decontamination of gravels contaminated with uranium

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Decontamination of gravels contaminated with uranium Gye-Nam Kim a,, Uk-Ryang Park a , Seung-soo Kim a , Wan-Suk Kim a , Jei-Kwon Moon a , Jae-hyuk Hyun b a Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yusong-gu, Daejeon 305-353, Republic of Korea b Environmental Engineering, Chungnam National University, 99 Daehak-ro, Yusong-gu, Daejeon 305-764, Republic of Korea article info Article history: Received 28 January 2014 Received in revised form 28 May 2014 Accepted 30 May 2014 Keywords: Decontamination Uranium Gravel Washing Electrokinetic Electrodialytic abstract Gravel washing equipment and electrokinetic–electrodialytic decontamination equipment were manu- factured to decontaminate gravel contaminated with uranium. The removal efficiency according to the gravel size and weight and the removal efficiency according to the lapsed time using the manufactured equipment were investigated through several experiments. The volume of gravel in the high uranium concentration group was about 10%, the rock types of which were quartz, lamprophyre, and schist. The larger the gravel size, the higher the contaminated concentration of gravel. The average uranium ( 238 U) concentration of gravel after the first washing was about 1.45 Bq/g, and the average removal efficiency of gravel after the third washing was about 37%. In addition, the removal efficiency of the contaminated gravel was not related to its size. The contaminated concentration of the gravel decreased with an increasing gravel weight. In addition, the removal efficiency of contaminated gravel was not related to its weight. When the electrokinetic–electrodialytic decontamination period of 5 days, 10 days, 15 days, and 20 days elapsed, the 238 U in the gravel was removed by about 40%, 65%, 72%, and 81%. The more the electrokinetic–electrodialytic decontamination time elapsed, the more the removal efficiency ratio of 238 U decreased. Finally, the gravel with a size of less than 10 cm was treated by soil washing and electrokinetic decontamination methods with soil, and gravel with size of more than 10 cm but less than 20 cm was treated by gravel washing and electrokinetic–electrodialytic decontamination methods. Gravel with a size of more than 20 cm is treated by a gravel washing method, and gravel contaminated with a high concentration of uranium was treated by crushing and ball mill washing methods. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The soil around nuclear facilities is contaminated with radionuclides during the operation and decommissioning of those facilities, especially during a nuclear accident such as the one at Chernobyl in Ukraine and the one at Fukushima in Japan. Korea has a lot of soil contaminated with uranium generated during its operation of nuclear facilities. Soil with a size of less than 10 cm is usually decontaminated using soil washing and electrokinetic technologies. However, it is difficult to use soil washing technology for decontamination of gravel with a size of more than 10 cm. It is impossible to scrub gravel in a washing tank, because the gravel sinks to the bottom (Fedje et al., 2013; Bisone et al., 2012; Gryschko et al., 2005; Tandy et al., 2004; Voglar and Lestan, 2013). In addition, when electrokinetic decontamination technology (Yang and Chang, 2011; Kaneta et al., 1992; Dong et al., 2005) is applied to gravel with a size of more than 10 cm, the removal effi- ciency of the radionuclides from the gravel is reduced, because the electro-osmotic flux at the surface of the gravel in an electrokinetic cell is reduced owing to a reduction of the particle surface area, which is attributable to the large size of the gravel (Yang and Chang, 2011; Kaneta et al., 1992; Dong et al., 2005; Kim et al., 2011; Kim et al., 2010, 2008). Meanwhile, there have been few studies on the decontamination of gravel with a size of more than 10 cm. The volume ratio of gravel whose size is more than 10 cm in the total volume of soil at KAERI was about 20%. Therefore, it is necessary to study the decontamination of gravel contaminated with radionuclides. Vitoria et al. used three mechanical gravel cleaning methods: (1) tractor rotovating, using a Dowdeswell Powervator 35 rotova- tor, with a width of 90 cm, behind a Ford 1220 four-wheel drive tractor, (2) high pressure jet washing, using a KEW 5203 KD pres- sure washer, in which water was pumped at 150 bar through a hand-held lance with jets of 5 mm and 1 mm diameter, and (3) http://dx.doi.org/10.1016/j.anucene.2014.05.031 0306-4549/Ó 2014 Elsevier Ltd. All rights reserved. Corresponding author. Tel.: +82 42 868 8674; fax: +82 42 868 2499. E-mail address: [email protected] (G.-N. Kim). Annals of Nuclear Energy 72 (2014) 367–372 Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene

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Page 1: Decontamination of gravels contaminated with uranium

Annals of Nuclear Energy 72 (2014) 367–372

Contents lists available at ScienceDirect

Annals of Nuclear Energy

journal homepage: www.elsevier .com/locate /anucene

Decontamination of gravels contaminated with uranium

http://dx.doi.org/10.1016/j.anucene.2014.05.0310306-4549/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +82 42 868 8674; fax: +82 42 868 2499.E-mail address: [email protected] (G.-N. Kim).

Gye-Nam Kim a,⇑, Uk-Ryang Park a, Seung-soo Kim a, Wan-Suk Kim a, Jei-Kwon Moon a,Jae-hyuk Hyun b

a Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yusong-gu, Daejeon 305-353, Republic of Koreab Environmental Engineering, Chungnam National University, 99 Daehak-ro, Yusong-gu, Daejeon 305-764, Republic of Korea

a r t i c l e i n f o

Article history:Received 28 January 2014Received in revised form 28 May 2014Accepted 30 May 2014

Keywords:DecontaminationUraniumGravelWashingElectrokineticElectrodialytic

a b s t r a c t

Gravel washing equipment and electrokinetic–electrodialytic decontamination equipment were manu-factured to decontaminate gravel contaminated with uranium. The removal efficiency according to thegravel size and weight and the removal efficiency according to the lapsed time using the manufacturedequipment were investigated through several experiments. The volume of gravel in the high uraniumconcentration group was about 10%, the rock types of which were quartz, lamprophyre, and schist. Thelarger the gravel size, the higher the contaminated concentration of gravel. The average uranium(238U) concentration of gravel after the first washing was about 1.45 Bq/g, and the average removalefficiency of gravel after the third washing was about 37%. In addition, the removal efficiency of thecontaminated gravel was not related to its size. The contaminated concentration of the gravel decreasedwith an increasing gravel weight. In addition, the removal efficiency of contaminated gravel was notrelated to its weight. When the electrokinetic–electrodialytic decontamination period of 5 days, 10 days,15 days, and 20 days elapsed, the 238U in the gravel was removed by about 40%, 65%, 72%, and 81%. Themore the electrokinetic–electrodialytic decontamination time elapsed, the more the removal efficiencyratio of 238U decreased. Finally, the gravel with a size of less than 10 cm was treated by soil washingand electrokinetic decontamination methods with soil, and gravel with size of more than 10 cm but lessthan 20 cm was treated by gravel washing and electrokinetic–electrodialytic decontamination methods.Gravel with a size of more than 20 cm is treated by a gravel washing method, and gravel contaminatedwith a high concentration of uranium was treated by crushing and ball mill washing methods.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The soil around nuclear facilities is contaminated withradionuclides during the operation and decommissioning of thosefacilities, especially during a nuclear accident such as the one atChernobyl in Ukraine and the one at Fukushima in Japan. Koreahas a lot of soil contaminated with uranium generated during itsoperation of nuclear facilities. Soil with a size of less than 10 cmis usually decontaminated using soil washing and electrokinetictechnologies. However, it is difficult to use soil washing technologyfor decontamination of gravel with a size of more than 10 cm. It isimpossible to scrub gravel in a washing tank, because the gravelsinks to the bottom (Fedje et al., 2013; Bisone et al., 2012;Gryschko et al., 2005; Tandy et al., 2004; Voglar and Lestan,2013). In addition, when electrokinetic decontamination technology

