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Adsorption on various leach container materials of plutonium and curium leached from nuclear waste glasses
NUCLEAR AND CHEMICAL WASTE MANAGEMENT, Printed in the USA. All rights reserved.
Vol. 8, pp. 45-54,
1988 Copyright
0191-X15X/88 $3.00 + .OO i 1988 Pergamon Press plc
ADSORPTION ON VARIOUS LEACH CONTAINER MATERIALS OF PLUTONIUM AND CURIUM LEACHED FROM NUCLEAR WASTE GLASSES
Tsunetaka Banba, Shingo Tashiro, Kiyoshi Nukaga, and Masayuki Nomura Deprrrtmrnt
of Environmrntui
Sufety
Reseurch,
Japan
Atomic
Energy
Research
Tamio Sagawa, Institute,
To&,
Ibaruki
319-l 1,
.Iupun
The adsorption tendency of leached-out ““a and 244Cm on the wall of leach containers, during the leaching of waste glasses, was studied. The test pieces of quartz glass, PFA Teflon, gold and stainless steel, which are candidates for a leach container, were immersed in deionized water with the waste glass containing 238pu or 244Cmin a Pyrex glass container at 100 “C, and then they were decontaminated with dilute nitric acid. The quartz glass was found to have the smallest contamination of 238Pu and 244Cm. The adsorption amounts of 244Cm on the PFA Teflon and quartz glass were approximately the same, and for 238Pu the Teflon showed about twice the amount of adsorption as that measured on the quartz glass. The gold and stainless steel showed 30-40 times the adsorption amount of 23% as that on the quartz glass. Adsorption and desorption of curium and plutonium were discussed in relation with difference of materials, time dependence and acidity of leachate. Curium, which has been previously leached out from waste glasses, showed a relatively simple adsorption and desorption behavior. In the case of plutonium, the colloidal species would take a large part in the adsorption and desorption processes. The relationship between the ratio of the colloidal to the ionic species of plutonium and the adsorption-desorption behavior was discussed. The observation of alpha autoradiographs elucidated that the ionic adsorption was desorbed more easily than the colloidal one.
ABSTRACT.
INTRODUCTION
nuclides easily. The leach rates measured from leachate concentrations can be corrected by the amount of plutonium (or curium) adsorbed on the wall of the leach container. The Material Characterization Center has proposed to use a leach container made from PFA Teflon (5). However, this selection of the material is not supposed to be done on the basis of the data on adsorptive properties of actinides. In this paper, the adsorption behavior on the surface of various leach container materials of plutonium and curium from the leachate was investigated in order to select a suitable material for the leach container and to make leaching data more accurate. Then we studied in detail the adsorption of 23% and 244Cm on the surface of quartz glass which was found to be the most suitable material for a leach container.
Since the actinide elements, such as plutonium and curium, have a potential hazard over a long period of tim’e, the leaching behavior of these actinides from high level radioactive waste forms has been studied intensively to evaluate the confinement ability of waste (1,2). However, some difficulties have been pointed out in obtaining accurate data in leaching experiments due to the large adsorption property of the actinides (3,4). The plutonium and curium leached into the solution would be adsorbed on the wall of leach containers during leaching of the waste glasses. Therefore, in the case of leaching experiments for the waste glass containing plutonium and curium, it would be essential to use a material which adsorbs a small amount of plutonium and curium from leachates and/or desorbs these RECEIVED 8 MAY
1987; ACCEPTED 2 NOVEMBER 1987.
EXPERIMENTAL
- The authors would like to thank Mr. Y. Tamura, T. Miyazaki, 0. Kikuchi, K. Futakami and Y. Kato for their technical assistance. It is also a pleasure to acknowledge the comments of Dr. S. Nakashima and Mr. H. Mitamura on the manuscript. Acknol+~lcdgements
Materials
The compositions 45
of waste glasses used for this
T. BANBA
46 TABLE Composition Content Component Additive SiO, B,G, Al@, CaO Na,O ZnO L&O
Waste Rb,O SrO y,o:, ZrO, Moo, MnO, Ag,G Cd0 SnO, Sb,G,
Content
Waste-Glass-l
Waste-Glass-2
45.2 13.9 4.9 4.0 9.8 2.5 2.0
45.2 13.9 4.9 4.0 9.8 2.5 2.0
Component
Containing
Adsorption
~,Gll Nd,G:, Sm@,, Bu,Gz GdtG, SeO, RuO, Fe@,, NiO Cr& P,G,
fiO2 CmO,
0.23 0.98 0.62 0.26 0.52 0.26 0.85
0.29 0.06 0.03 0.02 0.95 2.90 0.40 0.50 0.30
0.17 0.03 0.02 0.02 0.95 2.90 0.40 0.50 0.30
1.35 -
2.96
-
of Waste Glass 238Pu or 244Cm
Test for wartz
Glass
at Various Periods in Leachate
(lOO°C, 2 days)
(lOO'C, 1 hr to 5 days)
and Measurement /Zac
of
h at e
(pH meter. Gas-flow
Proportional
Alpha Autoradiography)
Desorption
Test (0.2H HN03 and D.2H HN03 + 0.02M HF Solution. 1 day, TOM
Observation
anb Measurement
temperature)
of
Test Pieces (Gas-flow Proportional
Counter,
Alpha Autoradiography)
FIGURE
0.98 0.62 0.45 0.90 0.44 1.48
A flow sheet of experimental procedures is shown in Fig. 1. Five pieces of each test material were
Adsorption
Test Pie:
Waste-Glass-2
Procedures
in
Observatfon
(wt%)
container material. These materials were purified by ultrasonic washing with acetone, ethyl-alcohol and finally distilled water.
