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Applied Radiation and Isotopes 67 (2009) 1526–1529

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Applied Radiation and Isotopes

0969-80

doi:10.1

� Corr

E-m

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

A comprehensive study on the laser decontamination of surfacescontaminated with Cs+ ion

B. Baigalmaa a,b, H.J. Won a,�, J.K. Moon a, C.H. Jung a, J.H. Hyun b

a Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong, Daejeon, Republic of Koreab Chungnam National University, 220 Gung-dong, Yuseong, Daejeon, Republic of Korea

a r t i c l e i n f o

Keywords:

Q-switched Nd:YAG laser

Ablation

Cs+

43/$ - see front matter & 2009 Elsevier Ltd. A

016/j.apradiso.2009.02.055

esponding author. Tel.: +82 42 8682331; fax:

ail address: nhjwon@kaeri.re.kr (H.J. Won).

a b s t r a c t

A Q-switched Nd:YAG laser with a 1064 nm, 450 mJ/pulse and 14 ns pulse width was employed to study

the decontamination characteristics of Type 304 stainless steel specimens contaminated with Cs+ ions.

The surrogate specimens were artificially contaminated with two kinds of premixed solutions. The laser

was irradiated for 10, 20 and 100 shots. The results were investigated using a SEM, EPMA and XPS. For

the surrogate specimen treated with the CsCl+KCl solution, more than 98% of the Cs+ ions were removed

during an irradiation of 100 shots. The specimen treated with the CsCl+KCl solution was easier to

decontaminate. By comparing the ratio of the O1s intensity to the Fe2p intensity of the XPS spectra, it

was found that the oxygen atoms that had evolved from the specimen treated with the CsNO3+KNO3

solution had decreased the laser’s decontamination performance.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

During the refurbishment of hot cells, decontamination is anessential step because it reduces the occupational exposure toworkers, and provides for a safe management of radioactivewastes. Korea Atomic Energy Research Institute (KAERI) hasoperated the DFDF (DUPIC Fuel Development Facility) since2000. All the DUPIC (Direct Use of PWR spent Fuel in CANDU)processes are executed in hot cells under a dry a condition.As the operation period increases, the need to repair thesehot cells also increases. The radioactivity of the hot cells in theDFDF is presumed to be very high and the predominantradionuclide is Cs-137. Before the refurbishment of the hot cells,the application of a remotely operated decontamination techniqueis required.

Laser cleaning is a relatively recent technique for removingpollutants from surfaces that is currently finding applications inmany fields (Koh and Sarady, 2003; Wesner et al., 1996; Craciun etal., 2002; Savina et al., 2000; Siatou et al., 2006; Kearns et al.,1998; Khalil and Sreenivasan, 2005). The merits of a laserdecontamination technique are a remote application, a highdecontamination factor, the generation of a small amount ofsecondary waste and a negligible occupational exposure toworkers. Kameo et al. (2004) reported that the decontaminationefficiency could be improved by employing an acid containingsodium silicate gel on a contaminated metal surface before a laser

ll rights reserved.

+82 42 8684797.

application. They presumed that the contaminated oxide layerwas dissolved by the acid and combined with the gel under a laserirradiation condition. Rafique et al. (2007) studied the XRD andSEM analyses of a laser irradiated cadmium. They used a pulsedNd:YAG laser (10 mJ, 12 ns and 1064 nm). From the test results,they reported that the hydrodynamic effects were apparent with aliquid flow which formed a recast material around the peripheryof a laser’s focal area. Dimogerontakis et al. (2005) studied athermal oxidation induced during a laser cleaning of analuminum-magnesium alloy. They used a Q-switched Nd:YAGlaser with a pulse duration of 10 ns and a wavelength of 1064 nm.For the surface analyses of the treated samples, an X-rayphotoelectron spectroscopy (XPS) and a secondary ion massspectroscopy were used. They found that a thermal oxidationtook place on the alloy during an irradiation in air with a laserenergy range from 0.6 to 1.4 J/cm2.

In a previous work (Won et al., 2007), the feasibility tests toselect a light source were performed using surrogate specimens.The tested light sources were a continuous type CO2 laser, acontinuous type Nd:YAG laser and a pulse type Nd:YAG laser. Itwas found that the pulse type Nd:YAG laser was the most efficientamong them.

The objective of this study is to evaluate the decontaminationefficiency of a fabricated Q-switched Nd:YAG laser system forsurrogate specimens artificially contaminated with Cs+ ions.Especially, the effects of the chlorine and nitrate anions on thelaser decontamination of the specimens were compared. Tounderstand the reactions happening during the laser irradiation,the specimens’ surfaces before and after a laser irradiation wereanalyzed by SEM, EPMA and XPS.

