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Leach test of spent ion exchange resins solidified in concrete
CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 375-380, 1992. Printed in the USA. 0008-8846/92 $5.00 + .00. Copyright © 1992 Pergamon Press Ltd.
LEACH TEST CONCRETE
OF
SPENT
ION EXCHANGE
RESINS
SOLIDIFIED
IN
H. Aalto' and A. Ipatti b
"Technical Research Centre of Finland, Reactor Laboratory, P.O. Box 200, SF-02151 Espoo, Finland b Imatran Voima Oy, Research and Development Division, P.O. Box 112, SF-01601 Vantaa, Finland.
Abstract This paper deals with the cementation of spent ion exchange resins arising at Loviisa NPS. The purpose of the experiment was to study the long-term stability of the solidification product. The spent resin used for the preparation of the test specimens was a mixture of bead-formed cation and anion resins. The leachant used in this experiment was local groundwater from the site of Loviisa NPS, equilibrated with granulated cement. The test method applied originates from Dr. Brodersen /1/. The radionuclides of interest to this dynamic long-term leach testing were Cs-134, Cs-137 originally present in spent resins and Sr-85, which was added to the wet resin mixture prior to cementation. After each leach period, gamma activities of leachant and granulate were measured. Based on gamma measurements, the penetration depths and cumulative fractional release rates for Cs-134, Cs-137 and Sr-85 have been calculated. Leach tests have been running for about 4 years and 19 leachant exchanges have been performed. Tests are still in progress and will be continued for up to 5 years.
1. I N T R O D U C T I O N Imatran Voima Oy (IVO) operates two 445 MWe PWR units in Loviisa. A solidification plant has been licensed but because the amount of low and intermediate level liquid waste accumulated has been considerably smaller than expected, the construction of the plant has not yet been of current interest. According to the final disposal concept, the solidified wastes will be disposed of on site. The repository will be constructed in the bedrock at a depth of 90 - 120 metres. Geological investigations have been performed at the site since 1979. IVO has studied solidification of liquid reactor waste with cement since 1976. The studies have primarily been carried out with inactive ion exchange resins, evaporator concentrates and lately also with inactive dregs arising through the microbial decomposition of organic dry wastes. 375
376
H. Aalto and A. Ipatti
Vol. 22, Nos. 213
In June 1987, three half-scale concrete containers were filled with solidified radioactive ion exchange resins. The aim of the test was to study the long-term durability of the waste packages immersed in local groundwater/2/. According to the test programme, smaller test specimens were also prepared in this connection for leach tests to be carded out at the Reactor Laboratory of the Technical Research Centre of Finland (VTT).
2. PREPARATON OF SPECIMENS Two batches of waste product mixtures corresponding to the waste product composition used in the half-scale solidification test were prepared with a universal mixer. The first batch was used for the preparation of test specimens PI21, PI22 and PI23. The composition of the second batch was identical, though a dose of 1 mCi Sr-85 was added to the wet resin before mixing. This batch was used for the preparation of test specimens PI51, PI52 and PI55. Composition of the waste product mixtures is presented in Table 1. Table 1 Composition of waste product mixtures used for preparation of test specimens Composition
kg/m 3
Weight parts
Ion exchange resins
361
1
Blast-furnace cement
1083
3
Ca(OH) 2
60
0.165
Additional water
276
0.765
Total
1780
4.93
In Table 1 the given weights and weight parts for ion exchange resins are valid for evensurface moist resins. The boric acid content of the resins was 55 g H3BOJkg resins. Taking into account the water content of the resin mixture, the ratio w/c was 0.49. The main radioactive nuclides and their concentrations were determined in the homogenized evensurface moist resin mixture. The concentration of Cs-134 was 5.52 GBq/m 3 and Cs-137 15.50 GBq/m 3. The cement used in solidification was a slow hardening blast-furnace slag cement having a slag content of 72 %. The test specimens were cast in disposable plastic moulds. Each of the specimens was provided with an acid-proof steel wire suspension hook. The dimensions of the cylindrical specimens were diameter 50 mm and height 50 mm. After casting, the specimens were pretreated for 7 days at 40°C and 100% humidity. After that the specimens were demoulded and stored at room temperature at 100% humidity for 11 weeks.
