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Leaching behavior of boric acid and cobalt from paraffin waste forms
ProgressinNuclearEnergy, Vol. 37,No. 1-4,pp.393-397.2000 Q 2000 Elsevier Science Ltd. All rights reserved
Pergamon
Printed in Great Britain 0149-1970/00/$ - see front matter
www.elsevier.com/locate/pnucene
PII: so149-1970(00)00077-9
LEACHING
BEHAVIOR OF BORIC ACID AND COBALT FROM PARAFFIN WASTE FORMS
JUYOULKIM, CHANGHYUN CHUNG
Department of Nuclear Engineering, Seoul National University San56-1 Shinrim-dong, Gwanak-gu, Seoul, 15 l-742, Korea Phone: +82-2-880-833 1, Fax: +82-2-889-2688, E-mail: [email protected]
CHANG LAK KIM
Nuclear Environment Technology Institute, Korea Electric Power Corporation 150 Dukjin-dong, Yusong-gu, Taejon, 305-353, Korea
ABSTRACT Ninety-day leaching test was performed to investigate the leaching characteristics of paraffin waste forms that had been recently generated in large quantities at Korean nuclear power plants(KNPPs). In the case where mixing weight ratio of boric acid to paraflin was 78122, which was adopted in concentrate waste drying system(CWDS) of KNPPs, the cumulative fraction leached(CFL) of boric acid and cobalt was about 5 1% and 61%, respectively. The compressive strengths of waste form before and after the leaching test exhibited 666psi(4,53MPa) and 232psi(1.58MPa), respectively. The CFLs of paraf?in waste form were well expressed by diffision-controlled dissolution model such as Gintsling-Brounshtein kinetics. The internal cross-sectional view of specimen after the test demonstrated the applicability of this unreacted shrinking core model to the leaching analysis of paraflin waste form. 0 2000 Elsevier Science Ltd. All rights reserved.
393
394
.I. Z Kim and C. H. Chung
1. INTRODUCTION Low- and intermediate-level radioactive wastes arising from the operation of nuclear power plants can be immobilized by various solidification techniques prior to disposal. Liquid radioactive wastes have been treated with filtration, ion exchange resin, evaporation, and so on. The remaining liquid concentrate wastes from evaporator have been immobilized by solidifying agents such as cement, bitumen, and polymer, and then they are filled and packaged in several kinds of containers. Because waste forms including radionuclides are immobilized with solidifying agents and finally disposed of, they can be safely stored in radioactive waste repository, isolated from biosphere for a long time. However, despite of engineered and natural barriers of radioactive waste repository, the radioactive waste forms would eventually be in contact with groundwater, and the release of radioactive species from the waste forms would occur by leaching mechanism. Therefore, acceptance criteria of radioactive waste forms have been developed to guarantee the long-term safety performance of radioactive waste repository (U.S. NRC, 1991). Low-level liquid borate wastes have been recently converted to parafIin waste forms by utilizing the concentrate waste drying system(CWLG) in Korean nuclear power plants(KNPPs) (Kim and Bae, 1997). Paraffin waste forms could be classified as class A unstable. Nevertheless CWDS had the advantages of high volume reduction, low cost, and simple manufacturing process. This study was performed to investigate the leaching characteristics of pat&in waste forms.