Test for
Various Materials
I
0.23
La@, CeO,
0.12 0.34 0.10 2.64 1.73 0.26 0.03 0.03 0.02 0.004
Preparation
Waste-Glass-
Waste TeO, cs,o BaO
study are shown in Table 1. Waste-glass-l and Waste-glass-2 contain 1.35 wt% plutonium oxide (231’u: 6.7 x 10’ Bq/g-glass) and 2.96 wt% curium oxide (244Cm: 4.03 x 10” Bq/g-glass), respectively. Plates (10 mm x 10 mm x 1 mm) of quartz glass (> 99 wt?J SiOJ, PFA Teflon, stainless steel (SUS 304L), and gold were tested as candidates of the
LLeachate
1
of Simulated High-Level Waste Glasses
(wt%)
0.12 0.34 0.18 2.64 1.73 0.26 0.03 0.03 0.02 0.004
ET AL.
1. Flow sheet for experimental
procedure.
Counter,
ADSORPTION
ON VARIOUS
LEACH
CONTAINER
MATERIALS
FIGURE 2. Apparatus for adsorption experiment.
immersed with a disc of waste glass in 1000 mL of distilled water in a Pyrex glass flask at 100°C for a period of two days. The test apparatus was placed in a glove box (see Fig. 2). This adsorption procedure was applied to the Waste-glass-l and Wasteglass-2, respectively. The radioactivity of both sides of the test pieces was measured with a gas flow proportional counter. The concentration of 23% or 24Tm, in each leachate, was calculated to be 1.4 x 10’ Bq/mL (5.7 x lo-‘M), 5.2 x lo2 Bq/mL (1.4 x lo-’ M), respectively, from the measurement of the alpha activity of an aliquot of the leachate. In order to study the desorption behavior of plutonium and curium, the test pieces which had adsorbed 23% or 244cm from the leachate were immersed in a solution containing 0.2 M HN03 at room temperature for a period of one day, and then in another solution containing 0.2 M HN03 and 0.02 M HF under the same conditions. In further experiments, three pieces, each of the quartz glass, were immersed with a disc of waste glass in 1000 mL of distilled water at 100°C for various periods (Fig. 1). The immersion period varied from 1 hour to 5 days. Thirty mL of leachate was also taken out for the measurement of the alpha activity and pH at each period. The tests were carried out for the Waste-glass-l and Waste-glass-2, respectively. Desorption in 0.2 M HN03 solution was also carried out. The pH was measured with a pH meter (TOA Electronics Ltd, Model HM-IOK). The distributions of “?Pu and 244Cm, adsorbed on test pieces from leachates, were examined by the alpha autoradiography (6) with cellulose nitrate films (KODAK, CNSS).
RESULTS AND DISCUSSION Adsorption and Desorption of Plutonium and Curium on Various Materials
Figure 3 shows the results of the adsorption and desorption of plutonium and curium on the test pieces. The pH values of leachates of the Wasteglass-l containing 23?Pu and of the Waste-glass-2 containing 244cm are 6.6 and 7.0, respectively. In Fig. 3, the figures represent the radioactivity (102Bq/cm2) of 239u or 244cm adsorbed initially on the surface of various materials and those remaining after the desorption. For all the materials, the adsorption amount of 244Cm is smaller than that of 23!Pu. The adsorption amount of 244Cmon the quartz glass is approximately the same as that on the PFA Teflon. On the other hand, in the case of 238pu, the quartz glass has the smallest amount of adsorption, and the Teflon shows about twice the amount of adsorption, as much as that measured on the quartz glass. The metals such as gold and stainless steel show 30-40 times the adsorption amount of 23?u as that on the quartz glass. The desorption ratio for 23%r and 24Bcm on the test pieces are summarized in Table 2. The desorption ratio is calculated by the following equation:
Desorption
ratio =
Desorption of z3xPu or “““Cm (Bq/cm’) Initial adsorption of *3sPu or 244Cm(Bq/cm2) x lOO(%).
Concerning the desorption of curium, 6O-80% of curium adsorbed on the quartz glass and Teflon are
48
T. BANBA ET AL
0
Pu Cm Quartz Glass
El : Before desorption
Pu Cm PFA Teflon
Pu
Cm
Pu Cm Stainless Steel
Gold
• lIil : After desorption by HNOs
El : After desorption by (HNOs + HF)
FIGURE 3. Radioactivity of 238puand *44cm adsorbed on various materials in leachates and remaining after the treatment ofdesorption.
desorbed by immersing them in an acid solution containing 0.2 M HN03, while the desorption ratios for metals are lower (20-40%). The curium still remaining on the test pieces is further desorbed by another acid solution containing 0.2 M HNOB and 0.02 M HF. As a result, it is found that the desorption ratio reaches more than 90% in every case. These results mean that the most part of 24“Cm adsorbed on various materials from the leachate is easily desorbed by the simple procedure. Therefore, one could obtain the leaching data without the effect of the curium sorption by rinsing the container with dilute acid solution, even though any of four materials is used for a leach container. Concerning plutonium, in general, it appears that the desorption is not easy. In the case of quartz glass, which has the smallest amount of adsorption in all test materials, no more than 51.4% of plutonium is desorbed by two stages of the desorbing procedure. Although 96.4% desorption ratio is obtained in the case of gold, the activity remaining TABLE Desoration
is about twice as much as that on the quartz glass because of a large amount of initial adsorption of plutonium (Fig. 3). As can be seen by the results presented above, the adsorption-desorption behavior differs between curium and plutonium. Therefore, further experiments were conducted to clarify the adsorptiondesorption behavior by using the quartz glass, which adsorbs the smallest amount of plutonium and curium. Time Dependence Glass
oj’Curium Adsorption
on Quartz
The concentration of curium and pH in leachate are shown in Fig. 4. In 5 days the concentration of curium and the value of pH increased from 1.9 x 10’ Bq/mL to 1.1 x lo3 Bq/mL and from 6.0 to 7.5, respectively. The increases of curium concentration and pH result from the release of curium and alkali metals such as sodium in the Waste-glass-2 into leachant. The adsorption ratio of curium from 2
Ratio C%) of *T’u and *“Em for Various
Materials
Quartz Glass
WA Teflon
Gold
Stainless Steel
HNO:,
76.2
61.6
27.2
38.6
HNO:, + HF
98.1
90.4
95.9
90.0
HNO:,
50.0
66.7
67.4
25.0
HNO,, + HF
51.4
64.2
96.4
77.8
49
ADSORPTION ON VARIOUS LEACH CONTAINER MATERIALS
f-
lo3
I
I
uq
!j 0;
lo2 -
I
80'
100
(hr)
FIGURE 5. Adsorption rate of *44cm on the quartz glass. Error bars show the standard deviation of each plot.