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B. Baigalmaa et al. / Applied Radiation and Isotopes 67 (2009) 1526–1529 1527

2. Experimental

2.1. Materials

Type 304 stainless steel specimens were used in the presentexperiment. The specimens were polished with abrasive papers.After they were washed with water and ethyl alcohol, they weredipped into an ultrasonic cleaner for 30 min and dried. Tosimulate a radioactive contamination in hot cells, the specimenswere respectively contaminated with two kinds of solutions thatcontained Cs+ ions. After a drying, the specimens’ surfaces wereanalyzed by EPMA. The relative atomic molar ratio of a specimen’ssurface before a laser irradiation is listed in Table 1. After a laserirradiation, the surface of a specimen was again analyzed by SEM,EPMA and XPS.

2.2. Experimental setup

Fig. 1 shows a schematic view of the experimental apparatus.The system emits a fundamental wavelength at 1064 nm and themaximum pulse energy is 450 mJ/pulse. The pulse width of theQ-switched Nd:YAG laser was fixed at 14 ns. All the tests wereperformed at a pulse energy of 450 mJ/pulse and a repetition rateof 10 Hz. Contaminated specimen was mounted on a stage thatallowed the specimen holder to move by 25 mm in the X, Y and Z

directions. A 16.5 cm focal length convex lens was used for aconvergence of a laser beam at a point on a target. The specimenwas irradiated by changing the number of laser shots.

Table 1The relative atomic molar ratio of a specimen’s surface before a laser irradiation.

N K O Si Cr Fe Ni

Type A 17.21 18.33 26.94 0.26 8.18 25.73 2.71

Type B 0.00 23.81 0.00 0.31 10.42 30.55 3.34

Fig. 1. Schematic view of the

Fig. 2. SEM micrographs of the specimen irradiated

3. Results and discussion

3.1. Decontamination characteristics

All of the exposed material was examined under SEM at amagnification of 100. The SEM micrographs of a specimen after 10shots of a laser irradiation are shown in Fig. 2. Fig. 2 shows thatthe metal surface was melted, and it formed a crater by a laserirradiation. Ablation can be based on a thermal activation only, ona direct bond breaking (photochemical ablation), or on acombination of these two factors (Miller and Richard, 1998.).Considering the wavelength of the Nd:YAG laser is in the IR regionof 1064 nm, the ablation of the materials was predominantlycaused by a thermal activation. The average atomic molar ratio ofthe Cs+ component was decreased to 0.08 for the Type A specimenand to 0.06 for the Type B specimen.

Fig. 3 shows the SEM micrographs of a specimen after 20 shotsof a laser irradiation. Comparing Fig. 3 with Fig. 2, it is found thatthe melting area affected by a laser irradiation was extended. Thehigh laser heating rate causes a significant amount of the metalarea to be melted. And, the advection of the molten metal caninduce a motion of the nonmolten particles by a surface tensionand viscous effects (Rafique et al., 2007). There is no severeformation of debris and the affected area is smooth. The averageatomic molar ratio of the Cs+ ion component was found todecrease to 0.04 and 0.03 for the Type A and Type B specimens,respectively.

Fig. 4 shows the SEM micrographs of a specimen after 100shots of a laser irradiation. Comparing Fig. 4 with Fig. 3, it is found

Cl Cs Remarks

0.00 0.64 Contaminated with 7.5�10�3 M CsNO3+1.6�10�1 M KNO3

30.41 1.16 7.5�10�3 M CsCl+1.6�10�1 M KCl

experimental equipment.

with 10 laser shots: (a) Type A and (b) Type B.

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Fig. 3. SEM micrographs of the specimen irradiated with 20 laser shots: (a) Type A and (b) Type B.

Fig. 4. SEM micrographs of the specimen irradiated with 100 laser shots: (a) Type A and (b) Type B.

00

5

10

15

20

Rem

oved

met

al v

olum

e, X

10-5

cm

3 14.3 J/cm2

57.3 J/cm2

229.2 J/cm2

1 2 3 4 5

B. Baigalmaa et al. / Applied Radiation and Isotopes 67 (2009) 1526–15291528

that debris is formed in the melting area. Smallman and Bishop(2002) explained that minute crystalline nuclei form at randompoints during the freezing of a molten material. These new nucleithen grow independently during the expansion of a surroundingmatrix, because of the removal of the thermal energy. The averageatomic molar ratio of the Cs+ ion component was found todecrease to 0.03 and 0.02 for the Type A and Type B specimens,respectively. For the Type B specimen, more than 98% of the Cs+

ions were removed.For the two kinds of specimens, the Cs+ ion was satisfactorily

removed by a laser irradiation. This can be explained by tworeasons: (1) as the Cs+ ion is a semi-volatile element, it isevaporated at a high temperature during a laser application; (2)Fe, Ni, and Cr which are the main elements of stainless steel areablated thermally by a laser irradiation. During the ablation of themain elements, the Cs+ ion was concurrently ablated.

