Leaching behavior of carbon-14 contained in portland cement

CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 381-386, 1992. Printed in the USA. 0008-8846/92 $5.00 + .00. Copyright © 1992 Pergamon Press Ltd.

LEACHING PORTLAND

BEHAVIOR CEMENT

OF

CARBON-14

CONTAINED

IN

Tsunetaka BANBA, Junko MATSUMOTO, and Susumu MURAOKA Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan

ABSTRACT MCC-1 static leaching experiments were carried out for a cementitious waste form in distilled water for up to 64 days at 5°C and 20°C in order to examine the leaching behavior of carbon-14. The complicated leaching behavior of carbon-14, meaning that the leached carbon-14 activity did not increase with (time)0.5, was attributable to the precipitation of calcite and the formation of colloidal particles in leachates, which were mainly dependent on the pH value and calcium concentration of leachate. The normalized elemental mass loss of carbon-14 was about 7.5 x 10-4 g/cm 2 at 20°C for 64 days, which was lower than those of cement constitute elements such as calcium, sodium and aluminum. Especially, the leach rate of aqueous carbon-14 was lower than that of carbon-14 in the suspended leachate by a factor of about 10.

INTRODUCTION The low-level radioactive wastes contain radionuclides with long half lives such as carbon-14 and actinides. As Kalinin [1] says, carbon-14 is one of the most hazardous radionuclides with global distribution, and is subject to isolation from the biosphere. Carbon-14 contained in the low-level radioactive waste is produced in reactors primarily by neutron activation of 14N(n,p), 13C(n,~t) and 170(n,tx) present in the fuel and the coolant/moderator. According to Kunz [2], the chemical form of carbon-14 released from the light water reactor (LWR) has the difference between pressurized water reactor (PWR) and boiling water reactor (BWR); The carbon-14 discharge from PWR is predominantly 14CH4, 14C2H 6 and other hydrocarbon gases. The 14CO2 fraction is 10 - 26%. For BWR the gaseous discharge is 95% 14CO2, 5% hydrocarbon gases. Since carbon-14 has such volatile chemical forms, carbon-14 released with low-level liquid and solid wastes at LWRs has appeared to be a small fraction, <5% of the gaseous release. Martin [3] has reported that the carbon-14 concentration of the concentrated dirty liquid wastes from the reactor is 0.08 + 0.008 nCi/g [3.0 + 0.3 Bq/g]. If this liquid waste is solidified into a cementitious form with a cement-fine aggregate-water (waste) ratio of 1:2:0.55, the carbon-14 concentration of the cement product becomes about 0.01 nCi/g [0.4 Bq/g]. On the other hand, the data by Bush [4] have shown that the carbon-14 concentration of cement products produced by retention of the isotope in the dessolver off-gases of a LWR fuel reprocessing plant is 50 - 100 lxCi/g [(0.2 - 3.7)x106 Bq/g]. This value is about 20,000 times 381

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Vol. 22, Nos.213

,h:~t of the reactor waste. In Japan, it is predicted that such a waste will arise with the future ,~peration of full-scale reprocessing plant. Therefore, the prediction of carbon-14 release from the waste form will be necessary for the safety evaluation of low-level radioactive waste ~:2positories. The leaching data of cementitious waste forms containing carbon-14 are important for understanding and predicting the long-term leaching behavior of carbon-14 in cementitious waste forms. Nevertheless, there are very few studies about the leaching behavior of carbon-14 contained in cementitious waste forms. In the present paper, the leaching data of carbon-14, which is incorporated into the cement as Na2CO3, are examined in connection with those of other elements and pH values of ~:.~~hates.

E~PERIMENTAL The samples of cementitious waste form were prepared by the following procedure: ~ x t u r e s of ordinary portland cement, standard sand and water, which contained 8.5 mg N~214CO3 per liter (1.8 x 108 Bq in carbon-14 radioactivity per liter), with a mass ratio of 1 : 2 : 0.55 were cast into cylindrical molds of 32 mm in diameter by 20 mm long and cured at room temperature in the sealed mold for 28 days then removed from the molds and sealed in polypropylen jars until they were tested. For the chemical analysis, a 14C-free sample was prepared in the same way as the preparation of test samples. Table I shows the compositions of the upper and lower parts of the sample in terms of weight percent of each component and the average of them. According to these data, the macroscopic homogeneity of the sample is good enough for the tests. All the samples were assumed to be of the same composition with the analyzed sample. The leaching tests were carried out with the MCC-1 static leach test method [5]. The samples were put into a polyethylene container filled with distilled water for each leaching period. The surface area (geometric surface area) to solution volume ratio was 0.12 cm -l. The tests were conducted at temperatures of 5°C and 20°C for up to 64 days. The two separate samples were used for each run. The samples were air-dried after removing them from the leachate. X-ray diffraction patterns of the surface of 14C-free samples, which were subjected to the leach tests under the same conditions as the tests for 14C-doped samples, were obtained with a Rigaku diffractometer using copper radiation. A Table I . Composition (wt~) of cement product. part of the leachates was subjected to the analyses Component Upper * Lower ~ Average without the pretreatment and the remainder was SI02 65.25 64.42 64.83 centrifuged in order to Ca0 18.15 18.70 18.43 A1203 3.33 3.46 3.39 separate the colloidal K20 1.06 1.07 1.07 particles with above 0.5 ~tm Fe.O 3 1.00 1.02 1.11 in diameter. The particle 0.48 0.50 0.49 size was determined by the Sa20 O. 34 O. 35 O. 35 calculation [6]. The TiO 2 O. 17 O. 17 O. 17 carbon-14 concentrations of Mn0 0.05 0.06 0.06 the leachates before and after C02 2.17 1.90 2.04 centrifuging were SO3 O. 50 O. 50 O. 50 determined by using the Ig.-Loss 9.72 9.78 9.75 liquid scintillation counter (Aloka LSC-3000). The The upper and lower p a r t s of a sample were carbon dioxide concentration analyzed.