I 0,
6.0 10
Time
100
(hr1
FIGURE 4. Concentration of *%rn and pH of leachate used for adsorption test. Plots indicate that the standard deviation is within the size of the plots.
leachate on the quartz glass is plotted against the time (Fig. 5). The adsorption ratio is defined by the following equation.
Adsorption
10
Time
ratio =
Surface concentration 244Cm(Bq/cm’)
of
Concentration of 244Cm in leachate (Bq/mL) The adsorption ratio of curium decreases with the of adsorption time (i.e., increasing concentration curium in leachate) (Fig. 4). This means that the adsorption amount of 24%m on the quartz glass is approximately constant in spite of the increase of curium concentration and pH in leachate. Alpha autoradiographs of 244cm, adsorbed on the surface of quartz glass, show an almost homogeneous alpha track distribution (Fig. 6), which is considered to indicate that the curium is adsorbed on the surface of quartz glass as ionic species as reported by Ichikawa et al. (7). This fact suggests that an ionic adsorption is predominant in the case of curium adsorption in leachate.
Time Dependence of Plutonium Adsorption on Quurtz Glass The concentration of plutonium and pH in leachate, and the adsorption rate of plutonium on the quartz glass are shown in Fig. 7. The properties of leachate are similar to those in the case of curium. The adsorption amount of plutonium increases with the adsorption time. Figure 8 shows the adsorption ratio of plutonium from leachate on the quartz glass in 5 days. The adsorption ratio of plutonium is approximately constant within 50 h and then increases rapidly. The constant adsorption ratio means that the adsorption of plutonium increases in proportion to the concentration of plutonium in leachate (Fig. 7). The increase in the adsorption ratio curve after 50 h is considered to be due to the following effects. The concentration of plutonium in leachate shows a low increasing rate after 50 h (Fig. 7) and the increasing rates of plutonium adsorption does not change in this region (Fig. 7) because of a time lag of the adsorption equilibrium. In fact, Ichikawa et al. has reported that at least a period of two days is required for the adsorption of plutonium to reach an equilibrium (7). Alpha autoradiographs of 23Kpuadsorbed on the surface of quartz glass are shown in Fig. 9. Some “stars” are observed in the autoradiographs. These “stars” are also observed on the other surfaces, (e.g., PFA Teflon and gold) as can be seen from Fig. 10. The origin of “stars” is considered to be plutonium adsorbed on the surface of a foreign contaminant, (i.e., pseudocolloids) because the size of “star” (-50 Frn diameter) is larger than that of plutonium polymer (<5.3 pm> (8) in aqueous solution and because in leachate there should be minute solid impurities, such as precipitates composed of leached elements. Ichikawa et al. have reported the same interpretation concerning the “stars” (7). The “stars” do not appear within 5 h and increase
50
T. BANBA
ET AL
FIGURE 6. Alpha autoradiographs of W2rn adsorbed on the surface of quartz glass in leachate for various periods of adsorption. Etching time is 10 minutes and exposure time is 3 hours.
rapidly at 10 h and then the number of “stars” keep the same level. This result implies that the pseudocolloids of plutonium occur rapidly at around 10 h, (i.e., at the pH of around 6.5) and then they do not increase. It is one of the reasons that the adsorption of plutonium cannot be explained by Freundlich’s isotherm (9).
Desorption Glass
of Curium
and Plutonium
on Quartz
Curium or plutonium adsorbed on the quartz glass for various lengths of time is desorbed by a nitric acid solution (0.2 M HNOJ. The results are shown in Fig. 11. Although the curium adsorbed on the quartz glass is desorbed more easily than the
ADSORPTION
ON VARIOUS
LEACH CONTAINER
MATERIALS
51
80
6.0 0
1
IO Ttme
100 (hr)
Time
(hr)
FIGURE 7. Concentration of 238puand pH of Ieachate used for adsorption test, and adsorption rate of “%‘u on the surface of quartz glass.Error bars show the standard deviation of each plot. Plots without bars indicate that the standard deviation is within the size of the plots.
plutonium, its desorption ratio decreases with increasing adsorption time. In the case of plutonium, there exists a minimum of the desorption ratio curve at the adsorption time around 10 h. This phenomenon should be related to different types of plutonium adsorption which can be observed from the alpha autoradiographs. The one occurs at low pH predominantly. This type of adsorption is distributed homogeneously on the surface of adsorbent and it is probably an ionic adsorption (7,lO). The other type of adsorption appears as the “star” in the autoradiograph and it mainly occurs at the pH of
Time
(hrl
FIGURE 8. Adsorption rate of *38p, on the quartz glass. Error bars show the standard deviation of each plot.