(Yang and Chang, 2011; Kaneta et al., 1992; Dong et al., 2005) isapplied to gravel with a size of more than 10 cm, the removal effi-ciency of the radionuclides from the gravel is reduced, because theelectro-osmotic flux at the surface of the gravel in an electrokineticcell is reduced owing to a reduction of the particle surface area,which is attributable to the large size of the gravel (Yang andChang, 2011; Kaneta et al., 1992; Dong et al., 2005; Kim et al.,2011; Kim et al., 2010, 2008). Meanwhile, there have been fewstudies on the decontamination of gravel with a size of more than10 cm. The volume ratio of gravel whose size is more than 10 cm inthe total volume of soil at KAERI was about 20%. Therefore, it isnecessary to study the decontamination of gravel contaminatedwith radionuclides.

Vitoria et al. used three mechanical gravel cleaning methods:(1) tractor rotovating, using a Dowdeswell Powervator 35 rotova-tor, with a width of 90 cm, behind a Ford 1220 four-wheel drivetractor, (2) high pressure jet washing, using a KEW 5203 KD pres-sure washer, in which water was pumped at 150 bar through ahand-held lance with jets of 5 mm and 1 mm diameter, and (3)

Page 2: Decontamination of gravels contaminated with uranium

Fig. 1. Gravel contaminated with uranium.

Table 1High and low concentration groups of contaminated gravel.

Concentration (Bq/g) High-concentration Low-concentration

1 5.32 0.822 7.68 1.473 4.56 2.184 6.72 1.75 7.25 1.85

Average 6.31 1.60

368 G.-N. Kim et al. / Annals of Nuclear Energy 72 (2014) 367–372

pump washing, using a Pacer pump with a 3 Hp Briggs and Strattonengine. The most effective method for cleaning river gravelsappeared to be pump washing (Victoria et al., 1999). Pereiraet al. used vegetable oil biodiesel for cleaning oiled shores. Pureoil biodiesels (rapeseed and soybean) were significantly moreeffective in the cleanup of oiled sands (up to 96%) than recycledwaste cooking oil biodiesel (70%) (Pereira and Mudge, 2004).Clement et al. used a physic-chemical method for the treatmentof dredged sediment. The positive effects of physic-chemicaltreatment are the reduction of sediment mass, materials easier tohandle, reduction of visual and odor nuisances, better settability,the removal of ammonia emissions and associated ecotoxicologicalrisks, removal of zinc and nickel emissions, and a reduction ofsediment toxicity to amphipods and chrironomids (Clementet al., 2010). Min et al. used thermal and mechanical treatmentto clean aggregates from concrete. Most pollutants are easilyseparated from the contaminated concrete waste, which concen-trates mainly in the porous fine cement powder. Removal onpollutants in concrete was influenced by the heating temperatureand crushed aggregate size. Heating temperature played an impor-tant role in moving the contaminants from the concrete waste(Min et al., 2010). Cho et al. used dry washing technology for thetreatment of polluted railroad ballast gravel. A dry washingmethod removes the pollutants on the surface of ballast gravelsby blasting media on the gravel. The technology efficientlyremoved heavy metals from contaminated gravel for a short time(Cho et al., 2012).

In this study, the gravel contaminated with uranium was sam-pled at an area near a nuclear facility in Korea. The contaminationcharacterization of gravels separated from soil was analyzed.The gravel washing equipment and electrokinetic–elctrodialyticdecontamination equipment were manufactured to decontaminatethe contaminated gravel. The removal efficiency according to thegravel size and weight, and the removal efficiency according tothe lapsed time by electrokinetic–electrodialytic equipment, wasinvestigated through several experiments. The optimum experi-ment conditions for uranium decontamination by the gravel wash-ing and electrokinetic–elctrodialytic decontamination equipmentwere found. Finally, a process to decontaminate the contaminatedgravel was developed for the self-disposal of radioactive gravelwaste on the basis of the experimental decontamination results.

