal efficiency of the radionuclides 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 solution 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 2 3 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 solution, namely, 1:7.5 and 1:10. The removal efficiencies of the radionuclides from the soil were measured. Simultaneously, the removal efficiencies of the radionuclides 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 mm was used for this experiment. Because the number of scrubbing repetitions plays an important role in enhancing the removal efficiency of the radionuclide, an experiment was executed with different numbers of scrubbing cycles to obtain the optimum removal efficiency of the radionuclides by the least number of scrubbing repetition. As the removal efficiency was changed with the concentration of the chemical agent, several washing experiments were executed within the range of M Figure 4. Design of the vertical plate for a sedimentation tank. of oxalic acid and 1 3 scrubbing cycles to determine the repetition number of scrubbing cycles and the optimum concentration. Effect of a Hydro-Cyclone Another experiment was performed to determine the effect of a hydro-cyclone located between the scrubber and the sedimentation. The soil in the sedimentation was slowly precipitated, while the soil in the hydro-cyclone was precipitated rapidly. Both soil samples were extracted and dried. Thereafter, their radioactivity levels were measured by a MCA (Multi Channel Analyzer) to determine the effect of the hydro-cyclone. A mixing tank (Figure 1) was used to dilute the mixing concentration of the soil, which makes the hydro-cyclone s separation work easier. The separated waste solution was transferred to the sedimentation tank, while the separated soil remained in a bowl attached at the bottom of the hydro-cyclone. Selection of a Method for Reduction of the Sedimentation Volume Because a large quantity of waste solution was generated from washing the contaminated soil in the scrubber, a large sized sedimentation tank was required. A sedimentation tank with vertical plates as shown in Figure 4 was manufactured for reduction of the sedimentation volume and the soil sedimentation time. The soil sediment time in the vertical plate sedimentation tank was compared with that in a conventional sedimentation tank, and the effect of an alum addition was estimated, as shown in Figure 5. Ion-Exchange to Reuse the Waste Solution Because the bulk of the waste solution was generated from washing the contaminated soil in a scrubber, an

5 410 Gye-Nam Kim, Wang-Kyu Choi, Chong-Hun Jung, and Jei-Kwon Moon Figure 5. Comparison of the general sedimentation tank and the vertical plate sedimentation tank. Figure 7. Radionuclide removal efficiency plotted as a function of the attrition scrubbing hours. equipment in the soil washing system are shown in Figures 6 to 11. Figure 6. Soil decontamination efficiencies of each chemical agent. ion-exchange experiment to reuse the waste solution was executed with a resin. The radionuclide-removal ability of a strong acid resin in a batch mode was analyzed. The waste-solution separated from the vertical plate sedimentation tank was passed through a column of strong acid resin for removal of the radionuclides. Then the first recycled waste solution was manufactured with the addition of some oxalic acid to the eluted waste solution. The first recycled waste-solution was reused for the TRIGA soil washing. The second recycled waste solution was manufactured using the same method as that for the first recycled waste solution. Next, the third and fourth recycled waste solutions were manufactured using the same method. Each recycled waste solution was analyzed to check its removal efficiency. Results and Discussion The soil contamination characteristics are given in Tables 1 to 3. The experimental results of the elemental Determination of a Suitable Size of Soil for a Washing Experiment The classification of the soil particles and concentrations of the radionuclides are shown in Table 1, which illustrates that the most abundant particle size ranges from to 1.0 mm. Because it is easier to decontaminate a soil having a particle size larger than 1.0 mm, the TRIGA soil particles having sizes that range from to 1.0 mm were chosen for the soil washing experiments. The content of the TRIGA soil components are described in detail in Table 2 and the physical characteristics are presented in Table 3. Determination of a Suitable Chemical Agent The results of the washing experiments with H 2 O, citric acid, citric acid+hno 3, NH 4 NO 3, FeCl 3, (COOK) 2 H 2 O, (NH 4 ) 2 SO 4, H 2 C 2 O 4 H 2 O, NaOH, and Na 3 PO 4 solutions are shown in Figure 6. When oxalic acid (H 2 C 2 O 4 H 2 O) was used as a washing agent, the removal efficiency of cesium was very high and that of cobalt was comparatively high. Oxalic acid is decomposed biologically, forms a stable metal-complex, and dissolves hydroxides [14,15]. What is more, its price is reasonable. Therefore, oxalic acid is an optimal chemical agent for soil washing. H 2 C 2 O 4 + 2Cs + Cs 2 C 2 O 4 ( ) + 2H + ( ) (1) H 2 C 2 O 4 + Co 2+ CoC 2 O 4 ( ) + 2H + ( ) (2) Determination of Optimum Scrubbing Time The measurement results of a soil radioactivity versus different scrubbing times are shown in Figure 7. The re-