6.5. It seems that this type of adsorption is pseudocolloidal (7) as mentioned above. Although there should be the adsorption of the colloidal plutonium hydroxide (called the true colloid) in the pH region from 8 to 9 (7,10), it is not evident in the present work whether it effects the adsorption on the surface of quartz glass or not. Figure 12 shows the relations between adsorption time and the number of alpha tracks of 238pumeasured from the autoradiographs of the quartz glass. The number of total tracks increases with the adsorption time. This is consistent with the measurement of the alpha activity shown in Fig. 7. On the other hand, the number of alpha tracks inside the “stars” is very few until 5 h of the adsorption time and increases rapidly at 10 h, and then it keeps approximately constant. Therefore, the ratio of the number of alpha tracks inside the “stars” to that of total alpha tracks has a maximum at 10 h. In Fig. 11, the minimum desorption ratio of plutonium is observed at the same adsorption time (around 10 h). These results imply that an ionic adsorption of plutonium is desorbed more easily than a pseudocolloidal one. Such undesorbability of colloidal adsorption has been also observed in the case of protactinium by Starik et al. (11). Figure 13 shows the autoradiographs of 23% on the quartz glass before and after the desorption. It is clear that the homogeneous tracks (ionic adsorption) decrease after the desorption. While the tracks in the “stars”
T. BANBA ET AL.
1 hr
2 hr
FIGURE 9. Alpha autoradiographs of 23% adsorbed on the surface Etching time is 10 minutes and exposure time is 24 hours.
(pseudocolloidal much.
adsorption)
do not change
very
CONCLUSIONS We have studied the adsorption-desorption behavior on various leach container materials of plu-
of quartz
glass
in leachate
for various
periods
of adsorption,
tonium and curium in the leachate. The results are as follows: 1. The amount of plutonium adsorbed on a quartz glass is the smallest, and is about half of that on PFA Teflon. Metals such as gold and stainless steel show more than 30 times adsorption amount of that on the quartz glass.
ADSORPTION
ON VARIOUS LEACH CONTAINER
MATERIALS
FIGURE 10. Alpha autoradiographs of ‘Tu adsorbed on the surfaces of PPA Teflon 48 hours. Etching time is 10 minutes and exposure time is 1 h.
2.
The adsorption of curium is observed as a homogeneous distribution which is desorbed easily. There is little difference of the amount of adsorption among the adsorbent materials. 3. Two types of plutonium adsorption are observed. One is an ionic adsorption which is distributed homogeneously on the surface of materials. ‘The other is a colloidal adsorption which appears as the “stars” in alpha autoradiographs. The ionic adsorption of plutonium is desorbed more easily than the colloidal one.
53
(A) and gold @I in leachates. Adsorption time is
concentration of radionuclide, pH and solution composition are varied with time) for the demonstration. We will continue testing to clarify the effects of the leachate composition on the sorption of plutonium and curium with a control on the sorption parameters.
70 25\”
In the present work, we carried out the adsorption tests under the dynamic condition (i.e., the
60 zz 50$ .-c 40 y” :
30 ; 20 10
1
/
,,,I
10
Time
2
IIll
100
(hr)
FIGURE 11. Relation between adsorption time and desorption ratio of ‘““Cm and z3Tu adsorbed on the quartz glass. Condition of desorption: immersion in 0.2 M HNOs for 1 day. Error bars show the standard deviation of each plot.
100
50 Time
‘;; .I2 ‘ctz
0
(hr)
FIGURE 12. Relations between adsorption time and the number of alpha tracks of 23% on the surface of quartz glass. Open circle represents the number of total tracks, black circle represents the number of tracks in the “stars,” and triangle represents the ratio of the latter to the former.
T. BANBA ET AL.
54
FIGXJRE 13. Alpha autoradiographs HNOzj (0.2 M) for I day.
of quartz glass immersed in leachate for IO hours before (A) and after (B) desorption of ?u
in
REFERENCES 1. Bates, J. K., Lam, D. J., and Steindler, M. J. Extended leach studies of actinide-doped SRL 131 glass. In: Scienrific Basis for Nuclear Wasre Management VI, Brookins, D. G. ed. p. 183. North-Holland, New York (1983). 2. Rees, T. F., Cleveland, J. M., and Nash, K. L. Leaching of plutonium from a radioactive waste glass by eight groundwaters from the western united states. Nucl. Technol. 70: 133 (1985). 3. Apted. M. J., McVary, G. L., and Wald, J. W. Release of actinides from defense waste glass under simulated repository conditions. Nucl. Technol. 73: 165 (1986). 4. Schramke, J. A., Simonson, S. A., and Coles, D. G. *?Ip and “9pu solution behavior during hydrothermal testing of simulated nuclear waste glass with basalt and steel. In: Scientific Basis for Nuclear Waste Management VIII. Jantzen, C. M., Stone, J. A., and Ewing, R. C. eds.. p. 343. Materials Research Society, Pittsburgh (1985). 5. Mendel, J. E. Nuclear waste materials handbook. DOE/TIC- 11400, Battelle Pacific Northwest Laboratories, Richland, WA (1982).
6. Nakamura, H. and Tashiro, S., eds., Progress report on safety research of high-level waste management for the period April 1985 to March 1986. JAERI-M 86-131, Japan Atomic Energy Research Institute, Tokai. Ibaraki, Japan (1986). 7. Ichikawa, F. Studies of the behavior of carrier-free radioisotopes VII. Adsorption and desorption of plutonium(IV) on glass and polyethylene. Radiochim. Acta. 22: 97 (1975). 8. Ichikawa, F. and Sato, T. On the particle size distribution of hydrolyzed plutonium(IV) polymer. J. Radiounal. Nacl.
Chem. 84: 269 (1984). 9. Zimon, A. D. Physical Chemistry for Radioactive Contamination and Decontaminarion. Atomizdat, Moscow, USSR (1975). 10. Grebenschikova, V. I. and Davydov, Y. P. Radiokhimiya 3: 155 (1961). 11. Starik, I. E., Sheidina, L. D., and Irumenkova, L. 1. Radiokhimiya I: 168 (1959).
