الفهرس 
1. Introduction
2. Disposal MediaOnly 14 pages are availabe for public view
 |  | from | 76 |  |  |
| | | from | 76 | | |
Abstract
EFFECT OF DIFFERENT MEDIA ON SOME PARAMETERS IN A SIMULATED DISPOSAL SITE K. El-Adham1, A.Y.El-Etre2, N.S. Mahmoud1, S.T.Atwa2, R. S. Zaky1 > 1.Nuclear and Radiological Regulatory Authority, Nasr City, Cairo 11762, Egypt1 2. Chemistry Department, Faculty of Science, Benha University, Benha, Egypt2 ABSTRACT. The present work studies the effect of chemical reaction on the migration behavior of radionuclides from a disposal site. The study considers the release from the cement blocks which will be investigated by the leaching tests, and the variation of five parameters; percentage change in weight, pH, Reduction potential (ORP), Conductivity (COND), and Total Dissolved Solids (TDS). These parameters are recorded as a function of time for blocks prepared with different water to cement ratios. These parameters are indicators of the change in the chemical reaction of the different medium. The experimental study is considered for 90 days. Keywords: Cement, disposal site, chemical reaction. INTRODUCTION Disposal is the emplacement of radioactive waste into a facility or a location with no intention of retrieving the waste. Disposal options are designed to contain the waste by means of passive engineered and natural features to isolate the waste from the accessible human environment to the extent necessitated to avoid associated hazard [1]. The safety of environment from a radioactive waste disposal site requires detailed knowledge about the possible release of radionuclides through the evaluation of safety performance of disposal. Mathematical models and computer codes are tools to evaluate the possible release rate of radionuclides from disposal site. Since the chemical reaction of radionuclides with the components of different media of the disposal site and with the surrounding environment are not considered in these mathematical models, the results obtained are significantly different from the real behavior of radionuclides. Therefore, the current study focuseson the effect of chemical reaction between different media (waste, cement, and soil) in the disposal system.
The study concentrates on two topics; the first is the release from the cement blocks which will be investigated by the leaching tests. The second is the migration of the released elements. This will be illustrated through studying the retention of the released elements by the soil in the hosting medium. The indicator parameters used to evaluate this study are; Reduction potential (ORP), Conductivity (COND), and Total Dissolved Solids (TDS) as a function of time for blocks with different water to cement ratios.
MATERIALS AND METHODS
Cement blocks were prepared with different water (distilled water (pH=6)) to cement (Portland cement) ratios (0.5, 0.6, 0.65, and 0.7). Some Cement blocks were prepared by mixing Cement with different salts (Fe as FeCl3 with concentrations 6.6×10-3 and 4.6×10-2 mole/L, Sr as SrCl2, with concentrations 1×10-3 and 1×10-2 mole/L and Co is added as CoCl2 with concentration 2.4×10-2 mole/L). The mixtures (Cement-salts) are poured into cubic molds with a volume of 1cm3, and cured for 28 days. X-Ray Diffraction (XRD) patterns are obtained by using PHILIPS X-ray unit (PW 1830), with diffractometer (PW 3710/31), and scintillation counter (PW 2563/00. Parameters such as pH, OPR, COND, and TDS are measured before and after 90 days using a digital pH-meter (Ultrameter TM, Myron L Company, USA) with an error of ± 0.01. The concentration of metals in the leachant is measured using Inductively Coupling Plasma (ICP) (Ultima 2 Jobin Yvon, France).
RESULTS AND DISCUSSIONS.
1- XRD Results.
Figures (1, 2), show the XRD patterns of cement and hydrated cement for 28 days, respectively. Results show that the composition of cement powder and hydrated cement is similar. The reaction of water with cement as shown in the equations leads to reducing the basal spacing [17]:
C3S + H2O C3S2Hx (Hydrated Gel) + Ca(OH)2 (1)
Primary Cementitious Product
Ca (OH) 2 Ca++ + 2(OH)- (2)
Ca++ + 2(OH)- + SiO2 (Soil Silica) CSH (3)
Secondary Cementitious Product
Ca++ + 2(OH)- + Al2O3 (Soil Alumina) CAH (4)
Also, it has been observed from XRD peaks that there is a change in the intensity and the number for cement before hydration [18, 19].