2. EXPERIMENTAL PROCEDURES Cylindrical waste forms with a diameter of 5cm and a height of 1Ocm were prepared. The operational temperature was maintained within the range of 120-140°C and the speed of stirrer was 600 r.p.m. Paraffin was physically mixed with boric acid and played a binding role within waste form. The mixing ratio of boric acid to paraffin was very important in order to make a homogeneous waste form because it made a difference between boric acid( 1.44) and paraffrn(0.933) in specific gravity. If the ratio of paraffin content was less than 15%, it was difftcult to make a waste form due to a low fluidity, while the phenomenon of stratification 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 ASTM C39-86 (1986). The test was applied to at least three specimens under each test condition of different mixing ratios of boric acid to parafhn. ANWANS-16.1 (1986) leaching standard procedure was used to investigate the leaching characteristics of paraffin waste form. The test was developed by an American Nuclear Society Standards Committee for the characterization of solidified low-level radioactive waste forms. This procedure uses demineralized water as the leachant and is conducted at a temperature of (22.5*5)“C. Sufficient leachant is used to provide a ratio of leachant volume to specimen external geometric surface area of (lOti.2)cm. The leachant is sampled and replaced at the following frequency; 2, 7, and 24h from the initiation of the test, then at 24h intervals for the next 4d, and then at 14, 28, and 43d intervals to extend the entire test to 90d. The paraBin waste form whose mixing ratio of boric acid to pa&in is 78122, which had been adopted in CWDS of KNPPs, was chosen for the leaching test. Non-radioactive species cobalt(I1) chloride hexahydrate whose amount corresponds to 0.24% of boric acid by weight was added to the specimen. The concentration of cobalt included in leachate was measured by means of inductive coupled plasma-mass spectroscopy(ICP-MS) and that of boric acid within leachate was analyzed by titration.
395
Leaching behaviour of boric acid and cobalt
3. RESULTS AND DISCUSSION The mixtures whose paraffin contents were within the range of 20-24% were easily poured into mold owing to good workability, and the compressive strength tests resulted in similar values of 622-673psi(4.23-4.58MPa). It was desirable to keep a pa&Fin weight ratio within the range of 20-24% in order to prepare a homogeneous waste form as the matters of fluidity and stratification were solved at a time. For the par&n content was 22%, the compressive strength before and after the leaching test of ninety days was 666psi(4.53MPa) and 232psi( lS8MPa), respectively. It was observed that the cumulative fraction leached(CFL) of boric acid and cobalt was about 5 1% and 61%, respectively, after ninety days as shown in Fig. l(a). The effective diffisivities of boric acid and cobalt were 1.5 X 1c6 cm’/sec and 5.7 X lo-* cm2/sec, respectively, which could be obtained from Eq. (1). They indicated that the leaching rates of pa&in waste form were considerably higher than those of cement and bitumen waste form with the effective difisivity of approximately 10-‘3-10-12 cm2/sec and l@‘*- IO-l7 cm’/sec, respectively, for cobalt. Fig. l(a) also showed that the CFL of cobalt was higher than that of boric acid only by 10% although the solubility of cobalt(0.33Sg/cm3 at 20°C) was about seven times higher than that of boric acid(0.0465g/cm3 at 20°C). These results could be explained by that the boric acid which occupied a large portion of paraffin waste form was easily dissolved from the surface of waste form by leachant, 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 the leachant because of the sufficient supply and the periodical replacement of leachant. Accordingly, the leaching of cobalt was thought to be mainly dependent on the dissolution of boric acid.
I .I.
.I. ??
Boric Acid
I.,
??
Boric Acid
??
Cobalt
80
90 100
I.,
,
1.
Time(day)
0
10
20
30
40 50 60 Time(day)
70
(c) (R is a correlation coefficient) Fig. 1 Cumulative Fraction Leached of Boric Acid and Cobalt Plotted as a Function of Time(a), Square Root of Time(b), and Gintsling-Brounshtein Kinetics as a Function of Time(c)
396
.I. Y. Kim and C. H. Chung
The leaching of paraffin waste form might appear to be controlled by conventional diffusion because the CFL values had linear relation to square root of time in Fig. l(b). But the conventional diffusion model which would have the maximum concentration at the center could not explain the photograph(Fig. 2) showing the shrinking dissolution front.