Time, min

Fig. 5. Relationship between the removed metal volume and the irradiation time.

3.2. Ablation of type 304 stainless steel

Removed volume of the Type 304 stainless steel specimenat 10 Hz of a repetition rate against the irradiation time underthree fluence conditions is shown in Fig. 5. The amount ofremoved metal is directly proportional to the application time at agiven fluence. The values of the apparent removal rate are4.01�10�6, 1.58�10�5 and 3.59�10�5 cm3/min at 14.3, 57.3and 229.2 mJ/cm2, respectively. The amount of the secondarywaste generated during the application of the decontaminationtechnology is heavily dependent on the methods, area, size andtime used. The irradiation time for the laser decontamination of

the Cs+ ion from a specimen’s surface was less than 10 s.Furthermore, the laser decontamination technique is selectivelyapplied to the contaminated area. The results indicate that thesecondary waste generated from the ablation of the stainless steelis negligible.

3.3. Surface reaction during a laser irradiation

To evaluate the chemical reactions happening during a laserirradiation, the chemical states of the surface atoms after an

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1200

Int

ensi

ty

Binding Energy (eV)

O1SOKLLNi2P

Fe2P

Cr2P

Ni2p

Int

ensi

ty

O1s

OKLL

Cr2p

Fe2P

1000 800 600 400 200 0

1200

Binding Energy (eV)

1000 800 600 400 200 0

Fig. 6. XPS spectrum of the metal surface after 420 shots of a laser irradiation: (a)

7.5�10�3 M CsNO3+1.6�10�1 M KNO3; (b) 7.5�10�3 M CsCl+1.6�10�1 M KCl.

B. Baigalmaa et al. / Applied Radiation and Isotopes 67 (2009) 1526–1529 1529

irradiation on the two kinds of specimens were analyzed by XPS.The results are shown in Fig. 6. A peak at 710.9 eV corresponds to2P3/2 of the Fe ion in Fe2O3 and at 530.3 eV it corresponds to O1s ofthe Fe2O3 or a metal oxide (Moulder et al., 1992). Especially, it hasbeen reported that a peak at 530.3 eV corresponds to the oxygenin Fe2O3 (Kameo et al., 2004).

A general expression for determining the atomic fraction of anyconstituent in a sample, Cx, is written as Eq. (1) (Moulder et al.,1992):

Cx ¼nXP

ni¼

IX

SX

P Ii

Si

(1)

where nX is the number of a given atom of an element per cm3 of asample, the S value is based on a peak area measurement and IX isthe number of photoelectrons of a element per second.

The ratio of the O1s intensity to the Fe2p intensity in Fig. 6(a) is3270.1 and that in Fig. 6(b) is 770.1. This means that moresurface oxygen ions in a specimen treated with the CsNO3+KNO3

solution are bonded with the other metal ions when comparedwith a specimen treated with the CsCl+KCl solution. Potassiumnitrate and potassium nitrite begin to decompose at 820720 K

(Kramer et al., 1982). The equation for the decomposition of KNO3

is

2KNO3 ! 2KNO2 þ O2 (2)

for a solid phase reaction. During a laser irradiation, O2 is evolvedon the surface of a specimen treated with the CsNO3+ KNO3

solution. From the SEM and EPMA analyses results, the specimenstreated with the CsNO3+KNO3 solution were more difficult todecontaminate. This can be ascribed to the formation of Cs2Ogenerated from the bonding of O2 with the cesium ions.

4. Conclusion

Application of a Q-switched Nd:YAG laser system for adecontamination of hot cell walls contaminated with radioactiveparticles is necessary to reduce the high radiation field. With theincrease of the application times for a laser shot, however, the debrisformation on the decontaminated surface was increased. In theexperimental range, more metal oxide was formed on the specimenstreated with the nitrate ion and it suppressed the decontaminationefficiency. Cs+ ions on the Type 304 stainless steel specimens,however, were satisfactorily removed by the light ablation methodwith a little generation of a secondary waste. Finally, the presentmethod also has a possibility to decontaminate the equipment in hotcells or to reduce the volume of radioactive wastes generated fromnuclear facilities with a small occupational exposure to workers.

Acknowledgment

This work has been carried out under the Nuclear R&DProgram funded by the Ministry of Science and Technology.

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