Figure (1). XRD Pattern of Cement Figure (2). XRD Pattern Hydrated Cement
The XRD patterns for solidified matrix containing Fe, Sr, Co, and mixture of salt are presented in Figures (3, 4, 5 and 6) respectively. It is shown that a slight shifting in basal spacing took place. Moreover, the change in diffraction intensity is appreciable. from Figure (3). It is clear there are new peaks appeared and some peaks are broadened. This can be explained by precipitation of Fe+2 ions on the surface of cement particles in the form of CSH gel (C-Fe-S-H) [19].
Figure (4) shows the XRD pattern of cement with SrCl2 additive. It is clear that there is a decrease in the intensity and in the number of peaks.
In case of Co+2 and mixture of salt (Figures (5, 6)) there is a shifting in basal spacing, new peaks appeared and a decrease in the number of peaks due to the incorporation of these ions into the lattices of CSH gel [19].
Figure (3) XRD Pattern Fe (II) Ions on Cement Figure (4) XRD Pattern Sr (II) Ions on Cement
Figure (5). XRD Pattern Co (II) Ions on Cement Figure (6). XRD Pattern Mixture of Salt on Cement
2- SEM Results
The images of hydrated cement, cement after addition of Fe2+, Sr2+, Co2+, and mixture of salts (Fe, Sr, and Co) are presented in Figures (7.A–E). Figure (7.A) shows portlandite needle-shaped silicate crystals which are the main crystalline products in the hydration of cement.
A – Cement B – Iron
C- Strontium
D- Cobalt E- Mixture of Salt
Figures (7.A–E) SEM Analyses
Figure (7.B) indicates that the surface of cement particles is covered by Fe+2. It may be explained by the fact that the hydration of cement can be modified by heavy metals due to coating around cement grains [20]. Also the structure remains crystalline. Figure (7.C) shows Sr+2 incorporation into the hydrated cement results in a poor crystalline structure. Figure (7.D) shows the morphology of cement after precipitation of Co+2. It is densely covered with portlandite or needle-shaped silicate crystals. Figure (7.E) shows the cement after precipitation metals salts of mixture. There is a clear indication for the precipitation of mixture of salt on cement. This phenomenon reveals a structure as a new bulky on the surface of cement. The mixture fixation on the cement is different from that of hydrated cement in the morphology.
3. Leaching Test.
3.1- Leachability of Cement Blocks without Additives.
Figure (8) illustrates the results of the leaching test for cement blocks without additives. The variation of the five parameters; % change in weight, pH, ORP, COND and TDS as a function of time for blocks with different water to cement ratio is shown.
Figure (8) illustrates the variation of the cement block weight with time. It is clear that the weight increases as the time increases. For the same period of the time, the weight increases as the W/C ratio increases. This can be attributed to the penetration of water through the pores of the blocks. It can also be said that as the W/C ratio increases the porosity of cement block increases. Penetration of water onto the pores is not a passive event but it indicates that some reactions take place. This is reflected on the change of the leachate characteristics (pH, ORP, COND and TDS). The general behavior of these parameters is an increasing function of time and of W/C ratio. This can be explained by the dissolution of one of the constituents of the cement [Ca(OH)2] in the water which penetrates into the pores leading to the increase of the concentration of this compound in the leachate [21].
Figure (8) Effect of Leaching Process on Some Parameters.
A- % Change in weight of Cement block with Time, B- Change in pH with Time,
B- Change of OPR with Time, D-and E- Change in COND and TDS with Time.
3.2- Leachability of Cement Blocks with Fe Additives.
Figure (9) shows the leaching results of cement blocks containing different concentrations of Ferric Chloride (FeCl3); 6.16×10-3, and 4.6×10-2 mole/L. Figure (9,A), shows the increase of the cement blocks weight as the leaching time increases. For the two FeCl3 concentrations, the effect of concentration on the weight is nearly negligible. The same behavior is realized for the pH, COND and TDS Figure (9,B,D,E). This indicates an increasing function of the time with a very slight effect of FeCl3 concentration. The ORP change as a function of time Figure (9,C) shows an appreciable effect of the FeCl3 concentration. For higher concentration the change of the ORP from negative values to positive values is higher for the higher concentration. This can be explained by the higher tendency for reaction in case of higher concentration than the lower concentrations [21].
Figure (9) Effect of Leaching Process on Cement Blocks
(W/C 0.5, mixed with FeCl3 at Different Concentrations).
A - % Change in Weight with Time, B- Change in pH with Time,
C- Change OPR with Time,D and E- Change of COND and TDS with Time.