Reacted Layer
Dissolution Front Fig. 2 External Shape(left) and Internal Section(right) of Paraffin Waste Form after the Leaching Test of 90 days The reaction initially occurred at the external surface of waste form and the dissolution front gradually moved inside leaving a reacted layer behind. The leaching rates of boric acid and cobalt were influenced by reacted layer depth as the reaction progressed. It was revealed that this reacted layer decreased the diffusive fluxes of leached boric acid and cobalt and controlled the overall dissolution rate. Therefore, the leaching mechanism of paraffin waste form was well explained by the diffusion-controlled dissolution model such as Gintsling-Brounshtein kinetics which was a kind of unreacted shrinking core model (Wen, 1968; Bamford and 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). This unreacted shrinking core model assumed that the reaction between solid and fluid was noncatalytic and solid particle was spherical. Although the specimen of this study was cylindrical, the above model could be applied to the leaching analysis of paraffin waste form because the leaching rates were constant along the all surfaces, i.e., top/bottom and lateral sides, of the cylindrical waste form. Fig. l(c) described the relation between dissolved fraction and reaction time by Eq. (1). Model predictions were excellently agreed with test data and overall reaction rate constants were obtained from the inclinations of each straight line.
4. CONCLUSIONS The leaching test of paraffin waste form was carried out according to ANSI/ANS-16.1 test procedure. For the waste form with the mixing ratio of 78122 between boric acid and paraffin, it was observed that about 51% and 61% of boric acid and cobalt, respectively, were released after ninety days. The compressive strength of waste form before and after the leaching test resulted in 666psi(4.53MPa) and 232psi(1,58MPa), respectively. The leaching rates of boric acid and cobalt were influenced by reacted layer depth as the reaction progressed and this reacted layer controlled the overall dissolution rate. It was concluded that the leaching mechanism of paraffin waste form was diffusion-controlled dissolution with the shrinking dissolution front.
Leaching behaviour of boric acid and cobali
397
REFERENCES American National Standards Institute (1986), Measurement of the Leachability of Solidified Low-level Radioactive Wastes by a Short-term Test Procedure, ANSVANS-16.1. American Society for Testing Materials (1986), Standard Test Method for Compressive Strength of Cylindrical 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 of International Symposium on Radiation Safety Management. U. S. Nuclear Regulatory Commission (1991), Low-level Waste Licensing Branch Technical Position on Waste Form, Rev. 1. Wen C.Y. (1968), Noncatalytic Heterogeneous Solid Fluid Reaction Models, Industrial and Engineering Chemistry, 60, 34.
Pergamon
Printed in Great Britain 0149-1970/00/$ - see front matter
www.elsevier.com/locate/pnucene
PII: so149-1970(00)00077-9
LEACHING
BEHAVIOR OF BORIC ACID AND COBALT FROM PARAFFIN WASTE FORMS
JUYOULKIM, CHANGHYUN CHUNG
Department of Nuclear Engineering, Seoul National University San56-1 Shinrim-dong, Gwanak-gu, Seoul, 15 l-742, Korea Phone: +82-2-880-833 1, Fax: +82-2-889-2688, E-mail: [email protected]
CHANG LAK KIM
Nuclear Environment Technology Institute, Korea Electric Power Corporation 150 Dukjin-dong, Yusong-gu, Taejon, 305-353, Korea
ABSTRACT Ninety-day leaching test was performed to investigate the leaching characteristics of paraffin waste forms that had been recently generated in large quantities at Korean nuclear power plants(KNPPs). In the case where mixing weight ratio of boric acid to paraflin was 78122, which was adopted in concentrate waste drying system(CWDS) of KNPPs, the cumulative fraction leached(CFL) of boric acid and cobalt was about 5 1% and 61%, respectively. The compressive strengths of waste form before and after the leaching test exhibited 666psi(4,53MPa) and 232psi(1.58MPa), respectively. The CFLs of paraf?in waste form were well expressed by diffision-controlled dissolution model such as Gintsling-Brounshtein kinetics. The internal cross-sectional view of specimen after the test demonstrated the applicability of this unreacted shrinking core model to the leaching analysis of paraflin waste form. 0 2000 Elsevier Science Ltd. All rights reserved.