2. Materials and methods

2.1. Characteristics of contaminated gravels

About 30% of the volume of soil contaminated with uranium,which was excavated at an area near a nuclear facility, was gravel.The gravel contaminated with uranium is shown in Fig. 1. High andlow concentration groups of contaminated gravels are shown inTable 1. The volume of gravel in the high uranium concentrationgroup was about 10%, the rock types of which were quartz, lamp-rophyre, and schist. The reason is considered to be that uraniumcan infiltrate into the deep side of those rocks, because quartz thatconsists of a crystal cluster, lamprophyre that consists of biotiteand amphibole, and schist that consists of biotite and muscoviteare easy to be split. Washing after using a crushing method shouldbe selected to decontaminate gravel in the high concentrationgroup for an improvement of the decontamination efficiency.

Fig. 2. Manufactured gravel washing equipment.

2.2. Manufacturing of gravel washing equipment

The gravel washing equipment was manufactured to wash thecontaminated gravel. The gravel washing equipment consisted ofa trammel, nozzle, gravel injection box, gravel collection box,

waste solution collection box, nitric acid solution box, and drumhoist, as shown in Fig. 2.

The trammel circulates gravels in its inside at a fixed rpm, andthe nozzles in the trammel spray a nitric acid solution to wash thecontaminated gravel. The gravel injection box injects contaminatedgravel into the trammel, and the gravel collection box collects thewashed gravel. The waste solution collection box collects wastesolution released during gravel washing, the nitric acid solutionbox supplies the nozzle in the trammel with nitric acid solution,and the drum hoist transports contaminated gravel to the gravelinjection box. Images taken before and after gravel washing areshown in Fig. 3.

The optimum experimental conditions of the gravel washingequipment were obtained through several experiments: the opti-mum rpm of the gravel washing equipment, and the optimum con-centration of nitric acid as a washing solution.

Page 3: Decontamination of gravels contaminated with uranium

Fig. 3. Before and after gravel washing.

G.-N. Kim et al. / Annals of Nuclear Energy 72 (2014) 367–372 369

2.3. Experiments to decontaminate gravels contaminated withuranium

Gravels near the uranium conversion facility in Korea AtomicEnergy Research Institute (KAERI) were contaminated withuranium. The size of the gravel was mainly 5–30 cm, and theweight was mainly 150–4000 g. The uranium concentration ofthe contaminated gravel was 0.5–10.0 Bq/g. The required uranium(238U) concentration for self-disposal was below 0.43 Bq/g. Thus, itwas necessary to reduce the uranium concentration of the gravelusing washing and electrokinetic decontamination.

2.4. Manufacturing electrokinetic–electrodialytic decontaminationequipment

Using electrokinetic decontamination technology, the removalefficiency of radionuclides from gravel decreased, because the elec-tro-osmotic flux at the surface of the gravel in the electrokineticcell decreased due to the large size of the gravel. In addition, thedecontamination period was lengthened because the electrolytein the anode room was contaminated with uranium owing to ahigher hydraulic conductivity of gravels in the gravel cell of theelectrokinetic equipment. Meanwhile, the electrodialytic methodhas generally been used for treating waste solution and soil reme-diation, which attaches an ion exchange membrane at the anoderoom or cathode room (Sun et al., 2012; Nystrom et al., 2005;Jensen et al., 2012; Jakobsen et al., 2004). Fig. 4 shows a schematicdiagram of electrokinetic–electrodialytic decontamination. Ananion exchange membrane was attached at the anode room parti-tion to prevent an infiltration of uranium ions. In this study, the

Fig. 4. Schematic diagram of electrokinetic–electrodialytic decontamination.

electrokinetic–electrodialytic decontamination equipment wasmanufactured as shown in Fig. 5 for a shortening of the decontam-ination period.