Vol.22, Nos.2/3
LEACHING,SPENTIONEXCHANGERESINS,CONCRETE
377
3. LEACH METHOD The test method applied was originally proposed by Dr. Brodersen for leaching bituminized specimens/1L In this method the leached activities can be divided into two components, namely activity in the leachant and activity in the granules at the bottom of the leach vessel. The leachant used in this experiment was local groundwater from the site of Loviisa NPS, equilibrated with granulated cement. By using equilibrated groundwater it is possible to avoid unexpected pH changes and thus stabilize the water chemistry during the course of the experiment. Test vessels were placed in an outer container with a tight lid containing a weak NaOH-solution. The NaOH-solution absorbs carbon dioxide from the air, thus preventing the formation of carbonate precipitates in the leachant during leach periods. The function of the cement granules at the bottom of the leach vessel is to stabilize the pH by buffering the OH--concentration of the leachant. Furthermore, the relatively large surface area of granules serves as a precipitation and/or adsorption surface for the leached radionuclides. The cement granules, grain size 0.25 - 2.0 mm, were made of hardened Portland cement mortar (1:1.8:0.43) by first crushing and then sieving the mortar with 0.25 mm and 2.0 mm sieves. Test arrangements are presented in Figure 1.
Leachant -NaOH (1%) . _ _ Leachedsample Sample supporter Cement granules (10g dry) _
.
_
1 Weighing in leachant
Sample is dried and weighed Fresh cement
I ~
~100ml transfer of old leachant
1 ~1
J
equilibrated
up to total i}1 water added amount
iii, 21.1. Old New Granules and the left over leachant are analysed Figure 1. Leach test arrangements according to Brodersen/1/.
378
H. Aalto mad A. Ipatti
Vol. 22, Nos. 213
4. RESULTS AND DISCUSSION The purpose of this experiment was to study the long-term stability and especially leaching behaviour of the ion exchange resin solidification product. Short-term leach tests are primarily used for quality control and for the comparison of properties of various solidification products. Through long-term leach tests, initial data can be acquired for safety assessment analysis of the final disposal concept. The chosen leach method offers laboratory test conditions which are as similar as possible to the conditions in the final repository. Cumulative penetration depths L,, often referred as CPi in literature published in the U.S.A., have been calculated according to Brodersen. Before calculation, all the measured activities must be corrected back to the initiation of the experiment. Penetration depths in specimen PI52 are presented for Sr-85, Cs-134 and Cs-137 as a function of the square root of time in Figure 2. Because Sr-85 has a half-life of 65 days, activity measurements were finished at 364 days. Cs-134 and Cs-137 activity measurements are being continued for up to 3.5 years. The form of the curves in Figure 2 reveal that the leaching of concrete waste products cannot be explained only by diffusion. Chemical reactions are still going on, changing the leaching behaviour of the specimen. Calculated penetration depths after a leaching period of 1 year on average are 0.054 cm for cesium and 0.030 cm for strontium and after 3.5 years for cesium 0.061 cm. Average leaching rates in the test given by the slopes of cumulative total activity curves versus the time at 100 d are 9.1"10.5 g/cm2"d, i.e. 5.2-10 .5 cm/d for cesium, and 6.8-10 .5 g/cm2-d, i.e. 3.9"10 .5 cm/d for strontium. The leaching rate for cesium at 3 years is 8.4"10 .6 g/cm2~l, i.e. 4.7"10 .6 cm/d.