393
394
.I. Z Kim and C. H. Chung
1. INTRODUCTION Low- and intermediate-level radioactive wastes arising from the operation of nuclear power plants can be immobilized by various solidification techniques prior to disposal. Liquid radioactive wastes have been treated with filtration, ion exchange resin, evaporation, and so on. The remaining liquid concentrate wastes from evaporator have been immobilized by solidifying agents such as cement, bitumen, and polymer, and then they are filled and packaged in several kinds of containers. Because waste forms including radionuclides are immobilized with solidifying agents and finally disposed of, they can be safely stored in radioactive waste repository, isolated from biosphere for a long time. However, despite of engineered and natural barriers of radioactive waste repository, the radioactive waste forms would eventually be in contact with groundwater, and the release of radioactive species from the waste forms would occur by leaching mechanism. Therefore, acceptance criteria of radioactive waste forms have been developed to guarantee the long-term safety performance of radioactive waste repository (U.S. NRC, 1991). Low-level liquid borate wastes have been recently converted to parafIin waste forms by utilizing the concentrate waste drying system(CWLG) in Korean nuclear power plants(KNPPs) (Kim and Bae, 1997). Paraffin waste forms could be classified as class A unstable. Nevertheless CWDS had the advantages of high volume reduction, low cost, and simple manufacturing process. This study was performed to investigate the leaching characteristics of pat&in waste forms.
2. EXPERIMENTAL PROCEDURES Cylindrical waste forms with a diameter of 5cm and a height of 1Ocm were prepared. The operational temperature was maintained within the range of 120-140°C and the speed of stirrer was 600 r.p.m. Paraffin was physically mixed with boric acid and played a binding role within waste form. The mixing ratio of boric acid to paraffin was very important in order to make a homogeneous waste form because it made a difference between boric acid( 1.44) and paraffrn(0.933) in specific gravity. If the ratio of paraffin content was less than 15%, it was difftcult to make a waste form due to a low fluidity, while the phenomenon of stratification 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 ASTM C39-86 (1986). The test was applied to at least three specimens under each test condition of different mixing ratios of boric acid to parafhn. ANWANS-16.1 (1986) leaching standard procedure was used to investigate the leaching characteristics of paraffin waste form. The test was developed by an American Nuclear Society Standards Committee for the characterization of solidified low-level radioactive waste forms. This procedure uses demineralized water as the leachant and is conducted at a temperature of (22.5*5)“C. Sufficient leachant is used to provide a ratio of leachant volume to specimen external geometric surface area of (lOti.2)cm. The leachant is sampled and replaced at the following frequency; 2, 7, and 24h from the initiation of the test, then at 24h intervals for the next 4d, and then at 14, 28, and 43d intervals to extend the entire test to 90d. The paraBin waste form whose mixing ratio of boric acid to pa&in is 78122, which had been adopted in CWDS of KNPPs, was chosen for the leaching test. Non-radioactive species cobalt(I1) chloride hexahydrate whose amount corresponds to 0.24% of boric acid by weight was added to the specimen. The concentration of cobalt included in leachate was measured by means of inductive coupled plasma-mass spectroscopy(ICP-MS) and that of boric acid within leachate was analyzed by titration.
395
Leaching behaviour of boric acid and cobalt
3. RESULTS AND DISCUSSION The mixtures whose paraffin contents were within the range of 20-24% were easily poured into mold owing to good workability, and the compressive strength tests resulted in similar values of 622-673psi(4.23-4.58MPa). It was desirable to keep a pa&Fin weight ratio within the range of 20-24% in order to prepare a homogeneous waste form as the matters of fluidity and stratification were solved at a time. For the par&n content was 22%, the compressive strength before and after the leaching test of ninety days was 666psi(4.53MPa) and 232psi( lS8MPa), respectively. It was observed that the cumulative fraction leached(CFL) of boric acid and cobalt was about 5 1% and 61%, respectively, after ninety days as shown in Fig. l(a). The effective diffisivities of boric acid and cobalt were 1.5 X 1c6 cm’/sec and 5.7 X lo-* cm2/sec, respectively, which could be obtained from Eq. (1). They indicated that the leaching rates of pa&in waste form were considerably higher than those of cement and bitumen waste form with the effective difisivity of approximately 10-‘3-10-12 cm2/sec and l@‘*- IO-l7 cm’/sec, respectively, for cobalt. Fig. l(a) also showed that the CFL of cobalt was higher than that of boric acid only by 10% although the solubility of cobalt(0.33Sg/cm3 at 20°C) was about seven times higher than that of boric acid(0.0465g/cm3 at 20°C). These results could be explained by that the boric acid which occupied a large portion of paraffin waste form was easily dissolved from the surface of waste form by leachant, 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 the leachant because of the sufficient supply and the periodical replacement of leachant. Accordingly, the leaching of cobalt was thought to be mainly dependent on the dissolution of boric acid.