3.3- Leachability of Cement Blocks Containing Sr additives.
Figure (10), illustrates the leachability of cement blocks containing different concentrations of SrCl2; 1×10-3, and 1×10-2 mole/L on the same parameters.
from Figure (10, A) it is clear that the weight change is an increasing function of time as well as of SrCl2 concentration. This can be explained by the release of Sr2+ ions into the solution which offers more pores filled by water. This also results in an increase in the values with the time in case of the higher concentration by the effect of Sr2+ ions and Ca2+ ions that released in the solution.
Concerning the change of the ORP with time, it goes from negative to positive values. The lower concentration shows the lower values than the higher concentration. This indicates that the higher concentration results in a higher release of free ions from the cement blocks into the solution. The same behavior is realized for both COND and TDS.
Figure (10) Effect of Leaching Process on Cement Blocks
(W/C 0.5 and mixed with SrCl2 with Different Concentrations).
A- % Change in Weight with Time, B- Change in pH with Time,
C-Change in OPR with Time, and D and E - Change in COND and TD with Time.
3.4 - Leachability of Cement Blocks Containing Co Additives.
Figure (11), shows the leachabiltiy results of cement blocks containing CoCl2 of concentration equals 2.4×10-2 mole/L. It can be shown that the increase in the weight reaches approximately 30% of the initial weight after the 100 days. This is the same case for all concentrations of FeCl3 and the higher concentration of SrCl2. The pH, ORP, COND and TDS are increasing functions with time but not similar in case of FeCl3 and SrCl2. This can be due to the co-precipitation of Cobalt into the pores which hinders the release of inorganic elements from cement blocks.
Figure (11) Effect of Leaching Process on Some Parameter Cement Blocks
(W/C 0.5, and mixed with CoCl2).
A- % Change in Weight with time, B- Change in the pH in the Solution with Time,
C- Change in OPR with Time, and D and E- Change in COND and TD Respectively with Time.
3.5- Leachability of Cement Blocks Containing a Mixture of Chlorides of Different Elements.
Figure (12) illustrates the results of the leaching process in the presence of a waste mixture containing; 4.6×10-2 mole/L FeCl3, 1×10-2 mole/L SrCl2, and 2.4×10-2 mole/L CoCl2. As shown in the Figure (11), there is a continuous increase in weight with the time. pH values are an increasing function of time. This is attributed to the presence of traces of Ca(OH)2, Fe ions and traces of Sr ions released from the cement blocks, pH reaches nearly 14.0. ORP increases with time from the negative to the positive values but lower than in case of FeCl3 and SrCl2 separately. Both of COND and TDS increase with time with higher values than in case of FeCl3 and SrCl2 separately.
Figure (12). Effect of Leaching Process on Cement Blocks
(W/C 0.5, and mixed with the three salts)
A- % Change in Weight with Time, B- Change in pH with Time,
C- Change in OPR with Time, and D and E- Change in COND and TDS with Time.
Chemical Analysis.
Table (2), lists the results of the chemical analysis of leachate from cement and sand clay after 21 days with different salts concentrations.
Table (2). Chemical Analysis of Salts in the Solution with Sand Clay and Cement Blocks.
Salt of Initial
Concentration (Con.)
(mole/L) in
Solution Retained
(Con.) in Soil
(mole/L) %
(Con.)
In
Solution Retained
(Con.) in Cement
(mole/L) %
(Con.)
In
Solution
Fe 4.6E-02 4.59E-02 0.001% 4.59E-02 0.001%
Sr 1.00E-02 5.421E-03 45.87% 7.49E-03 25.14%
Co 1.00E-02 9.99E-03 0.001% 9.99E-03 0.001%
Mixture
Fe
Sr
Co
4.6E-02
1.00E-02
1.00E-02
4.59E-02
8.63E-03
9.983E-03
0.001%
13.69%
0.002%
4.59E-02
8.66E-03
1.0 E-02
0.001%
13.35%
0000
Understanding the long-term behavior of contaminants is of more importance in disposal site. The pervious results indicates that when water containing contaminants gets in contact with soil a part of the contaminants is retained by the soil and other part remains in the water. This phenomenon is quantified by the distribution coefficient (Kd).