PENETRATION DEPTH
0.08 0.06 i ~~¢:~j~:~ Ln CM 0.04[ 0.04 ~ I 0.02 ~ = ~ ~ i 0
0
= o
sr~s Cs-134
.t,
Cs-137
10 20 30 SQRT TIME (DAYS)
40
Figure 2. Cumulative penetration depths for Sr-85, Cs-134 and Cs-137 in specimen PI52. Cumulative leach fractions in percentage units as a function of the square root of time are presented in Figure 3. After a leaching period of 1 year on average 6.5% of the initial cesium activity has been leached out, after 3 years 7.3%. The released strontium activity was 3.6% after 1 year.
Vol. 22, Nos. 2t3
LEACHING,SPENTION EXCHANGERESINS,CONCRETE
379
CUMULATIVE RELEASE PERCENTAGE 8.00 6.00 Ai / AO % 4.00 2.00 J,
Cs-137
0.00 0
10
20
30
40
SQRT TIME (DAYS) Figure 3.
Cumulative release fractions in percentage units for Sr-85, Cs-134 and Cs-137 in specimen PI52.
The leach method applied in this study offers a possibility to divide the released activities into two components, namely activity in the leachant and activity in the granulate. "F~=c cumulative distribution of Sr-85 is presented in Figure 4. As expected, practically all of t.~:.~ strontium activity is in leachant. The same is also the case for cesium activities. It means the, neither precipitation nor adsorption plays any significant role for strontium and cesiu~, activities in the pH range 12.0 - 12.5.
DISTRIBUTION OF Sr-85 80000 60000 Bq
40000 20000 =
0 0
5
10
o
Leachant
-"
Granulate
•it
Total
-J"
15
SQRT TIME (DAYS)
Figure 4. Distribution of Sr-85 between leachant and granulate.
=
=1
20
H. Aalto and A. Ipatti
380
Vol. 22, Nos. 213
The local groundwater from the site of Loviisa NPS used in this study is a granitic groundwater which can be characterized by relatively high concentrations of chloride, 5300 ppm, and sulphate, 650 ppm. This is true also for sodium, 2400 ppm, calcium 565 ppm and magnesium 292 ppm. Bicarbonate concentration is 90 ppm and acidity pH 7.4. According to Cembureau's classification, the groundwater is weakly aggressive /3/. This classification does not take into account the chloride concentration. However, the high concentration as in this study's 5300 ppm may be detrimental to concrete reinforcement. The calculated leach rates are in the same order of magnitude as referred to in the literature/4,5/. The test result are conservative because the initial test specimens chosen were not of the best possible quality according to visual inspection. However, after a leach period of 3.5 years no form of degradation has been observed.
5. CONCLUSIONS The purpose of this experiment was to study the long-term stability and especially leaching behaviour of the ion exchange resin solidification product. IVO has developed and characterized the waste product and it has been tested in conditions which are as similar as possible to the local repository conditions. The test method used in this study offered possibilities to simulate the required specified conditions. Based on the results, the cumulative leach fraction for cesium seems to be about two times greater than the corresponding value for strontium. When comparing the calculated leach rate for cesium at 100 days to that at 3 years, the difference is about one order of magnitude. Further improvements concerning cesium release can be achieved by using additives. This leach test will be continued for up to 5 years to gain more basic data for long-term prediction and modelling.
6. REFERENCES
.
A. Ipatti, Cement solidification of spent ion exchange resins - Intermediate results of a half-scale experiment. Nuclear Waste Commission of Finnish Power Companies. Report YJT-90-19. 1990. K. Brodersen, Proposed procedure for leaching of bituminized resins in cementconditioned water. NKA report AVF(82)212 (Draft), 1982. Cembureau Recommendation. Use of concrete in aggressive environments. The European Cement Association. 1st edition 1978. D. Laske, W. HiJbner, M. Kiinzle, Verfestigung aktiver PowdexIonenaustauschharze aus Siedewasser-Reaktoren. NAGRA Technischer Bericht 83-19. 1983. A. Muurinen, P. Uotila, J. Rantanen, Leach testing on simulated waste solidified in cement. (In Finnish). Nuclear Waste Commission of Finnish Power Companies. Report YJT-82-38. 1982.