I .I.
.I. ??
Boric Acid
I.,
??
Boric Acid
??
Cobalt
80
90 100
I.,
,
1.
Time(day)
0
10
20
30
40 50 60 Time(day)
70
(c) (R is a correlation coefficient) Fig. 1 Cumulative Fraction Leached of Boric Acid and Cobalt Plotted as a Function of Time(a), Square Root of Time(b), and Gintsling-Brounshtein Kinetics as a Function of Time(c)
396
.I. Y. Kim and C. H. Chung
The leaching of paraffin waste form might appear to be controlled by conventional diffusion because the CFL values had linear relation to square root of time in Fig. l(b). But the conventional diffusion model which would have the maximum concentration at the center could not explain the photograph(Fig. 2) showing the shrinking dissolution front.
Reacted Layer
Dissolution Front Fig. 2 External Shape(left) and Internal Section(right) of Paraffin Waste Form after the Leaching Test of 90 days The reaction initially occurred at the external surface of waste form and the dissolution front gradually moved inside leaving a reacted layer behind. The leaching rates of boric acid and cobalt were influenced by reacted layer depth as the reaction progressed. It was revealed that this reacted layer decreased the diffusive fluxes of leached boric acid and cobalt and controlled the overall dissolution rate. Therefore, the leaching mechanism of paraffin waste form was well explained by the diffusion-controlled dissolution model such as Gintsling-Brounshtein kinetics which was a kind of unreacted shrinking core model (Wen, 1968; Bamford and 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). This unreacted shrinking core model assumed that the reaction between solid and fluid was noncatalytic and solid particle was spherical. Although the specimen of this study was cylindrical, the above model could be applied to the leaching analysis of paraffin waste form because the leaching rates were constant along the all surfaces, i.e., top/bottom and lateral sides, of the cylindrical waste form. Fig. l(c) described the relation between dissolved fraction and reaction time by Eq. (1). Model predictions were excellently agreed with test data and overall reaction rate constants were obtained from the inclinations of each straight line.
4. CONCLUSIONS The leaching test of paraffin waste form was carried out according to ANSI/ANS-16.1 test procedure. For the waste form with the mixing ratio of 78122 between boric acid and paraffin, it was observed that about 51% and 61% of boric acid and cobalt, respectively, were released after ninety days. The compressive strength of waste form before and after the leaching test resulted in 666psi(4.53MPa) and 232psi(1,58MPa), respectively. The leaching rates of boric acid and cobalt were influenced by reacted layer depth as the reaction progressed and this reacted layer controlled the overall dissolution rate. It was concluded that the leaching mechanism of paraffin waste form was diffusion-controlled dissolution with the shrinking dissolution front.
Leaching behaviour of boric acid and cobali
397
REFERENCES American National Standards Institute (1986), Measurement of the Leachability of Solidified Low-level Radioactive Wastes by a Short-term Test Procedure, ANSVANS-16.1. American Society for Testing Materials (1986), Standard Test Method for Compressive Strength of Cylindrical 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 of International Symposium on Radiation Safety Management. U. S. Nuclear Regulatory Commission (1991), Low-level Waste Licensing Branch Technical Position on Waste Form, Rev. 1. Wen C.Y. (1968), Noncatalytic Heterogeneous Solid Fluid Reaction Models, Industrial and Engineering Chemistry, 60, 34.