Vol. 22, Nos. 2/3

LEACHING,CARBON-14,CEMENT

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Fig.1. 14C and CO2 concentrations of leachates versus time at 5 and 20°C. Error bars attached to the average show the values of two sets of data in each run. of the leachates was measured by the carbon dioxide electrode (Orion Research Inc. MODEL 95-02). The calcium, silicon and sodium concentrations of the leachates were determined by the Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES: Shimadzu Co. GEW-170P) and Atomic Absorption Spectrometry (AAS: Shimadzu Co. AA-646). The pH values of the leachates were measured with the pH-meter (Horiba Inc. MODEL F-13 ). The leachates were handled in a carbon dioxide-free glove box, in which the atmosphere was replaced with argon gas. R E S U L T S AND DISCUSSION The carbon-14 and carbon dioxide concentrations of the leachates as a function of time at temperatures of 5°C and 20°C are shown in Fig.1. The carbon-14 is rapidly leached from the sample during the first 2 days. After 4 days of leaching, the carbon-14 leachate concentration decreases significantly and turns the increasing lO" 1 , , tendency again after 20 days. Since this behavior ! appears to be the same at temperatures of 5°C and 20°C, the leaching data at 20°C are discussed ea hereinafter. The drop in the carbon dioxide ~" 1°'2 ~ t ' ~ ' " = N'a concentration curve is visible in the region of low carbon-14 concentration, although a slight _ variation occurs. In Fig.2, the calcium, sodium,

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Fig.3. pH values of leachates versus time at 20°C. Error bars attached to the average show the values of two sets of data in each run.

384

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Vol. 22, Nos. 213

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silicon and aluminum concentrations of the leachates are plotted as a function of time at 20°C. The pH values of the leachates rise from approximately 12.2 at the first one day to 12.8 at 64 days as shown in Fig.3 and may be seen to increase with increases in the calcium, sodium and aluminum concentrations.

The complicated leaching behavior of carbon-14 can be accounted for as follows. The Fig.4. Parts of X-ray diffraction pattern from the capillary porosity in a concrete is precipitates on the surface of the 14C-free sample dependent on the water-cement leached at 20°C for 64 days. This pattern ratio [1]. During casting and includes calcite (C) and quartz (Q) phases. subsequent maturation, water is released and the water-cement ratio in the upper layers increases by comparison with the bulk, so when those layers set they become more porous. The higher the water-cement ratio, the more extended the layer with elevated porosity [1]. The initial increasing parts of carbon-14 concentration as well as other elements reflect the leaching from the surface layers. As the concentrations of calcium and carbon dioxide exceed the solubility product of CaCO3, the value of which is 10-9 - 10-8, owing to the significant release of calcium (Fig.2), the calcium carbonate containing carbon-14 precipitates in the leachate and the carbon-14 leachate concentration decreases after 4 days. The precipitation of calcium carbonate leads to a decrease in carbon dioxide and calcium leachate concentrations at the same periods. For example, the drops in carbon dioxide and calcium concentrations and pH of the 8 day-leachate imply that the amount of precipitates surpass those of carbon dioxide and calcium leached from the sample. The chemical equilibrium of this leachate provided by the geochemical code PHREEQE [7] also showed that the leachate was almost saturated with respect to the calcium carbonate (CaCO3). The presence of calcite in the precipitates has been confirmed by the X-ray diffraction analyses (Fig.4). The pH value of leachate rises with the leaching time (Fig.3), especially after 20 days the pH values are beyond 12.5. Under the high pH condition [8], the presence of colloidal particles in leachates may affect the carbon-14 concentration. The leaching behavior of carbon-14 will hereinafter be discussed from the standpoint of the colloid formation. 0

20.0

25.0 30.0 Degrees 28

35.0 (CuKa)

40.0

The carbon-14 and calcium leachate concentrations at 20°C before and after centrifuging are shown in Fig.5. As can be seen from Fig.5, actually the colloidal particles have been formed from the first day of leaching and almost all of them probably consist of calcium carbonates. Because concerning the other elements the difference in both the concentrations was hardly discernible. The concentration of aqueous carbon-14 is undoubtedly low and varies much less with time, compared with the carbon-14 concentration of leachate before centrifuging. These data are consistent with the interpretation of leaching behavior of carbon-14 as mentioned above. Namely the complicated curve of carbon-14 concentration of non-centrifuged leachates is attributable to the behavior of suspended colloidal particles. In the region of low carbon-14 concentration after 4 days, the suspended particles grow in the leachate with an increase in calcium concentration and then a large part of them precipitate.

