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Applied Radiation and Isotopes 67 (2009) 1534–1539

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Applied Radiation and Isotopes

0969-80

doi:10.1

� Corr

E-m

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

Trapping behavior of gaseous cesium by fly ash filters

J.M. Shin �, J.J. Park, K.C. Song, J.H. Kim

Korea Atomic Energy Research Institute, 150 Dukjin-dong, Yuseung-gu, Daejeon 305-353, Republic of Korea

a r t i c l e i n f o

Keywords:

Fly ash filter

Trapping

Gaseous cesium

43/$ - see front matter Crown Copyright & 2

016/j.apradiso.2009.02.070

esponding author. Tel.: +82 42 868 8848; fax:

ail address: jmshin@kaeri.re.kr (J.M. Shin).

a b s t r a c t

The high volatility of a gaseous form and its high chemical reactivity make a cesium emission control

very difficult work. In this study, fly ash filters were tested for the removal of gaseous cesium from a hot

flue gas under air and hydrogen conditions at 700–1000 1C. Tests were performed by using a simulated

gaseous cesium volatilized from Cs2SiO3 in a two-zone furnace. Fly ash filter was found to be the most

promising filter for trapping the gaseous cesium. The results of the trapping tests are presented, along

with the effects of the temperature, superficial gas velocity, and carrier gas on the cesium trapping

quantity.

Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved.

1. Introduction

The Direct Use of spent PWR fuel in CANDU (DUPIC) reactortechnology adopts an Oxidation and REduction of OXide fuel(OREOX) treatment as a principal dry process for recycling spentPWR fuel. Key parameters which influence the DUPIC fuelfabrication characteristics are thought to be both the powderproperties and the amount of fission products in a spent fuel (Choiet al., 2006; Westphal et al., 2006; Yang et al., 2006).

During the OREOX and sintering steps of the DUPIC fuelfabrication process, volatile fission gases (tritium, carbon-14,krypton, iodine, etc.) and semi-volatile fission gases (cesium,ruthenium, etc.) can be generated. The operating conditions forthe OREOX process are 450 1C in air and 700 1C in an Ar–4% H2

atmosphere, respectively. In addition, the operating condition forthe sintering process is 1800 1C in an Ar–4% H2 atmosphere.

High temperature processes for a spent fuel result in thegeneration of a highly radioactive gaseous waste containing 137Cs(half-life; 30 years) and other decay products. Among the gaseouswastes, cesium is one of the highly radioactive wastes and heatgenerating fission products. The high volatility of a gaseous form andits high chemical reactivity make a cesium emission control verydifficult work.

Potential waste forms for 137Cs storage include borosilicateglass, glass ceramics, cesium loaded zeolites, and cesiumaluminosilicates. Prior investigations have involved studies onthe preparation, leachability, and thermal stability of cesiumaluminosilicate materials. Particularly the phases of CsAlSiO4,CsAlSi5O12, and CsAlSi2O6 (pollucite) have been considered.Gallagher et al. (1977) described the systematic study of the

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preparation of CsAlSiO4. Kormanei and Roy (1983) studied theexchange behavior of synthetic cesium aluminosilicates, andCerny (1978) discussed a pollucite alteration from a wastedisposal point of a view.

Of these cesium aluminosilicates, pollucite offers the followingadvantages. The pollucite structure can accommodate more than40 wt% Cs into its structure and, as a result, will produce a highlydense waste form. As a pollucite waste form is denser than theglass-ceramic or zeolite based alternatives, significant cost-savings result due to the smaller volume of the material to bestored. In addition, the measured and calculated solubility ofpollucite as well as the leachability and mobility of Cs in apollucite structure are approximately three orders of a magnitudeless than that measured for silicate glass (Anchell et al., 1994).

Some attempts have been made on the use of a fly ash and a flyash filter to trap gaseous cesium (Park et al., 1996; Shin and Park,2001; Shin et al., 2007; Westphal et al., 2007). The previousstudies manufactured a fly ash filter to have base sites on itssurfaces to trap the gaseous cesium. In these works, a fly ash filterwas found to be a promising filter for trapping gaseous cesium asa pollucite phase. Recently, Shin et al. (2005) analyzed the cesiumtrapping characteristics of a fly ash filter by changing the reactiontemperature (500–1000 1C under the air condition), carrier gasand gas velocity (2–20 cm/s under the air condition). However,there is little information about the cesium trapping character-istics of a fly ash filter under air and hydrogen atmospheres inorder to apply it to a DUPIC off-gas treatment system.