Vol. 8, pp. 45-54,
1988 Copyright
0191-X15X/88 $3.00 + .OO i 1988 Pergamon Press plc
ADSORPTION ON VARIOUS LEACH CONTAINER MATERIALS OF PLUTONIUM AND CURIUM LEACHED FROM NUCLEAR WASTE GLASSES
Tsunetaka Banba, Shingo Tashiro, Kiyoshi Nukaga, and Masayuki Nomura Deprrrtmrnt
of Environmrntui
Sufety
Reseurch,
Japan
Atomic
Energy
Research
Tamio Sagawa, Institute,
To&,
Ibaruki
319-l 1,
.Iupun
The adsorption tendency of leached-out ““a and 244Cm on the wall of leach containers, during the leaching of waste glasses, was studied. The test pieces of quartz glass, PFA Teflon, gold and stainless steel, which are candidates for a leach container, were immersed in deionized water with the waste glass containing 238pu or 244Cmin a Pyrex glass container at 100 “C, and then they were decontaminated with dilute nitric acid. The quartz glass was found to have the smallest contamination of 238Pu and 244Cm. The adsorption amounts of 244Cm on the PFA Teflon and quartz glass were approximately the same, and for 238Pu the Teflon showed about twice the amount of adsorption as that measured on the quartz glass. The gold and stainless steel showed 30-40 times the adsorption amount of 23% as that on the quartz glass. Adsorption and desorption of curium and plutonium were discussed in relation with difference of materials, time dependence and acidity of leachate. Curium, which has been previously leached out from waste glasses, showed a relatively simple adsorption and desorption behavior. In the case of plutonium, the colloidal species would take a large part in the adsorption and desorption processes. The relationship between the ratio of the colloidal to the ionic species of plutonium and the adsorption-desorption behavior was discussed. The observation of alpha autoradiographs elucidated that the ionic adsorption was desorbed more easily than the colloidal one.
ABSTRACT.
INTRODUCTION
nuclides easily. The leach rates measured from leachate concentrations can be corrected by the amount of plutonium (or curium) adsorbed on the wall of the leach container. The Material Characterization Center has proposed to use a leach container made from PFA Teflon (5). However, this selection of the material is not supposed to be done on the basis of the data on adsorptive properties of actinides. In this paper, the adsorption behavior on the surface of various leach container materials of plutonium and curium from the leachate was investigated in order to select a suitable material for the leach container and to make leaching data more accurate. Then we studied in detail the adsorption of 23% and 244Cm on the surface of quartz glass which was found to be the most suitable material for a leach container.
Since the actinide elements, such as plutonium and curium, have a potential hazard over a long period of tim’e, the leaching behavior of these actinides from high level radioactive waste forms has been studied intensively to evaluate the confinement ability of waste (1,2). However, some difficulties have been pointed out in obtaining accurate data in leaching experiments due to the large adsorption property of the actinides (3,4). The plutonium and curium leached into the solution would be adsorbed on the wall of leach containers during leaching of the waste glasses. Therefore, in the case of leaching experiments for the waste glass containing plutonium and curium, it would be essential to use a material which adsorbs a small amount of plutonium and curium from leachates and/or desorbs these RECEIVED 8 MAY
1987; ACCEPTED 2 NOVEMBER 1987.
EXPERIMENTAL
- The authors would like to thank Mr. Y. Tamura, T. Miyazaki, 0. Kikuchi, K. Futakami and Y. Kato for their technical assistance. It is also a pleasure to acknowledge the comments of Dr. S. Nakashima and Mr. H. Mitamura on the manuscript. Acknol+~lcdgements
Materials
The compositions 45
of waste glasses used for this
T. BANBA
46 TABLE Composition Content Component Additive SiO, B,G, Al@, CaO Na,O ZnO L&O
Waste Rb,O SrO y,o:, ZrO, Moo, MnO, Ag,G Cd0 SnO, Sb,G,
Content
Waste-Glass-l
Waste-Glass-2
45.2 13.9 4.9 4.0 9.8 2.5 2.0
45.2 13.9 4.9 4.0 9.8 2.5 2.0
Component
Containing
Adsorption
~,Gll Nd,G:, Sm@,, Bu,Gz GdtG, SeO, RuO, Fe@,, NiO Cr& P,G,
fiO2 CmO,
0.23 0.98 0.62 0.26 0.52 0.26 0.85
0.29 0.06 0.03 0.02 0.95 2.90 0.40 0.50 0.30
0.17 0.03 0.02 0.02 0.95 2.90 0.40 0.50 0.30
1.35 -
2.96
-
of Waste Glass 238Pu or 244Cm
Test for wartz
Glass
at Various Periods in Leachate
(lOO°C, 2 days)
(lOO'C, 1 hr to 5 days)
and Measurement /Zac
of
h at e
(pH meter. Gas-flow
Proportional
Alpha Autoradiography)
Desorption
Test (0.2H HN03 and D.2H HN03 + 0.02M HF Solution. 1 day, TOM
Observation
anb Measurement
temperature)
of
Test Pieces (Gas-flow Proportional
Counter,
Alpha Autoradiography)
FIGURE
0.98 0.62 0.45 0.90 0.44 1.48
A flow sheet of experimental procedures is shown in Fig. 1. Five pieces of each test material were
Adsorption
Test Pie:
Waste-Glass-2
Procedures
in
Observatfon
(wt%)
container material. These materials were purified by ultrasonic washing with acetone, ethyl-alcohol and finally distilled water.