The distribution coefficient (Kd) is the retention of salts on 1g of solid phase divided by the quantity of the same salt in 1 ml of liquid phase. Kd is an important parameter to understand the behavior of radionuclides in a media. It is difficult to take Kd values from literature (generic values). When this is the only alternative, it introduces a great deal of uncertainty. Therefore, the following experiments are used to determine the distribution coefficient of salts.
Figure (13), shows the retention of salts on soil and cement in case of the presence of a single salt in the solution and in case of using a mixture of salts. As shown, the retained concentration of salts on the soil and cement are similar in both cases of using a single or mixed salts except with Sr salt, which shows lower value on soil than that on cement (in case of a single salt). The comparison of the quantity retained of salts in case of a single and a mixture illustrates in Figure (14).
انت
Figure (13). Comparison between the Retention
Concentration of Salts on the Soil and Cement.
a- Single Salt, b- Mixed Salt.
Figure (14). Comparison between the Retention
Concentration in Case of Single and Mixed Salt.
a- In Soil, b- In Cement.
CONCLUSIONS
Because all components of the disposal system are porous materials, the natural and engineered barriers are used to decrease the leakage of waste into the surrounding environment. The safety performance of disposal site is carried out through the safety assessment methodologies. Mathematical models and computer codes are the tools to evaluate the possible release rate of radionuclides from disposal site. Since the chemical reaction of radionuclides with different media of the disposal site the components of the system with the surrounding environment are not considering in these mathematical models. The results obtained are significantly different than the real behavior of radionuclides. Therefore, the current study focuses on the effect of chemical reactions between different media (waste, cement, and soil) in the disposal system by considering some parameters such as; pH, ORP, COND, and TDS.
XRD analysis indicates that the salts incorporated into the cement have a strong effect on the crystalline form. Additionally, SEM analyses provide very valuable insight onto the physical/chemical characterization of the microstructure and morphology of the solids. from the leaching processes, all cement blocks weight increase with time due to the absorption of water inside the pores and pH rises as a result of releasing Ca(OH)2 from cement blocks. Since the potential reduction depends on pH, an increase of ORP occurs with time for all cement blocks. Also both of the COND and TDS increase with time due to the release of inorganic elements from cement blocks. On the other hand, reactions of metals ions with soil are important in determining their effects on the environment. Accordingly, the results of soil reactions with leaching solution shows that the equilibrium of the reaction dependent on time and concentration of elements.
Finally, from experimental, the retention values of Fe and Co in case of single salt on soil have nearly the same values with cement. On the other hand, the retained quantity of Sr as a single salt is lower on soil than on cement. Meanwhile the retained values of mixed salts show nearly same values in both cases on soil and cement. Additionally, the value of distribution coefficient experimentally calculated is higher than the generic values. Except in the case of Sr, Kd is nearly the same value in both cases.
The chemical reaction occuring in the disposal site medium and the surrounding geosphere affects the migration behavior of radionuclide. The present study is considered as a first part to study some chemical indicators. Further studies will be followed.
REFERENCES
1- IAEA safety standards series No. SSR-5, disposal of radioactive waste. Specific safety requirements, Vienna, 2011.
2- Hidrothermal alteration of “La Serrata” bentonita (Almeria, Spain) by alkaline solutions S. Ramírez, J. Cuevas, R. Vigil, and S. Leguey,. Applied Clay Science 21, (2002) 257-269.
3- Testing methodology for pH determination of cementitious materials. Application to low pH binders for use in HLNWR. R&D on low-pH cement for a geological repository A. Hidalgo, J.L. García, M.C. Cruz, L. Fernández, and C. Andrade,. Workshop Junio (2005) Madrid.
4- Formulating a low-alkalinity cement for radioactive waste repositories.C. Cau Dit Coumes, S. Courtois, S. Leclercq, X. Bourbon, Atalante June 21-25 (2004) 3-6.
5- Low-pH grouting cements-results of leaching experiments and modelling. R&D on low-pH cement for a geological repository. U. Vuorinen, and J. Lehikoinen, Workshop Junio (2005) Madrid.
6- Selective stabilization of deep core drilled boreholes using low-pH cement. R&D on low-pH cement for a geological repository. T. Hugo-Persson, B. Lagerblad, and C. Vogt. Workshop Junio (2005) Madrid.
7- A study of the effects of an alkaline plume from a cementitious repository on geological materials. M.C. Braney, A. Haworth, N.L. Jefferies, and A.C. Smith, Journal of Contaminant Hydrology, 13 (1993) 379-402.
