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Pergamonwww.elsevier.com/locate/pnucene

P r o g r e s si n N u c l e a rEnergy, Vol. 3 7 , N o . 1 - 4 , p p . 3 9 3 - 3 9 7 . 2 0 0 0Q 2 0 0 0 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0149-1970/00/$ - see front matter

PII: so149-1970(00)00077-9

LEACHING BEHAVIOR OF BORIC ACID AND COBALTFROM PARAFFIN WASTE FORMS

JUYOULKIM, CHANGHYUN CHUNG

Department of Nuclear E ngineering, Seoul National UniversitySan56-1 Shinrim-dong, G wanak-g u, Seoul, 15 l-742, K oreaPhone: +82-2-88 0-833 1, Fax: +82-2-889 -2688, E-mail: kimjuyou@ gong.snu.ac.kr

CHANG LAK KIM

Nuclear Environment Technology Institute, Korea Electric Power Corporation150 Dukjin-dong, Yusong-gu, Taejon, 305-353, Korea

ABSTRACTNinety-day leaching test was performed to investigate the leachingcharacteristics of paraffin waste forms that had been recently generated in large

quantities at Korean nuclear power plants(KNPP s). In the case where m ixingweight ratio of boric acid to paraflin w as 78 122, which w as adopted in concentratewaste drying system(CW DS) of KNP Ps, the cumulative fraction leached(CFL) ofboric acid and cobalt w as about 5 1% and 61%, respectively. The compressivestrengths of waste form before and after the leaching test exhibited666psi(4,53M Pa) and 232psi(1.58M Pa), respectively. The CFLs of paraf?in wasteform were well expressed by diffision-controlled dissolution m odel such asGintsling-Brounshtein kinetics. The internal cross-sectional view of specimenafter the test demonstrated the applicability of this unreacted shrinking core modelto the leaching analysis of paraflin waste form. 0 2000 Elsevier Science Ltd. Allrights reserved.

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394 .I. Z Kim and C. H. Chung

1. INTRODUCTIONLow- a nd intermediate-level radioactive wastes arising from the operation of nuclear power p lants can beimmobilized by various solidification techniques prior to disposal. Liquid radioactive wastes have beentreated with filtration, ion exchange resin, evaporation, and so on. The remaining liquid concentrate wastes

from evaporator have been imm obilized by solidifying agents such as cement, bitumen, and polymer, a ndthen they are filled and packaged in several kinds of containers. Because waste forms includingradionuclides are imm obilized with solidifying agents and finally disposed of, they can be safely stored inradioactive waste repository, isolated from biosphere for a long time. However, despite of engineered andnatural barriers of radioactive waste repository, the radioactive waste forms w ould eventually be in contactwith groundwater, and the release of radioactive species from the waste forms w ould occur by leachingmechan ism. Therefore, acceptance criteria of radioactive waste forms have been developed to guarantee thelong-term safety performance of radioactive waste repository (U.S. NR C, 1991).

Low-level liquid borate wastes have been recently converted to parafIin waste forms by utilizing theconcentrate waste drying system(CWLG ) in Korean nuclear power plants(KNP Ps) (Kim and Bae, 1997).Paraffin waste forms could be classified a s class A unstable. Nevertheless CW DS h ad the advantages of highvolume reduction, low cost, and simple manufacturing process. T his study was performed to investigate theleaching characteristics of pat&in waste forms.

2. EXPERIMENTAL PROCEDURESCylindrical waste forms w ith a diameter of 5cm and a height of 1O cm were prepared. The operational

temperature was maintained within the range of 120-140 C and the speed of stirrer was 600 r.p.m. Paraffinwas physically mixed with boric acid and played a binding role within waste form. The mixing ratio of boricacid to paraffin was very imp ortant in order to make a homogeneous waste form because it made adifference between boric ac id( 1.44) and paraffrn (0.933) in specific grav ity. If the ratio of paraffin contentwas less than 1 5%, it was difftcult to make a waste form due to a low fluidity, w hile the phenomenon ofstratification began to occur when the ratio of paraffin content was more than 25%.

The compressive strength test was performed to confirm the integrity of waste forms according to ASTMC39-86 (1986). The test was applied to at least three specimens under each test condition of different mixingratios of boric acid to parafhn.

ANW AN S-16.1 (1986) leaching standa rd procedure w as used to investigate the leaching characteristicsof paraffin waste form. The test was developed by an American Nuclear Society Standards Committee forthe characterization of solidified low-level radioactive waste forms. This procedure uses demineralized wateras the leachant and is conducted at a temperature of (22.5*5) C. Sufficient leachant is used to provide a ratioof leachant volume to specimen external geometric surface area of (lOti.2)cm. The leachant is sampled andreplaced at the following frequency; 2, 7, and 24h from the initiation of the test, then at 24h intervals for thenext 4d, and then at 14, 28, and 43d intervals to extend the entire test to 90d. The paraBin waste form whosemixing ratio of boric acid to pa&in is 78122 , which had been adopted in CW DS of KNP Ps, wa s chosen forthe leaching test. Non-radioactive species cobalt(I1) chloride hexahydrate whose am ount corresponds to0.24% of boric acid by weight was added to the specimen. The concentration of cobalt included in leachatewas measured by means of inductive coupled plasm a-mas s spectroscopy(ICP-MS) and that of boric acidwithin leachate was analyzed by titration.