Test for
Various Materials
I
0.23
La@, CeO,
0.12 0.34 0.10 2.64 1.73 0.26 0.03 0.03 0.02 0.004
Preparation
Waste-Glass-
Waste TeO, cs,o BaO
study are shown in Table 1. Waste-glass-l and Waste-glass-2 contain 1.35 wt% plutonium oxide (231’u: 6.7 x 10’ Bq/g-glass) and 2.96 wt% curium oxide (244Cm: 4.03 x 10” Bq/g-glass), respectively. Plates (10 mm x 10 mm x 1 mm) of quartz glass (> 99 wt?J SiOJ, PFA Teflon, stainless steel (SUS 304L), and gold were tested as candidates of the
LLeachate
1
of Simulated High-Level Waste Glasses
(wt%)
0.12 0.34 0.18 2.64 1.73 0.26 0.03 0.03 0.02 0.004
ET AL.
1. Flow sheet for experimental
procedure.
Counter,
ADSORPTION
ON VARIOUS
LEACH
CONTAINER
MATERIALS
FIGURE 2. Apparatus for adsorption experiment.
immersed with a disc of waste glass in 1000 mL of distilled water in a Pyrex glass flask at 100°C for a period of two days. The test apparatus was placed in a glove box (see Fig. 2). This adsorption procedure was applied to the Waste-glass-l and Wasteglass-2, respectively. The radioactivity of both sides of the test pieces was measured with a gas flow proportional counter. The concentration of 23% or 24Tm, in each leachate, was calculated to be 1.4 x 10’ Bq/mL (5.7 x lo-‘M), 5.2 x lo2 Bq/mL (1.4 x lo-’ M), respectively, from the measurement of the alpha activity of an aliquot of the leachate. In order to study the desorption behavior of plutonium and curium, the test pieces which had adsorbed 23% or 244cm from the leachate were immersed in a solution containing 0.2 M HN03 at room temperature for a period of one day, and then in another solution containing 0.2 M HN03 and 0.02 M HF under the same conditions. In further experiments, three pieces, each of the quartz glass, were immersed with a disc of waste glass in 1000 mL of distilled water at 100°C for various periods (Fig. 1). The immersion period varied from 1 hour to 5 days. Thirty mL of leachate was also taken out for the measurement of the alpha activity and pH at each period. The tests were carried out for the Waste-glass-l and Waste-glass-2, respectively. Desorption in 0.2 M HN03 solution was also carried out. The pH was measured with a pH meter (TOA Electronics Ltd, Model HM-IOK). The distributions of “?Pu and 244Cm, adsorbed on test pieces from leachates, were examined by the alpha autoradiography (6) with cellulose nitrate films (KODAK, CNSS).
RESULTS AND DISCUSSION Adsorption and Desorption of Plutonium and Curium on Various Materials
Figure 3 shows the results of the adsorption and desorption of plutonium and curium on the test pieces. The pH values of leachates of the Wasteglass-l containing 23?Pu and of the Waste-glass-2 containing 244cm are 6.6 and 7.0, respectively. In Fig. 3, the figures represent the radioactivity (102Bq/cm2) of 239u or 244cm adsorbed initially on the surface of various materials and those remaining after the desorption. For all the materials, the adsorption amount of 244Cm is smaller than that of 23!Pu. The adsorption amount of 244Cmon the quartz glass is approximately the same as that on the PFA Teflon. On the other hand, in the case of 238pu, the quartz glass has the smallest amount of adsorption, and the Teflon shows about twice the amount of adsorption, as much as that measured on the quartz glass. The metals such as gold and stainless steel show 30-40 times the adsorption amount of 23?u as that on the quartz glass. The desorption ratio for 23%r and 24Bcm on the test pieces are summarized in Table 2. The desorption ratio is calculated by the following equation:
Desorption
ratio =
Desorption of z3xPu or “““Cm (Bq/cm’) Initial adsorption of *3sPu or 244Cm(Bq/cm2) x lOO(%).
Concerning the desorption of curium, 6O-80% of curium adsorbed on the quartz glass and Teflon are
48
T. BANBA ET AL
0
Pu Cm Quartz Glass
El : Before desorption
Pu Cm PFA Teflon
Pu
Cm
Pu Cm Stainless Steel
Gold
• lIil : After desorption by HNOs
El : After desorption by (HNOs + HF)
FIGURE 3. Radioactivity of 238puand *44cm adsorbed on various materials in leachates and remaining after the treatment ofdesorption.
desorbed by immersing them in an acid solution containing 0.2 M HN03, while the desorption ratios for metals are lower (20-40%). The curium still remaining on the test pieces is further desorbed by another acid solution containing 0.2 M HNOB and 0.02 M HF. As a result, it is found that the desorption ratio reaches more than 90% in every case. These results mean that the most part of 24“Cm adsorbed on various materials from the leachate is easily desorbed by the simple procedure. Therefore, one could obtain the leaching data without the effect of the curium sorption by rinsing the container with dilute acid solution, even though any of four materials is used for a leach container. Concerning plutonium, in general, it appears that the desorption is not easy. In the case of quartz glass, which has the smallest amount of adsorption in all test materials, no more than 51.4% of plutonium is desorbed by two stages of the desorbing procedure. Although 96.4% desorption ratio is obtained in the case of gold, the activity remaining TABLE Desoration
is about twice as much as that on the quartz glass because of a large amount of initial adsorption of plutonium (Fig. 3). As can be seen by the results presented above, the adsorption-desorption behavior differs between curium and plutonium. Therefore, further experiments were conducted to clarify the adsorptiondesorption behavior by using the quartz glass, which adsorbs the smallest amount of plutonium and curium. Time Dependence Glass
oj’Curium Adsorption
on Quartz
The concentration of curium and pH in leachate are shown in Fig. 4. In 5 days the concentration of curium and the value of pH increased from 1.9 x 10’ Bq/mL to 1.1 x lo3 Bq/mL and from 6.0 to 7.5, respectively. The increases of curium concentration and pH result from the release of curium and alkali metals such as sodium in the Waste-glass-2 into leachant. The adsorption ratio of curium from 2
Ratio C%) of *T’u and *“Em for Various
Materials
Quartz Glass
WA Teflon
Gold
Stainless Steel
HNO:,
76.2
61.6
27.2
38.6
HNO:, + HF
98.1
90.4
95.9
90.0
HNO:,
50.0
66.7
67.4
25.0
HNO,, + HF
51.4
64.2
96.4
77.8
49
ADSORPTION ON VARIOUS LEACH CONTAINER MATERIALS
f-
lo3
I
I
uq
!j 0;
lo2 -
I
80'
100
(hr)
FIGURE 5. Adsorption rate of *44cm on the quartz glass. Error bars show the standard deviation of each plot.