2.5. Radioactivity measurement for gravel

The original uranium concentration for gravel was measuredusing a Multi-Channel Analyzer (MCA) with a standard tube of1000 cc, QCY48 (Amersham), manufactured by the Korea ReachInstitute Standards and Sciences. The MCA operates in a pulseheight analyzer mode. The scintillation counter measures the pulseheight distribution from a gamma ray source. The amplitude of anincoming analog pulse is digitized by an analog digital converter(ADC), and the digital value is used as the address of the incre-mented memory location. Thus, the screen display of the numberof counts vs. the channel number is really a histogram of thenumber of counts vs. the pulse height, i.e., a pulse height spectrum.The time required to measure the radioactivity concentration of agravel sample using the MCA was estimated to be 4–8 h.

The original radioactivity concentration of the washed gravelsbefore electrokinetic–electrodialytic decontamination was calcu-lated. The electrokinetic–electrodialytic experiment was stoppedat pre-determined interim times, the gravel samples wereextracted from the gravel cell, and the radioactivity concentrationof the gravel samples were then measured using the MCA. Thegravel samples were returned to their original locations inthe gravel cell, and the experiment was continuously restarted.The removal efficiency of the nuclides was calculated as a ratioof the original gravel concentration, and the gravel concentrationswere measured at pre-determined interim times. After completionof the electrokinetic–electrodialytic decontamination experimentsfor radioactive gravels with initial concentrations of 0.5–6.5 Bq/g,the residual concentration of gravel was calculated. Finally, thedecontamination period required for decontaminating the initialgravel concentration to a clearance concentration level (238U:0.43 Bq/g) was estimated with the experimental electrokinetic–electrodialytic results.

3. Results and discussion

From the results of the washing experiments with gravel wash-ing equipment, it was concluded that the optimum rpm of thegravel washing equipment was 10 ppm, the optimum concentra-tion of nitric acid as a washing solution was 1.0 M, and the opti-mum spray rate by nozzle was 50 L/min. Table 2 shows theresults of the removal efficiency according to the gravel size bywashing using the manufactured gravel washing equipment. Thelarger the gravel size, the more the contaminated concentrationof gravel decreased. The reason is considered to be that the uncon-taminated volume in a larger size of gravel is larger, because it isdifficult for uranium to infiltrate into the deep side of gravel. Theaverage concentration of gravel after the first washing was about

Fig. 5. Manufactured electrokinetic–electrodialytic decontamination equipment.

Page 4: Decontamination of gravels contaminated with uranium

Table 2Removal efficiency according to the gravel size by gravel washing.

Gravel size Ci (Bq/g) 1st 2nd 3rd Removal efficiency (%)

5 cm (154 g) 2.25 1.04 1.01 1.0 565 cm (266 g) 2.93 2.14 2.10 2.09 2910 cm (766 g) 2.54 1.83 1.76 1.76 3110 cm (784 g) 2.12 1.67 1.61 1.60 2515 cm (944 g) 1.9 1.32 1.27 1.22 3615 cm (1074 g) 1.2 0.68 0.67 0.65 46

Average 2.16 1.45 1.40 1.39 37.2

Table 3Removal efficiency according to the gravel weight by gravel washing (drum 1).

Gravel weight (g) Ci (Bq/g) 1st 2nd Removal efficiency (%)

1328 0.97 0.91 0.9 71526 3.71 2.1 1.6 431656 1.6 1.2 1.18 261876 0.61 0.4 0.37 392200 0.4 0.2 0.18 502252 0.72 0.32 0.3 583242 0.76 0.72 0.54 29

Average 1.25 0.84 0.72 42

Table 4Removal efficiency according to the gravel weight by gravel washing (drum 2).