8- Modelling of the corrosion of the cement paste by deionized water.F. Adenot, and M. Buil, Cement and Concrete Research, 22 (1992) 489-496.
9- Cements in radioactive waste disposal. F.P. Glasser, and M. Atkins Material Research Society Bulletin, 12 (1994) 33-39
10- Physico-chemical transformation of sulfated compounds during the leaching of highly sulfated cemented wastes. P. Lovera, F. Adenot, M. Jorda, and R. Cabrillac, Cement and Concrete Research, 57 (27) (1997) 1523-1532.
11- Long-term behaviour of cement pastes used for nuclear waste disposal: review of physico-chemical mechanisms of water degradation.P. Faucon, F. Adenot, J.F. Jacquinot, J.C. Petit, R. Cabrillac, and M. Jorda, Cement and Concrete Research, 28 (1998) 847-857.
12- Lippincott, A.editor (1997). Atlas of Idaho’s wildlife. The Idaho Department of Fish and Game, The Nature Conservancy, & Idaho Gap Analysis Project, joint publishers. Groves, C. R., Butterfield, B., Lippincott, A., Csuti, B., & Scott, J. M.; -Biology section: amphibian, reptile, bird, and mammal base information.
13- Concrete Liquid Retaining Structures Design, Specification and Construction. Green, J. K., and P. H. Perkins. Applied Science Publishers Ltd., London, England, 1979.
14- Oxidation-Reduction Potential for Water Disinfection Monitoring, Control, Trevor V. Suslow, and Documentation, University of California Davis, http://anrcatalog.ucdavis. 2004edu/pdf/8149.
15- Conductivity Analyzers and Their Application. Gray and James R. (2004). In Down, R.D; Lehr, J.H. Environmental Instrumentation and Analysis Handbook. Wiley. pp. 491–510. ISBN 978-0-471-46354-2. Retrieved 2009-05-10.
16- Handbook of Drinking Water Quality (2nded.). John Wiley and Sons. DeZuane, John (1997). ISBN 0-471-28789-X.
17- Interferences of Cement Based- Solidification/Stabilization and Heavy Metals: A Review, Khitam Abdulhussein Saeed, Khairul Anuar Kassim, and Amin Eisazadeh, EJGE Vol. 17 [2012], Bund. S.
18- X-Ray Diffraction and SEM Investigation of Solidification/ Stabilization of Nickel and Chromium Using Fly ash. RANJANA A. PATILand SANGESH P. ZODAPE. ISSN: 0973-4945; CODEN ECJHAO. E-Journal of Chemistry. 2011, 8(S1), S395-S403.
19- Cement-Based Solidi_cation/Stabilization of Heavy Metal Contaminated Soils with the Objective of Achieving High Compressive Strength for the Final Matrix. H. Ganjidoust, A. Hassani1 and A. Rajabpour Ashkiki. Transaction A: Civil Engineering Vol. 16, No. 2, p 107{115}.Sharif University of Technology, April 2009.
20- Leachability of Pb- Doped solidified waste forms using Portland cement and calcite .II Investigation of SEM\EDS. Dong JIN Lee. Envion.Eng. Res. Vol.9, No., 2, pp. 66-74, 2004. Korean society of envriomrntal engineers.
21- Application Bulletin. Oxidation reduction potential (OPR)\REDOX and free chlorine. Water Quality Instrumentation Accuracy Reliability Simplicity Printed in U.S.A.2012.
22- Cement/clay interactions – A review: Experiments, natural analogues, and modeling, Eric C. Gaucher , Philippe Blanc, BRGM, Avenue C. Guillemin, BP 6009, 45100 Orleans Cedex, France,2006
23- Environmental attack on concrete, D. Bonen, and S.L. Sarkar, Proc 16th Eng Found Conf, Am Soc Civil Eng New York, 1994, pp. 11–23.
24- Improved Sorption Ability for Radionuclides by Cementitious Materials, K. Noshita, T. Nishi, and M. Matsuda, WM’98 Proc, Tucson, AZ, USA, 1998, pp. 1880–1886.
25- Reactions of tobermorite gel with aluminates, ferrites, and sulfates, L.E. Copeland, E. Bodor, T.N. Chang, and C.H. Weise, J. PCA Res.Dev. Lab. 9 (1) (1967) 61–74.
26- Subsurface cobalt-60 migration from a low- level waste disposal site. J. L. Young, 1984.