Surface and Coatings Technology 171(2003) 317–320

0257-8972/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972(03)00293-7

Decontamination of radioactive metal surface by atmospheric pressureejected plasma source

Yong-Hwan Kim*, Yoon-Ho Choi, Ji-Hun Kim, Jongkyu Park, Won-Tae Ju, Kwang-Hyun Paek,Y.S. Hwang

Nuclear Plasma Experiment Laboratory, Dept. of Nuclear Engineering, Seoul National University, Seoul, South Korea

Abstract

An atmospheric pressure ejected plasma source has been developed and applied to the decontamination of cobalt-containedoxide layer on metal surfaces. A helium-based discharge with relatively low gas temperature of approximately 200;300 8C hasbeen used for the decontamination. Small amounts of CF and O gases are added into helium plasmas as reactive species for4 2

carbonylation and fluorination of cobalt. Treatments are performed with various operating parameters such as RF power, treatmenttime and CFyO gas flow ratio and so on. Decontamination ratios of approximately 95% have been achieved through both4 2

gasification and solidification. With the increased reactive gases, the decontamination processes are expedited via gasification,and decontamination ratios solely by gasification have reached up to approximately 70%, indicating the necessity of extra effortof removing solidified powder components.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Atmospheric pressure plasma; Decontamination; Cobalt

1. Introduction

Since Co-60 is identified as a primary cause ofradioactivity build-up in nuclear power plants, removalof cobalt from metal surface is the main issue in thedevelopment of surface decontamination techniquew1x.In this study, an atmospheric pressure ejected plasmasource has been developed and evaluated for the decon-tamination of cobalt-contained oxide layer on the metalsurface. Current decontamination processes such as wet-chemical process, mechanical machining and high pres-sure gas phase decontamination processesw2,3x havedisadvantages of generating large amounts of secondarywaste. A dry process such as vacuum plasma processmay avoid the secondary waste problemw4x, but proc-essing volume is very limited in this process sinceplasma-processing volume is limited by the size ofplasma vacuum chamber. To overcome these disadvan-tages, plasma decontamination technique operating atatmospheric pressure has been emerging as a promisingnew technologyw5x.

*Corresponding author. Present address: Bienroder Weg 54E, Seoul,South Korea; Tel.:q31-30-2532964; fax:q31-30-2543165.

E-mail address: scmagnet@nownuri.net(Y.-H. Kim).

Decontaminating cobalt from the metal surfaces canbe performed by plasma chemical reactions of generat-ing either any volatile compounds or easily removablepowders. Chemical properties of those chemical com-pounds are listed in Table 1. The volatile cobalt com-pound of Co (CO) can be obtained through2 8

carbonylation in the presence of plasmas with carbonmonoxide. Also, easily removable powders such asCoF and CoF can be formed through fluorination in2 3

fluorine plasmas. These plasma chemical reactions areas follows:

* w xCarbonylation: CoqCO™Co CO volatileŽ .82

* w xFluorination: CoqF™CoF , CoF solid2 3

These two important reactions can be strongly acti-vated by generating stable atmospheric plasma discharg-es since carbon monoxide as well as fluorine radicalscan be generated in helium-based plasmas with O and2

CF as reactive gases. At atmospheric pressures, stable4

plasmas with such reactive gases are difficult to obtain,without sufficient helium gases in most cases. Argon-air based plasma torch which generates a kind of arcplasma with relatively high gas temperatures is observedto be very unstable when CF gases are introduced.4

318 Y.-H. Kim et al. / Surface and Coatings Technology 171 (2003) 317–320

Table 1Chemical properties of cobalt and cobalt compounds

Molecular Melting point Boiling pointformula (8C) (8C)

Co 1495 2927Co (CO)2 8 51 (decomposes)CoF2 1127 1400CoF3 927

Fig. 2. AES result: formation of cobalt oxide.

Fig. 1. (a) Schematic diagram of experimental setup and(b) plasmasource.

2. Experimental setup and procedure

Experimental setup is shown in Fig. 1a, showing atest sample in front of an atmospheric plasma sourcewith evacuating system present. The plasma source ispowered by 13.56 MHz, 1 kW radio frequency(RF)source via matching box, and gas flow rates are con-trolled with a mass flow controller(MFC). Helium gas(10 l per min) as a carrier gas and a small amount ofCF and O gases of approximately 1% of helium gas4 2 as reactive species are fed through MFC. Reaction

products are discharged to outside through the ventingsystem.The atmospheric pressure ejected plasma source is