LEACH TEST CONCRETE
OF
SPENT
ION EXCHANGE
RESINS
SOLIDIFIED
IN
H. Aalto' and A. Ipatti b
"Technical Research Centre of Finland, Reactor Laboratory, P.O. Box 200, SF-02151 Espoo, Finland b Imatran Voima Oy, Research and Development Division, P.O. Box 112, SF-01601 Vantaa, Finland.
Abstract This paper deals with the cementation of spent ion exchange resins arising at Loviisa NPS. The purpose of the experiment was to study the long-term stability of the solidification product. The spent resin used for the preparation of the test specimens was a mixture of bead-formed cation and anion resins. The leachant used in this experiment was local groundwater from the site of Loviisa NPS, equilibrated with granulated cement. The test method applied originates from Dr. Brodersen /1/. The radionuclides of interest to this dynamic long-term leach testing were Cs-134, Cs-137 originally present in spent resins and Sr-85, which was added to the wet resin mixture prior to cementation. After each leach period, gamma activities of leachant and granulate were measured. Based on gamma measurements, the penetration depths and cumulative fractional release rates for Cs-134, Cs-137 and Sr-85 have been calculated. Leach tests have been running for about 4 years and 19 leachant exchanges have been performed. Tests are still in progress and will be continued for up to 5 years.
1. I N T R O D U C T I O N Imatran Voima Oy (IVO) operates two 445 MWe PWR units in Loviisa. A solidification plant has been licensed but because the amount of low and intermediate level liquid waste accumulated has been considerably smaller than expected, the construction of the plant has not yet been of current interest. According to the final disposal concept, the solidified wastes will be disposed of on site. The repository will be constructed in the bedrock at a depth of 90 - 120 metres. Geological investigations have been performed at the site since 1979. IVO has studied solidification of liquid reactor waste with cement since 1976. The studies have primarily been carried out with inactive ion exchange resins, evaporator concentrates and lately also with inactive dregs arising through the microbial decomposition of organic dry wastes. 375
376
H. Aalto and A. Ipatti
Vol. 22, Nos. 213
In June 1987, three half-scale concrete containers were filled with solidified radioactive ion exchange resins. The aim of the test was to study the long-term durability of the waste packages immersed in local groundwater/2/. According to the test programme, smaller test specimens were also prepared in this connection for leach tests to be carded out at the Reactor Laboratory of the Technical Research Centre of Finland (VTT).
2. PREPARATON OF SPECIMENS Two batches of waste product mixtures corresponding to the waste product composition used in the half-scale solidification test were prepared with a universal mixer. The first batch was used for the preparation of test specimens PI21, PI22 and PI23. The composition of the second batch was identical, though a dose of 1 mCi Sr-85 was added to the wet resin before mixing. This batch was used for the preparation of test specimens PI51, PI52 and PI55. Composition of the waste product mixtures is presented in Table 1. Table 1 Composition of waste product mixtures used for preparation of test specimens Composition
kg/m 3
Weight parts
Ion exchange resins
361
1
Blast-furnace cement
1083
3
Ca(OH) 2
60
0.165
Additional water
276
0.765
Total
1780
4.93
In Table 1 the given weights and weight parts for ion exchange resins are valid for evensurface moist resins. The boric acid content of the resins was 55 g H3BOJkg resins. Taking into account the water content of the resin mixture, the ratio w/c was 0.49. The main radioactive nuclides and their concentrations were determined in the homogenized evensurface moist resin mixture. The concentration of Cs-134 was 5.52 GBq/m 3 and Cs-137 15.50 GBq/m 3. The cement used in solidification was a slow hardening blast-furnace slag cement having a slag content of 72 %. The test specimens were cast in disposable plastic moulds. Each of the specimens was provided with an acid-proof steel wire suspension hook. The dimensions of the cylindrical specimens were diameter 50 mm and height 50 mm. After casting, the specimens were pretreated for 7 days at 40°C and 100% humidity. After that the specimens were demoulded and stored at room temperature at 100% humidity for 11 weeks.