I 0,
6.0 10
Time
100
(hr1
FIGURE 4. Concentration of *%rn and pH of leachate used for adsorption test. Plots indicate that the standard deviation is within the size of the plots.
leachate on the quartz glass is plotted against the time (Fig. 5). The adsorption ratio is defined by the following equation.
Adsorption
10
Time
ratio =
Surface concentration 244Cm(Bq/cm’)
of
Concentration of 244Cm in leachate (Bq/mL) The adsorption ratio of curium decreases with the of adsorption time (i.e., increasing concentration curium in leachate) (Fig. 4). This means that the adsorption amount of 24%m on the quartz glass is approximately constant in spite of the increase of curium concentration and pH in leachate. Alpha autoradiographs of 244cm, adsorbed on the surface of quartz glass, show an almost homogeneous alpha track distribution (Fig. 6), which is considered to indicate that the curium is adsorbed on the surface of quartz glass as ionic species as reported by Ichikawa et al. (7). This fact suggests that an ionic adsorption is predominant in the case of curium adsorption in leachate.
Time Dependence of Plutonium Adsorption on Quurtz Glass The concentration of plutonium and pH in leachate, and the adsorption rate of plutonium on the quartz glass are shown in Fig. 7. The properties of leachate are similar to those in the case of curium. The adsorption amount of plutonium increases with the adsorption time. Figure 8 shows the adsorption ratio of plutonium from leachate on the quartz glass in 5 days. The adsorption ratio of plutonium is approximately constant within 50 h and then increases rapidly. The constant adsorption ratio means that the adsorption of plutonium increases in proportion to the concentration of plutonium in leachate (Fig. 7). The increase in the adsorption ratio curve after 50 h is considered to be due to the following effects. The concentration of plutonium in leachate shows a low increasing rate after 50 h (Fig. 7) and the increasing rates of plutonium adsorption does not change in this region (Fig. 7) because of a time lag of the adsorption equilibrium. In fact, Ichikawa et al. has reported that at least a period of two days is required for the adsorption of plutonium to reach an equilibrium (7). Alpha autoradiographs of 23Kpuadsorbed on the surface of quartz glass are shown in Fig. 9. Some “stars” are observed in the autoradiographs. These “stars” are also observed on the other surfaces, (e.g., PFA Teflon and gold) as can be seen from Fig. 10. The origin of “stars” is considered to be plutonium adsorbed on the surface of a foreign contaminant, (i.e., pseudocolloids) because the size of “star” (-50 Frn diameter) is larger than that of plutonium polymer (<5.3 pm> (8) in aqueous solution and because in leachate there should be minute solid impurities, such as precipitates composed of leached elements. Ichikawa et al. have reported the same interpretation concerning the “stars” (7). The “stars” do not appear within 5 h and increase
50
T. BANBA
ET AL
FIGURE 6. Alpha autoradiographs of W2rn adsorbed on the surface of quartz glass in leachate for various periods of adsorption. Etching time is 10 minutes and exposure time is 3 hours.
rapidly at 10 h and then the number of “stars” keep the same level. This result implies that the pseudocolloids of plutonium occur rapidly at around 10 h, (i.e., at the pH of around 6.5) and then they do not increase. It is one of the reasons that the adsorption of plutonium cannot be explained by Freundlich’s isotherm (9).
Desorption Glass
of Curium
and Plutonium
on Quartz
Curium or plutonium adsorbed on the quartz glass for various lengths of time is desorbed by a nitric acid solution (0.2 M HNOJ. The results are shown in Fig. 11. Although the curium adsorbed on the quartz glass is desorbed more easily than the
ADSORPTION
ON VARIOUS
LEACH CONTAINER
MATERIALS
51
80
6.0 0
1
IO Ttme
100 (hr)
Time
(hr)
FIGURE 7. Concentration of 238puand pH of Ieachate used for adsorption test, and adsorption rate of “%‘u on the surface of quartz glass.Error bars show the standard deviation of each plot. Plots without bars indicate that the standard deviation is within the size of the plots.
plutonium, its desorption ratio decreases with increasing adsorption time. In the case of plutonium, there exists a minimum of the desorption ratio curve at the adsorption time around 10 h. This phenomenon should be related to different types of plutonium adsorption which can be observed from the alpha autoradiographs. The one occurs at low pH predominantly. This type of adsorption is distributed homogeneously on the surface of adsorbent and it is probably an ionic adsorption (7,lO). The other type of adsorption appears as the “star” in the autoradiograph and it mainly occurs at the pH of
Time
(hrl
FIGURE 8. Adsorption rate of *38p, on the quartz glass. Error bars show the standard deviation of each plot.