Therefore, it is necessary to evaluate the cesium trappingcharacteristics in terms of the temperature, superficial gasvelocity, and carrier gas for such applications. The purpose ofthis study is to analyze the effects of a carrier gas, a superficial gasvelocity, and the reaction temperature on the cesium trappingquantity. Reaction products formed by the reaction of the gaseouscesium compounds with the fly ash filters were investigated by

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J.M. Shin et al. / Applied Radiation and Isotopes 67 (2009) 1534–1539 1535

using X-ray diffractometry (XRD analyzer, Simens D-5000) andenergy dispersive X-ray (EDX spectrometer scanning electronmicroscope, Jeol JSM-5300).

2. Experimental

2.1. Materials

Fly ash from a Boryung coal fired power plant was used as afilter material. It was mainly composed of SiO2, Al2O3, Fe2O3, andothers. The mole ratio of the Si/Al contained in the fly ash wasabout 2.1.

To manufacture a ceramic foam filter, fly ash and polyvinylalcohol as a binder were mixed together to make any uniform slipsolution. This slip solution was impregnated with a polyurethanesponge and any surplus slip was removed from the sponge. The

Fig. 1. Photograph of a fly ash filter.

COOLERCARRIE

4%H2/Ar Air

CARRIERGAS

VENT

COOLINGWATER(1) OUT

THERMOCOUPLEFOR T1

CESIUMGENERATOR

COOLINWATER(2)

Fig. 2. Schematic diagram of the experimenta

sponge used in this study was 25 ppi (pores per inch). And then itwas dried at 105 1C and sintered at 1200 1C. The heating rate wasconstantly maintained at 5 1C/min and the sintering time was30 min. The fly ash filter had an inner diameter of 45 mm, athickness of 10 mm, and an average weight of about 7 g. This filteralso had a 10 m2/g specific surface area and a 30% porosity. Fig. 1shows a photograph of the fly ash filter used in this study.

2.2. Methods

A schematic diagram of the experimental apparatus fortrapping the gaseous cesium is shown in Fig. 2. Cesium source,which was placed in an alumina boat at the center of the firstfurnace, was used to generate a controlled source of volatilecesium, which was scheduled to pass through the fly ash filtersmounted in the second furnace. The filters were tightly packed inan alumina tube of the furnace by using an alumina sheet. Thecesium containing a carrier gas was passed through a filter bed offly ash. The experiments were conducted under air and hydrogen(Ar/4% H2) conditions because volatile cesium is released duringthe OREOX and sintering steps while manufacturing DUPIC fuel.

Cesium silicate glass was used as a cesium source. Cesiumsilicate glass was prepared by quantitatively mixing CsNO3 andSiO2 to have a composition of Cs2SiO3 (Cs2O � SiO2). The mixturewas reacted at 1100 1C for 30 min, cooled, and powdered for theexperiment. The samples were also kept in desiccators, becausethey are very hygroscopic. All the experiments were conducted inair and hydrogen (Ar/4% H2) conditions, respectively.

In order to investigate the cesium trapping characteristics of afly ash filter as a function of the reaction temperature, experi-ments were conducted by heating the trapping zone from 700 to1000 1C for 3 h at a 0.2 m/s superficial gas velocity under air andhydrogen (Ar/4% H2) conditions, and then cooling it down to roomtemperature. Source temperature of the cesium silicate wasconstantly maintained at 1000 1C. The filter depth was 50 mm.Each experiment was initiated at first by starting the carrier gasflow to purge the system. The preheated furnace temperature wasthen raised to heat the fly ash filters. Heating of the cesium silicateglass was subsequently started to vaporize the cesium monoxide

FORR GAS

COOLINGWATER(2) IN

COOLINGWATER(1) IN

THERMOCOUPLEFOR T2

FILTER SUPPORTER

THERMOCOUPLEFOR T3

G OUT

l apparatus for trapping gaseous cesium.

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(Cs2O), which was carried by the flue gas through the fly ash filterbed. Cesium trapping quantity was calculated by measuring theweight change of the filter before and after trapping the gaseouscesium. In order to understand the effect of the gas velocity on thecesium trapping quantity of the fly ash filter, the air velocity wasvaried between 0.05 and 0.8 m/s. The trapping temperature wasconstantly maintained at 800 1C and the trapping time was 3 hunder the air and hydrogen conditions, respectively.

XRD (Siemens, D-5000) technique was applied to analyze thereaction products on the fly ash filters. The used X-ray was Cu Karay, and the scanning rate was 21/min. The angle, 2y, was withinan angle of 15–601. EDX (Jeol, JSM-5300) was also used to observethe reaction products of the fly ash filter with gaseous cesium.