Vol.22, Nos.2/3
LEACHING,SPENTIONEXCHANGERESINS,CONCRETE
377
3. LEACH METHOD The test method applied was originally proposed by Dr. Brodersen for leaching bituminized specimens/1L In this method the leached activities can be divided into two components, namely activity in the leachant and activity in the granules at the bottom of the leach vessel. The leachant used in this experiment was local groundwater from the site of Loviisa NPS, equilibrated with granulated cement. By using equilibrated groundwater it is possible to avoid unexpected pH changes and thus stabilize the water chemistry during the course of the experiment. Test vessels were placed in an outer container with a tight lid containing a weak NaOH-solution. The NaOH-solution absorbs carbon dioxide from the air, thus preventing the formation of carbonate precipitates in the leachant during leach periods. The function of the cement granules at the bottom of the leach vessel is to stabilize the pH by buffering the OH--concentration of the leachant. Furthermore, the relatively large surface area of granules serves as a precipitation and/or adsorption surface for the leached radionuclides. The cement granules, grain size 0.25 - 2.0 mm, were made of hardened Portland cement mortar (1:1.8:0.43) by first crushing and then sieving the mortar with 0.25 mm and 2.0 mm sieves. Test arrangements are presented in Figure 1.
Leachant -NaOH (1%) . _ _ Leachedsample Sample supporter Cement granules (10g dry) _
.
_
1 Weighing in leachant
Sample is dried and weighed Fresh cement
I ~
~100ml transfer of old leachant
1 ~1
J
equilibrated
up to total i}1 water added amount
iii, 21.1. Old New Granules and the left over leachant are analysed Figure 1. Leach test arrangements according to Brodersen/1/.
378
H. Aalto mad A. Ipatti
Vol. 22, Nos. 213
4. RESULTS AND DISCUSSION The purpose of this experiment was to study the long-term stability and especially leaching behaviour of the ion exchange resin solidification product. Short-term leach tests are primarily used for quality control and for the comparison of properties of various solidification products. Through long-term leach tests, initial data can be acquired for safety assessment analysis of the final disposal concept. The chosen leach method offers laboratory test conditions which are as similar as possible to the conditions in the final repository. Cumulative penetration depths L,, often referred as CPi in literature published in the U.S.A., have been calculated according to Brodersen. Before calculation, all the measured activities must be corrected back to the initiation of the experiment. Penetration depths in specimen PI52 are presented for Sr-85, Cs-134 and Cs-137 as a function of the square root of time in Figure 2. Because Sr-85 has a half-life of 65 days, activity measurements were finished at 364 days. Cs-134 and Cs-137 activity measurements are being continued for up to 3.5 years. The form of the curves in Figure 2 reveal that the leaching of concrete waste products cannot be explained only by diffusion. Chemical reactions are still going on, changing the leaching behaviour of the specimen. Calculated penetration depths after a leaching period of 1 year on average are 0.054 cm for cesium and 0.030 cm for strontium and after 3.5 years for cesium 0.061 cm. Average leaching rates in the test given by the slopes of cumulative total activity curves versus the time at 100 d are 9.1"10.5 g/cm2"d, i.e. 5.2-10 .5 cm/d for cesium, and 6.8-10 .5 g/cm2-d, i.e. 3.9"10 .5 cm/d for strontium. The leaching rate for cesium at 3 years is 8.4"10 .6 g/cm2~l, i.e. 4.7"10 .6 cm/d.