composed of two coaxial electrodes. Outer ground elec-trode made of aluminum is covered with ceramic tubeas a dielectric barrier to avoid arcing at high RF power.Both aluminum and stainless steel have been tested asinner hot electrode materials, however, the stainless steelelectrode makes the gas temperature too high since heatremoval cannot be done because of its low thermalconductivity. Therefore, most experiments are performedwith the aluminum electrode as a hot electrode. Surfacesof the aluminum electrode have been anodized to gen-erate stable glow plasma, and both electrodes are cooledwith water. Gap distance between the inner hot electrodeand ceramic tube was 2 mm because arc transitionoccurs at low RF power if the gap distance is too small,and plasma becomes also unstable if the gap distance istoo large. Gas distributor was located inside the plasmasource to supply gas uniformly. Details of plasma sourceare shown in Fig. 1b.Test samples for decontamination experiments are

prepared with AISI 304 stainless steel plate as a basematerial to coat a cobalt oxide film on it. The baseplates are cleaned with ultrasonic cleaning in the acetonebath for 10 min to remove oil from machining, andapplied with a drop(approx. 10ml) of Co-59 solutionwhich contains 1% nitric acid. Then, the cobalt oxidefilm has been formed on the plate by heat treatment inthe electrical furnace at the temperature of 5008C for 2h. After heat treatment, samples are cleaned with ultra-sonic cleaning in the distilled water bath for 5 min toremove cobalt that has failed to form oxide layer. Theformation of Cobalt-oxide layer has been verified withAuger Electron Spectroscopy(AES) as shown in Fig.2.

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Fig. 3. Experimental procedure.

Fig. 4. Dependency of decontamination ratio on the processing time,power 400 W, He 10 slm:(a) CF 60 and O 60 sccm;(b) CF 804 2 4

and O 60 sccm;(c) CF 80 and O 80 sccm.2 4 2

Fig. 5. Variation between gasification and solidification: power 400W, He 10 slm:(a) CF 60 and O 60 sccm;(b) CF 80 and O 604 2 4 2

sccm;(c) CF 80 and O 80 sccm.4 2

Decontamination experiments are performed with var-ious operating parameters such as gas compositions andprocessing time. After the plasma treatments, the decon-tamination ratios are quantified with ICP-AES throughacid extraction of isotopes. To quantify the relativecontribution to the decontamination between gasificationand solidification, two analyzing processes are per-formed; process(A) without the ultrasonic cleaning andthe other process(B) with the cleaning as shown in theFig. 3. The process(B) measures the decontaminationonly through generating the volatile compound, so calledgasification.

3. Experimental results

3.1. Dependency of total decontamination ratio onprocessing time and gas composition

When the applied RF power has been increased,decontamination ratios measured after 5 min have beenincreased with the RF power. Further decontaminationexperiments are performed at 400 W, the maximumpower within the limit of stable discharges withoutstreamers for various gas compositions. As shown inFig. 4, decontamination ratios increase with more reac-tive gases, especially with more CF gases at the initial4

phase. Reactive gases are confirmed to be expeditingdecontamination speed although decontamination ratiosare saturated at the value of approximately 95% after10 min for all cases. However, there was an erosion

problem of surrounding metal structures and electrodeswith higher flow rate of CF , so the flow rate of CF4 4

cannot be increased further.

3.2. Variation between gasification and solidification

Contributions of gasification and solidification aredifferentiated, respectively. For initial 5 min, total decon-tamination ratios are increased as more reactive gasesare fed as shown in Fig. 5. Especially, decontaminationthrough gasification is increased with the added reactivegases more than that of solidification is decreased.Therefore, processing time seems to be reduced byadding more reactive gases mainly through fast gasifi-cation. However, when process time is increased to 10min, total decontamination ratios are saturated at thesame level for all cases.Considering the same saturated level even for the less

reactive gas case, strong gasification processes arebelieved to be followed after the initial relatively strong

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solidification with low gasification. It is indicative thatthere are some expedited gasification reactions throughsolidification. However, total decontamination ratios arestill saturated at approximately the same level for allcases after 10 min.

4. Conclusions

Overall decontamination ratios of approximately 95%are achieved through solidification and gasification withan atmospheric-pressure ejection plasma source. How-ever, decontamination ratios solely by gasification areincreasing up to approximately 70% with the increasedreactive gases, indicating the necessity of extra effort ofremoving solidified powder components. As flow ratesof reactive gas are increased, initial processing speed isincreased with more gasification processes than solidi-fication processes while maximum amounts of reactivegases are limited by the material erosion problem.Relationship between decontamination ratios and plasma

parameters need to be established with plasma diagnos-tics to optimize the processing conditions and the designparameters of plasma source.

Acknowledgments

This research is supported by the Korea Ministry ofScience and Technology.

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