3. Results and discussion

3.1. Trapping characteristics

As the mole ratio of Si/Al contained in the fly ash was about 2.1,the fly ash filter was suitable for trapping the gaseous cesium as apollucite (CsAlSi

2O

6) phase. When the fly ash was manufactured

into a filter, the hardness of the filter was very firm. Themanufactured fly ash filter had pores interconnected with eachother throughout the whole structure of the filter to maintain asmooth flow of the flue gas.

Fig. 3 shows the cesium trapping quantity plotted against thetrapping time in hours under the air and hydrogen conditions. Ascan be seen in Fig. 3, the trapping quantity of the gaseous cesiumby the fly ash filter was increased linearly with the trapping time.This suggests that under the given experimental conditions, therate of the cesium trapping quantity by the fly ash filter is notcontrolled by the mass transfer of the cesium vapor from the bulkof the flue gas to the external surfaces of the fly ash filter. Fig. 3also shows that the trapping quantity of the gaseous cesium bythe fly ash filter was also increased with an increasing trappingtime under the given experimental conditions. This might be dueto the fact that the reaction rate was increased with an increasingfilter bed temperature and trapping time. The trapping quantity ofthe gaseous cesium by the fly ash filter was also not dependent onthe types of carrier gas as shown in Fig. 3. Above results indicatethat the cesium trapping quantity by the fly ash filter was very

Time, hr0.0 0.5 1.0 1.5 2.0 2.5 3.0

Ces

ium

trap

ping

qua

ntity

, g-C

s 2O

/g-fi

lter

0.00

0.05

0.10

0.15

0.20

0.25

0.30

700°C, air 800°C, air900°C, air1000°C, air700°C, hydrogen800°C, hydrogen900°C, hydrogen1000°C, hydrogen

Fig. 3. Effect of the filter bed temperature on the cesium trapping quantity as a

function of the trapping time.

sensitive to the trapping temperature and time regardless of thetypes of carrier gas.

Fig. 4 shows the cesium trapping quantity plotted against thefilter depth under the air and hydrogen conditions. The trappingquantity of the gaseous cesium by the fly ash filter was increasedwith an increasing trapping temperature at the same filter depthas shown in Fig. 4. Fig. 4 also shows that the cesium trappingquantity and cesium trapping area by the fly ash filter were alsoincreased with a decreasing filter depth. In the gas–solidreactions, such as the system involved in the present study, theeffect of the superficial gas velocity of the flue gas on the rate ofthe cesium trapping quantity provides another means of measur-ing the role played by the mass transfer of cesium vapor from thebulk of the flue gas to the external surfaces of the fly ash filter. Forthe mass transfer controlled reactions, an increase in the super-ficial gas velocity increases the mass transfer coefficient, therebyincreasing the extent of a resultant reaction. Based on thesereasons, a series of tests were conducted at 800 1C using asimulated flue gas at superficial gas velocities from 0.05 to 0.8 m/s.

Fig. 5 shows that the cesium trapping quantity of the fly ashfilter was decreased with an increasing superficial gas velocityregardless of the types of carrier gas.

Fig. 6 shows the cesium trapping quantity plotted against thesuperficial gas velocity under the air and hydrogen conditions. Ascan be seen in Fig. 6, the trapping quantity of the gaseous cesiumby the fly ash filter was decreased nonlinearly. This might be dueto the fact that the contact time between the fly ash filter and thegaseous cesium was decreased with an increasing gas velocity.This suggests that under the experimental conditions, the rate ofthe cesium trapping quantity by the fly ash filter is not controlledby the mass transfer of the cesium vapor from the bulk of the fluegas to the external surfaces of the fly ash filter. Rather the rate ofthe cesium trapping quantity is controlled by either a diffusion ofthe cesium vapor through the internal pores, a trapping of thecesium vapor on the active sites of the fly ash filter, or thechemical kinetics.

Fig. 7 shows a photograph of the fly ash filters after trappingthe gaseous cesium at 800 1C under the hydrogen condition at a0.2 m/s superficial gas velocity. When gaseous cesium was reacted

Filter depth, m0.01 0.02 0.03 0.04 0.05

Ces

ium

trap

ping

qua

ntity

, g-C

s 2O

/g-fi

lter

0.00

0.05

0.10

0.15

0.20

0.25

0.30

700°C, air800°C, air900°C, air1000°C, air700°C, hydrogen800°C, hydrogen900°C, hydrogen1000°C, hydrogen

Fig. 4. Effect of the filter bed temperature on the cesium trapping quantity as a

function of the filter depth.

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with the fly ash filter under the hydrogen condition the color ofthe fly ash filter changed from brown to black. It may indicate thatthe trapping characteristics of the gaseous cesium by the fly ashfilter could be changed with the carrier gas.