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Leaching behaviour of boric acid and cobalt

3. RESULTS AND DISCUSSION395

The mixtures whose paraffin con tents were within the range of 20-24% were easily poured into moldowing to good w orkability, and the compressive strength tests resulted in similar values of622-673 psi(4.23-4.58M Pa). It was desirable to keep a pa&F in weight ratio within the range of 20-24% inorder to prepare a homogeneous waste form as the matters of fluidity and stratification were solved at a time.For the par&n content w as 22%, the compressive strength before and after the leaching test of ninety dayswas 666psi(4.53M Pa) and 232psi( lS8MP a), respectively.

It was observed that the cumulative fraction leached(CFL) of boric acid and cobalt wa s about 5 1% and61% , respectively, after ninety days as show n in Fig. l(a). The effective diffisivities of boric acid and cobaltwere 1.5 X 1c6 cm/sec and 5.7 X lo-* cm2/sec, respectively, which could be ob tained from Eq. (1). Theyindicated that the leaching rates of pa&in waste form were considerably higher than those of cement andbitumen waste form w ith the effective difisivity of approximately 10-3-10-12 cm2/sec and l@*- IO-l7cm/sec, respectively, for cobalt. Fig. l(a) also showed that the CFL of cobalt was higher than that of boricacid only by 10% although the solubility of cobalt(0.33Sg/cm3 at 20C) was about seven times higher thanthat of boric acid(0.0465g/cm 3 at 20C). These results could be explained by that the boric acid whichoccupied a large portion of paraffin waste form was easily dissolved from the surface of waste form byleachant, and the cobalt immobilized within paraffin waste form was leached out in company with boric acid.The dissolution rates of boric acid and cobalt were not influenced by their concentration changes within theleachant becau se of the sufficient supply and the periodical replacement of leachant. Accordingly, theleaching of cobalt was thought to be mainly dependent on the dissolution of boric acid.

?? Boric Acid

Time(day)

.I. I .I. I., I., , 1.?? Boric Acid?? Cobalt

0 10 20 30 40 50 60 70 80 90 100Time(day)

(c) (R is a correlation coefficient)Fig. 1 Cum ulative Fraction Leached of Boric Acid and Cobalt Plotted as a Function of Time(a),Square R oot of Time(b), and Gintsling-Brounshtein Kinetics as a Function of Time(c)

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396 .I. Y. Kim and C. H. Chung

The leaching of paraffin waste form might appear to be controlled by conventional diffusion because theCFL values had linear relation to squ are root of time in Fig. l(b). But the conventional diffusion modelwhich would have the maximu m concentration at the center could not explain the photograph(Fig. 2)showing the shrinking dissolution front.

Reacted Layer

Dissolution FrontFig. 2 External Shape(left) and Internal Section(right) of Paraffin Waste Formafter the Leaching Test of 90 days

The reaction initially occurred at the external surface of was te form and the dissolution front gradua llymoved inside leaving a reacted layer behind. The leaching rates of boric acid and cobalt were influenced byreacted layer depth as the reaction progressed. It was revealed that this reacted layer decreased the diffusivefluxes of leached boric acid and cobalt and controlled the overall disso lution rate. Therefore, the leachingmechan ism of paraffin waste form was well explained by the diffusion-controlled dissolution model such asGintsling-Brounshtein kinetics which was a kind of unreacted shrinking core model (W en, 1968; Bamfordand Tipper, 1969).

l-3(1-X)2/3 +2(1-X)=KDt (I)where, X was dissolved fraction, Ku overall reaction rate constant(day-), and t reaction time(day). Thisunreacted shrinking core m odel assum ed that the reaction between solid and fluid was noncatalytic and solidparticle was spherical. Although the specimen of this study was cylindrical, the above model could beapplied to the leaching analysis of paraffin waste form because the leaching rates were constant along the allsurfaces, i.e., top/bottom and lateral sides, of the cylindrical w aste form. Fig. l(c) described the relationbetween dissolved fraction and reaction time by Eq. (1). Model p redictions were excellently agreed with testdata and overall reaction rate constants were obtained from the inclinations of each straight line.

4. CONCLUSIONSThe leaching test of paraffin waste form was carried out according to ANSI/AN S-16.1 test procedure. Forthe waste form with the mixing ratio of 78122 between boric acid and paraffin, it was observed that about51% and 61% of boric acid and cobalt, respectively, were released after ninety day s. The compressivestrength of waste form before and after the leaching test resulted in 666psi(4.53M Pa) and 232psi(1,58MPa ),respectively. The leaching rates of boric acid and cobalt were influenced by reacted layer depth as thereaction progressed and this reacted layer controlled the overall dissolution rate. It was concluded that theleaching me chanism of paraffin waste form was diffusion-controlled dissolution with the shrinking

dissolution front.

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Leaching behaviour of boric acid and cobali

REFERENCES397

American National Standards Institute (1986), Mea surem ent of the Leacha bility of Solidified Low-levelRadioactive Wastes by a Short-term Test Procedure, ANSVANS-16.1.American Society for Testing Materials (1986), Standard Test Method for Compressive Strength ofCylindrical Concrete Specimens, ASTM C39-86.Bamford C.H. and Tipper C. F. H. (1969), The i%eo?y ofKinetics, Comprehensive Chemical Kinetics, Vol.II ,Elsevier Scientific Publishing Company, Amsterdam.Kim H.G. and Bae H.I. (1997), Experiences for Concentrated Waste Drying System, Proceedings ofInternational Symposium on Radiation Safety Management.U. S. Nuclear Regulatory Com mission (1991), Low-level Waste Licensing B ranch Technical Position onWaste Form, Rev. 1.Wen C.Y. (1968), Noncatalytic Heterogeneous Solid Fluid Reaction M odels, Industrial and EngineeringChemistry, 60, 34.