Gravel weight (g) Ci (Bq/g) 1st 2nd Removal efficiency (%)

1214 3.68 1.86 2.03 451244 1.5 1.47 1.23 181382 6.49 4.26 3.68 431432 0.89 0.68 0.67 251458 1.2 0.67 0.59 511760 1.45 0.99 0.98 322728 0.95 0.85 0.84 123250 0.73 0.63 0.54 264440 0.71 0.28 0.27 624906 0.79 0.54 0.47 41

Average 1.84 1.22 1.13 39

370 G.-N. Kim et al. / Annals of Nuclear Energy 72 (2014) 367–372

1.45 Bq/g. The average removal efficiency of the gravel after thethird washing was about 37% and the removal efficiency of thethird gravel washing was very little in comparison with those ofthe first and second washings. In addition, the removal efficiencyof the contaminated gravel was not related to its size.

Table 3 shows the results of the removal efficiency according tothe gravel weight by washing the gravel in drum 1 using the

Fig. 6. Images of grave

manufactured gravel washing equipment. Table 4 shows theresults of the removal efficiency according to the gravel weightby washing the gravel in drum 2 using the manufactured gravelwashing equipment. The higher the gravel weight, the more thecontaminated concentration of the gravel decreased. The reasonis considered to be that the uncontaminated volume in a higherweight of gravel is larger, because it is difficult for uranium toinfiltrate into a deep side of gravel. The contamination concentra-tion and removal efficiency of the gravel depend on the rock type.It is impossible to decontaminate deeply contaminated gravel bywashing or electrokinetic–electrodialytic decontamination. Theoptimum number of washings for contaminated gravel is consid-ered to be two. In addition, the removal efficiency of contaminatedgravel was not related to its weight. Fig. 6 shows images of thegravel used in Table 4.

3.1. Ball mill washing after crushing

Gravel in the high concentration group should use a washingafter a crushing work to improve the decontamination efficiency.Gravel contaminated with a high concentration of uranium wascrushed to below 1 mm size, as shown in Fig. 7. The crushed gravelwas then put in a ball mill, and washed and crushed for 3–4 h. Theremoval efficiencies of gravel by ball mill washing are shown inTable 5. The average removal efficiency after the second washingby a ball mill was more than 90%.

3.2. Electrokinetic–electrodialytic decontamination

Uranium (UO22+) in the contaminated gravel in the electrokinetic–

electrodialytic decontamination equipment was removed by electro-osmosis, electro-migration, and a hydraulic pressure flow, as in thefollowing equation:

j ¼ ½ðko þ kmÞRrI þ khrp�C � Ds2rC; ð1Þ

where j is the molar flux of the species per unit pore area, ko theelectro-osmotic permeability, km the electro-migration coefficient,R the electric resistance, I the electric current, kh the hydraulic per-meability, P the pressure, C the molar concentration, D the diffusioncoefficient, and s is a non-dimensional tortuosity.

To increase the removal velocity of radionuclides from gravel,electrokinetic–electrodialytic decontamination equipment wasmanufactured. This equipment mixed the electrokinetic conceptand electrodialytic concept. That is, an anion exchange membranewas attached at the net plate of the anode room of the electroki-netic equipment to prevent the transfer of positive ions into the

l used in Table 4.

Page 5: Decontamination of gravels contaminated with uranium

Fig. 7. Crushing and ball mill washing for gravel contaminated with high concentration.

Table 5Removal efficiencies of gravel by ball mill washing.