PENETRATION DEPTH
0.08 0.06 i ~~¢:~j~:~ Ln CM 0.04[ 0.04 ~ I 0.02 ~ = ~ ~ i 0
0
= o
sr~s Cs-134
.t,
Cs-137
10 20 30 SQRT TIME (DAYS)
40
Figure 2. Cumulative penetration depths for Sr-85, Cs-134 and Cs-137 in specimen PI52. Cumulative leach fractions in percentage units as a function of the square root of time are presented in Figure 3. After a leaching period of 1 year on average 6.5% of the initial cesium activity has been leached out, after 3 years 7.3%. The released strontium activity was 3.6% after 1 year.
Vol. 22, Nos. 2t3
LEACHING,SPENTION EXCHANGERESINS,CONCRETE
379
CUMULATIVE RELEASE PERCENTAGE 8.00 6.00 Ai / AO % 4.00 2.00 J,
Cs-137
0.00 0
10
20
30
40
SQRT TIME (DAYS) Figure 3.
Cumulative release fractions in percentage units for Sr-85, Cs-134 and Cs-137 in specimen PI52.
The leach method applied in this study offers a possibility to divide the released activities into two components, namely activity in the leachant and activity in the granulate. "F~=c cumulative distribution of Sr-85 is presented in Figure 4. As expected, practically all of t.~:.~ strontium activity is in leachant. The same is also the case for cesium activities. It means the, neither precipitation nor adsorption plays any significant role for strontium and cesiu~, activities in the pH range 12.0 - 12.5.
DISTRIBUTION OF Sr-85 80000 60000 Bq
40000 20000 =
0 0
5
10
o
Leachant
-"
Granulate
•it
Total
-J"
15
SQRT TIME (DAYS)
Figure 4. Distribution of Sr-85 between leachant and granulate.
=
=1
20
H. Aalto and A. Ipatti
380
Vol. 22, Nos. 213
The local groundwater from the site of Loviisa NPS used in this study is a granitic groundwater which can be characterized by relatively high concentrations of chloride, 5300 ppm, and sulphate, 650 ppm. This is true also for sodium, 2400 ppm, calcium 565 ppm and magnesium 292 ppm. Bicarbonate concentration is 90 ppm and acidity pH 7.4. According to Cembureau's classification, the groundwater is weakly aggressive /3/. This classification does not take into account the chloride concentration. However, the high concentration as in this study's 5300 ppm may be detrimental to concrete reinforcement. The calculated leach rates are in the same order of magnitude as referred to in the literature/4,5/. The test result are conservative because the initial test specimens chosen were not of the best possible quality according to visual inspection. However, after a leach period of 3.5 years no form of degradation has been observed.
5. CONCLUSIONS The purpose of this experiment was to study the long-term stability and especially leaching behaviour of the ion exchange resin solidification product. IVO has developed and characterized the waste product and it has been tested in conditions which are as similar as possible to the local repository conditions. The test method used in this study offered possibilities to simulate the required specified conditions. Based on the results, the cumulative leach fraction for cesium seems to be about two times greater than the corresponding value for strontium. When comparing the calculated leach rate for cesium at 100 days to that at 3 years, the difference is about one order of magnitude. Further improvements concerning cesium release can be achieved by using additives. This leach test will be continued for up to 5 years to gain more basic data for long-term prediction and modelling.
6. REFERENCES
.
A. Ipatti, Cement solidification of spent ion exchange resins - Intermediate results of a half-scale experiment. Nuclear Waste Commission of Finnish Power Companies. Report YJT-90-19. 1990. K. Brodersen, Proposed procedure for leaching of bituminized resins in cementconditioned water. NKA report AVF(82)212 (Draft), 1982. Cembureau Recommendation. Use of concrete in aggressive environments. The European Cement Association. 1st edition 1978. D. Laske, W. HiJbner, M. Kiinzle, Verfestigung aktiver PowdexIonenaustauschharze aus Siedewasser-Reaktoren. NAGRA Technischer Bericht 83-19. 1983. A. Muurinen, P. Uotila, J. Rantanen, Leach testing on simulated waste solidified in cement. (In Finnish). Nuclear Waste Commission of Finnish Power Companies. Report YJT-82-38. 1982.