Vol. 22, Nos. 213

LEACHING,CARBON-14,CEMENT

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Time [days]

Fig.5. ~4C and Ca concentrations of leachates at 20°C before and after centrifuging. Consequently, the carbon-14 concentration of suspended leachates decreases. A slow increase in carbon- 14 concentration after 32 days of leaching results from an increase in the amount of suspended particles due to the high pH of leachates. In this region the suspended particles probably consist of colloidal compounds such as Ca(OH)2 (portlandite) and CaO-SiO2-H20 (calcium silicate hydrate), because of the slow decreases in the calcium and silicon concentrations found during the periods of 32 to 64 days (Fig.2). It is possible to assume, therefore, that there must be partially adsorbed carbonate species (including 14C) present on the surface of the colloidal particles. The leach rate in the present study is represented by the normalized elemental mass loss:

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where NL i is the normalized elemental mass loss of element 'T' (g/cm2), Mi is the mass of element 'T' in the leachate (g), F i is the fraction of element 'T' in the pristine cementitious sample (unitless), and SA is the surface area (cm 2) of the sample. NL i indicates which mass of cement would dissolve, if the entire cement were to behave like the element i. For elements which dissolve congruently with the reacted cement mass, all of NL i must be equal. Figure 6 shows the normalized elemental mass losses of some typical elements at 20°C. The different NL i values for elements imply that the cementitious waste form dissolves incongruently. The dried cementitious product normally consists of a solid

~Aqueou~Ca c0 10.2

E '°

,

"~ 10-4 N ~ A q u e o u s Z

C-14

10"5 0

20 40 60 Time [days]

80

Fig.6. Normalized elemental mass losses of selected elements of the cement product in distilled water at 20°C with time. Error bars attached to the average show the values of two sets of data in each run.

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T. Bmaba et el.

Vol. 22, Nos. 213

matrix of hydrated calcium silicates, Ca(OH)2, aluminates and fine aggregates, and an aqueous pore fluid. The solid phase is not itself homogeneous [8]. Such a chemical nature of cements seems to cause the incongruent dissolution of the cementitious sample. The high leach rate (NL) of sodium may result from the release of pore fluid, which contains mainly Na + and K ÷ with OH- as the counter ion as reported by Glasser et al. [8]. The low leach rate of silicon can be attributed to the low solubility of quartz which is a main component of sand (about 93 wt%). The leach rate of carbon-14 appears to be relatively low, especially the leach rate of aqueous carbon-14 is comparable to that of silicon. This is due to the high pH and high calcium concentration of leachate as described previously. The leaching of carbon-14 contained in a cementitious waste form is affected by the chemistry of leachate as mentioned above. According to Ogawa [9], the carbon-14 was leached from the cementitious waste form with a diffusion control under the experimental condition of the IAEA recommended leach test method [10], which requires the renewal of leachant daily during the first week and at intervals of 7 days during the subsequent tests. However, the present results indicate that under the MCC-1 static leach test condition the saturation-controlled leaching behavior of carbon-14 results from the precipitation of calcium carbonate and the formation of colloidal particles in the leachate. Since it is considered that the repository condition, where the water could not be replaced frequently, is close to the MCC-1 condition, the long-term leaching rate of carbon-14 cannot be predicted by the previous ionic-transport models based on the diffusion process [11] alone. Therefore, it is necessary to develop the prediction model with speciation programs which are capable of evaluating solution compositions including pH, and solid phases precipitated from solution. ACKNOWLEDGEMENT We would like to thank K. Takashima, Y. Ohuchi and K. Kuramoto for the use of ICP-AES, AAS and X-ray diffractometer, respectively. We are also indebted to Y. Ono for the use of PHREEQE code. REFERENCES 1. N.N. Kalinin, A.N. Elizarova, and A.G. Tutov, Radiokhimiya, 31, 165 (1989). 2. C. Kunz, Health Physics, 49, 25 (1985). 3. J.E. Martin, Health Physics, 50, 57 (1986). 4. R.P. Bush et al., AERE-R 10543 (Revised) (1984). 5. Materials Characterization Center, Nuclear Waste Materials Handbook, DOE/TIC-11400 (1981). 6. F. Ichikawa and T. Sato, J. Radioanalytical and Nucl. Chem., 84, 269 (1984). 7. D.L. Parkhurst et al., PB81-167801 (1980). 8. F.P. Glasser et al., Mat. Res. Soc. Symp. Proc. Vol.44, Material Research Society, 849 (1985). 9. H. Ogawa et al., J. At. Energy Soc. Japan, 30, 684 (1988) (in Japanese). 10. E.D. Hespe, At. Energy Rev. 9, 195 (1971). 11. M.J. Bell, ORNL-TM-3232 (1971).