Gravel weight (g) Ci (Bq/g) 1st 2nd Removal efficiency (%)

550 4.8 0.95 0.36 92.5650 3.6 0.75 0.32 91.1850 3.45 0.64 0.27 92.2

G.-N. Kim et al. / Annals of Nuclear Energy 72 (2014) 367–372 371

anode room, thus preventing the solution in the anode room frombeing contaminated with UO2

2+. The pure solution in the anoderoom can shorten the time required to remove uranium fromgravel surface. The experimental electrokinetic–electrodialyticconditions were as follows. The gravel volume in the gravel cellwas 400 L, the electric current was 150–200 A, the electric voltagewas 15–20 V, the electrolyte inflow rate was 130–160 mL/min, thetemperature in electrokinetic–electrodialytic experiment wasbelow 65 �C, and L (electrolyte volume, mL)/S (gravel weight, g)in the gravel cell was about 0.33. Table 6 shows the removal effi-ciency of uranium from gravel according to the lapsed time by

Table 6Removal efficiency according to the lapsed time by electrokinetic–elec

Lapsed time Origin (Bq/g) 5 Days (%)

Removal efficiency 2.3 421.7 391.3 38

Average 1.7 40

Fig. 8. The decontamination process for

electrokinetic–electrodialytic equipment. When the decontamina-tion period of 5 days, 10 days, 15 days, and 20 days elapsed, 238Uin the gravel was removed by about 40%, 65%, 72%, and 81%. Themore the decontamination time elapsed, the more the removalefficiency ratio of 238U decreased. In addition, the more the initialconcentration of 238U increased, the more the removal efficiencyof 238U increased.

A process to decontaminate contaminated gravel was devel-oped for the self-disposal of radioactive gravel waste on the basisof preceding experimental decontamination results. The decon-tamination process for gravel contaminated with uranium isshown in Fig. 8. Gravel contaminated with uranium can be treatedby four methods. First, gravel with a size of less than 10 cm is trea-ted by soil washing and electrokinetic decontamination methodswith soil. Second, gravel with a size of more than 10 cm but lessthan 20 cm is treated by gravel washing and electrokinetic–elec-trodialytic decontamination methods. Third, gravel with a size ofmore than 20 cm is treated by a gravel washing method. Fourth,gravel contaminated with a high concentration of uranium is trea-ted by crushing and ball mill washing methods.

trodialytic equipment.

10 Days (%) 15 Days (%) 20 Days

67 74 83% (0.39 Bq/g)64 71 80.6% (0.31 Bq/g)63 70 80% (0.26 Bq/g)

65 72 81% (0.32 Bq/g)

gravel contaminated with uranium.

Page 6: Decontamination of gravels contaminated with uranium

372 G.-N. Kim et al. / Annals of Nuclear Energy 72 (2014) 367–372

4. Conclusions

The volume of gravel in the high concentration group was about10%, the rock types of which were quartz, lamprophyre, and schist.The larger the gravel size, the more the contamination concentra-tion of the gravel decreased. The average concentration of gravelafter the first washing was about 1.45 Bq/g. The average removalefficiency of gravel after the third washing was about 37%, andthe removal efficiency of the third gravel washing was very littlein comparison with those of the first and second washings. Inaddition, the removal efficiency of the contaminated gravel wasnot related to its size. The higher the gravel weight, the more thecontamination concentration of the gravel decreased. The contam-ination concentration and removal efficiency of the gravel dependon the rock type. The optimum number of washings for contami-nated gravel is considered to be two. In addition, the removalefficiency of contaminated gravel is not related to its weight. Whenthe electrokinetic–electrodialytic decontamination period of5 days, 10 days, 15 days, and 20 days elapsed, 238U in gravel wasremoved by about 40%, 65%, 72%, and 81%. The more the electroki-netic–electrodialytic decontamination time elapsed, the more theremoval efficiency ratio of 238U decreased. Conclusively, gravelwith a size of less than 10 cm should be treated by soil washingand electrokinetic decontamination methods with soil, and gravelwith a size of more than 10 cm but less than 20 cm should betreated by gravel washing and electrokinetic–electrodialyticdecontamination methods. Gravel with a size of more than 20 cmshould be treated by a gravel washing method, and gravel contam-inated with a high concentration of uranium should be treated bycrushing and ball mill washing methods.

Acknowledgement

This project was carried out under the Nuclear R&D Program byMOST in KOREA.

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