6 Development of a Washing System for Soil Contaminated with Radionuclides Around TRIGA Reactors 411 Figure 8. Radionuclide removal efficiency plotted as functions of the oxalic acid concentration and the ratio of the soil weight to the volume of oxalic acid. Figure 9. Radionuclide removal efficiency plotted as functions of the number of scrubbing repetitions and the oxalic concentration. moval efficiency of the radionuclide in the soil increased upon increasing the scrubbing time up to 4 h, but it leveled off thereafter. Because the overall soil washing time increases with an extension of the scrubbing time, 4 h was the optimal scrubbing time. Determination of the Mixing Ratio of the Soil Weight to the Volume of Oxalic Acid Solution The removal efficiencies of 137 Cs and 60 Co are plotted as functions of the oxalic acid concentration and the mixing ratios of the soil weight to the volume of oxalic acid solution in Figure 8. The removal efficiency of 137 Cs at the 1:10 mixing ratio was 10 % higher than that at 1:7.5. Even if the volume of the requested oxalic acid solution increases, the ratio of 1:10 remained the optimum mixing ratio. Also, the increasing rate of the removal efficiency decreased remarkably after the oxalic acid concentration Figure 10. Effect of the use of a hydro-cyclone on the removal efficiency. reached 0.5 M, as shown in Figure 8. Therefore, concentrations of M were optimal for washing. Determination of the Repetition Number of Scrubbing Cycles and the Chemical Agent Concentration Results of the experiments to determine the number of scrubbing cycles and the optimum concentration are shown in Figure 9. When the TRIGA soil was washed through a scrubbing process of 1 3 cycles with 0.5 M oxalic acid, the removal efficiencies of 137 Cs and 60 Co were 47, 71, and 79 % and 78, 92, and 95 %, respectively. Also, the removal efficiencies of 137 Cs and 60 Co with 1.0 M oxalic acid were 50, 77, and 87 % and 82, 93, and 97 %, respectively. Therefore, to obtain removal efficiencies greater than 75 % for 137 Cs and 60 Co, scrubbing for more than 2 cycles with 1.0 M oxalic acid or for more than 3 cycles with 0.5 M oxalic acid is required. Thus, we conclude that 2 cycles of scrubbing with 1.0 M oxalic acid will avoid the generation of a considerable amount of waste solution. Effect of Hydro-Cyclone The effect of the addition of a hydro-cyclone unit between the scrubber and the sedimentation was investigated. Results of the radioactivity level of both soil samples, which were obtained from sedimentation processes performed with and without a hydro-cyclone unit are shown in Figure 10. The removal efficiencies of 137 Cs and 60 Co in the soil precipitated in the hydro-cyclone were 47 and 78 %, respectively, while in the sedimentation they were 10 and 50 %, respectively. The reason for this behavior is probably that Cs 2 C 2 O 4 and CoC 2 O 4 formed as in Equations (1) and (2) and precipitated and mixed with the soil in the sedimentation. In other words, 137 Cs and 60 Co in the hydro-cyclone were removed with the waste solution prior to the formation of Cs 2 C 2 O 4 and CoC 2 O 4.

7 412 Gye-Nam Kim, Wang-Kyu Choi, Chong-Hun Jung, and Jei-Kwon Moon Figure 11. Sedimentation time change plotted with respect to the sedimentation conditions. Reduction of the Sedimentation Volume The sedimentation time in the general sedimentation tank was ca. 8 h, while that in the vertical plate sedimentation tank was 2.5 h, as shown in Figure 11. In the case of alum addition in the general sedimentation tank the sedimentation time was 1.5 h, while that in the case of alum addition in the vertical plate sedimentation tank was only 1.0 h. Therefore, alum addition in the vertical plate sedimentation tank appeared to be the most economical case. In the vertical plate sedimentation tank, the waste solution entered through an inflow into the sedimentation system and then flowed upward and spread out at the top of the sedimentation tank and finally reached the bottom of the sedimentation tank, as shown in Figure 4. The recycled waste solution in this case was produced by removing the fine particles from the waste solution. Because the ph of the recycled waste solution was 1.1, the removal efficiency of the recycled waste solution was similar to that of the original oxalic acid solution. Ion-Exchange for Reuse of Waste Solution The waste solution of oxalic acid was analyzed for its metal content. The analysis results showed that cesium and cobalt formed chelate complexes with oxalate ions and Si, Al, Ca, Mg, Fe, Mn, etc., and they were dissolved in the waste solution. Fe and Si remained after passage through a column of strong acid resin. Each recycled waste-solution was investigated for its removal efficiency; the experimental results are shown in Figure 12. The removal efficiency of cobalt and cesium by each recycled waste-solution was similar to that of the original oxalic acid solution. Meanwhile, because the metals in the mixture of soil and oxalic acid reacted with oxalic acid and forms oxalate precipitates, the concentration of oxalic acid in solution was reduced and the ph increased. After passing the mixture through a column of strong Figure 12. Removal efficiencies of recycled oxalic acid in terms of the cycle time. acid resin, the ph of the waste solution increased again. Therefore, it is not necessary to add oxalic acid to decrease the ph of the recycled waste solution. Only % of an original oxalic acid solution should be added to supplement the quantity that was removed by soil absorption during soil washing. Conclusion Our fabricated soil washing system with a hydro-cyclone was very effective for decontaminating the radionuclides in the TRIGA soil. The size of the contaminated TRIGA soil ranged from very fine to more than 1.0 mm. The volume of soil particles larger than 1.0 mm was %, and the volume of soil particles smaller than 0.06 mm was less than %, but the volume of medium-sized soil particle was %. The radioactive concentration was strongest in the soil particle smaller than mm, as predicted. Oxalic acid was the best chemical agent for washing, especially for cobalt. A scrubbing time of 4 h was the optimum time to obtain a removal efficiency of more than 75 % for 137 Cs and 60 Co. A mixing ratio of the soil weight to the volume of the oxalic acid solution of 1:10 was the best for washing; two scrubbing cycles with 1.0 M oxalic acid avoided the generation of a considerable amount of waste solution. The removal efficiency with a hydro-cyclone was 30 % higher than that without. Vertical plates and alum had important roles in reducing the sedimentation time. The waste solution could be reused up to five times after passage through a column of a strong acid resin.

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