6.5. It seems that this type of adsorption is pseudocolloidal (7) as mentioned above. Although there should be the adsorption of the colloidal plutonium hydroxide (called the true colloid) in the pH region from 8 to 9 (7,10), it is not evident in the present work whether it effects the adsorption on the surface of quartz glass or not. Figure 12 shows the relations between adsorption time and the number of alpha tracks of 238pumeasured from the autoradiographs of the quartz glass. The number of total tracks increases with the adsorption time. This is consistent with the measurement of the alpha activity shown in Fig. 7. On the other hand, the number of alpha tracks inside the “stars” is very few until 5 h of the adsorption time and increases rapidly at 10 h, and then it keeps approximately constant. Therefore, the ratio of the number of alpha tracks inside the “stars” to that of total alpha tracks has a maximum at 10 h. In Fig. 11, the minimum desorption ratio of plutonium is observed at the same adsorption time (around 10 h). These results imply that an ionic adsorption of plutonium is desorbed more easily than a pseudocolloidal one. Such undesorbability of colloidal adsorption has been also observed in the case of protactinium by Starik et al. (11). Figure 13 shows the autoradiographs of 23% on the quartz glass before and after the desorption. It is clear that the homogeneous tracks (ionic adsorption) decrease after the desorption. While the tracks in the “stars”
T. BANBA ET AL.
1 hr
2 hr
FIGURE 9. Alpha autoradiographs of 23% adsorbed on the surface Etching time is 10 minutes and exposure time is 24 hours.
(pseudocolloidal much.
adsorption)
do not change
very
CONCLUSIONS We have studied the adsorption-desorption behavior on various leach container materials of plu-
of quartz
glass
in leachate
for various
periods
of adsorption,
tonium and curium in the leachate. The results are as follows: 1. The amount of plutonium adsorbed on a quartz glass is the smallest, and is about half of that on PFA Teflon. Metals such as gold and stainless steel show more than 30 times adsorption amount of that on the quartz glass.
ADSORPTION
ON VARIOUS LEACH CONTAINER
MATERIALS
FIGURE 10. Alpha autoradiographs of ‘Tu adsorbed on the surfaces of PPA Teflon 48 hours. Etching time is 10 minutes and exposure time is 1 h.
2.
The adsorption of curium is observed as a homogeneous distribution which is desorbed easily. There is little difference of the amount of adsorption among the adsorbent materials. 3. Two types of plutonium adsorption are observed. One is an ionic adsorption which is distributed homogeneously on the surface of materials. ‘The other is a colloidal adsorption which appears as the “stars” in alpha autoradiographs. The ionic adsorption of plutonium is desorbed more easily than the colloidal one.
53
(A) and gold @I in leachates. Adsorption time is
concentration of radionuclide, pH and solution composition are varied with time) for the demonstration. We will continue testing to clarify the effects of the leachate composition on the sorption of plutonium and curium with a control on the sorption parameters.
70 25\”
In the present work, we carried out the adsorption tests under the dynamic condition (i.e., the
60 zz 50$ .-c 40 y” :
30 ; 20 10
1
/
,,,I
10
Time
2
IIll
100
(hr)
FIGURE 11. Relation between adsorption time and desorption ratio of ‘““Cm and z3Tu adsorbed on the quartz glass. Condition of desorption: immersion in 0.2 M HNOs for 1 day. Error bars show the standard deviation of each plot.
100
50 Time
‘;; .I2 ‘ctz
0
(hr)
FIGURE 12. Relations between adsorption time and the number of alpha tracks of 23% on the surface of quartz glass. Open circle represents the number of total tracks, black circle represents the number of tracks in the “stars,” and triangle represents the ratio of the latter to the former.
T. BANBA ET AL.
54
FIGXJRE 13. Alpha autoradiographs HNOzj (0.2 M) for I day.
of quartz glass immersed in leachate for IO hours before (A) and after (B) desorption of ?u
in
REFERENCES 1. Bates, J. K., Lam, D. J., and Steindler, M. J. Extended leach studies of actinide-doped SRL 131 glass. In: Scienrific Basis for Nuclear Wasre Management VI, Brookins, D. G. ed. p. 183. North-Holland, New York (1983). 2. Rees, T. F., Cleveland, J. M., and Nash, K. L. Leaching of plutonium from a radioactive waste glass by eight groundwaters from the western united states. Nucl. Technol. 70: 133 (1985). 3. Apted. M. J., McVary, G. L., and Wald, J. W. Release of actinides from defense waste glass under simulated repository conditions. Nucl. Technol. 73: 165 (1986). 4. Schramke, J. A., Simonson, S. A., and Coles, D. G. *?Ip and “9pu solution behavior during hydrothermal testing of simulated nuclear waste glass with basalt and steel. In: Scientific Basis for Nuclear Waste Management VIII. Jantzen, C. M., Stone, J. A., and Ewing, R. C. eds.. p. 343. Materials Research Society, Pittsburgh (1985). 5. Mendel, J. E. Nuclear waste materials handbook. DOE/TIC- 11400, Battelle Pacific Northwest Laboratories, Richland, WA (1982).
6. Nakamura, H. and Tashiro, S., eds., Progress report on safety research of high-level waste management for the period April 1985 to March 1986. JAERI-M 86-131, Japan Atomic Energy Research Institute, Tokai. Ibaraki, Japan (1986). 7. Ichikawa, F. Studies of the behavior of carrier-free radioisotopes VII. Adsorption and desorption of plutonium(IV) on glass and polyethylene. Radiochim. Acta. 22: 97 (1975). 8. Ichikawa, F. and Sato, T. On the particle size distribution of hydrolyzed plutonium(IV) polymer. J. Radiounal. Nacl.
Chem. 84: 269 (1984). 9. Zimon, A. D. Physical Chemistry for Radioactive Contamination and Decontaminarion. Atomizdat, Moscow, USSR (1975). 10. Grebenschikova, V. I. and Davydov, Y. P. Radiokhimiya 3: 155 (1961). 11. Starik, I. E., Sheidina, L. D., and Irumenkova, L. 1. Radiokhimiya I: 168 (1959).