3.2. X-ray diffraction

XRD analysis was used to identify the final products formed bytrapping the gaseous cesium on the fly ash filter at 800 1C. The

Filter depth, m0.00 0.01 0.02 0.03 0.04 0.05

Ces

ium

trap

ping

qua

ntity

, g-C

s 2O

/g-fi

lter

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.05m/sec, air0.10m/sec, air0.20m/sec, air0.40m/sec, air0.80m/sec, air0.05m/sec, hydrogen0.10m/sec, hydrogen0.20m/sec, hydrogen0.40m/sec, hydrogen0.80m/sec, hydrogen

Fig. 5. Effect of the superficial gas velocity on the cesium trapping quantity as a

function of the filter depth.

Superficial gas velocity, m/sec0.0 0.2 0.4 0.6 0.8

Trap

ping

rate

of c

esiu

m,

g-C

s 2O

/g-fi

lter/h

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Fig. 6. Effect of the superficial gas velocity on the trapping rate of cesium.

Fig. 7. Photographs of the fly ash filters after trapping g

reacted surfaces of the fly ash filter at 800 1C of a trappingtemperature under the hydrogen condition revealed XRD peaksthat indicate cristobalite, quartz, mullite, and pollucite phases, asshown in Fig. 8. Also, analysis of the fly ash filters exposed tocesium vapors indicated the formation of a pollucite. Based on theXRD results, the following reaction scheme is proposed fortrapping of cesium:

Cs2SiO3 ðamorphousÞ þ 4% H2=Ar ðgÞ

! Cs2O ðgÞ þ SiO2 ðamorphousÞ þ 4% H2=Ar ðgÞ (1)

Cs2O ðgÞ þ Al2O3 � 4 SiO2 ðsÞ

! Cs2O � Al2O3 � 4 SiO2 ðsÞ (2)

aseous cesium at 800 1C in the hydrogen condition.

20 30 40 50 60

Q:Quartz M:MulliteP:Pollucite C:Cristoballite

Cou

nts

2 �

M

Q

Q

Q

PP

PP

5 F

2 F

1 F

C

C

C

C

C

Fig. 8. X-ray diffraction patterns of filters trapped that cesium at 800 1C in the

hydrogen condition. 1F: front-side of the first filter unit, 2F: front-side of the

second filter unit and 5F: front-side of the fifth filter unit.

Fig. 9. Cesium concentration map of the cross section of a fly ash filter.

ARTICLE IN PRESS

SEM image of a fly ash filter after trapping cesium ( X 1,000 )

SEM-EDX analysis

Fig. 10. SEM-EDX analysis of a fly ash filter after trapping cesium in the hydrogen condition.

J.M. Shin et al. / Applied Radiation and Isotopes 67 (2009) 1534–15391538

From the above equations, we may conclude that the formationof pollucite occurred when gaseous cesium was reacted with thealuminosilicates of the fly ash filters at 800 1C in the hydrogencondition.

To further understand the trapping characteristics on thesurface of a fly ash filter after trapping cesium in a hydrogencondition, a fly ash filter flake that had trapped gaseous cesiumwas mounted in epoxy and analyzed by the SEM and EDXprocedures.

A cesium map of the flake shows that the cesium concentrationis concentrated on the fly ash filter surface as shown in Fig. 9.

3.3. SEM-EDX analysis

From an identification of the SEM-EDX analysis results shownin Fig. 10, a microscopic observation of the fly ash filter aftertrapping gaseous cesium, taken at a 1000�magnification,indicates that it has a pollucite phase of bulky crystals with arough surface and a mullite phase of needle-like crystals. An EDXanalysis of the fly ash filter surface indicates that the 29 wt%cesium concentration is close to the 58 wt% value calculated bythe composition of pure pollucite from the postulated reactionmechanisms, by assuming a complete conversion. This indicatesthat the surface of the fly the ash filter was converted by theresultant reaction to form pollucite.

4. Conclusions

Experimental results obtained from this study show that flyash filters are extremely effective at immobilizing gaseous cesiumunder air and hydrogen conditions at 700–1000 1C. The results ofthe effect of the temperature, superficial gas velocity, and carrier

gas on the cesium trapping quantity provide promise for the useof a fly ash filter as a material for fixing gaseous cesium. Its hightrapping quantity for cesium, the hardness of the filter, the simpleprocess of the filter, and its abundance are good characteristics forits application to an off-gas treatment system. These resultsindicate great promise for the use of a fly ash filter as a potentialfilter for trapping gaseous cesium.

Acknowledgment

This work was performed under the Long-term Nuclear R&DProgram sponsored by the Ministry of Science and Technology.

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